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 ES Home > Vol. 3, No. 2 > Art. 5

Copyright ©1999 by The Resilience Alliance*

The following is the established format for referencing this article:
Rockström, J., L. Gordon, C. Folke, M. Falkenmark, and M. Engwall. 1999. Linkages among water vapor flows, food production, and terrestrial ecosystem services. Conservation Ecology 3(2): 5. [online] URL: http://www.consecol.org/vol3/iss2/art5/


A version of this article in which text, figures, tables, and appendices are separate files may be found by following this link.

Research

Linkages Among Water Vapor Flows, Food Production, and Terrestrial Ecosystem Services

Johan Rockström1,2, Line Gordon2, Carl Folke2,3, Malin Falkenmark4,5, and Maria Engwall2


1RELMA; 2Stockholm University; 3Beijer International Institute of Ecological Economics; 4Swedish Natural Science Research Council; 5Stockholm International Water Institute (SIWI)




ABSTRACT

Global freshwater assessments have not addressed the linkages among water vapor flows, agricultural food production, and terrestrial ecosystem services. We perform the first bottom-up estimate of continental water vapor flows, subdivided into the major terrestrial biomes, and arrive at a total continental water vapor flow of 70,000 km3/yr (ranging from 56,000 to 84,000 km3/yr). Of this flow, 90% is attributed to forests, including woodlands (40,000 km3/yr), wetlands (1400 km3/yr), grasslands (15,100 km3/yr), and croplands (6800 km3/yr). These terrestrial biomes sustain society with essential welfare-supporting ecosystem services, including food production. By analyzing the freshwater requirements of an increasing demand for food in the year 2025, we discover a critical trade-off between flows of water vapor for food production and for other welfare-supporting ecosystem services. To reduce the risk of unintentional welfare losses, this trade-off must become embedded in intentional ecohydrological landscape management.

KEY WORDS: catchment management, ecohydrological landscape, evapotranspiration, food production, freshwater management, global freshwater assessment, resilience, terrestrial ecosystem services, trade-offs, water use efficiency, water vapor flows.

Published August 6, 1999.


INTRODUCTION

Earth is a human-dominated planet. The well-being of humanity is intimately dependent upon the ecological life-support systems now undergoing rapid changes (Vitousek et al. 1986, Lubchenco 1998). The capacity of ecological systems to continuously supply a flow of nature's services to humanity is largely taken for granted (de Groot 1992, Daily 1997), despite the fact that this capacity is increasingly becoming a limiting factor for socioeconomic development (Odum 1989, Folke 1991, Jansson et al. 1994).

In many areas, both locally and regionally, available freshwater is already a limiting factor for industrial development, household needs, and irrigation of crops (Gleick 1993, Falkenmark 1997). An estimated 25% of the world's food market is at present driven by water scarcity, i.e., food is imported due to insufficient irrigation water for local food production (Postel 1998). A recent analysis indicates that 55% of the world population in 2025 will live in countries incapable of self-sufficient food production, due to lack of water for irrigated agriculture (Falkenmark 1997). Furthermore, water quality deterioration caused by human activities is diminishing the quantity of freshwater available to society (Lundqvist 1998). Recent estimates indicate that humanity appropriates for industry, households, and irrigated agriculture 54% of the global accessible runoff flow (Postel et al. 1996).

However, freshwater - the bloodstream of the biosphere - is also needed to drive critical processes and functions in forests, woodlands, wetlands, grasslands, croplands, and other terrestrial systems, and to maintain them resilient to change. These systems generate numerous essential ecosystem services, including biomass production in agriculture and forestry (Costanza et al. 1997). Surprisingly, past international global freshwater assessments of whether or not humanity is heading toward regional and even a global water crisis, have neglected the water vapor flows supporting the generation of ecosystem services (Gleick 1993, UN-SEI 1997). Generally, it is only the liquid runoff water, moving across the continents in rivers and as groundwater flow, that is perceived as the freshwater resource for socioeconomic development. Even if there is reason to be concerned over future liquid water use, by far the largest proportion of terrestrial production of food, biomass, and the generation of other ecosystem services originates from rain-fed land use. As an example, around two-thirds of the world's food, harvested from 83% of the world's croplands, is derived from rain-fed production (Gleick 1993).

In this article, we perform the first bottom-up calculation of continental water vapor flows. The estimate is generalized from field studies of water vapor flows from different biomes, focusing in particular on croplands, grasslands, forests, woodlands, and wetlands, biomes of great significance for the generation of terrestrial ecosystem services. The estimate includes calculations of a range of water requirements for terrestrial biomes, depending on water management and annual climatic variations.

We begin to address the complex, but largely neglected, issue of the interplay among water vapor flows, agricultural food production, and the generation of ecosystem services in terrestrial biomes. Our findings highlight the fact that the critical issue of how to feed a growing human population through agricultural food production cannot be tackled in isolation from the freshwater-dependent generation of ecosystem services in the surrounding landscape.


INVISIBLE GREEN WATER VAPOR AND VISIBLE BLUE LIQUID WATER

In the Introduction, we distinguished between water vapor flows and liquid water flows. In the literature on water and food production, water vapor and liquid water are sometimes called green water and blue water, respectively (Falkenmark 1995). Both concepts provide useful tools for the analysis of local, regional, and global flows in the hydrologic cycle. Liquid (blue) water flow is the total runoff originating from the partitioning of precipitation at the land surface (forming surface runoff ) and the partitioning of soil water (forming groundwater recharge) (Fig. 1). Water vapor (green) is the return flow of water to the atmosphere as evapotranspiration (ET), which includes transpiration by vegetation and evaporation from soil, lakes, and water intercepted by canopy surfaces (Rockström 1997). We regard ET as the result of the work of the whole ecosystem, including the resilience it needs for securing the generation of ecosystem services in the long run.


Fig. 1. The hydrological cycle, showing the repartitioning of rainfall into vapor and liquid freshwater flow (modified from Jansson et al. 1999).

GIF image file (51 K)


Previous estimates, e.g., L'vovich and White (1990), have calculated ET indirectly as the difference between precipitation, P, over continents (110,305 km3/yr) and total runoff, R, (38,230 km3/yr), arriving at 72,075 km3/yr. It should, however, be noted that in areas where data on rainfall and river flow did not exist, the estimates were done using the six component model developed by L'vovich (1979), in which runoff is estimated from regression curves related to rainfall, and the partitioning between drainage and evapotranspiration is through proportionality curves specific for different biomes.


ESTIMATING WATER VAPOR FLOWS OF MAJOR TERRESTRIAL BIOMES

The distribution of ecosystems on a global scale is, to a large extent, governed by climatic factors including water availability, but it can also be influenced by natural or human-induced disturbance regimes. When water is in free supply, the ET from a complete green canopy of standard crop can be predicted directly from climatic factors (Thornwaite and Mather 1955, Penman 1963). This is called potential ET. The actual ET of an ecosystem, however, is dependent on (1) the water supply, limited by the amount of precipitation, on-flow of water, and ability to store water in the system; and (2) the processes in an ecosystem that modify the amount of water flowing in and out from the system. These processes, necessary for the generation of ecosystem services, include the development of deep or shallow rooting structures, transformation of topography, and changes in size of leaf area, and they are largely dependent on the quality of the soil. The ET of an ecosystem is thus not only a factor of climate but also a result of the ability of the biota to modify the available water flow.

We based our calculations on spatial coverage, multiplied by annual ET (in millimeters peryear) of each system, and subdivided them as far as possible according to ecosystem properties influencing ET, i.e., primarily vegetation cover and climate (Table 1).


Table 1. Total water vapor estimates with the classification of biomes and vegetation subgroups.

Biome Vegetation subgroups Climatic zone Land surface a nk Actual evapotranspiration (mm/yr) Water vapor estimates (km3/yr)
      1000 km2   Mean Low High Mean Low High
Forest, woodlands taiga boreal 11,560 3 401 380 420 4636 4393 4855
  predominantly coniferous temperate 3500 4 487 395 580 1705 1383 2030
  predominantly deciduous temperate 8500 4 729 588 964 6199 4998 8194
  woodland/woody savannah temperate 5200 3 416 300 530 2165 1560 2756
  forest, dry/deciduous/seasonal tropical/subtropical 7400 2 792 783 800 5857 5794 5920
  forest, wet tropical/subtropical 5300 3 1245 880 1493 6600 4664 7913
  savannah/woodland, dry tropical/subtropical 12,700 2 882 870 894 11,201 11,049 11,354
  savannah/woodland, wet tropical/subtropical 1300 3 1267 1100 1500 1647 1430 1950
Subtotal 40,009 35,271 44,972
Wetland bog boreal 651 3 221 200 260 144 130 169
  bog temperate 488 4 674 456 1020 329 223 498
  swamp temperate 41 3 843 670 720 35 27 30
  swamp subtropical 16 5 1127 930 1277 18 15 20
  swamp b tropical 508 1 1656 1408 1904 841 715 967
Subtotal 1366 1110 1684
Grasslands cool grassland mostly temperate 6940 16 410 130 633 2843 900 4393
  mountainous grassland temperate 650 4 655 430 951 426 280 618
  warm and hot grassland mostly tropical 17,300 7 599 403 862 10,356 6967 14,913
  mountainous grassland c tropical 650 1 600 402 798 390 261 519
  dry shrubland tropical 4000 2 270 225 315 1080 900 1260
Subtotal 15,095 9308 21,702
Production (103Mg/yr) d n Water Use Efficiency (m3/Mg) e
Croplands cereals, grain temperate 790,476 15 1309 539 2643 1095 564 1919
  cereals, grain tropical 625,409 10 1438 591 4369 764 555 1178
  cereals, total DM f temperate 4011 19 438 240 646 56 31 83
  cereals, total DM f tropical 664,404 3 331 271 372 235 217 247
  cotton lint   18,509 3 5454 4227 6313 101 78 117
  cotton seed   86,925 1 2083 1667 2500 181 144 217
  fibers   5541 4 574 278 870 3 2 5
  forage   725,032 19 934 172 2810 641 249 1403
  fruit temperate 208,348 2 269 163 375 58 34 78
  fruit tropical 232,748 3 259 150 350 60 35 81
  natural rubber/gums   6088 2 30,137 29,167 31,108 183 178 189
  nuts g   6929 1 415 200 1080 2 1 6
  oil-bearing crops temperate 35,454 3 1892 1530 2117 64 54 72
  oil-bearing crops tropical 55,225 2 3083 2667 3500 71 70 92
  oil palm h   6,604,000 km2 1 1500 mm 1250 mm 1750 mm 242 212 278
  pulses, dry seed temperate 43,493 3 3355 1731 5833 157 75 197
  pulses, dry seed tropical 166,338 5 1866 1250 3003 283 214 370
  pulses, green seed temperate 9326 2 1149 583 1714 12 5 16
  rice   540,838 4 1099 839 1404 594 454 759
  roots and tubers temperate 558,137 7 286 139 402 144 94 217
  roots and tubers g tropical 330,786 1 616 369 1299 204 122 430
  roots, tubers for fodder f temperate 11,105 8 326 157 616 4 2 7
  spices i   4091 0 1000 800 1500 4 3 6
  stimulant crops   791 3 4515 2083 6983 33 28 74
  sugar cane   1,120,898 3 123 100 163 138 112 182
  vegetables j   549,683 6 147 35 500 75 22 242
Subtotal 5404 3552 8427
Total 61,879 49,280 76,800

a Land surfaces for forests and grasslands are derived from Olson et al. (1983); land surfaces for wetlands are from Matthews (1983).

b Low/high values are based on the mean +/- 15%.

c Low/high values are based on the mean +/- 33%, based on the average standard deviation of the other subgroups in grasslands.

d Production data are from the FAO (Faostat 1997).

e Note that only the aggregate average WUE values for similar crops are presented in Table 1. For example, the WUE values and the water vapor estimates for cereals/temperate in Table 1 are derived from individual values for each major cereal (wheat, barley, oats, rye, and buckwheat).

f The WUE was calculated based on the total dry matter yield.

g The mean WUE comes from only one article; the low and high are the variations within that article.

h For oil palms, which are harvested on the same area for oil, kernels, and fruit, the total freshwater used was calculated by

area harvested = oil produced (Mg)/production of oil (Mg/ha)

total water vapor used = area harvested (km2) x ET from palm stands (m)

Palm oil production (6,603,778 Mg) was collected from the FAO (Faostat 1997). The production was assumed to be 1.75 Mg/ha (Mémento de l´Agronome 1984). The ET from palm stands is 1500 mm (Jackson 1989). The variation of ET was assumed to be 125 mm. The minimum and maximum calculation for oil palm was therefore based on the ET rates of 1375 mm and 1725 mm. The same values were used for low/high calculations.

i Spices is a small group, covering only about 0.045% of the total global area harvested each year (Faostat 1997). There is also a large variation in species composition, as well as in parts of the plant used for production measurement. The WUE was, therefore, based on a qualified assumption. The WUE was assumed to be 1000 m3/Mg. The minimum was assumed to be 800 m3/Mg, the maximum as 1500 m3/Mg, and the standard deviation as 200 m3/Mg. Spices are often just a small part of a plant, why the WUE will be higher if the whole plant is considered.

j For vegetables that were produced as feed, the WUE was calculated based on the ”grain” yield of that specific crop or group of crops.

k The number of references.


For croplands, a somewhat different method was used, because they are located in a wide range of climatic regions (from tropic to boreal and from arid to humid), vary highly in production intensities, and there are detailed data on production (yield x surface area) and water requirements of production. By crop production, we refer to actual harvest and not to potential crop production. Biomass production in croplands is roughly linearly proportional to ET for constant hydroclimatic conditions, when water is not limiting growth (Sinclair et al. 1984). The slope of the relationship between biomass growth and ET is defined as the water use efficiency (WUE). WUE has, however, been defined in various ways in the literature, commonly as the amount of transpired water per yield unit, or the amount of water applied (through irrigation) per yield unit.

We have used this relationship in the calculation of water vapor flow from croplands, multiplying the annual crop production (in megagrams per year of harvested economic biomass; 1 Mg = 1 ton) by WUE estimates (cubic meters per megagram). The WUE data for each subgroup in Table 1 derive from a broad number of sources (n in Table 1; fully specified in Appendix 1).

The actual water vapor flow for each subsystem will vary in space and over time, due to climatic fluctuations, different biotic and abiotic conditions, and different land management practices. We have taken into account the effects of such variations on water vapor flows by calculating a high and low estimate for each subgroup, based on the lowest and the highest ET or WUE data.

Grasslands, wetlands, woodlands, and forests

Surface extensions of the biomes shown in Table 1 were derived from those in Carbon in Live Vegetation of Major World Ecosystems (Olson et al. 1983), except for wetlands, for which we used Matthews' (1983) Global Database on Distribution, Characteristics, and Methane Emission of Natural Wetlands.

Grasslands include all noncultivated formations with <10% tree canopy cover, thus including natural grazing land, pastures, and shrubland. Woodland is a wide description, with various densities of trees and tree canopy coverage between 10% and 99%. Forests are defined by tree canopy coverage of 100% (Olson et al. 1983). Wetlands include bogs/fens and swamps/marshes, and are here defined as permanently or seasonally inundated areas, forested or nonforested.

Based on the subclasses from Olson et al. (1983), some reclassifications were made (Table 1). In wetlands and forests/woodlands, vegetation type and climate interact in the generation of ET (Mitch and Gosselink 1983, Nulsen et al. 1986); these biomes were thus classified in subgroups according to those variables. For grasslands, we have assumed that total ET depends primarily on climatic factors rather than on vegetation cover, although the relation between evaporation and transpiration can vary (Penning de Vries and Djitèye 1982, Liang et al. 1989). For warm and hot grasslands with annual precipitation of P < 600 mm/yr, we assume that ET = P (i.e., that there is no liquid water flow). This assumption is valid for dry grasslands on a large spatial scale (le Hourerou 1984). For grassland systems with P > 600 mm/yr, 20% runoff is assumed. The ET data for each subgroup derive from a broad number of peer-reviewed sources (indicated under n in Table 1 and fully specified in Appendix 2).

Croplands

Agricultural ET was estimated from mean crop production data over a period of five years (1992 -1996) using individual crop data from FAO (Faostat 1997). The time span was included to reduce the effects of interannual yield fluctuations.

WUE data were collected for each major food crop. All crops were classified into 16 subgroups according to key parameters influencing WUE, i.e., hydroclimate, plant community, and the harvested part of plant (grain, fiber, fruit, etc.). Special attention was given to ensure that the WUE values corresponded to the economic yield registered in Faostat. WUE data from several research sites were included for each major crop and subgroup in order to reflect the variability in ET for different agricultural settings (see Appendix 1).

These calculations for croplands cover ET requirements to produce the harvested economic yield. Added to this flow is the ET water from other non-economic vegetation in agricultural lands. Here, non-economic vegetation includes weeds and vegetation in open drainage ditches, green enclosures, and wind breaks. This vegetation can, however, support ecological services in that it can, for example, contribute to nutrient retention in the landscape and provide a habitat for insects that may be important for pollination and predation of pests (Matson et al. 1997). Earlier efforts at estimating this share of the water cycle are very rudimentary. For example, the total net primary production (NPP) in croplands used in Postel et al. (1996) and Postel (1998), and based on Vitousek et al. (1986) and Ajtay et al. (1979), includes only NPP from crops grown for harvest. The assumption that the total annual ET, based on this NPP multiplied with a global average WUE, would reflect the actual ET from the world's croplands seems to be a very rough estimate. We have not found any global estimate of NPP in croplands coming from weeds, drains, ditches, etc., nor an estimate that covers the ET from this production. Thus, we assume that 10% of the average annual rainfall over land surfaces (834 mm/yr), i.e., roughly 80 mm/yr, supports non-economic biomass growth in agricultural lands. Even though our estimate, based on the assumption that 10% of the precipitation on croplands supports such production, is crude, it seems more reasonable than previous estimates.

Results of water vapor estimates of major terrestrial biomes

The estimate resulted in a total water vapor flow from forests, woodlands, wetlands, grasslands, and croplands of 63,200 km3/yr (Table 2). We estimated the total water vapor flow from grasslands to be 15,100 km3/yr (range 9300 to 21,700 km3/yr); from forests and woodlands to be 40,000 km3/yr (range 35,300 - 45,000 km3/yr); and from wetlands to be1400 km3/yr (range of 1100 - 1700 km3/yr) (Table 1). The total water vapor flow in the world's croplands for crop production was estimated as 5400 km3/yr, with low/high values ranging from 3600 to 8400 km3/yr. Adding ET for non-economic plant growth on agricultural lands of 1300 km3/yr gives a mean water vapor flow of 6700 km3/yr, ranging from 4900 to 9800 km3/yr.


Table 2. Bottom-up estimate of global water vapor flows from the continents.

Water vapor source Our estimates (km3/yr) Earlier estimates (km3/yr) References
Major terrestrial biomes        
Croplands   6800 2285 - 5500 Postel et al. (1996), Shiklomanov (1996), Postel (1998)
Temperate and tropical grasslands   15,100 5800 Postel et al. (1996)
Temperate and tropical forests, woodlands, and taiga   40,000 6800 Postel et al. (1996)
Bogs, fens, swamps, and marshes   1400    
  Subtotal 63,200 14,885 - 18,100  
Other systems        
Green areas in urban settlements   100 100 Postel et al. (1996)
Upstream rural water use   210    
Lake evaporation   600 600 L´vovich (1979)
Evaporation from large reservoirs (>100 x 106 m3)   130 130 L´vovich and White (1990)
Evaporation from small reservoirs (<100 x 106 m3)   30    
Tundra and deserts   5700    
  Subtotal 6800 830  
  Total 70,000 15,715 - 18,930  


We believe our estimates to be conservative, especially for agriculture. Our ET estimates for crop production only relate to harvested yield after reduction for threshing and post-harvest losses (which can amount to > 20% of the ET-demanding crop on the farmer's field). The WUE data used in this article originate from research stations that generally have more favorable cultivation conditions than does the farmer, which results in higher WUE values than under on-farm conditions. Our data show that, on average, some 1400 m3 of ET flow is needed to produce 1 Mg of cereal grain in the tropics. There are, however, many research findings suggesting that WUE is much lower in farmers' fields, often amounting to some 3000 - 6000 m3/Mg (Dancette 1983, Rockström et al. 1998). This is explained by relatively lower soil fertility, higher runoff losses, and less advanced land management practices on-farm, and will result in lower yields (< 1000 kg/ha in sub-Saharan Africa) and higher soil evaporation losses. This low WUE in agriculture is reflected by the high estimate in Table 1 of 8427 km3/yr for crops.

The aggregate estimate in Table 1 ranges from 49,000 to 77,000 km3/yr, which is roughly a deviation of 14,000 km3/yr from the mean. The large fluctuations in water vapor flow within the subgroups mainly reflect four different sources of variation: location, climatic fluctuations, land management, and random error. It is worth mentioning that the fluctuation of annual rainfall over land surfaces is of the same order of magnitude as our estimated water vapor fluctuations, and varies between 90,000 and 120,000 km3/yr. The considerable variations in water vapor use suggest that mean water vapor estimates, especially for agriculture, are of limited interest in assessing regional and global freshwater needs. The range includes parameters that we cannot influence (e.g., soil properties and hydroclimatic fluctuations), but also factors that we can influence through integrated land and freshwater management.

The large range also indicates that there is an important potential for improving WUE in agriculture. Crop management, such as choice of cultivars, planting density, crop protection, and soil and water management, will affect the ratio between ET and yield and, thereby, WUE. In soil and water management, care must be taken with nutrients and soil structure in order to minimize the effects of erosion and runoff. Variations in WUE for a specific crop species also illustrate the capacity of a certain crop to grow in a spectrum of hydroclimates (e.g., maize from humid to semiarid tropics).

Postel et al. (1996) estimated the freshwater requirements for the annual human appropriation of net primary production of grasslands to be 5800 km3/yr, and of harvested forest products to be 6800 km3/yr. Postel (1998) also estimated the annual human appropriation for total food production (including croplands, grazing lands, irrigation water losses, and aquaculture) to be 13,800 km3/yr.

In summary, earlier estimates suggest that humans depend on some 14,900-15,800 km3/yr (Table 2) of water vapor to support human-appropriated primary production. This corresponds to 21-22% of the top-down estimate by L'vovich and White (1990) of water vapor flow from continents (72,075 km3/yr). Our results indicate that the major terrestrial biomes appropriate as much as 88% of this water vapor flow.


ESTIMATING TOTAL WATER VAPOR FLOWS FROM CONTINENTS

As shown in Table 2, our estimated average water vapor flow from croplands, forests, woodlands, grasslands, and wetlands amounts to 63,200 km3/yr. By adding water vapor flows from remaining continental systems, we perform, to our knowledge, the first bottom-up calculation of total water vapor flows from continents. The estimate is generalized from field studies of water vapor flows from different biomes.

Evapotranspiration from green areas in urban settlements has been estimated at 100 km3/yr (Postel et al. 1996), and vapor flows from lakes account for an estimated 600 km3/yr (L'vovich 1979). Added to this is the complex grey zone of domestic water use by rural societies. The magnitude of this upstream rural water evaporating after use is difficult to estimate. If 82% of the population in developing countries (estimated from WRI 1994 and FAO 1995) is assumed to have a daily need, for domestic purposes, of 150 l p/d, an estimated 180 km3/yr is appropriated. The suggested domestic daily water use of 150 l p/d is taken as an aggregate of Shuval's estimate of roughly 25 m3 p-1 yr-1 (= 68 l p-1 d-1) needed for basic small-scale production of legumes, livestock, and chicken around homesteads in arid regions (Lundqvist and Gleick 1997), and Gleick's suggested basic household need of water amounting to 50 l p/d (Gleick 1996). 20 l p/d were added in order to reflect the water demand for animals in pastoral communities and large-scale livestock raising.

L'vovich and White (1990) estimated that the volume of water in small reservoirs amounts to some 5% of the volume in large reservoirs (about 5500 km3 when full). Based on this, we have estimated the vapor flow from small reservoirs as 30 km3/yr, by assuming an average depth of small reservoirs to 3 m and a vapor flow of 400 mm/yr. In Table 2, we include the vapor flow from small reservoirs in upstream rural water use. Large reservoirs (with a storage capacity > 100 x 10 6 m3) return an estimated 130 km3/yr of vapor flow to the atmosphere (L'vovich and White 1990).

Tundra and deserts, covering some 31 x 10 6 km2 of land (Olson et al. 1983), with an average annual ET of 180 mm (Frank and Inouye 1994), return approximately 5730 km3 water to the atmosphere each year. These biomes play a role in global climate and support local human populations and biota.

Adding evaporation from lakes, large and small reservoirs, and ET flow from green areas in human settlements, tundra, and deserts, and upstream rural water use gives a total water vapor flow of about 70,000 km3/yr (Table 2). This implies that our estimate generalized from field data of water vapor flows from a diversity of systems has captured 97% of previous global top-down and indirect ET estimates from continents. It should be noted, however, that this range might vary between 56,000 and 84,000 km3/yr (51-76% of annual mean rainfall) just by taking into account the variation of the major biomes (Table 1).

How much of this freshwater flow does humanity depend upon for terrestrial ecosystem services? Because ecosystems are complex systems linked dynamically across spatial and temporal scales, it is difficult to judge human water vapor dependence on a global level. There are those who believe that such a dependence should only be attributed to a particular service or to marginal changes in freshwater requirements between services and other human uses of freshwater. There are others who would argue that the water vapor requirement of the whole ecosystem is necessary for the generation of ecosystem services, at least in a longer term and sustainability perspective. In the following section, we will discuss interrelations between freshwater and terrestrial ecosystem services, and illuminate the many welfare-supporting ecosystem services that depend on complex ecosystem dynamics, which, in turn, depend on the bloodstream of the biosphere.


INTERRELATIONS BETWEEN WATER VAPOR FLOWS AND TERRESTRIAL ECOSYSTEM SERVICES

Physical and chemical processes provided by freshwater are fundamental. Water constitutes an essential building block in all terrestrial production, contributes to the processes that generate ecosystem services, and provides crucial interconnections within and between ecosystems. It works as a carrier of solutes, plays a key role in global, regional, and local climate regulation, and sets the ecohydrological conditions for biological diversity in any habitat.

Freshwater availability is a prerequisite in the production (e.g., crops, timber, cattle), information (e.g., nature experiences, aesthetic information), and regulation (e.g., formation of topsoil, sequestering of CO2, assimilation of nutrients) functions of the environment (de Groot 1992). These functions are defined as ecosystem services and include ecological processes that produce, directly or indirectly, goods and services from which humans benefit (Daily 1997).

Crops, trees, cattle, and other biomass production depend on accessible renewable freshwater. Nature requires water for food web support to wildlife and for maintenance of habitats in which they live. The processes of topsoil formation in forests and croplands and nutrient retention in wetlands involve water. Grassland systems develop patchy dynamics that respond to water availability by redistributing water and nutrients in the landscape for improved performance (Walker 1993). Ecosystem services of tropical rain forests depend both on water transpired by vegetation and on evaporation that supports species adapted to a moist environment.

Ecosystems are interconnected by liquid water and water vapor flows. Forests are linked to other systems such as grasslands and wetlands, both directly and indirectly, in ways in which freshwater plays a critical role. Freshwater directly transports mineral nutrients and organic matter between systems. Indirectly, freshwater supports services across ecosystems, such as the spreading of seeds, both directly by water and indirectly as water is needed to sustain a habitat for mobile organisms that spread seeds, and to sustain a habitat for bees and other insects that are important for pollination. The biota play an important role in the regulation of atmospheric water by redirecting liquid water to water vapor flow, thereby recycling it to local rainfall. This can be of great significance, e.g., in the Sahel region where > 90% of the rainfall appears to be attributed to ET flow from vegetated land surfaces (Savenije 1995). Furthermore, terrestrial ecosystems contribute to freshwater quality through biochemical processes such as denitrification and other forms of microbiological activity, and by facilitating infiltration, thereby moderating river flow seasonality, erosion, and flooding.

Freshwater is also required for ecosystem resilience. Resilience is the buffer capacity to disturbance performed by functional groups of species linked in complex temporal and spatial webs of interactions (Peterson et al. 1998). Dynamics of ecosystems (Holling 1986) and variability in water flow patterns can interact and respond to each other with feedback mechanisms at different temporal and spatial scales (Mitch and Gosselink 1983, Swank et al. 1988). Forest fires can cause huge runoff increases that may impact on downstream systems, as experienced in Australia (E. O'Laughlin, Canberra, Australia, personal communication). Resilience makes it possible for a forest to absorb a fire and maintain the potential to reorganize and recover, thereby continuing to supply ecosystem services essential to society, and also to reduce negative effects on downstream water-ecosystem services for other human uses. Similarly, grasslands have adapted to disturbances such as invasion of grazers or insects, fire, and periods of flooding or drought, and need the dynamic interactions of biological diversity to respond in a resilient fashion to these disturbances (Walker 1993). Freshwater is a key driverin these dynamics.

Putting freshwater in such an ecological context and in the light of data in Tables 1 and 2 suggests that the degrees of freedom for production of life support for the expanding world population is limited. There will be fundamental trade-offs between food production and other welfare-supporting ecosystem services in terms of available freshwater.


FRESHWATER, FOOD, AND ECOSYSTEM SERVICES FOR A GROWING HUMAN POPULATION

The per capita dependence on water vapor for production of food in croplands is roughly 1180 m3/yr, based on a population of 5.7 billion people in 1995 (UN 1997). Recognizing that the human population probably will reach 6 billion within the next few months, we used data from 1995, as they can be compared with the data on crop production that we have used, which refer to the years 1992-1996. Future demand for food will involve an increased appropriation in terms of additional water vapor flow for crop production. (Grasslands also provide food in terms of animal protein. However, because grazing is only one of multiple functions in the grassland system, and is also a process within the system, it would be misleading to try to estimate how much of the 2650 m3 p-1 yr-1 of water vapor estimated here from grasslands is attributed to cattle production).

L'vovich and White (1990) have estimated the changes in runoff during the past 300 years (1680-1980) caused by redirections of liquid water to water vapor flows through irrigation. Their results suggest that the water vapor flows have increased from 86 km3/yr to 2570 km3/yr during this period. In the coming 100 years, they estimate a further doubling in response to food production needs. Considerable changes in water vapor and liquid water flow patterns seem unavoidable.

Using the human population increase reported by the United Nations (UN 1997), i.e., an increase of 2.6 to 8.3 billion in 2025, and assuming a current per capita water vapor use for crop production, we calculate an additional water need of 3100 km3/yr in 2025. This would imply a total crop water demand in 2025 of about 9,800 km3/yr, a 31% increase in freshwater demand for crop production. Could we appropriate this amount of freshwater in a trade-off-free manner toward other terrestrial biomes? We have identified three possible options.

The first option, propagated by international organizations (e.g., FAO, UNDP, IIMI), is to increase irrigated agriculture. According to Shiklomanov (1997), the increase in ET in irrigated agriculture by 2025 would amount to 425 km3/yr, or about 14% of the additional freshwater demand. Because increased irrigation implies liquid-to-vapor redirection of freshwater and, thereby, a continuation of river depletion, the scope for solving future food shortages through irrigation alone, without causing severe impacts elsewhere, seems limited (Leah 1995).

The second option is to improve rain-fed agriculture (Falkenmark et al. 1998). There seem to be two major avenues. The first is to improve water-use-efficiency in crop production by redirecting in-field evaporation to transpiration within croplands, i.e., increasing the yields with the same amount of water vapor flow. It seems reasonable to assume a 10% overall increase in WUE as a result of e.g., better crop varieties, improved farming practices, soil fertility management, and soil and water conservation measures. This would diminish the future water needs by about 300 km3/yr. The second avenue is to redirect evaporating surface runoff for use in croplands. This option concerns water that now runs off from croplands and evaporates in areas of low biomass productivity and degraded lands, predominantly in semiarid and arid regions, i.e., water that never reaches rivers and does not contribute to the generation of ecosystem services. This water could be captured by surface-water harvesting and used for supplementary irrigation during dry spells (Rockström and Valentin 1997). This measure would not only conserve water but alsowould conserve soil by diminishing erosion caused by surface water runoff. A first-cut estimate of this option is arrived at by a comparison between surface runoff on a local scale from croplands vs. runoff on a continental scale, assuming an even distribution of croplands globally. The amount of water available for redirection from evaporating surface runoff in semiarid and arid regions for use in croplands is hard to estimate. We assumed that the difference in surface runoff coefficients between field scale and continental scale for croplands in Africa, Asia, and South America is attributed to evaporating surface runoff. An even distribution of croplands on the different continents was assumed. Croplands cover 10.5% of the global terrestrial area. The runoff water from croplands available for surface water harvesting in Africa, Asia, and South America would then be roughly 300 km3/yr. See Appendix 3 for data and references.

Thus, it may very well be that developments in irrigated and rain-fed agriculture cannot cover the full need of increased water appropriation for food production, actually only about one-third or 1000 km3/yr out of 3100 km3/yr, according to our first-cut estimate. Desalinization of seawater for food production is not a viable solution because the costs would be several factors higher than the price of the crops.

It seems as though the final option to feed another 2.6 billion world inhabitants until AD 2025, is to redirect substantial amounts of water vapor flows from other biomes to croplands. Intensifying the conversions of forests, woodlands, and, to some extent, grasslands and wetlands, to croplands in the tropics and subtropics is a likely development scenario. Assuming that the main part of the remaining freshwater demand would be appropriated from tropical/subtropical systems, their water vapor flows would decrease by 5.5% in only 25 years. Because most of the population growth will occur in the tropical region, this is also where the increase in food production primarily will occur. Thus, we divided the 2100 km3/yr of additional water vapor needed by the total water vapor from our estimates in tropical grasslands, forests, woodlands, and wetlands, which amounts to 38,000 km3/yr, thus resulting in a 5.5% increase.

There is a severe risk that further land use change to capture freshwater for crop production will lead to increasingly fragile, less diverse systems with lower resilience, and will cause subsequent erosion of ecosystem services. Will such redirections of water vapor increase or decrease total human well-being? The results of our estimate, in the light of an expanding human population and escalating globalization, illustrate that we are facing major challenges in freshwater-land use management. Management must explicitly deal with what we call the increasing water vapor-related scarcity. This "new scarcity," which concerns the critical trade-off between water vapor for ecosystem services generated by terrestrial biomes and water vapor for food production, has not been sufficiently addressed in freshwater assessments.


INTENTIONAL ECOHYDROLOGICAL LANDSCAPE MANAGEMENT

The critical trade-off between use of water vapor for food production to a growing world population or for welfare-supporting ecosystem services must be addressed in a conscious way. Proper attention must be paid to side effects generated by land use change. Modifications of ecosystems will alter water flows, and redirection of water flows will modify ecosystem services. There are numerous intentional local and sector-based land use decisions that have caused unintentional ecologically and water-driven side effects. Such effects are generally discussed under the term "environmental impacts," without perception of the causes behind them.

Ecologically driven side effects of land use conversion, such as shifts in key functional groups of species or loss of resilience, can change ecological and hydrological preconditions for the generation of ecosystem services. For example, movements of organisms in the landscape may change, and thereby impact on ecosystem services such as pest control, pollination, and seed dispersal by birds, bats, mammals, and insects (Baskin 1997, Bisonette 1997). Ecologically driven side effects can impact on processes of significance to the surrounding region (such as denitrification by wetlands), or processes performed on a local scale, but valued at a global scale (such as sequestration of CO2 by forests). These side effects may accumulate and transfer to the landscape and further, to a regional and even to a global scale (Holling 1994).

Freshwater-driven side effects of human activities caused by land use conversions can also change ecological and hydrological preconditions for the generation of ecosystem services. Such side effects are linked to interventions with the water partitioning process, and are propagated downstream or downwind by the water cycle. They may involve river depletion, altered relations between storm flow and low flow, and consequences for water-dependent downstream activities such as direct water uses, or ecosystem services generated by riparian wetlands and aquatic ecosystems. For example, land-clearing in southwestern Australia caused a rising water table and a threat of saline groundwater seepage into ephemeral watercourses that fed drinking water reservoirs. In the Murray Darling basin and the Hungarian Great Plain, deforestation caused widespread water-logging. Land conversion may also have atmospherically transferred consequences on downwind rainfall (Savenije 1995).

Our scenario of freshwater needs for food production for the additional world population indicates that substantial amounts of freshwater will have to be redirected to croplands from other terrestrial biomes. Increased irrigation and land conversions will produce costly side effects on the capacity of both aquatic (Postel and Carpenter 1997) and terrestrial ecosystems to generate ecosystem services. With a sectoral management and a business-as-usual approach, regional conflicts will probably grow rapidly. Instead of passively allowing unintentional impacts to develop, as in the past, an ability to manage the overall catchment, or the ecohydrological landscape, in an intentional manner must be developed.

A few cases of intentional ecohydrological landscape management have been reported from Australia and South Africa, recognizing the interdependence among liquid/vapor freshwater flows, ecosystems services, and human well-being. In Australia, an agreement has been signed between a forest firm and Melbourne City on increasing the rotation time in an upland forest to improve the water source for the city (Jayasuria 1994). In South Africa a permit system has been in operation for several decades, by which the "costs" of afforestation, in terms of river depletion, are estimated (van der Zel 1997). Moreover, the South African fynbos restoration project involves systematic reduction of the invasion of highly water-consuming alien vegetation. The fynbos catchment is seen as an integrated whole, and governance rests on combined ecological and hydrological knowledge and understanding (van Wilgen et al. 1996).


CONCLUSIONS

We have estimated the total water vapor flow from continental ecosystems to be 70,000 km3/yr, based on generalized field data. Our result captures 97% of the evapotranspiration branch (72,075 km3/yr) of global freshwater budgets (L'vovich and White 1990). A large part of our water vapor flow (63,200 km3/yr, or 90%) is attributed to forests, woodlands, wetlands, grasslands, and croplands. These terrestrial biomes sustain society with essential welfare-supporting ecosystem services, including food production.

We do not know the actual freshwater requirements for generating key terrestrial ecosystem services appropriated by the present global human population. To what extent freshwater can be used more efficiently in existing ecosystems is also an open question. Future understanding of complex behavior and interactions within and between ecosystems and freshwater flows may improve this knowledge. We can, however, conclude that earlier global freshwater assessments, which have focused their analysis on the runoff branch of freshwater (e.g., Gleick 1993, UN 1997), have seriously underestimated the human dependence on renewable freshwater flows. Water perceived as unused or even invisible on a human-dominated planet, to a large extent, is already in use for ecosystem support and services to social and economic development.

What are the implications of our results for the management of freshwater, food production, and terrestrial ecosystem services in a world of an expanding human population, intensification in global affairs, and ecological systems undergoing rapid change? Obviously, a shift in perception and approach to water management is necessary. Water is not just an economic commodity to be engineered as input in food production or industrial activities. Water is a fundamental force in ecological life-support systems on which social and economic development depend. Freshwater flows, crop production, and other terrestrial ecosystem services are interconnected and interdependent. Therefore, water appropriation for crop production to a growing human population should no longer be viewed in isolation from potential impacts of freshwater re-directions. It may lead to erosion of critical and welfare-supporting ecosystem services in both terrestrial and aquatic systems, and potential conflicts between upstream and downstream users.

Land use choices are also water choices, and will always lead to alterations in the flow of freshwater and ecosystem services elsewhere. This trade-off is made explicit in our scenario of freshwater for crop production to support a growing human population. It has to become embedded in the management of dynamic freshwater ecosystem linkages, in what we call the ecohydrological landscape. The challenge is immense and will require co-management at catchment levels, often crossing administrative and even national boundaries.


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Acknowledgments:

This article is the product of an interdisciplinary team work, where the contributions of the authors successively grew out from their starting contributions (Falkenmark's and Folke's conceptual bridge building, Rockström's methodology development and Gordon's and Engvall's in depth data analysis). Gordon's work is supported by the Swedish Council for Forestry and Agricultural Research (SJFR), and Folke's partly by the Pew Scholars program of The Pew Charitable Trusts.


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APPENDIX 1

Total water vapor flows from croplands, with data and references for calculation and classification of subgroups.

Subgroup Climatic
zone
Crop Yielda
(103Mg/yr)
n WUE
(m3/Mg)
Water vapor flow
(km3/yr)
References
Mean Low High Mean Low High
Cereals grain temperate     15 1309 539 2643        
barley 159,021 6 1070 539 1575 170, 7 85, 71 250, 39 1, 16, 29, 30, 23, 35
oats 32,710 1 1368     44, 75 17, 63 86, 45 35
wheat 557,420 8 1482 787 2643 826, 19 438, 70 1473, 14 6, 13, 14, 16, 23, 35, 44
rye 25,054         32, 81 13, 50 66, 21  
buckwheat 2892         3, 79 1, 56 7, 64  
triticale 6341         8, 30 3, 42 16, 76  
cereals nes* 2098         2, 75 1, 13 5, 54  
mixed grain 4940         6, 47 2, 66 13, 6  
Cereals grain tropical     10 1438 591 4369        
maize 533,732 4 1151 938 1456 614, 56 500, 37 777, 7 13, 22, 34, 41
millet/sorghum 91,188 6 1629 591 4369 148, 56 53, 85 398, 39 13, 16, 34, 41, 44
quinoa 33         0, 5 0, 2 0, 14  
fonio 203         0, 29 0, 12 0, 89  
canary seed 253         0, 36 0, 15 1, 11  
Cereals, total
DMb
temperate     19 438 240 646       1, 2, 6, 12, 14, 20, 23, 26, 29, 30, 45
rye grass for forage 4011         1, 76 0, 96 2, 59  
straw husks 124,221         54, 44 29, 85 80, 20  
Cereals, total
DMb
tropical     3 331 271 372        
maize for forage 470,124 2 361 349 372 169, 48 164, 7 174, 89 12, 45
sorghum for forage 50,545 1 271     13, 70 13, 70 18, 80 45
green corn (maize) 7270         2, 40 1, 97 2, 70  
forage products nes 136,465         49, 20 36, 98 50, 77  
Nutsc     1 415 200 1080        
karite nuts (sheanuts) 620         0, 26 0, 12 0, 67  
brazil nuts 57         0, 0 0, 0 0, 0  
kolanuts 303         0, 26 0, 12 0, 67  
cashew nuts 670         0, 2 0, 1 0, 6  
chestnuts 497         0, 13 0, 6 0, 33  
tung nuts 631         0, 28 0, 13 0, 72  
almonds 1212         0, 21 0, 10 0, 54  
walnuts 1002         0, 26 0, 13 0, 68  
pistachios 386 1 415 200 1080 0, 16 0, 8 0, 42 25
hazelnuts (filberts) 629         0, 42 0, 20 1, 8  
areca nuts (betel) 487         0, 16 0, 8 0, 42  
nuts nes 434         0, 26 0, 13 0, 68  
Pulses dry
seed
temperate     3 3355 1731 5833        
beans, dry 17,142 2 2116 1731 2500 36, 27 29, 68 42, 85 5, 13
broad beans, dry 3227         10, 83 5, 59 18, 82  
peas, dry 12,960 1 5833     75, 60 22, 44 75, 60 13
lentils 2773         9, 30 4, 80 16, 18  
vetches 1010         3, 39 1, 75 5, 89  
lupines 1407         4, 72 2, 44 8, 20  
string beans 1352         4, 54 2, 34 7, 89  
pulses, nes 3623         12, 15 6, 27 21, 13  
Pulses dry
seed
tropical     5 1866 1250 3003        
soybeans 124,318 2 1607 1250 1964 199, 80 155, 40 244, 20 3, 13
pigeon peas 3192         5, 96 3, 99 9, 59  
bambara beans 54         0, 10 0, 7 0, 16  
chickpeas 7673         14, 32 9, 59 23, 04  
cow peas, dry 2331         4,35 2,91 7, 0  
groundnuts in shell 27,514 3 2039 1458 3003 56, 9 40, 13 82, 63 9, 13, 34
castor beans 1257         2, 35 1, 57 3, 77  
Pulses green
seed
temperate     2 1149 583 1714        
beans, green 3470 1 583     2, 2 2, 2 5, 95 13
peas, green 4901 1 1714     8, 40 2, 86 8, 40 13
broad beans, green 955         1, 10 0, 56 1, 64  
Roots and
tubers
temperate     7 286 139 402        
potatoes 285,968 4 246 196 402 70, 41 56, 17 114, 96 13, 42
roots and tubers nes 4166         1, 19 0, 58 1, 67  
sugar beets 267,498 3 268 139 373 71, 77 37, 15 99, 78 10, 13
sugar crops nes 504         0, 14 0, 7 0, 20  
Roots and
tubersc
tropical     1 616 369 1299        
sweet potatoes 130,426         80, 32 48, 13 169, 38  
cassava 163,097 1 616 369 1299 100, 44 60, 18 211, 81 48
yautia (cocoyam) 185         0, 11 0, 7 0, 24  
taro (coco yam) 5520         3, 40 2, 4 7, 17  
yams 31,557         19, 43 11, 64 40, 98  
Roots and
tubers for
fodderb
temperate     8 326 157 616       10, 12, 42, 46, 48
beets for fodder 10,991         3, 58 1, 73 6, 77  
swedes for fodder 114         0, 4 0, 2 0, 7  
Fruit temperate     2 269 163 375        
apples 49,422         13, 28 8, 3 18, 53  
pears 11,737         3, 15 1, 91 4, 40  
sour cherries 1348         0, 36 0, 22 0, 51  
cherries 1751         0, 47 0, 28 0, 66  
watermelons 38,820 1 163     6, 31 6, 31 14, 56 13
peaches and nectarines 10,531         2, 83 1, 71 3, 95  
plums 6684         1, 80 1, 9 2, 51  
stonefruit nes fresh 330         0, 9 0, 5 0, 12  
strawberries 2539         0, 68 0, 41 0, 95  
raspberries 319         0, 9 0, 5 0, 12  
gooseberries 192         0, 5 0, 3 0, 7  
currants 691         0, 19 0, 11 0, 26  
blueberries 152         0, 4 0, 2 0, 6  
cranberries 228         0, 6 0, 4 0, 9  
berries nes 297         0, 8 0, 5 0, 11  
grapes 56,737 1 375     21, 28 9, 22 21, 28 13
figs 1101         0, 30 0, 18 0, 41  
fruit fresh nes 25,468         6, 84 4, 14 9, 55  
Fruit tropical     3 259 150 350        
persimmons 1373         0, 35 0, 21 0, 48  
cashew apple 1306         0, 34 0, 20 0, 46  
bananas 53,734 1 276     14, 81 8, 6 18, 81 13
plantains 28,145         7, 28 4, 22 9, 85  
oranges 56,021         14, 48 8, 40 19, 61  
mandarins, clementines, etc. 14,763         3, 82 2, 21 5, 17  
lemons and limes 8803         2, 28 1, 32 3, 8  
grapefruit and pomelo 4781         1, 24 0, 72 1, 67  
citrus fruit nes 3995 1 350     1, 40 0, 60 1, 40 13
quinces 323         0, 8 0, 5 0, 11  
apricots 2360         0, 61 0, 35 0, 83  
mangos 18,408         4, 76 2, 76 6, 44  
avocados 208         0, 5 0, 3 0, 7  
pineapples 11,441 1 150     1, 72 1, 72 4, 0 13
dates 4278         1, 11 0, 64 1, 50  
kiwi fruit 917         0, 24 0, 14 0, 32  
papaya 5612         1, 45 0, 84 1, 96  
kapok fruit 437         0, 11 0, 7 0, 15  
fruit, tropical nes 7417         1, 92 1, 11 2, 60  
coffee, green 5770         1, 49 0, 87 2, 2  
cocoa beans 2655         0, 69 0, 40 0, 93  
Oil-bearing
crops
temperate     3 1892 1530 2117        
 non-wooded rapeseed 29,595 2 1780 1530 2029 52, 67 45, 28 60, 5 7, 35
mustard seed 469 1 2117     0, 99 0, 72 0,99 47
hempeed 36         0, 7 0, 5 0, 8  
linseed 2351         4, 45 3, 60 4, 98  
poppy seed 41         0, 8 0, 6 0, 9  
oilseeds nes 1458         2, 76 2, 23 3, 9  
vegetable tallow 117         0, 22 0, 18 0, 25  
tallowtree seeds 781         1, 48 1, 19 1, 65  
melon seed 607         1, 15 0, 93 1, 28  
Oil-bearing
crops
tropical     2 3083 2667 3500        
 non-wooded safflower seed 753 1 2667     2, 1 2, 1 2, 63 13
sunflower 23,004 1 2667     61, 34 61, 34 80, 51 13
sesame seed 2432         7, 50 6, 49 8, 51  
stillinga oil 117         0, 36 0, 31 0, 41  
 woody tropical                    
coconuts d 10.3 x106 km2   1320 mm 1200 mm 1500 mm 136, 17 123, 79 154, 74  
palm oil, palm kernels, oil palm fruit d 6.6 x106 km2   1500 mm 1250 mm 1750 mm 99, 6 82, 55 115, 57  
olives 11,100 1 583 500 667 6, 47 5, 55 7, 40 13
Fibers       4 574 278 870       2, 6, 12
flax fiber and tow 571         0, 33 0, 16 0, 50  
kapokseed in shell 328         0, 19 0, 9 0, 29  
hemp fiber and tow 102         0, 6 0, 3 0, 9  
jute 2619         1, 50 0, 73 2, 28  
jute-like fibers 618         0, 35 0, 17 0, 54  
ramie 106         0, 6 0, 3 0, 9  
sisal 333         0, 19 0, 9 0, 29  
agave fibers nes 55         0, 3 0, 2 0, 5  
coir 172         0, 10 0, 5 0, 15  
abaca (manila hemp) 106         0, 6 0, 3 0, 9  
fiber crops nes 423         0, 24 0, 12 0, 37  
kapok fiber 107         0, 6 0, 3 0, 9  
Vegetablese       6 147 35 500        
onions + shallots, green 3287 1 113     0, 37 0, 12 1, 64 13
onions, dry 33765         4, 95 1, 19 16, 88  
leeks + other alliac.veg. 1493         0, 22 0, 5 0, 75  
garlic 9211         1, 35 0, 32 4, 61  
carrots 15,226         2, 23 0, 54 7, 61  
chicory roots 399         0, 6 0, 1 0, 20  
tomatoes 80,192 2 83 74 92 6, 63 5, 90 7, 35 13, 33
pumpkins squash gourds 9060         1, 33 0, 32 4, 53  
cucumbers and gherkins 21,192         3, 11 0, 75 10, 60  
eggplants 10,591         1, 55 0, 37 5, 30  
chillies + peppers, green 12,955 1 500     6, 48 0, 46 6, 48 13
okra 1274         0, 19 0, 4 0, 64  
cantaloupes + melons 15,216         2, 23 0, 53 7, 61  
cabbages 44,618 1 67     2, 97 1, 57 22, 31 13
artichokes 1163         0, 17 0, 4 0, 58  
asparagus 2763         0, 41 0, 10 1, 38  
lettuce 12,722 1 35     0, 45 0, 45 6, 36 36
spinach 5648         0, 83 0, 20 2, 82  
cauliflower 11812         1, 73 0, 42 5, 91  
vegetables, fresh nes 186,836         27, 39 6, 57 93, 42  
cabbage for fodder 2184         0, 32 0, 8 1, 9  
pumpkins for fodder 743         0, 11 0, 3 0, 37  
turnips for fodder 2336         0, 34 0, 8 1, 17  
leaves and tops vines 19461         2, 85 0, 68 9, 73  
vegetables, canned nes 807         0, 12 0, 3 0, 40  
carobs 249         0, 4 0, 1 0, 12  
carrots for fodder 90         0, 1 0, 0 0, 4  
vegetables + roots for fodder 44,393         6, 51 1, 56 22, 20  
Spicesf     0 0 1000 800 1500        
peppermint 55         0, 5 0, 4 0, 8  
pyrethrum, dried 18         0, 2 0, 1 0, 3  
pepper 229         0, 23 0, 18 0, 34  
pimento allspice 1896         1, 90 1, 52 2, 84  
vanilla 5         0, 0 0, 0 0, 1  
cinnamon (canela) 66         0, 7 0, 5 0, 10  
cloves, whole + stems 133         0, 13 0, 11 0, 20  
nutmeg, mace, cardamom 59         0, 6 0, 5 0, 9  
anise, badian, fennel 182         0, 18 0, 15 0, 27  
ginger 591         0, 59 0, 47 0, 89  
spices nes 856         0, 86 0, 68 1, 28  
Forage       19 934 172 2810        
hay, non-leguminous 87,514         81, 72 15, 1 245, 89  
hay (unspecified) 59,689         55, 74 10, 24 167, 71  
grasses nes for forage 23,2015 6 758 429 1031 175, 95 99, 58 239, 26 24, 37, 43
clover for forage 64,155 8 1117 172 2810 71, 67 11, 0 180, 26 2, 24, 31, 32
alfalfa for forage 160,767 5 890 573 1432 143, 1 92, 12 230, 17 8, 13, 20, 32, 45,
leguminous f. forage 49,601         46, 32 8, 51 139, 37  
hay (clover lucerne) 4991         4, 66 0, 86 14, 2  
range pasture 59,800         55, 84 10, 26 168, 2  
improved pasture 6500         6, 7 1, 11 18, 26  
Stimulant       3 4515 2083 6983        
tea 2587 2 5730 4478 6983 14, 83 11, 58 18, 7 40, 28
tobacco leaves 7217 1 2083     15, 3 15, 3 50, 40 13
mate 668         3, 2 1, 39 4, 67  
hops 122         0, 55 0, 25 0, 85  
Natural
rubber/gum
      2 30,137 29,167 31,108        
natural rubber 6065 2 30,137 29,167 31,108 182, 80 176, 91 188, 68 11, 50
natural gums 22         0, 68 0, 66 0, 70  
Sugar cane   sugar cane 1,120,898 3 123 100 163 137, 84 111, 53 182, 15 13, 15, 49
Rice   rice paddy 540,838 4 1099 839 1404 594, 34 453, 65 759, 36 13, 34, 38, 44
Cotton seed   cotton seed 86,925 1 2083 1667 2500 181, 9 144, 88 217, 31 13
Cotton lint   cotton lint 18,509 3 5454 4227 6313 100, 96 78, 24 116, 84 17, 19, 27
Subtotal:               5410 3591 8442  
Reference numbers: (1) Andersen et al. 1992; (2) Armstrong et al. 1994; (3) Ashley 1983; (4) Barker et al. 1989; (5) Barros and Hanks 1993; (6) Beech and Leach 1989; (7) Bhan et al. 1980; (8) Bolger and Matches 1990; (9) Boote 1983; (10) Brown et al. 1987; (11) Bucks et al. 1985; (12) Black 1971; (13) Doorenbos and Kassam 1979; (14) Entz and Fowler 1991; (15) Gascho and Shih 1983; (16) Gregory 1988; (17) Grimes et al. 1969; (18) Hattendorf et al. 1988; (19) Hearn 1980; (20) Heichel 1983; (21) Heitholt 1989; (22) Hillel and Guron 1973; (23) Imtiyaz et al. 1982; (24) Johnsson 1994; (25) Kanber et al. 1993; (26) Kirkham and Kanemasu 1983; (27) Lascano et al. 1994; (28) Laylock 1964; (29) Lopez-Castaneda and Richards 1994; (30) Mahalakshmi et al. 1994; (31) Oliva et al. 1994; (32) Power 1991; (33) Pruitt et al. 1984; (34) Rockström 1992; (35) Scott and Sudmeyer 1993; (36) Shih and Rahi 1984; (37) Shih and Snyder 1985; (38) Shih et al. 1983; (39) Shih 1988; (40) Stephens and Carr 1991; (41) Stewart et al. 1975; (42) Tanner 1981; (43) Thomas 1984; (44) Turner and McCauley 1983; (45) Waldren 1983; (46) Winter 1988; (47) Yadav et al. 1994; (48) Yao and Goué 1992; (49) Yates and Taylor 1986; (50) Bucks et al. 1984. For full citations, see Appendix 4.

Footnotes:

a Yield data for individual crops were collected from Faostat (1997)
b The WUE was calculated based on the total dry matter yield.
c The mean WUE comes from only one article why the low and high values are the variations within that article.
d The total water vapor flow from oil palm and coconuts was calculated as

total water vapor flow (km3/ yr)= area harvested (km2) x ET (m) from palm stands
The ET from palm stands is 1500 mm and for coconut it is 1320 (Jackson 1989). The low/high was assumed to be 1250 mm and 1750 mm (oil palm) and 1200 and 1500 mm (coconuts). The area of coconut production was collected from Faostat. For oil palms, the area harvested (the same area is also harvested for oil kernels and fruit) was calculated by
area harvested (ha/yr) = oil produced (Mg/yr)/production of oil (Mg/ha)
Palm oil production (6603778
Mg) was collected from Faostat. The production was assumed to be 1.75 Mg/ha (Mémento de l´Agronome 1984).
e For vegetables that were produced as feed, the WUE was calculated based on the “grain“ yield of that specific crop or subgroup, not the total biomass.
f Spices is a small group with roughly 0.045% of the total global area harvested each year (Faostat 1997). Within this subgroup there is a large variation in species composition as well as in parts of plant used for production measurement. The WUE was therefore based on a qualified assumption of 1000 m3/Mg. The low was assumed to 800 m3/Mg and the high to 1200 m3/Mg. This is higher than the 500 m3/Mg that Postel et al. (1996) used as an average global WUE value. Because spices are often just a small part of a plant, the WUE will be higher.
* nes = not elsewhere specified or included (abbreviation from FAO Stat. 1997.)

APPENDIX 2

Total water vapor flow from forests/woodlands, wetlands, and grasslands, with data and references for calculation and classification of subgroups.

Biome Subgroup Climatic
zone
Areaa
(103 km2)
nb ET
(mm/yr)
Water vapor flow
(km3/yr)
Referencesc
Mean Low High Mean Low High
Forest taiga boreal 11,560 3 401 380 420 4636 4393 4855  
          420           L'Vovich (1979)
          403           Black et al. (1996)
          380           Frank and Inouye (1994)
  predominantly coniferous temperate 3500 4 487 395 580 1705 1383 2030  
          543           Frank and Inouye (1994)
          395           Tiktak and Bouten (1994)
          430           Running et al. (1989)
          580           Yin (1993)
  predominantly deciduous temperate 8500 4 729 588 964 6199 4998 8194  
          588           Frank and Inouye (1994)
          620           Yin (1993)
          745           Luxmoore (1983)
          964           Moran and O'Shaugnessy (1984)
  woodland/woody savanna temperate 5200 3 416 300 530 2165 1560 2756  
          300           Angell and Miller (1994)
          530           L'Vovich (1979)
          419           Joffre and Rambal (1993)
  dry/deciduous/seasonal tropical/subtropical 7400 2 792 783 800 5857 5794 5920  
          783           San José et al. (1995)
          800           L'Vovich (1979)
  wet tropical/subtropical 5300 3 1245 880 1493 6600 4664 7913  
          880           L'Vovich (1979)
          1363           Frank and Inouye (1994)
          1493           Leopoldo et al. (1995)
  savanna/woodland, dry tropical/subtropical 12,700 2 882 870 894 11,201 11,049 11,354  
          870           L'Vovich (1979)
          894           Frank and Inouye (1994)
  wet tropical/subtropical 1300 3 1267 1100 1500 1647 1430 1950  
          1100           L'Vovich (1979)
          1500           L'Vovich (1979)
          1200           L'Vovich (1979)
Subtotal     55,460         40,009 35,271 44,972  
                       
Wetland bog boreal 651 3 221 200 260 144 130 169 Frank and Inouye (1994)
          202            
          200           Rouse (1982)
          260           L'Vovich (1979)
  bog temperate 488 4 674 456 1020 329 223 498  
          456           Boeye and Verheyen (1992)
          490           Mitsch and Gosselink (1993)
          730           Gilvear et al. (1993)
          1020           Mitsch and Gosselink (1993)
  swamp temperate 41 3 843 670 720 35 27 30  
          670           Mitsch and Gosselink (1993)
          1139           Gehrels and Mulamoottil (1990)
          720           Mitsch and Gosselink (1993)
  swamp subtropical 16 5 1127 930 1277 18 15 20  
          930           Mitsch and Gosselink (1993)
          1032         Yin and Brook (1992)
          1317           Dolan et al. (1984)
          1080           Mitsch and Gosselink (1993)
          1277           Abtew (1996)
  swamp d tropical 508 1 1656 1408 1904 841 715 967 Schaeffer-Novelli et al. (1990)
          1656            
Subtotal     1704         1366 1110 1684  
                       
Grassland cool grassland temperate 6940 16 410 130 633 2843 900 4393  
          130           Branson et al. (1969)
          190           Sims et al. (1978)
          205           Liang et al. (1989)
          276           Bokhari and Singh (1974)
          339           Scott and Sudemyer (1993)
          413           Frank and Inouye (1994)
          417           Sims et al. (1978)
          422           Roberts and Roberts (1992)
          450           L´vovich (1979)
          450           Sims et al. (1978)
          450           Stephenson (1990)
          480           Bokhari and Singh (1974)
          530           Sims et al. (1978)
          571           Frank and Inouye (1994)
          600           Stephenson (1990)
          633           Bokhari and Singh (1974)
  warm and hot
grassland e
tropical 17,300 7 599 403 862 10,356 6967 14,913  
          403           Le Houerou (1984)
          466           Lieth (1975)
          500           L´vovich (1979)
          596           Misra (1979)
          655           Carlson et al. (1990)
          708           Laurenroth (1979)
          862           Weltz and Blackburn (1995)
  montane grassland temperate 650 4 655 430 951 426 280 618  
          430           Sims et al. (1978)
          440           Sims et al. (1978)
          799           Holdsworth and Mark (1990)
          951           Holdsworth and Mark (1990)
  montane grassland f tropical 650 1 600 402 798 390 261 519  
          600           L´vovich (1979)
  dry shrubland tropical 4000 2 270 225 315 1080 900 1260  
          225           Stephenson (1990)
          315           Stephenson (1990)
Subtotal     29,540         15,095 9308 21,702  
a Surface areas for grasslands and forest/woodlands are based on Olson et al.(1983), and for wetlands on Matthews (1983), because total spatial coverage of wetlands corresponds roughly with Olson's database, whereas Matthews' has a finer classification of wetlands categories.
b Here, n refers to the number of references.
c For references, see Appendix 4.
d Low/high values are based on a coefficient of variation of +/- 15%.
e When annual precipitation P < 600 mm/yr, we assumed that ET = P (i.e.. that there is no blue water flow). This assumption is valid for dry grasslands on a large spatial scale (Le Houerou 1984). For grassland systems with P > 600 mm/yr, 20% runoff was assumed. These assumptions were made due to lack of data on ET from grasslands in tropical regions.
f Low/high values are based on a coefficient of variation of +/- 33%, which is the average standard deviation of the grassland subgroups with more than two references.


APPENDIX 3

Estimating evaporating surface runoff from croplands.

The amount of water available for redirection from evaporating surface runoff in semiarid and arid regions for use in croplands is hard to estimate. We assumed that the difference in surface runoff coefficients between field scale and continental scale for croplands in Africa, Asia, and South America is attributed to evaporating surface runoff. An even distribution of croplands on the different continents was assumed. Croplands cover 10.5% of the global terrestrial area.

Continent Precipitationa Surface
runoffa
Runoff
coefficient,
continental
scale
Runoff
coefficient,
field scale
Difference in
surface runoff
Evaporating surface
runoff
(10.5% croplandsb)
km3 /yr km3 /yr % % km3 /yr km3 /yr
Africa 20780 2480 12 20 1676 176
Asia 32140 9130 28 30 512 54
South America 29355 6450 22 25 889 94
Sum           324
a Data from L´vovich and White (1990).
b The global cropland area is roughly 10.5% of the global terrestrial area.



APPENDIX 4

Cited literature in appendices.

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Andersen, M. N., C. R. Jensen, and R. Lösch. 1992. The interaction effects of potassium and drought in field-grown barley. I. Yield, water-use efficiency and growth. Acta Agriculturae Scandinavica 42:34-44.

Angell, R. F., and R. F. Miller. 1994. Simulation of leaf conductance and transpiration in Juniperus occidentalis. Forest Science 40:5-17.

Armstrong, E. L., J. S. Pate, and D. Tennant. 1994. Water use and root growth in field pea. Austrailian Journal of Plant Physiology 21:517-532.

Ashley, D. A. 1983. Pages 389-422 in I. D. Teare and M. M.Peet, editors. Crop-water relations. John Wiley, New York, New York, USA.

Baker, R. E., A. B. Frank, and J. D. Berdahl. 1989. Cultivar and clonal differenceces for water-use efficiency and yield in four forage grasses. Crop Science 29:58-61.

Barros, L.C.G., and R. J. Hanks. 1993. Evapotranspiration and yield as affected by mulch and irrigation. Agronomy Journal 85:692-697.

Beech, D. F., and G. J. Leach. 1989. Comparative growth, water-use, and yield of chickpea, safflower, and wheat in south-eastern Queensland. Australian Journal of Experimental Agriculture 29:655-662.

Bhan, S., M. Balaraju, and V. Ram. 1980. Water use, yield, and quality of rapeseed as influenced by spacing, irrigation and time of harvest when raised in a multiple-cropping system. Indian Journal of Agricultural Science 50:760-763.

Black, C. C. 1971. Ecological implications of dividing plants into groups with distinct photosynthetic production capabilities. Advances in Ecological Research 7:87-110.

Black, T. A., G. Denhartog, H. H. Neuman, P. D. Blanken, P. C. Yang, C. Russell, Z. Nesic, X. Lee, S. G. Chen, R. Staehler, M. D. Novak. 1996. Annual cycles of water vapour and carbon dioxide fluxes in and above a boreal aspen forest. Global Change Biology 2-3:219-229.

Boeye, D., and R. F Verheyen. 1992. The hydrological balance of a groundwater discharge fen. Journal of Hydrology 137:149-163.

Bokhari, U. G., and J. S Singh. 1975. Standing state and cycling of nitrogen in soil-vegetation components of prairie ecosystems. Annual Botany 39:273-285.

Bolger, T. P., and A. G. Matches. 1990. Water-use efficiency and yield of safoin and alfalfa. Crop Science 30:143-148.

Boote, K. J. 1983. Pages 255-286 in I. D. Teare and M. M.Peet, editors. Crop-water relations. John Wiley, New York, New York, USA.

Branson, F. A, R. F. Millerand, and I. S. McQueen. 1969. Plant communities and associated soil and water factors on shale-derived soils in northeastern Montana. Ecology 51:391-407.

Brown, S. C., P. J.Gregory, and A. Wahbi. 1987. Pages 275-283 in J. P. Srivastava, E. Porceddu, E. Acevedo, and S. Varma, editors. Drought tolerance in winter cereals. John Wiley, Chichester, UK.

Bucks, D. A., F. S. Nakayama, and O. F. French. 1984. Water management for guayule rubber production. Transactions of the ASAE. 27:1763-1770.

Bucks, D. A., F. S.Nakayama, O. F. French, W. W. Legard, and W. L Alexander. 1985. Irrigated guayule - production and water use relationships. Agricultural Water Management 10:95-102.

Carlson, D. H., T. L. Thurow, R. W. Knight, and R. K. Heitschmidt. 1990. Effect on honey mesquite on the water balance of Texas Rolling Plains rangeland. Journal of Range Management 43:491-496.

Dolan, T. J., A. J. Hermann, S. E. Bayley, and J. Zoltek. 1984. Evapotranspiration of a Florida, U.S.A., freshwater wetland. Journal of Hydrology 74:355-371.

Doorenbos, J., and A. H. Kassam. 1979. Yield response to water. FAO Irrigation and Drainage Paper, Rome, Italy.

Entz, M. H., and D. B. Fowler. 1991. Agronomic performance of winter versus spring wheat. Agronomy Journal 83:527-532.

FAO. 1995. World agriculture: towards 2010. N. Alexandratos, editor. John Wiley, Chichester, UK.

Faostat. 1997. Electronic database available on the internet http://apps.fao.org FAO, Statistics Division, Rome, Italy. [Data taken 09/26/97.]

Frank, D. A., and R. S. Inouye. 1994. Temporal variation in actual evapotranspiration of terrestrial ecosystems: patterns and ecological implications. Journal of Biogeography 21:401-411.

Gascho, G. J., and S. F. Shih. 1983. Pages 445-480 in I. D. Teare and M. M.Peet, editors. Crop-water relations. John Wiley, New York, New York, USA.

Gehrels, J., and G. Mulamoottil. 1990. Hydrologic processes in a southern Ontario wetland. Hydrobiologia 208:221-234.

Gilvear, D. J., R. Andrews, J. H.Tellam, J. W. Lloyd, and D. N. Lerner. 1993. Quantification of the water balance and hydrogeological processes in the vicinity of a small groundwater-fed wetland, East Anglia, UK. Journal of Hydrology 144:311-334.

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Gregory, P. J. 1988. Pages 171-175 in Proceedings of a Conference on Dryland Farming, Amarillo/Bushland, Texas, August 1988. Challenges in dryland agriculture: a global perspective. Texas A & M University Press, Texas, USA.

Grimes, D. W., H. Yamada, and W. L. Dickens. 1969. Functions for cotton (Gossypium hirsutum L.) production from irrigation and nitrogen fertilization variables: I. Yield and evapotranspiration. Agronomy Journal 61:769-773.

Hattendorf, M. J., M. S. Redselfs, B. Amos, L. R. Stone, and R. E Gwin, Jr. 1988. Comparative water use characteristics of six row crops. Agronomy Journal 80:80-85

Hearn, A. B. 1980. Water relationships in cotton. Outlook Agriculture 10:159-166.

Heichel, G. H. 1983. Pages 127-156 in I. D. Teare and M. M.Peet, editors. Crop-water relations. John Wiley, New York, New York, USA.

Heitholt, J. J. 1989. Water use efficiency and dry matter distribution in nitrogen and water-stressed winter wheat. Agronomy Journal 81:464-469.

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Holdsworth, D. K., and A. F. Mark. 1990. Water and nutrient input : output budgets. Effects of plant cover at seven sites in upland snow tussok grasslands of Eastern and Central Otago, New Zealand. Journal of the Royal Society of New Zealand 20:1-24.

Imitiyaz, M., K. J. Kristensen, and V. Overgaard Mogensen. 1982. Influence of irrigation on water extraction, evapotranspiration, yield and water use efficiency of spring wheat and barley. Acta Agriculturae Scandinavica 32:263-271.

Jackson, I. J. 1989. Climate, water and agriculture in the tropics. Longman Scientific and Technical, New York, New York, USA.

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Address of Correspondent:
Carl Folke
Natural Resources Management
Department of Systems Ecology
Stockholm University
S-106 91 Stockholm, Sweden
and
Beijer International Institute of Ecological Economics
Royal Swedish Academy of Sciences
PO Box 50005, S-10405 Stockholm, Sweden
Phone: +46 8 164217
Fax: +46 8 158417
calle@system.ecology.su.se

*The copyright to this article passed from the Ecological Society of America to the Resilience Alliance on 1 January 2000.

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