Table 2. Examples of human actions in river basins, their water-related consequences, and negative impacts.

Action Water-related consequences Negative impacts
Urbanization: water supply from the upstream part of the basin or from the groundwater system Increase in impervious areas
Increase in peak discharge and surface runoff
Point and diffuse source pollution
Decrease in groundwater recharge

Higher flood risk
Possible water quality problems downstream
Intensification of agriculture: high amounts of fertilizers, pesticides, and herbicides in use; irrigation; high livestock densities

Higher runoff in the nongrowing season
Lower runoff in the growing season
Associated diffuse source pollution
Eutrophication of waterbodies
Salinization
Extensification of agriculture: use of more land for agricultural production, ploughing of extensive areas, irrigation

Higher runoff Diffuse source pollution
Forestation Increase in water storage
Enhanced evaporative losses
Reduced runoff
Reduced total annual streamflow

Depend on local conditions
Dam construction: reservoirs and impoundments for flow regulation to meet human requirements, water supply, hydropower production, flood control, and navigation

Changes in downstream flow
Streamflow regulation, i.e., storing water when abundant and releasing when scarce
Flood control
Barrier to movement of fish and to transport of sediment
Change in water regime downstream with possible consequences for water quality
Channelization by straightening, shortening, and converting natural channels to concrete ones Reduction in the times to peak
Faster conveyance
Direct impacts: increased flow velocities, enhanced stream power, and higher risk of floods
Indirect impacts: reduced water storage in watersheds, reduced self-purification capacity of rivers, elimination of wetlands

Irrigation Persistent water loss through deep percolation, causing local groundwater table to rise
Entrance of natural salts into soil horizon close to the surface by capillary rise
Irrigation with salt water
Salt washes off the land during stormflow or enters river with baseflow

Soil and river salinization resulting from irrigation
Excessive water withdrawals from groundwater Lower groundwater table More dry conditions during the vegetation period