Environmental impacts of irrigation are the changes in quantity and quality of soil and water as a result of irrigation and the ensuing effects on natural and social conditions at the tail-end and downstream of the irrigation scheme.

The impacts stem from the changed hydrological conditions owing to the installation and operation of the scheme.

An irrigation scheme often draws water from the river and distributes it over the irrigated area. As a hydrological result it is found that:

• the downstream river discharge is reduced
• the evaporation in the scheme is increased
• the groundwater recharge in the scheme is increased
• the level of the water table rises
• the drainage flow is increased

These may be called direct effects.

The effects thereof on soil and water quality are indirect and complex, waterlogging and soil salination are part of these, whereas the subsequent impacts on natural, ecological and socio-economi cconditions is very intricate.

Irrigation can also be done extracting groundwater by (tube) wells. As a hydrological result it is found that the level of the water descends. The effects may be water mining, land/soil subsidence, and, along the coast, saltwater intrusion.

Irrigation projects can have large benefits, but the negative side effects are often overlooked.The lower the irrigation efficiency, the higher are the losses. Although fairly high irrigation efficiencies of 70% or more (i.e. losses of 30% or less) can be obtained with sophisticated techniques like sprinkler irrigation and drip irrigation, or by precision land levelling for surface irrigation, in practice the losses are commonly in the order of 40 to 60%.

The effects of irrigation on water table, soil salinity and salinity of drainage and groundwater, and the effects of mitigation measures can be simulated and predicted using agro-hydro-salinity models.