<Li> In Peru some 0.3 million ha of the 1.05 million ha of irrigable land suffers from degradation (see Irrigation in Peru). </Li> <Li> Estimates indicate that roughly one - third of the irrigated land in the major irrigation countries is already badly affected by salinity or is expected to become so in the near future . Present estimates for Israel are 13% of the irrigated land, Australia 20%, China 15%, Iraq 50%, Egypt 30% . Irrigation - induced salinity occurs in large and small irrigation systems alike </Li> <Li> FAO has estimated that by 1990 about 52 million ha of irrigated land will need to have improved drainage systems installed, much of it subsurface drainage to control salinity </Li> <Ul> <Li> The downstream drainage water quality may deteriorate owing to leaching of salts, nutrients, herbicides and pesticides with high salinity and alkalinity . There is threat of soils converting into saline or alkali soils . This may negatively affect the health of the population at the tail - end of the river basin and downstream of the irrigation scheme, as well as the ecological balance . The Aral Sea, for example, is seriously polluted by drainage water . </Li> <Li> The downstream quality of the groundwater may deteriorate in a similar way as the downstream drainage water and have similar consequences </Li> </Ul>

Where has irrigation been used by humans to alter biomes