in areas with decreasing precipitation as predicted by the climate models, soil moisture may be substantially reduced. with this in mind, agriculture in most areas already needs irrigation, which depletes fresh water supplies both by the physical use of the water and the degradation agriculture causes to the water. irrigation increases salt and nutrient content in areas that would not normally be affected, and damages streams and rivers from damming and removal of water. fertilizer enters both human and livestock waste streams that eventually enter groundwater, while nitrogen, phosphorus, and other chemicals from fertilizer can acidify both soils and water.