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Soil salinization. A Colorado farm soil with white salt residues from irrigation.

출처 : Environment (9th) by Peter H. Raven (2015) Wiley - 284page

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Agriculture with its widespread clearing of native vegetation and replacement with annual crops and pastures has changed the hydrological balance of the landscape. Perennial native vegetation uses most of the rainfall via direct interception by leaves and exploitation of soil moisture by deep root systems. Shallow rooted annual crops and pastures use less rainfall, increasing recharge to groundwater and causing watertables to rise. Dryland salinity can appear anywhere from 20 to 100 years following clearing, depending on the type of groundwater flow system that is operating. Unfortunately dryland salinity is occuring in nearly all dry regions using irrigation schemes. This not only increases salinity in groundwater but also in soil layers, reducing field productivity. Its process is illustrated in the diagram above.

Irrigation is linked to salinization of groundwater and soil layers in a number of ways. All irrigation water contains dissolved salts derived as it passes over and through the land, and rain water also contains some salts. These salts are generally in very low concentration in the water itself. Firstly, in some cases when excess water is supplied to crops it evaporates away leaving salt deposits in surface layers. These can later be leached into water systems. Secondly, irrigation can lead to waterlogging as described above. This occurs when irrigation systems over load the soil with water it cannot utilize. Thirdly, ineffective irrigation in upland farms can lead to saline seapage, whereby saline groundwater flows downslope as throughflow to salinize lowland farmland and water systems. The problem of salinisation is well documented in the Murray-Darling drainage basin, but can generally considered ubiquitous to intensive irrigation schemes. Finally, salinization is also considered a problem in dams associated with irrigation schemes. In this case, increased evaporation rates lead to higher salt levels in the reservoir. When this released it increase salt levels in rivers. Dams also act to cool down water, so as water is released it also inputs unnaturally cold water into the river, which has significant impact on the health and ecology of the river.

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