ALL irrigation water contains dissolved salts derived as it passed over and through the land, and rain water also contains some salts. These salts are generally in very low concentration in the water itself. However, evaporation of water from the dry surface of the soil leaves the salts behind. Salinization is especially likely to become a problem on poorly drained soils when the groundwater is within 3 metres or less of the surface (depending on the soil type). In such cases, water rises to the surface by capillary action, rather than percolating down through the entire soil profile, and then evaporates from the soil surface.
Salinization is a worldwide problem, particularly acute in semi-arid areas, which use large amounts of irrigation water and are poorly drained. These conditions are found in parts of the Mideast, in China’s North Plain, in Soviet Central Asia, in the San Joaquin Valley of CA, and in the Colorado River Basin. Salinization reduces crop productivity. In the U.S., salinization may be lowering crop yields on as much as 25-30% of the nation’s irrigated lands. In Mexico, salinization is estimated to be reducing grain yields by about 1 million tonnes per year, or enough to feed nearly million people. In extreme cases, land is actually being abandoned because it is too salty to farm profitably.
Salinity has been associated with irrigated agriculture since its early beginnings. One reason is that irrigation often exacerbates the effects of salinity, which occurs naturally. 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 India range from 27% to 60% of the irrigated land, Pakistan 14%, Israel 13%, Australia 20%, China 15%, Iraq 50%, Egypt 30%. Irrigation-induced salinity occurs in large and small irrigation systems alike. In recent years, many farmers have been abandoning their rice fields in Sahelian irrigation schemes due to the incidence of salinity.
Salinity is often linked with the rise of groundwater tables resulting from excess irrigation and poor drainage in large-scale, perennial irrigation systems. The resulting shallow water tables bring salts to the upper layers of the soil profile. That salinity can also be induced by the use of pumped groundwater of marginal or poor quality has been realized only more recently. In these cases, the physical process underlying salinization is the absence of a downward soil water flux of sufficient magnitude to leach the salts from the root zone.
Saline soils contain sufficient soluble salts to adversely affect the growth of most plants. With a predominance of sodium on the exchange complex and a low concentration of salts in the infiltrating water, the infiltration rate and permeability can be severely, and in some cases, irreversibly reduced. Leaching and drainage cause salt loading of the water resource into which the effluent is discharged. The volume needing disposal can be reduced through improved irrigation management and reuse of drainage outflow for irrigation.
Irrigation/agronomic practices interventions
1. Minimize water losses in the on-farm distribution system.
2. Improve irrigation systems performance to minimize deep percolation and surface runoff.
3. On-farm watercourse improvement and precision land leveling.
4. Implement more efficient irrigation methods (e.g. drip instead of surface irrigation).
5. Minimize sediment concentration in runoff water.
6. Grow different crops or introduce different crop rotations (i.e., less-water demanding crops, more drought- and salt-tolerant crops).
7. Irrigate according to reliable crop water requirement estimates and leaching requirement calculations.
8. Manage fertilizer programs so as to minimize nutrients available for detachment and transport.
9. Apply soil amendments and reclamation practices.
The potential to increase substantially the irrigated area of the world is limited. Gains from new capacity are expected to be largely offset by losses such as salinization, as well as retirement of areas being irrigated by pumping water in excess of rates of recharge. In fact, most new water capacity is predicted to come from increasingly efficient use of existing supplies rather than harnessing of new supplies. On the other hand, managing existing irrigation projects so as to minimize their environmental impact is a requirement for long-term sustainability of irrigated agriculture. Both, improved water use efficiency and environmental stewardship, are indeed complementary goals.
