25. Use of Marginal-Quality Waters for Sustainable Crop Production

Existing high-quality water supplies are not sufficient to sustain irrigated agricultural production in arid and semiarid regions. Use of marginal-quality waters and reuse of agricultural drainage waters and treated municipal wastewaters are currently the only economic options for maintaining production in these regions. Marginal-quality waters are characterized by one or more of the following: elevated salinity, sodicity, pH, alkalinity, dissolved organic matter, and toxic elements such as boron and selenium. These water characteristics have a potentially adverse impact on crop productivity as well as soil physical properties. However, these waters can generally be used with proper management considerations. Model simulations of irrigation with saline waters confirm field data indicating that existing leaching guidelines overestimate water quantities needed for salinity control in the root zone. Leaching recommendations must also consider efficient water use, costs of water, nutrients, and disposal of drainage water as well as crop production. The sodicity hazard associated with application of saline water has been generally overlooked, due primarily to lack of consideration of the adverse impact of even small quantities of rain on physical properties of the soil surface. Recent long-term studies on infiltration of degraded waters indicate that the sodicity hazard to soil physical properties is greater than currently considered and that surface addition of gypsum may need to be more generally utilized. The utility of computer modeling as an aid to irrigation management is further demonstrated by the simulation of intermittent use of high-boron waters currently deemed unsuitable for irrigation. Future salinity research is critical to meet food demands of arid and semiarid regions of the world, especially for development of new varieties improving the salt and boron tolerance of sensitive species and improved prediction of plant production under multiple stress conditions (salinity water nutrient and toxic element).