Dryland management for salinity control

Abstract

The objective of managing the lands in a watershed to maintain or enhance a dependable water yield of low salinity differs fundamentally from that of enhancing agricultural production in situ. The challenge is to devise strategies compatible with both. Vegetative management to increase evapotranspiration reduces salt emissions; it also reduces water yield and, if achieved by forestation, agricultural production. However, U.S. experience indicates that crop selection to increase water use in recharge areas is an effective practice to ameliorate downslope saline seeps. It appears the physico-chemical principles that control salt and water flow through geologic systems, and the effects of vegetation thereon, are well established. This is true, at least, for systems where the predominant salt is NaCl derived from deposition in rainfall. The mathematical tools to make use of these principles are also adequate. The data base, however, frequently is not sufficient to describe the system, nor is our ability to make the necessary field measurements at a reasonable cost. Aside from economic considerations, potential solutions for dryland salinity problems must be related to the specific site conditions. They may include interception drainage, drainage of water from perched water tables, reduction of hydrostatic pressure in artesian systems, as well as soil and crop management systems. The viability of these (or other) solutions can only be assessed after adequate delineation of the site conditions, including identification of the recharge area, description of the subsurface conditions with evaluation of the hydraulic properties of the aquifer materials traversed by the flux, and sufficient information to derive the flow paths. In addition, the time dependence of the flow system must be considered. Whereas flow problems have most often been solved in terms of potential distributions, it will be helpful to pay more explicit attention to velocity fields and transit times. Examples of specific situations, real or imagined, will be used to illustrate the points made above. A parallel will be drawn with similar problems under irrigated agriculture.