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000060 4 Therefore, a proper relation between irrigation management and drainage must be maintained in order to prevent irrigated lands from becoming salt affected and waterlogged. The amount of water applied should be sufficient to supply the crop and satisfy the leaching requirement but not enough to overload the drainage system. It is important to recognize that inefficient irrigation is the major cause of salinity and shallow water tables in most irrigation projects of Colorado, and of the world for that matter, and that the need for drainage can be reduced through improvements in irrigation management and through reuse of drainage water for irrigation. Ways to improve irrigation efficiency and to enhance drainage water reuse should usually be sought first before the drainage capacity is increased. Where the drainage waters can be intercepted before being returned to lower lying lands or to surface or ground waters, this should be done to prevent them from salinizing these precious resources. Intercepted saline drainage water can be desalted and reused, disposed of by pond evaporation or by injection into some isolated deep aquifer, or it can be used as a water supply where use of saline water is appropriate. The latter approach offers many advantages, as discussed next. 3. Intercept, Isolate and Reuse Drainage Water for Irrigation Agricultural drainage usually finds its way naturally into fresh-water supplies, but sometimes by design with the intent to conserve water, to increase water use efficiency or to gain additional water to enable the expansion of irrigation. Such "blending" occurs repeatedly in the Arkansas and South Platte River systems and, though it enhances river flows, is probably counter productive. The return of saline waters to good-quality water supplies, even when sufficient dilution occurs to keep the salinity ofthe mixture within apparently safe quality-limits, reduces the quantity of the total water supply that can be used in consumptive processes (such as the growth of salt-sensitive crops) that are limited by salt concentration. More consumptive use (hence, crop production) can generally be obtained from the two waters by keeping them separated. A strategy for such "non-blended" use is described later. Those who advocate blending should consider the potential deleterious effect it can have upon the usabilitv of the total water supply, not just what effect it has on the volume of available water. The ultimate goal of irrigation/drainage management should be to minimize the amount of water unnecessarily extracted from the projects good-quality water supply and to maximize the utilization of the extracted portion during irrigation use, so that as much of it as possible is consumed in transoiration (hence producing biomass) and as little as possible is wasted and discharged as drainage. Towards this goal, to the extent that the drainage water from a field or project still has value for transpirational use by a crop (i.e., the crop is sufficiently salt-tolerant to be able to extract the water from the saline solution at a rate fast enough to meet its transpirational requirement), it should be intercepted and used again for irrigation before ultimate disposal. This will reduce drainage and the associated potential for salinization, as welI as increase the available supply of water for irrigation. It will also reduce the waterlogging and overall amount of soil salinity degradation in the associated region.