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24 Estimating Economic Impacts of Salinity... N -J o ..... While these tolerance levels must be con- sidered as estimates and should serve only as a general guide, research into crop yields in the lower Colorado River Basin indicates that such thresholds are not unreasonable. It is notewor- thy that the U.S. Soil Conservation Service clas- sifies soil salinity in the upper soil layer (above 8 inches) as slightly saline if the saturation level is less than 4,000 micromhos per centimeter (Il-mhos/cm) - about 2,560 mgiL TDS, showing how the differing requirements of the soil-water users can lead to different kinds of defmition for soil salinity. Salinity Effects on Crop Yields While Maas has determined approximate threshold levels above which salinity damage may begin, Dr. James Rhoades, also of the U.S. Salinity Laboratory in Riverside, has set himself to the task of dealing with growing crops in salty water or soil. While his test programs often require the availabilty of very high quality water during germination (water not available to farmers in the Lower Colorado Basin), he has tested a variety of crops with a variety of more readily available farm management practices to conclude that it is possible to grow crops at much higher salinity levels present in irrigation water and soil than previously thought possible. The historical record of crop yield per acre seems to support the position of Dr. Rhoades that some crops can be grown with fairly salty irrigation water. Crop production at higher salinity levels presupposes fairly sophisticated irrigation and management practices. In reality, only 20 per- cent of the Imperial Irrigation District is under sprinkler irrigation. Throughout that area and in much of the Lower Colorado Basin, flood irriga- tion is still the method of choice, since it re- quires less capital investment and lower operation and maintenance costs than sprinkler irrigation. However, flood irrigation is more difR ficult to schedule at the precise time demanded by the condition of the soil water, so crop yield may be reduced compared with yields using sprinkler irrigation. Moreover, highly saline water, under flood irrigation, requires more freR quent applications to accomplish needed leach- ing. The more frequently a farmer must irrigate, the higher are his labor costs. Since so few farms are automated, farmers are very happy with water below 600 mgiL TDS since it requires many fewer labor-intensive irrigations. Other Factors Affecting Crop Yields Clearly, salinity can affect crop yield, accord- ing to the U.S. Department of Agriculture. Most farmers and agricultural agents agree. A former Imperial County Agricultural Commis- sioner states that lower TDS levels increase the suitability of soil structure for crops and allow for better leaching and thus a more complete use of irrigation water for crop growth. Yet, as Maas reiterates, it is not clear at what precise salinity level crop yield is affected. There ap- pears to be an inverse relationship between yield and salinity levels in the period 1971-1984 but this relationship is far from regular. What relationship can be drawn between TDS and yield is subject to distortion by the other factors, such as variations in rainfall and temperature, the presence of pests, and variations in farm management practices. Economics of crop prices also can affect yield, as a former County Commissioner points out. When market prices are down, not all crops are harvested, thus skewing the acreage yields reported in some places. Recognizing that these other factors affect the salinity/yield relationship, it is believed that their impact can be largely overcome by use of empirical data from all of the major agricultural counties in the Lower Colorado River Basin over a period of 14 years (1971-84) which will include variation in climatic conditions, market condi- tions, and farm management practices throughout the region. Estimating Current and Future (1987-2010) Salinity Damages to Agriculture For purposes of estimating current and future salinity damages to agriculture from use of Colorado River water for irrigation, an cmpiri- cal model was developed which relates crop losses to the differential yield that can be expected at two levels of salinity: the first, a