WSP10974 - Laserfiche WebLink

Home Browse Search

e - ~. - \.: Fanners now overirrig;l.te; during the early months of the irrigation season when surplus water is available, in an effort to minimize, as much as possible, the effects of the late-season difficiencies. Distribution of water in accordance with crop needs will enable farmers to reduce or eliminate this excessive spring irrigation. The result will be increased crop yields with li tUe more total stream depletion than at present. During years of critically low runoff, excess spring snowmelt has escaped downstream but after project construction this excess water will be stored in the reservoir. Storage will result in a substantial increase in the available supply for the project for those years. The resulting water supply in such years will be approximately equal to that which has existed in the average or better than average years without the project. The planned capaci ty of Crawford Reservo ir is only sli ghtly in excess of the average project requirements for late-season water. For the normal range of operating condi tions, the carty-over wi 11 vary from nothing to about 4,000 acre feet. A project operations study has been made by the Bureau of Reclamation. Review of this study shows that under project operations, with condi tions of weather and runoff similar to those of the study period of 1942 to 1956 (including the lowest runoff in the history of the project area), only three years out of 15 would have a less than adequate water supply. Runoff records for the .9ni th Fork Ri ver are of short duration. Comparing these records with nearby longer period records indicates that, if the project had operated during the 58-year period of 1904 to 1957, there would have been only six years wi th water supplies so short as to adversely affect the project. Since the reservoi r capacity is limited to annual storage and will not provide regulation of runoff from one year to another, it will be important that farm operators wi thin the project have avai 1able each year some fore- cast of the prospective supplies. Such water supply forecasts are usually available from the Soil Conservation Service Snow Survey and Water Supply Forecast program. They make it possible for the farmer to adjust his operations to fit the prospective water supply, thus minimizing the economic effects of a defficient supply. There are no snow courses on the Smi th Fork watershed at present. I t would appear that in order to provide an adequate basis for estimating snow cover and forecasting water supplies, existing networks should be extended to provide two or three snow courses on the watershed, wi th corresponding soil moisture measuring stations. Findings Based on a weighted average seasonal consumptive use of 18.7 inches per acre and an estimated on-farm irrigation efficiency of 42 percent, the estimated farm irrigation requirement per irrigated acre is 44.4 inches of water. Proposed project facilities will deliver an average of 42.6 inches of irrigation water at the fann headgate, thus meeting 96 percent of the average requirements of the project lands. - 16 -