Introduction Irrigation has long been a means of increasing crop production on land otherwise deemed underproductive. This is the case in many arid portions of the western United States. However, what many irrigators might consider adequate water application could be incorrectly estimated. The problems associated with inefficient application techniques of irrigation water can often be easily identified and corrected with additional water. Overirrigation, on the other hand, is often difficult to detect as the effects are not as easily identified and quantified by the grower.
The fact that agriculture is the major consumptive use of the water resources of Wyoming (Wyoming State Engineers Office, 1973) and the United States (Murray Sleeves, 1972) suggests that improving irrigation application techniques could benefit people in agriculture as well as society in general. If irrigators employed more efficient practices, some of the water presently diverted by agriculture for surface irrigation would be available for other uses, including additional irrigation development. However, it should be pointed out that poor irrigation efficiency does not imply that the excess water is lost. In fact, much of this water is returned to some water course for reuse, but it is likely to be lower in quality.
Improving irrigation efficiency implies irrigation scheduling, which would improve the timing and amounts of water applications. Jensen (1972) stated that little change has occurred in irrigation scheduling practices in the western United States during the 25 years previous to the early 1960's. The two major reasons for lack of change were: "(1) The needs of managers of irrigated farms and the acceptability of suggested scheduling procedures have not been adequately evaluated; (2) the cost of irrigation water often has not been significant, and (3) indirect costs such as yield reductions caused by delayed irrigations and additional nitrogen requirements created by excessive water application are not easily recognized or quantified. Also, crop and soil damage costs encountered on lower-lying areas by excessive water use on upper areas are not always borne by the upper area irrigators".
Given the low cost of water relative to labor and capital. it appears that irrigators substitute water for other resources. The problem then is to determine whether or not present water applications result in significant economic losses to the irrigators. Economic losses to individual farmers associated with improper water applications could arise from:
If these losses occur, the question is: would reduction of these losses through improved irrigation efficiency and drainage offset costs of the improvements?
Preliminary work by the U.S. Bureau of Reclamation, Riverton Project, Wyo., has provided evidence that some of the excessive water application losses discussed previously do in fact occur. Seeped lands on parts of the project indicate rising water tables. Higher levels of nitrates and total dissolved solids (TDS) in drain water compared to levels in the applied water imply soil nutrient leaching. Bureau personnel have monitored alfalfa fields on the Riverton Project, where as much as 3.35 ha-m (11 acre-feet) of water were applied during the growing season. Alfalfa, a high water-using crop, requires .6 ha-m (2 acre-feet per acre) during a normal year (Trelease et al., 1970).
Inefficient use of irrigation water resulting in high delivery requirements does not seem to be an isolated problem. Irrigation deliveries along the Big Horn River in north-central Wyoming range from 1.0 to 2.1 ha-m (dark, 1972). Houk (1951) reported irrigation efficiencies for major farm crops in the western U.S. ranging from 20 to 50%. The U.S. Department of Agriculture (1960) reported an average farm irrigation efficiency for the western United States of 47%, with 70 to 75% farm efficiency attainable. The U.S. Bureau of Reclamation, Region 6 (1972), reported results of specific studies on the Midvale Project near Riverton, Wyo., where farm irrigation efficiencies ranged from 13.4 to 69.7% with an average of 24.3%. Water management studies by the U.S. Bureau of Reclamation, Region 7 (1972), on parts of the Bostwick Irrigation District No. 2, Kansas, found farm irrigation efficiencies ranging from 22 to 50% with an average of 41.6%.
The data within this report help identify the economic losses which may occur with overirrigation. In addition, insights into potential improvements in irrigation scheduling are discussed.
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