RESEARCH BRIEFS Wyoming Water Resources Center

Evaluation of Irrigation Diversions and Return Flows

Investigator: Victor Hasfurther, Department of Civil Engineering and Wyoming Water Resources Center, University of Wyoming.

Purpose: Ranching and farming are two of the oldest and largest users of water in Wyoming accounting for approximately ninety-five percent of the total amount of water withdrawn from streams and aquifers. However, only a portion of the water withdrawn for ranch and farm irrigation is consumptively used and unavailable for other uses. The rest is returned to streams or aquifers by overland flow and by percolation through the soil to an aquifer which stores or returns it to the stream system. Benefits of irrigation include recharge of alluvial (shallow) aquifers that serve as underground storage reservoirs, increased likelihood of maintaining instream flow and a steadier more reliable source of water downstream resulting from the return flow pattern of an interactive stream-aquifer system. An understanding of the quantity and timing of return water flow is critical for effective water management for downstream water users.

Methods: A return flow study was conducted on the New Fork River drainage near Pinedale (west-central), Wyoming between 1984 and 1988. The study encompassed a 28 square mile area, from the New Fork Lakes south to State Highway 191, over an unconfined alluvial aquifer. In this area there is a surface elevation drop of 270 feet in 10.25 miles and the aquifer is approximately 30 feet thick at the center of the valley. A network of 32 stream gauges, 25 wells and three precipitation gauges were either installed or available in the study area. A water budget analysis, in conjunction with a finite-difference groundwater modeling program, was utilized to determine the interaction of irrigation applications and the stream aquifer system in this area.

Results: Analysis of the water balance and stream-aquifer flow in the study area indicate that, over the four year study period, seventy-two percent of the water diverted for flood irrigation (62,400 ac-ft) returns to the stream system within one year. The stream-aquifer interaction is especially rapid during the high water table, aquifer recharge periods in the spring and summer. Approximately ninety percent (40,200 ac-ft) of the total return flow water volume returns to the stream surface system during this time period. The remaining ten percent (4,700 acre feet) of the total volume of return flow water is released slowly from aquifer storage to re-enter the stream system during the normally low stream flow winter months. Although the delayed return flow water quantity is relatively insignificant because of the high flow rate of this aquifer, it still aids greatly in maintaining a more consistent stream flow during the winter months.

Conclusions that can be drawn from the annual flow analysis of this study area are that during an average precipitation year approximately seventy-five percent of the water diverted for flood irrigation can be expected to return to the system as overland and stream-aquifer flow. In an average year, overland return flow accounts for 42 percent and underground return flow accounts for 33 percent of the total quantity of return flow and the alluvial aquifer acts as a 24,000 acre foot underground reservoir. During drier years, when more of the water diverted is "lost" to consumptive use by evapotranspiration, only approximately fifty percent of the water diverted for flood irrigation can be expected as return flow.

Overall, these results indicate that the practice of flood irrigation in this area has some beneficial effects on the downstream system by returning high spring flows at a slower, more constant rate over a larger portion of the spring and summer months (greater than would occur naturally if the spring runoff were not stored in the aquifer). In addition, return flow water released from aquifer storage is beneficial to downstream users because (1) evaporation losses are reduced by the underground storage of return flow water, (2) return flow aquifer recharge stores water without the expense of dam construction, (3) return flow release in this area is sufficiently rapid to enable downstream use during the same irrigation season, and (4) the land surface above the alluvial aquifer if able to be put to a productive land use.

Future Work: This study is a single case but is believed to be representative of alluvial valley situations in many similar areas in Wyoming (e.g. Big Piney, Star Valley). Studies need to be conducted on other types of stream-aquifer interaction systems which exist in the State of Wyoming, where the geology and soils are different.

Publications:

Hasfurther, V.R., H.W. Peck and S.A. Mizell. 1986. "Modeling an Alluvial Aquifer with Flood Irrigation." Water Forum '86: World Water Issues in Evolution, 1:320.

Peck, Hilaire W. 1985. "A Model for an Alluvial Aquifer Receiving Recharge from Flood Irrigation." M.S. Thesis, Department of Civil Engineering, University of Wyoming. 107 pp.

Wetstein, John H. 1989. "Return Flow Analysis of a flood Irrigated Alluvial Aquifer." M.S. Thesis, Department of Civil Engineering, University of Wyoming. 312 pp.

Wetstein, J. and V.R. Hasfurther. 1989. "Response of a Surface Aquifer to Flood Irrigation." Proceedings of the ASCE Specialty Conference, National Water Conference. pp. 367-374.

Wetstein, J.H., V.R. Hasfurther and G.L. Kerr. 1989. "Return Flow Analysis of a Flood Irrigated Alluvial Aquifer: Final Report to Wyoming Water Research Center and Wyoming Water Development Commission.

RESEARCH BRIEFS are published by the Wyoming Water Resources Center with funds provided in part by the US Geological Survey, Department of Interior, as authorized by the Water Resources Research Act of 1984. The research on which this report is based was financed in part by the US Geological Survey, Department of Interior and Wyoming Water Resources Center. The views expressed do not necessarily represent those of the Department of Interior or the WWRC. Persons seeking admission, employment, or access to programs at the University of Wyoming shall be considered without regard to race, color, national origin, sex, age, religion, political belief, handicap, or veteran status.

RB92-103

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