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cc ltL4 Irrigation Seepage Irrigation water is applied to crops in the Rio Grande inner valley through a distribution system of canals, laterals, and ditches. Water that is applied to fields in the Rio Grande inner valley, but is not evapotranspired, becomes recharge to the aquifer system. Irrigation seepage in the Albuquerque area was estimated using the Kernodle and others (1995) model to be about 9,600 acre-feet for the year ending in March 1994. Kernodle and others (1995, p. 17-18, 110) asswned the irrigation seepage rate to be 1 acre-foot per acre per year, based. on 3 acre-feet per acre per year being applied. Bjorklund and Maxwell (1961, p. 53) estimated that about one-third of the irrigation water applied reaches the water table and that 3 acre-feet per acre per year is usually applied to most crops (excluding alfalfa, which requires more than 4 acre-feet per acre per year). The Bureau of Reclamation (1994a, table 8) has estimated the rate of this recharge to range from 0.1 0 to 1.22 acre-feet per acre per year in the Rio Grande inner valley, depending on crop type and soil series. Total applied irrigation water ranged from 1.1 to 4.2 acre-feet per acre per year, depending on crop type and soil series. The average for all crops and soil series was 2.5 acre- feet per acre per year. By assuming that irrigation practices are consistent with those estimated by the Bureau of Reclamation (1994a), reasonable estimates of the rate and distribution of irrigation seepage that recharges the aquifer from cropland can be made based on the estimates described above and the distribution of crops. Digital data that include the distribution of irrigated areas in the inner valley of the Albuquerque Basin at a source scale of 1:24,000 are available from the National Biological Service for 1935, and digital data at a source scale of 1:12,000 are available from the Bureau of Reclamation for 1955, 1975, and 1992. The 1992 data distinguish crop type, Distribution of crops by county is available from the New Mexico Crop and Livestock Reporting Service (1962-94). Distribution of soil series in the Albuquerque area is reported by the U.S, Department of Agriculture (1977). About 14,000 acres of cropland were irrigated in the Albuquerque area in 1975 and about 9,600 were irrigated in 1992 (Bureau of Reclamation digital data). Hansen (in press) estimated that about half of the recharge from the Rio Grande surface-water system between Bernalillo and lsleta Pueblo comes from canal and irrigation seepage. Reduction of irrigated cropland in the Albuquerque area may, therefore, have a significant effect on recharge from the inner valley (Steve Hansen, written commun., 1995), which would influence ground-water/surface-water interaction. It is, therefore, essential to docwnent changes in the amount and distribution of irrigated cropland in the Rio Grande inner valley. The measurement of water diverted into the canal system at Angostura by the MRGCD and their proposed measurements of return flow to the river would enable the separation of river- channel seepage from canal, drain, and irrigation seepage. Comparing return-flow measurements with diversion measurements during the irrigation season would show net seepage in canals, drains, and irrigated areas, and return-flow measurements during the winter would show drain seepage. This information is essential for adjusting the ground-water-flow model, thus improving the estimates of irrigation seepage as well as canal and drain seepage. Septic-Field Seepage Water from septic leach fields that reaches the water table is recharge to the aquifer system. Septic-field seepage in the Albuquerque area was estimated using the Kernodle and others (1995) model to be about 6,000 acre-feet for the year ending in March 1994. This number was based on an assumed seepage rate of 75 gallons per person per day in areas not served by sewer systems (Kernodle and others, 1995, p. 18-19, 110), Siegrist and others (1976, p. 536-537) calculated wastewater production from 11 rural Wisconsin households to average 43 gallons per 28