INTERNAL DRAINAGE UNDER IRRIGATION OF TWO SLOWLY PERMEABLE SUBSOILS OF THE NORTHERN GREAT PLAINS T. P. Trooien, G. A. Reichman MEMBER ASAE ABSTRACT. Lack of water limits crop yield in the semiarid northern Great Plains but many soils of the northern Great Plains have been thought to be nonirrigable because of their slowly permeable subsoils. We measured the internal drainage in response to two levels of nitrogen fertility and three levels of irrigation or overirrigation — one, two, or three times the calculated evapotranspiration — of corn (Zea mays L.) at two sites for five and six years. Both of our sites had subsoils classified as slowly permeable; one site had alluvial subsoil, the other had glacial till subsoil. For sufficient nitrogen fertility (200 kg NI ha) at the glacial till site, drainage between 1 July and harvest from the 2ET treatment was 109 mm greater than from the lET treatment; drainage from the SET treatment was 197 mm greater than from the 2ET treatment. Insufficient nitrogen fertility (100 kg NI ha) reduced crop water use and resulted in greater drainage. At the glacial till site, the average internal drainage between 1 July and harvest from all three irrigation treatments at 100 kg NI ha was 100 mm greater than at 200 kg NI ha. The maximum five-year average drainage amount between 1 July and harvest at the till site was 471 mm, more water than the average applied to the lET treatments during the same period. Tensiometer measurements occasionally indicated saturation at the 2.0-m depth at the alluvial site, but not at 0.9 or 1.S5 m; saturation was never encountered at the till site at 0.9,1.S5, or 2.0 m. Based on the measured drainage rates and lack of perched water table development, the soils tested in this study can drain enough water to permit successful well-managed irrigation. Keywords, Irrigation, Drainage, Nitrogen, Corn. W ater limits crop growth and yield in the semiarid northern Great Plains. Irrigation can overcome this lack of water, increasing net returns and permitting greater flexibility in crop selection. Leitch et al. (1991) reported that irrigation could increase net returns by $86 to $267/ha in three areas of North Dakota. But many soils of the northern Great Plains have been considered nonirrigable due to low hydraulic conductivity of their subsoils. A site has been considered nonirrigable when it has a barrier at a depth of 1.8 m or less. The U.S. Department of Interior, Bureau of Reclamation (1978) defines a barrier zone as a layer that has a hydraulic conductivity of 20% or less of the weighted hydraulic conductivity of the overlying layers. We established two research sites that have barriers within 1.8 m of the soil surface. The internal drainage rates at both research sites were judged great enough to prevent water table development under well-managed irrigation, based on tests without growing crops (Doering et al., 1986; Trooien and Reichman, 1990). Even 60 d after the last water application, the internal drainage rates were still 0.6 mm / d at a site with slowly permeable glacial till Article was submitted for publication in October 1992; reviewed and approved for publication by the Soil and Water Div. of ASAE in March 1993. Trademarks and company names are included for the benefit of the reader and do not imply endorsement or preferential treatment of the product by USDA. The authors are Todd P. Trooien, Agricultural Engineer, and George A. Reichman, Soil Scientist (retired), USDA-Agricultural Research Service, Northern Great Plains Research Laboratory, Mandan, ND. starting at a depth of 1.0 m (Trooien and Reichman, 1990) and 0.8 mm/d at a site with slowly permeable alluvium starting at a depth of 1.5 m (Doering et al., 1986). Moreover, irrigated lands in southern Alberta similar to one of our sites have experienced few problems due to soil permeability. Chang and Oosterveld (1981) concluded that their 13 sites underlain by slowly permeable glacial tills had sufficient internal drainage to prevent waterlogging under normal irrigation. The successful irrigation in Alberta and previous internal drainage results at our sites suggest that the barrier criterion, based on hydraulic conductivity, may not be applicable to our sites. Bender et al. (1983) noted that the most important factor in the irrigation of many soils of eastern South Dakota is the capacity of the soils to remove excess water from the irrigated area. One danger of irrigating soils with slowly permeable subsoils is the development of a water table within the crop root zone to the point that the water table impedes crop root function. Previous water table studies showed that com {Zea mays L.) yield is reduced when the water table is within 0.30 to 0.76 m of the soil surface. In a southeastern North Dakota lysimeter study on sandy soils, the greatest com yield was measured when the water table was 1 m deep (Benz et al., 1985); a shallower water table, 0.46 m deep, reduced com yield. Com yields were near the maximum when the water table was deeper, but more irrigation was required. Elsewhere, Goins et al. (1966) measured reduced com yield when the water table was shallower than 0.76 m on loamy fine sand and loam soils and when the water table was shallower than 0.30 m on silty clay loam soils. Williamson and van Schilfgaarde (1965) also measured reduced com yields when the water table was shallower than 0.76 m. On glacial till soils in VOL. 36(3):709-715 - MAY-JUNE 1993 709