Effects of Subsurface Drainage on Evapotranspiration for Corn and Soybean Crops in Southeastern North Dakota

A field experiment was conducted during the 2009 and 2010 growing seasons to determine the effect of subsurface drainage (SSD) on evapotranspiration (ET) and crop coefficients (K-c) for a farm field in the Red River Valley of North Dakota. The total area of the field was 44 ha, half of which had subsurface drainage installed in the fall of 2002 at an approximate depth of 1.1m and a spacing of 18.3m. Corn (Zea mays) was planted in 2009 and soybean (Glycine max) in 2010. Evapotranspiration rates were measured in both the SSD and surface drained [or undrained (UD)] by using the eddy covariance (EC) method. The changes in water table and soil moisture content were monitored continuously in both fields. The K-c for corn and soybean was developed by using the ET measured by the EC system, and the reference ET was estimated by using the American Society of Civil Engineers Environmental and Water Resources Institute alfalfa reference method. As expected, the use of SSD affected the ET in a seasonal pattern and the ETwas crop dependent. Seasonally, higher ETwas observed during spring and fall in the UD field attributable to shallower water table and higher soil moisture content. In the summer, a higher ETwas found in the SSD field. The higher ET in the UD field in spring and fall, which was 109 and 191 mm in 2009 and 2010, compared with 105 and 176 mm in 2009 and 2010 in the SSD field, did not offset the higher ET in the SSD field in the summer, which was 310 and 351 mm in 2009 and 2010, compared with 249 and 324 mm in 2009 and 2010 in the UD field. For July and August, the ET in the SSD field was 31% greater in 2009 for corn and 14% greater in 2010 for soybean than that in the UD crop fields. For the entire growing season, the ET in the SSD field was 16% higher in 2009 and 7% higher in 2010 compared with the UD field. During the peak growing season (July), the K-c was greater in the SSD field, with peak values of 0.70 for corn and 0.76 for soybean, but in the UD field, the peak K-c values were only 0.54 for corn and 0.65 for soybean. DOI: 10.1061/(ASCE)IR.1943-4774.0000508. (C) 2012 American Society of Civil Engineers.