Evapotranspiration and water use efficiency of continuous maize and maize and soybean in rotation in the upper Midwest US

This study examined evapotranspiration (ET) from no-till, rainfed maize and soybean during three growing seasons (May-Sep) of normal rainfall years (2009, 2010, 2011) and a drought year (2012) in Michigan, USA, based on daily soil water uptake measured by time-domain reflectometry at multiple depths through the root zone. During normal rainfall years, growing-season ET was similar between continuous maize (mean standard deviation: 471 +/- 47 mm) and maize in rotation (469 +/- 51 mm). During the drought year, ET decreased by only 3% for continuous maize but by 20% for maize in rotation. During the normal rainfall years, ET for soybean (453 +/- 34 mm) was statistically indistinguishable from ET for maize, and was lower during the drought year (333 mm). Water use efficiency (WUE), calculated from harvest yield (grain + corn stover) and ET, was 25.3 +/- 4.2 kg ha(-1) mm(-1) for continuous maize and 27.3 +/- 3.1 kg ha(-1) mm(-1) for maize in rotation during the normal rainfall years, whereas WUEs for both continuous maize and maize in rotation were much lower in the 2012 drought year (14.0 and 15.5 kg ha(-1) mm(-1), respectively), coincident with lower production. Soybean had a much lower WUE than maize during the three normal years (6.95 +/- 0.96 kg ha(-1) mm(-1)) and the drought year (4.57 kg ha(-1) mm(-1)), also explained by lower yield. Both maize and soybean tended to use all available water in the soil profile; there was no consistent difference in ET between these crops, while yield varied markedly from year to year.