Biophysical regulation of evapotranspiration in semiarid croplands

Water vapor exchange is a key component in terrestrial surface water balance. However, the biophysical regulation processes that control evapotranspiration (ET) in semiarid farmland areas have not been fully resolved. In this study, seasonal and interannual variances in ET and their environmental controls from 2009 to 2011 were investigated in a semiarid, rain-fed farmland using eddy covariance. One moderately dry year (2010) and two extremely dry years (2009 and 2011) were observed; however, 2009 (an extremely dry year) had the same weather condition as that of 2010 (a moderately dry year), and 2011 set out as different from the other two years. The results indicated that the maximum daily ET in the 2009 (4.4 mm d−1) and 2010 (4.7 mm d−1) growth seasons was higher than that in 2011 (3.0 mm d−1). Cumulative annual ET values were 310.6, 318.0, and 210.2 mm in 2009 to 2011, which were 109%, 96%, and 72% of the precipitation (PPT) received, respectively. The contribution of PPT in summer to the annual ET was more than that in autumn, and the extremely dry climate during the pregrowth stage and early spring period restricted the annual ET in 2011. The lowest annual ET in 2011 was caused by low daytime ET rather than nighttime ET. The strong power relationship (R2 = 0.85) between the decoupling coefficient (Ω) and the canopy conductance (gc) indicated that ET was sensitive to gc, and gc control on ET increased as the day progressed. The physiological controls of ET were associated closely with soil moisture, and drought had a strong effect on the ET response to environmental factors, such as incoming shortwave radiation and vapor pressure deficit. These results have important potential implications for understanding and modeling water cycle feedback to the increasing drought climate in the Loess Plateau characterized by serious soil and water loss.