Evapotranspiration partitioning and energy budget in a rainfed spring maize field on the Loess Plateau, China

Understanding land-atmosphere interactions in rainfed spring maize fields is important for elucidating land surface processes on the Loess Plateau. In this study, water vapor and energy flux observations were used to investigate the evapotranspiration (ET) partitioning and energy budget in a rainfed spring maize field on the Loess Plateau in 2015 and 2016. Over this 2-year period, the total ET was 851 mm, slightly higher than the total precipitation (847 mm). Plant transpiration (T) accounted for 59% and 56% of ET during the 2015 and 2016 growing seasons, respectively. The most important factor controlling daily ET and soil evaporation was net radiation (Rn), and the most important factor controlling daily T was the green leaf area index (GLAI) during the growing season; Rn also controlled ET during the non-growing season. Downward longwave radiation offset 83% of upward longwave radiation in both years; 19% and 18% of downward shortwave radiation was reflected back to the atmosphere by the land surface in 2015 and 2016, respectively. During the growing season, latent heat flux was the largest consumer of Rn; during the non-growing season, sensible heat flux was the dominant consumer of Rn. Soil heat flux (G) and imbalance energy (Imb) did not exhibit obvious seasonal variation pattern, and G/Rn and Imb/Rn values were relatively low during different periods. Midday evaporative fraction (EF), daily crop coefficient (Kc), midday Priestley-Taylor coefficient (α), and midday surface conductance (gs) increased linearly with increasing GLAI, and the midday Bowen ratio (β) decreased linearly with increasing GLAI during the growing season. Midday EF, daily Kc, and midday α increased exponentially with increasing midday gs, and midday β decreased exponentially with increasing midday gs during the growing season and the non-growing season; the threshold value of midday gs was approximately 8 mm s−1 in the spring maize field.