Evapotranspiration from a Mongolian steppe under grazing and its environmental constraints

The magnitude and seasonal dynamics of evapotranspiration (ET) for a steppe in central Mongolia was estimated over a full year period (from 25 March 2003 to 24 March 2004, 366 days) using the eddy covariance (EC) technique. The steppe, typical of central Mongolia, is dominated by temperate C3 plants and experiences moderate grazing. The environmental constrains over ET for the steppe were evaluated by examining the responses of ET to biotic (leaf area index) and abiotic (atmospheric evaporative demand and soil moisture condition) factors. Seasonal variations in ET followed closely the variation in leaf area index. Change in soil moisture was the most important environmental factor controlling the dynamics of ET in this grassland ecosystem, indicated by the strong susceptibility of ET and the Priestley-Taylor parameter ((alpha), calculated as the ratio of the measured ET to the equilibrium ET) to soil water content. The (alpha) also showed a distinct seasonal variability, but its value on average was lower than 0.5 during most of the measurement period, suggesting that the steppe was limited in water supply. The maximum daily ET rate was 2.8 mm d^-1. Cumulative ET during the study period estimated directly by the EC method was 163 mm, which was 66% of the precipitation received at the site during the same time period (248 mm).