Seasonal and interannual variations in water vapor exchange and surface water balance over a grazed steppe in central Mongolia

This study explored the seasonal and interannual variation in water vapor exchange and surface water balance over a grazed steppe in central Mongolia through analysis of 4 years (2003-2006) of flux data obtained via the eddy covariance method. Annual precipitation (PPT) in 2003 measured 239 mm which is 32% above the 10-year (1983-2002) average of 181 mm. By contrast, PPT for the other 3 years of the study fell below the 10-year average, measuring 159 mm in 2004, 110 mm in 2005, and 119 mm in 2006. The annual evapotranspiration (ET) for each of the study years measured 156, 160, 153, and 101 mm, respectively, and the peak value of ET during the growing season varied from 2.2 to 3.2 mm d⁻¹. At the study site, the ratio of ET to the equilibrium ET (ET_eq) was usually lower than 0.5 during the growing season, which reflects the significant effect of water shortage on ET. The large seasonal variation in canopy surface conductance (g_s), caused by variation in soil water content (SWC) and vapor pressure deficit (VPD), was the major factor affecting ET. The annual ET/PPT was 0.65 in 2003, 1.01 in 2004, 1.39 in 2005, and 0.85 in 2006. The stored soil water (especially at a depth of 0-30 cm) resulting from autumnal precipitation of the previous year remained frozen for about 5 months, from winter through early spring. This stored water had a considerable effect on plant growth during the following spring. For the central Mongolian steppe, there was a high correlation between the mean normalized difference vegetation index (NDVI) and total precipitation during the growing season (May-September) as well as during the preceding 9 months (August-April). This correlation reflects the important contribution of precipitation input and stored soil water during the previous year to ET.