SIMULATING SEASONAL AND INTERANNUAL VARIATIONS OF ECOSYSTEM EVAPOTRANSPIRATION AND ITS COMPONENTS IN INNER MONGOLIA STEPPE WITH VIP MODEL

Aims Evapotranspiration (ET) plays an important role in arid and semiarid temperate grassland where water availability is a major limiting factor for ecosystem functions. Understanding temporal variation of ET can help explain the surface-atmosphere interaction and its ecological function in grassland ecosystems. Partitioning total ET into its components of evaporation from soil (E) and transpiration from plants (T) is important for understanding the biotic and abiotic factors that control water balance. Our objectives were to simulate the seasonal and interannual variations of ET and its components, analyze the contribution of the components to ET and analyze influencing factors.
Methods We used flux data derived from eddy covariance technology over Inner Mongolia steppe (43°32′ N, 116°40′ E), measured LAI and MODIS data from 2003 to 2005 and parameterized VIP (Vegetation interface processes) model to simulate ET of the grassland. The results were validated using half-hourly latent heat fluxes (LE) and net radiation (R_n) estimated from eddy covariance measurements.
Important findings VIP model can effectively simulate latent heat fluxes of the grassland (R²=0.80). In 2003 and 2004, precipitation (P) was near average and annual ET was 337 and 338 mm, respectively, which were greater than P. In the drier year of 2005, annual ET was 223 mm, which was higher than P. On average, E and T made relatively equivalent contributions to ET. About 83% of annual ET occurred during the growing season. E was the primary component of ET before June and was exceeded by T after that. The monthly totals of both ET and T reached maxima in July and August. Total ET during July and August accounted for 43% of the annual amount. ET was strongly correlated with LAI and moderately correlated with P. E changed little during the growing season, and the difference in ET was accounted for T.