Grazing intensity effects on the partitioning of evapotranspiration in the semiarid typical steppe ecosystems in Inner Mongolia

Grazing intensity effects on the evapotranspiration (ET) partitioning into evaporation (E) and transpiration (T) were investigated using eddy covariance measurement data during the growing season (May to September) from 2005 to 2008 over typical steppes in Inner Mongolia, China. Four grazing intensities were examined (UG79, ungrazed since 1979; WG, winter grazed; CG, continuously grazed; HG, heavily grazed). Soil water content (SWC) significantly influenced daily T/ET by affecting T and E in different ways. On a monthly scale, T/ET at UG79 (0.46-0.77) increased with vegetation growth regardless of the uneven distribution of precipitation. The mean T/ET at UG79 for the growing season ranged from 0.50 to 0.74 for 2005 to 2008. In the normal precipitation year (2006), WG and HG reduced T/ET because they significantly reduced T through the removal of leaf area. In the dry year (2005), this negative effect on T was depressed by summer droughts, while WG and HG decreased E primarily due to the decreased interception of rainfall by the canopy. The grazing effect on T/ET was opposite to that in 2006. These results indicate that grazing effects on ET partitioning could vary with soil water conditions in the semiarid typical steppe ecosystems.