Evapotranspiration and Soil Moisture Dynamics in a Temperate Grassland Ecosystem in Inner Mongolia , China

Precipitation, evapotranspiration (ET), and soil moisture are the key controls for the productivity and functioning of temperate grassland ecosystems in Inner Mongolia, northern China. Quantifying the soil moisture dynamics and water balances in the grasslands is essential to sustainable grassland management under global climate change. We conducted a case study on the variability and characteristics of soil moisture dynamics from 1991 to 2012 by combining field monitoring and computer simulation using a physically based model, MIKE SHE, at the field scale. Our long-term monitoring data indicated that soil moisture dramatically decreased at 50, 70, and 100 cm depths while showing no obvious trend in two other layers (0-10 cm and 10-20 cm). The MIKE SHE model simulations matched well to measured daily ET during 2011-2012 (correlation coefficient R = 0.87; Nash-Sutcliffe NS = 0.73) and captured the seasonal dynamics of soil moisture at five soil layers (R = 0.36-0.75; NS = 0.06-0.42). The simulated long-term (1991-2012) mean annual ET was 272 mm, nearly equal to the mean precipitation of 274 mm, and the annual precipitation met ET demand for only half of the years during 1991 to 2012. Recent droughts and lack of heavy rainfall events in the past decade caused the decreasing trend of soil moisture in the deep soil layers (50-100 cm). Our results show that precipitation dominated the water balances in the grassland, and the balance between ET and precipitation explained the seasonal changes of soil water content in different soil layers. We conclude that the persistent drought and change in precipitation patterns of decreasing large storm events as well as total rainfall were the main causes for the observed drying trend in soils in the 2000s, although we could not exclude other impacts from regional hydrologic alterations, such as groundwater overdraft.