Three-year Variations of Water, Energy and CO$_2$ Fluxes of Cropland and Degraded Grassland Surfaces in a Semi-arid Area of Northeastern China

Based on 3 years (2003--05) of the eddy covariance (EC) observations on degraded grasslandand cropland surfaces in a semi-arid area of Tongyu (44 25'N, 122 52'E, 184 m a.s.l.),Northeast China, seasonal and annual variations of water, energy and CO2 fluxes have been investigated.The soil moisture in the thin soil layer (at 0.05, 0.10 and 0.20 m) clearly indicates the pronounced annualwet-dry cycle; the annual cycle is divided into the wet (growing season) and dry seasons (non-growingseason). During the growing season (from May to September), the sensible and latent heat fluxes showed alinear dependence on the global solar radiation. However, in the non-growing season, the latent heat fluxwas always less than 50 W m-2, while the available energy was dissipated as sensible, rather thanlatent heat flux. During the growing season in 2003--05, the daily average sensible and latent heat fluxes werelarger on the cropland surface than on the degraded grassland surface. The cropland ecosystem absorbed more CO2than the degraded grassland ecosystem in the growing season in 2003--05. The total evapotranspiration on thecropland was more than the total precipitation, while the total evapotranspiration on the degraded grasslandwas almost the same as the total annual precipitation in the growing season. The soil moisture had a goodcorrelation with the rainfall in the growing season. Precipitation in the growing season is an importantfactor on the water and carbon budget in the semi-arid area.