Sub-area

A runoff-evaporation (RE) land surface hydrological model, which was developed by Hu et al. (2004) and Tang et al. (2004), was used to calculate the water balances inside the two sub-areas. A schematic diagram of the RE model is presented in Fig. 4. The model consists of three land use components: irrigation area, native plants land and wasteland with the associated surface and ground water dynamic relationships defining the interactions among them. The irrigation area is described by seven crop types: cotton, wheat, paddy, corn, melon, gardens and irrigated pasture. The three components are connected through a ditch system (surface water) and groundwater exchange. The field infiltration and phreatic evaporation are considered because the water table is shallow in the study area. The water balance of the ditch system, irrigation area and non-irrigation area are then calculated according to the model.