Irrigation area

According to the RE model, the water balance of the irrigation area can be summarized as:

D_o + P_i + G_i - E_i - D - X - S_is - S_ig = 0 (4)

P_iprecipitation over irrigation area G_igroundwater recharge to irrigation area E_ievaporation from irrigation area Ddrainage Xgroundwater exchange between irrigation and non-irrigation area S_issoil water storage change in irrigation area S_iggroundwater storage change in irrigation area where, P_i is precipitation over irrigation area, G_i is groundwater recharge to irrigation area, E_i is evaporation from irrigation area, D is drainage, X is groundwater exchange with non-irrigation area, S_is is soil water storage change in irrigation area, and S_ig is groundwater storage change in irrigation area. Groundwater recharge to irrigation area consists of irrigation ditch penetration, river seepage and groundwater recharge from mountain torrent and spring. Evaporation from irrigation area (E_i) is characterized by reference evapotranspiration, together with crop factor and soil moisture. The reference evapotranspiration is calculated from the FAO Penman-Monteith method (Richard et al., 1998). The integrative crop factor over the irrigation area is computed as the sum of crop types weighted by the respective area fractions. An irrigation area model with two unsaturated layers and one groundwater layer was used to estimate irrigation area evaporation affected by soil moisture, and the change in soil water storage in irrigation area ( S_is) was characterized in the model. The field infiltration and phreatic evaporation are considered in the model because the water table depth is shallow in the study area. This water exchange between the unsaturated zone and the groundwater also changes the groundwater storage in the irrigation area ( S_ig) and the water table. The drainage (D) is therefore estimated from the difference between water table and drainage level in the following way (McDonald et al., 1988):

D = (5)

drainage coefficient A_ddrainage area hwater table level h_dthe bottom level of drainage where, is a drainage coefficient, describing water table decrease ratio due to the difference between water table and drainage level, A_d is drainage area, h is water table level, and h_d is the bottom level of drainage. The drainage coefficient () depends highly on the underlying soil conductivity and the average distance between the drainage ditches. It is a regionalized parameter which represents the drainage system state. The drainage area (A_d) means the area where groundwater can recharge the drainage. Because the drainage system is fully developed, the whole irrigation area is taken as drainage area in this study. The groundwater exchange with non-irrigation area (X) is also estimated based on the water table difference between irrigation and non-irrigation area.