A computer model is developed to find the drain spacing using dynamic equilibrium and peak irrigation period and steady state concepts. Then sensitivity analysis is done to study the impact of drain depth, soil and crop type, salinity of irrigation water and irrigation scheduling on the drain spacing calculated with the dynamic equilibrium .Three different soil textures and three crops, wheat, potato, corn are chosen with surface irrigation.
The drain spacing is influenced by the crop and soil type. In general, the drain spacing with the dynamic equilibrium is higher than that of the peak irrigation period and, less than the steady state concept. The most influential factor on the drain spacing, is deep percolation losses for each irrigation, followed by hydraulic conductivity of soil, depth of impermeable layer, permissible water table depth, drain depth and salinity of irrigation water. The drain spacing for wheat is more sensitive to the change in drain depth, permissible water table depth, depth of impermeable layer, salinity of irrigation water and hydraulic conductivity of the soil, while that for potato is more sensitive to the change in deep percolation losses .