Effect of Drain Spacing on Water Table Depth and Soil Salinity

A computer model was made to find drain spacing with Hooghout equation then SALTMOD was applied for water and salt balances of four different soils. Crop rotation with alfalfa in 50% of the land as continuous crop(in winter and summer) and 50% of the land wheat in winter then maize in summer season. Drain spacing can be increased than that calculated with Hooghout equation according to soil type and critical water table depth, So salinity, and water table depth were increased without exceeding root zone depth, in clay loam soil. Whereas it's increase has little effect on loamy sand soil, therefore, the best drain spacing can be chosen according to acceptable salinity in the soil . SALTMOD gives, depending on cumulative Gumbel distribution, a reasonable indication of the predicted change in soil salinity. Under the proposed conditions of irrigation and drainage after, 10 years , the root zone salinity in 80% of area irrigated with 0.5 ds/m water, was predicted to increase in summer to ,9.46, 8,7.53, 4.3 ds/m in the clay loam, loam, sandy loam, loamy sand soil respectively. Keywords :SALTMOD , root zone salinity ,water table depth.