Abstract

The sub surface flows resulting by rainfall are one of the main sources of supplying water. In these conditions, swap of agricultural lands in different seasons is one of the main problems of farmers which are caused by an increase in the level of subsurface water or impropriety of the soil texture. In such conditions, the sodium and salty soil cause a decrease in the efficiency of crops harvest. In addition, water collected from rainfall is one of water resource in urban and non-urban areas, therefore determining the drainage coefficient for estimating drainage water of it is very important.  Therefore, in this study, different methods of determining the drainage coefficient in sodium and salty soil have been investigated. The obtained results have shown that the maximum of drainage coefficient in steady method is 2.4 mm/day. However, this coefficient changes throughout the day according to the irrigation calendar method from the minimum 0.37 to the maximum 2.49 mm/day. The results have indicated that the amount of the obtained drainage coefficient by steady method is always more than the amount obtained by unsteady method. But the amount of this coefficient is the equal in the beginning of summer using both methods. Therefore, using steady method causes an increase in the estimated subsurface flow amount so it is suggested to use the unsteady method to estimate the drainage coefficient.