Effects of evaporation rate on groundwater level and drainage coefficient by means of HYDRUS-2D software (Case study: Interception drain of Qazvin plain)

Fresh groundwater overexploitation causes many problems the most important of which are saline water expansion to upstream lands and enhancing salt concentration in shallow groundwater conditions. Interceptor drains can control salinity expansion and decrease saline water hydraulic gradient to fresh water resources. Also, in arid and semi-arid regions evaporation plays an important role in decreasing hydraulic head. In this research HYDRUS-2D has been used in order to assess the effect of evaporation on groundwater depth and drainage coefficient in Abyek interception drain. This salt marsh has located in the southeast of Qazvin and the purpose of installing an interceptor drain in this area is to lower the saline groundwater level and control the salinity expansion till upstream lands gradually improve by soil leaching due to rainfall and groundwater flow and drainage outflow. Simulation has been done in 90 days for three soil texture: clay, loam and sand and four evaporation rate including: low, medium, high (0.2, 1, 2.5 ) and without evaporation. Results showed that in clay evaporation has a significant role in causing a decrease in water table level and drainage coefficient. As the evaporation rate increased from 0 to 0.2  groundwater level drop increased 16.5% and drainage coefficient decreased 20.6%. in loamy soil these values were 1.9% and 1.7% respectively. As a result, the effect of evaporation is less in loamy soil and low evaporation rate can be ignored. Also, low and medium evaporation rates in sandy soils are ineffective in causing ground water level decrease and reducing drainage coefficient. While high evaporation rate with 6.9% increase in water level fall and 15.7% decrease in drainage coefficient is effective and should be measured accurately and consider in simulation. The results of this study are consistent with the work of researchers who have worked in related fields and it is suggested that special attention should be paid to effect of evaporation in drainage studies specially in clay soils.