Study of Common Drainage Equations’ Accuracy in Determination of Subsurface Drains Distance in Moghan region

Due to low rainfall and high evaporation in climatic conditions of Iran, improper management of irrigation practices has resulted to salinity and waterlogging of many lands. By attention to importance of the research on drainage issue and drainage problems of 20,000 ha agricultural lands in Moghan region, this research was conducted to identify the appropriate equation for determining subsurface drains’ spacing (s.d.s.) in an experimental field with the area of 18 ha, located in section 807, branch 3, of Agro-Industry of Moghan. Three drainage systems with spaces of 50, 100 and 150 meters between the drainage pipes having 200 meters’ lengths, were constructed in the experimental field. After beginning the irrigation, the measurements of water levels and drainage out-flows were performed. Comparison of the methods for determining the suitable distances between the subsurface drains showed that the Hammad, Bouwer & van schilfgard and vanschilfgard equations showed the most accurate results with 3, 6 and 7 average deviation percentages from subsurface drains’ actual spaces, respectively. Among the conventional equations for determining the s.d.s., the Bouwer & Vanschilfgard, Dumm, Glover and Vanschilfgard equations were recommended, respectively. The accuracy of these equations did not have a considerable difference with each other. Although, the efficiencies of the established drainage systems with 50 and 150 meters’ distances between drains were approximately identical, a system with 100 meters’ distance for drains was recommended to increase the degree of reliability.