An Estimation of Appropriate Depth and Spacing of Subsurface Drains Based on Drainage Water Quality

The dominant traditional view to designing drainage systems explains drainage goals only in terms of crop yield increase and rhizosphere improvement while ignoring the environmental impacts of drainage projects. Nowadays, this view has been changed into a new one in which both environmental and agricultural purposes are taken into consideration. Depth and spacing of subsoil drains are important parameters in designing drainage systems, the proper determination of which can improve the system's performance. In this research, experimental and numerical Visual MODFLOW models were employed for simulation of water flow and salt transfer to pipe drains; and the effects of depth and drain spacing on quantity and quality of drain water were considered in either model. On the basis of the results obtained from the experimental model and the numerical simulation of flow for various depths and spacing of drains, a formula was presented and calibrated. Chemical characteristic indices of drain water (EC and TDS) can be determined based on this formula, considering drains depth and spacing, hydraulic head, recharge rate, and index quantities of chemical characteristics of inflow and of groundwater. This formula was validated through a comparison of computed and observed hydraulic heads for various placements positions of drains on the basis of two statistical characteristics of RSME and R2. The obtained mean values of 0.154 cm and 0.994 for these statistical indices, respectively, indicated the acceptable accuracy of the formula.