Simulation of quality and quantity of outflow from subsurface drains, using system dynamics

In many irrigated areas, shallow water table causes water logging and salinity of the soils, thus usually subsurface drainage systems are used to control this situation. In most cases, subsurface drainage systems discharge a large amount of water and salt to the environment, hence drainage water management is necessary to prevent this situation. Drainage is an action that ensures the sustainable use of land, in addition to increasing the performance. The traditional view in the drainage systems’ design, has known the drainage aim, unique to increase the yield and improve the grown environment and does not examined the environmental impacts of drainage projects. Today, this attitude has lost its position in the new approaches, in addition to agricultural and industrial purposes, environmental goals of the drainage projects are also examined. In this study the quantity and quality of output drained water from the drainpipes as compared to the time were simulated using the method system dynamics analysis, and were compared to in vitro model data. The effect of changing the depth of drainage pipes, distances between drainage on the drained water volume, and the amount of output salt were investigated. For the experiments in this study, the physical laboratory model of Water Resources, in the Faculty of Agriculture, University of Tabriz was used. Physical model is a rectangle cube-shaped container, metal-glass, with 2 m length, 1 m width and 0.5 m height. In order to determine the position of water table in the soil, 20 piezometers were put at the bottom of the model. Four reticulated metal pipes with a diameter of 2 cm were embedded as drainage pipes in the model. Tests were performed with depths of 0.4, 0.3 and 0.2 m from the soil surface, and three distances of 1.4, 1 and 0.6 m between the drains. In the upper part of the model, a sprinkler irrigation system consisted of four lateral and eight water sprayers, were placed at the height of 0.6 m above the soil surface: they were used to produce precipitation and irrigation practices. In this research, the VENSIM programming software, which is one of the most appropriate programs for dynamic evaluation of systems was used for modeling. This software shows the performance of the system during the simulation, with repeated solving of various equations in the system, by limited different methods. In order to evaluate the performance of the systems, various tests on water table level, drainage water quantity, salinity of drained water and groundwater salinity were done. The water table level increases through time and remains constant at a steady state. The output discharge from drains also increases during time and reaches to a steady state by fixing the water table and reducing its fluctuations. The salinity of drained and groundwater decreased during the experiment and approached to the amount of irrigation water salinity (0.45 dS/m), due to the mixing of irrigation water with low salinity groundwater with the higher salinity. The salinity of output drainage also decreases causing reduction of the groundwater salinity. Drainage volume, and the amount of drainage water EC, were measured in order to investigate the effect of changing the depth and distance between drainage pipes on the quality and quantity of the drainage water in each establishment. By increasing the depth and distance between drainage tubes, the length and depth of the water lines, towards drain pipes, increases and the share of groundwater participating in the output drainage increases, which leads to the increase of output’s salinity. This increase in depth and distance, also increases the soil storage volume in the drainage pipes, and increases the discharge of drainage water. The simulation data were compared with the measured values and correlation coefficients for output discharge, salinity of drainage water, water table level and groundwater salinity in trial mode (drain spacing: 1.4 m, depth: 0.4 m), which were 0.75, 0.92, 0.98 and 0.97, respectively. In addition, the values of the RMSE statistical indicator for these parameters were obtained as 0.1875 mm/min, 0.1684 dS/m, 0.0081 m and 0.0825 dS/m, respectively, indicating that the simulation results have the higher accuracy. The evaluation of quantity and quality of drains’ discharge cumulatively shows that the increase in drains’ depth and spacing causes the increase of the volume of drainage water and salt.