Estimation of Soil Hydraulic and Solute Transport Parameters Using HYDRUS-2D (Case Study: Qazvin Interception Drain)

The recent growth in population has led to an increase in food supply requirements which, in turn, has resulted in an increase in groundwater exploitation. In areas where fresh and saline aquifers are adjacent, the overexploitation of fresh groundwater leads to a decrease in fresh ground water level and, as a result hydraulic gradient of saline groundwater towards fresh groundwater increases. Also, a decrease in fresh groundwater level leads to a decrease in the plain discharge. This way, the saline water cannot exit the plain and pressure head rises leading to extra waterlogging and salinization. Interception drain is an effective solution to intercept saline flow and discharge it out of the plain. In effect, the drainage system reduces saline water table level and prevents waterlogging and salinization. Salt marshes of Qazvin plain, located in the south-east of Qazvin, is facing the same problem and saline soil areas in the region have accordingly increased 10000 ha in recent years (Saman Abrah and Kamab Pars engineers, 2010). An interception drain system was thus implemented in the study area in order to reduce the saline water table level and prevent waterlogging and salinization. Following the construction of the drain in the region, 99 observation wells were also drilled around the drain in 9 sections (A-I) such that each row has 11 observation wells so as to monitor the groundwater fluctuation and salinity. At each section, 7 wells were located on the upslope and the remaining 4 wells were on the downslope (Sotoodehnia et al, 2014). However, it takes a lot of money and time to measure water table level and salinity from observation wells continuously. Modelling is indeed a cost-effective way to predict the water table level and salinity in future. HYDRUS is a powerful tool that can be used to simulate and monitor the drain situation reliably, but it should be calibrated for the study area. Calibration of HYDRUS means estimating soil hydraulic and solute transport parameters that can be done with HYDRUS inverse solution ability. Using HYDRUS-2D, Abbasi and Tajik (2007) estimated both soil hydraulic and solute transport parameters in the field scale. They also compared the measured and simulated values of soil moisture and salinity. Though soil moisture was almost overestimated, and salinity was almost underestimated, the results showed that the errors were small and the model was accurately calibrated (Abbasi and Tajik, 2007). The main objective of this study was thus to assess HYDRUS-2D ability for simulating solute transport in a very saline aquifer in the study area, and also to consider saturated and unsaturated regions simultaneously. Moreover, this study employed HYDRUS-2D to simulate salinity transport in the areas where the amounts of salinity was extremely high. According to Latifi et al. (2020), evaporation is an effective factor in groundwater fluctuations.This study also examines the effect of evaporation and aquifer thickness in an inverse solution. Aquifer thickness and evaporation have an important effect on the inverse solution due to the statistical indexes involved. Thus, some statistical indexes were carried out to assess the accuracy of calibration, and the results showed that the model was calibrated accurately.