Groundwater recharge modeling using semi-distributed SWAT Model, case study: Marvast Plain

Groundwater recharge or deep drainage or deep percolation is a hydrologic process where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. Groundwater recharge depends on several factors such as infiltration capacity, stochastic characteristics of rainfall, and climate factors. Groundwater recharge is of great importance especially in semiarid regions. In arid and semi-arid regions of the world, groundwater serves as an essential alternative to surface water resources for water supply purposes. It plays a significant role in meeting the water demands of man and the ecosystem and is perceived as the panacea to the looming water scarcity scare. Determination of recharge quantity provides worthy help for managers in water resources management. Ground water recharge includes recharge as a natural part of the hydrologic cycle and human-induced recharge, either directly through spreading basins or injection wells, or as a consequence of human activities such as irrigation and waste disposal. The Soil and Water Assessment Tool (SWAT) is a river basin scale model developed to quantify the impact of land management practices in large, complex watersheds. SWAT is a public domain hydrology model with the following components: weather, surface runoff, return flow, percolation, evapotranspiration, transmission losses, pond and reservoir storage, crop growth and irrigation, groundwater flow, reach routing, nutrient and pesticide loading, and water transfer. SWAT is a continuous time model that operates on a daily time step at basin scale. Its objective is to predict the long-term impacts of management and of the timing of agricultural practices within a year (i.e., crop rotations, planting and harvest dates, irrigation, fertilizer, and pesticide application rates and timing). It can be used to simulate at the basin scale water and nutrients cycle in landscapes whose dominant land use is agriculture. It can also help in assessing the environmental efficiency of best management practices and alternative management policies. In this study, the hydrologic process of the Marvast basin was simulated using SWAT Model in order to determine the amount of groundwater recharge in Marvast plain. In this way, firstly, the required maps i.e. slope, soil and land use maps were produced. In order to produce land use maps, panchromatic and multi-spectral imagery were fused to enhance the spectral and spatial resolution of Landsat imagery. In the next step, the fused imagery was used to produce land use maps using pixel based and object oriented image processing techniques. The slope map was produced using digital elevation model. The soil map was also produced using soil profiles in the regions. The requisite climatic data were also imported to the model with a daily scale. According to the importance of irrigation and its effect on evapotranspiration and groundwater recharge, irrigation amounts ​​were also considered importing irrigation plan in SWAT Model. Afterwards, the model was calibrated using SWAT CUP software and the SUFI-2 algorithm. Finally, the verification showed that the model with Nash-Sutcliff of 0.59, coefficient of determination of 0.83 and the root mean square error of 0.05 has a relatively good performance. The results showed that object oriented image processing technique outperformed pixel based technique. It was shown that the amount of groundwater recharge was 27082602 cubic meters and the irrigation water return coefficient is 34%. It was confirmed that SWAT Model has a relatively good performance for groundwater recharge modeling. Improving the cropping pattern, preventing development of unauthorized wells and excessive groundwater withdrawals, as well as proper irrigation systems, can be effective in reducing the groundwater storage deficiency and preventing an increase in water resource crisis. This study showed that this model is not efficient for short term runs, however, its performance is better for long term runs. It is suggested that the SWAT and MODFLOW Model be used together to study both surface and underground currents. Also, lysimeters or SWAP Model can be used to better determine the amount of return flow and groundwater recharge.