Investigating the effect of cropping pattern on changes in underground water level in a part of Mashhad-Chenaran plain aquifer

Introduction

Management of underground water resources in arid and semi-arid regions is of particular importance. Because the drinking water of a huge part of the countries that live in arid and semi-arid regions relies on underground water sources. The ever-increasing need for water following severe socio-economic changes in the country along with successive droughts in recent years has led to the expansion of the use of deep, semi-deep wells and dams. More extraction of underground water by digging deep wells has caused a drop in the underground water level.The underground water level is one of the most important parameters affecting the qanat discharge, preventing land subsidence and permanent rivers recharge. Using new methods to know the changes in the underground water level and simulating the effects of different management methods can be of great help in managing the water resources of an aquifer. Recently, the use of different models and simulation of aquifers has been able to help mankind to achieve its management goals, and groundwater modeling has become an important tool for managing water resources in aquifers. Since Khorasan Razavi province is located in the arid and semi-arid climate zone and exploitation of underground water has a great effect in providing part of the water needed for agriculture, drinking and livestock drinking Management of this valuable resource is essential. But for management, a series of tools are needed to predict and simulate the effects of various factors. One of these tools is model and modeling, which can achieve acceptable results by using some measurable parameters.

Materials and Methods

Mashhad-Chenaran plain is under the main basin of Kashfroud River, which has an area of 990,914 hectares, is located in the north of Razavi Khorasan province, and is under the basin of Qaraqom watershed. Mashhad-Chenaran plain includes a part of Mashhad city, Targaba-Shandiz, Chenaran and a part of south Qochan. The GMS model was used to investigate the effect of implementing the cultivation pattern and its effect on the changes in the underground water level in the northern part of the Mashhad-Chennaran aquifer. The required layers of GMS model are: a: layer of observation wells b: layer of harvesting values, c: layer of feeding aquifer d: layer of boundary of aquifer and e: layer of bottom rock of aquifer. In the simulation of the studied area, the size of the cells in the entire studied area was considered to be 300 x 300 meters. According to the available data and information for running the model in steady state, September 2006 was entered as the initial hydraulic load. Then, the LPF package was used in MODFLOW model to solve the groundwater flow and level, and the groundwater flow of the studied area was simulated in two stable and unstable states.

Results and Discussion

After preparing the coverage needed for the model, the model was calibrated in steady state. After calibrating the model in the steady state by introducing stress periods, the GMS model was calibrated in the unsteady state. The results showed that the GMS model has simulated the water level with proper accuracy after calibration. Examining the chart of observed and simulated values shows that in some of the investigated piezometers, the simulated groundwater level is higher than the observed ones, and the model tends to overestimate.To implement the management scenario, six percent of the water used in all the wells used in the agricultural sector was reduced and the model was re-implemented. The results showed that the six percent reduction of water consumption in the agricultural sector on the level of piezometers was about 5 to 15 cm on average and caused the rise of the underground water level in the studied areas.

Conclusion

One of the main sources of water supply for agriculture, animal husbandry and drinking in Mashhad-Chenaran Plain region is underground water sources. But in recent years, due to excessive exploitation of underground water resources and climate changes, the level of underground water has decreased and has had many economic-social consequences. Therefore, for the optimal management and exploitation of this underground water source, it is necessary to know the changes in the underground water level and predict the effects of various factors on it. One of the tools that can be used to determine the changes in the underground water level and the effect of various factors on it are computer models. One of the important tools for managing underground water resources are the models designed for this issue, therefore the MOFLOW model used in this research has a good ability to simulate the level of underground water in the studied areas in permanent and non-permanent conditions. Examining the chert of the observed and simulated balance by the GMS model showed that a six percent reduction in water withdrawal in the agricultural sector increases the level of underground water, but it does not compensate for the amount of damage caused to the aquifer due to excessive withdrawal and only reduces its effect.