تاثیر الگوی کشت بر تغییرات تراز آب زیرزمینی در بخشی از آبخوان دشت مشهد-چناران

بخش اعظم منابع آب زیرزمینی برای تولیدات کشاورزی استفاده می‌شود. یکی از برنامه‌هایی که می‌تواند در کاهش برداشت آب زیرزمینی برای کشاورزی کمک کند برنامه‌ الگوی کشت ابلاغی است. در این برنامه علاوه بر تامین امنیت غذایی به بهره‌وری آب و کاهش مصرف آن توجه شده است. هم‌چنین، تراز آب زیرزمینی یکی از مهم‌ترین متغیرهای تاثیرگذار بر آب‌دهی قنوات، جلوگیری از فرونشست زمین و تغذیه‌ رودخانه‌های دائمی است. لذا هدف از این پژوهش بررسی تاثیر اجرای الگوی کشت روی تغییرات تراز آب زیرزمینی با استفاده از مدل gms در بخشی از آبخوان دشت مشهد-چناران است. برای این منظور از مدل modflow در بسته‌ نرم‌افزاری gms استفاده شد و لایه‌ چاه‌های مشاهده‌ای، لایه‌ مقادیر برداشت، لایه‌ تغذیه‌ آبخوان، لایه مرز آبخوان، لایه‌ سنگ کف آبخوان برای مدل تهیه و به‌عنوان پوشش به مدل معرفی شد. سپس مدل واسنجی و ارزیابی شد و مقادیر تراز آب زیرزمینی شبیه‌سازی شد. پس از شبیه‌سازی تراز آب زیرزمینی سناریوی مدیریتی کاهش شش درصد آب مصرفی که در برنامه‌ الگوی کشت به آن اشاره ‌شده است به مدل gms وارد شد و میزان تاثیر آن روی تراز آب زیرزمینی برآورد شد. نتایج نشان داد مدل gms توانایی خوبی برای شبیه‌سازی تراز آب زیرزمینی در محدوده‌های مورد مطالعه دارد و مقدار mae و rmse به‌ترتیب 75/0 و 88/0 برای حالت پایدار و در حالت ناپایدار به‌ترتیب 40/1 و 20/2 به‌دست آمد. هم‌چنین، نتایج نشان داد کاهش شش ‌درصدی آب مصرفی در بخش کشاورزی روی تراز پیزومترها به‌طور متوسط حدود پنج تا 15 سانتی‌متر بوده است و باعث بالا آمدن تراز آب زیرزمینی در محدوده‌های مورد مطالعه شده است. نتایج حاصل از مدل‌سازی اعمال سناریوهای مدیریتی روی منابع آب زیرزمینی می‌تواند ابزاری مناسب برای مدیران در برنامه‌ریزی دقیق و بهره‌برداری بهینه و منطبق بر پایداری آبخوان باشد.

effect of cropping pattern on changes in groundwater level in a part of mashhad-chenaran plain aquifer

introduction management of groundwater resources in arid and semi-arid regions is particularly importantbecause the drinking water of a large part of the countries that are in arid and semi-arid areas relies on groundwater sources. the ever-increasing need for water due to severe socio-economic changes in the country and successive droughts in recent years has led to the expansion of deep and semi-deep wells and dams. excessive extraction of groundwater by digging deep wells has caused a drop in the groundwater level. the groundwater level is one of the most important parameters affecting qanat discharge, preventing land subsidence, and ensuring permanent river recharge. using new methods to know the changes in the groundwater level and simulating the effects of different management methods can be of great help in managing aquifer water resources. recently, the use of different models and simulations of aquifers has helped researchers achieve better management goals, and groundwater modeling has become an important tool for managing aquifer water resources. since khorasan razavi province is located in the arid and semi-arid climatic zone, the exploitation of groundwater plays a significant role in providing part of the water needed for agriculture, drinking, and livestock farming. thus, management of this valuable resource is essential. however, for effective management, a series of tools are required to predict and simulate the effects of various factors. one of these tools is modeling, which can yield acceptable results using some measurable parameters.materials and methods mashhad-chenaran plain falls under the main basin of the kashfroud river, which has an area of 990,914 ha and is located in northern khorasan razavi province within the qaraqom watershed. mashhad-chenaran plain includes a part of mashhad city, targaba-shandiz, chenaran, and a section of southern quchan. the gms model was used to investigate the effect of implementing the cultivation pattern and its effect on groundwater level changes in the northern part of the mashhad-chennaran aquifer. the required layers of the gms model are: (a) observation well layer, (b) harvesting values layer, (c) aquifer recharge layer, (d) aquifer boundary layer, and (e) bottom rock layer of aquifer. in the simulation of the studied area, the cell size throughout the studied area was set at 300 × 300 m. according to the available data and information for running the model in a steady state, the initial hydraulic load was set to september 2006. then, the lpf package was used in the modflow model to solve the groundwater flow and level, and the groundwater flow of the studied area was simulated in both stable and unstable states.results and discussion after preparing the coverage needed for the model, the model was calibrated in a 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 accurately simulated the water level after calibration. a review of the observed and simulated values chart shows that in some of the piezometers studied, the simulated groundwater level was higher than the observed one, and the model tended to overestimate the levels. to implement the management scenario, water used in all all agricultural wells was reduced by six percent, and the model was re-run. the results showed that the 6% reduction in water consumption in the agricultural sector led to an increase in the piezometers levels by approximately 5 to 15 cm on average, resulting in a groundwater level rise in the studied areas.conclusion one of the primary sources of water supply for agriculture, animal husbandry, and drinking in the mashhad-chenaran plain region is groundwater. however, in recent years, due to excessive exploitation of groundwater resources and climate changes, t groundwater levels has declined, causing significant socio-economic impacts. therefore, for better management and sustainable utilization of this groundwater source, it is necessary to monitor groundwater level fluctuations and predict the effects of various factors on these changes. one of the best tools for determining groundwater level fluctuations and assessing the impact of different factors is computer modeling. groundwater models play crucial roles in managing these resources, and the modflow model used in this research proved effective in simulating groundwater levels in both steady and unsteady conditions. analysis of the observed and simulated balance using the gms model demonstrated that a six percent reduction in agricultural water withdrawal could raise groundwater levels. however, this increase does not fully compensate for the damage caused by excessive withdrawal and only mitigates its effects.