Although nature has the ability to deal with change, it cannot tolerate the growth of industries, deforestation and degradation of the environment. These huge and abrupt changes have caused to increasing the destruction of nature in recent decades, and increasing greenhouse gases. Ground water plays an important role in sustaining ecosystems and enabling human adaptation to climate variability and climate change. Groundwater resources are the largest store of fresh water in arid and semi-arid regions. In the areas where surface water resources are limited and inaccessible for people, groundwater can be considered as a secure resource. Climate change caused fluctuations in the ground water. Many wells dried up and some of them have lost their advantage due to the lack of available water. Climate change through changing climatic variables (including temperature, precipitation and evapotranspiration) directly affects surface water resources in many parts of the country. Besides, there have been many related problems such as drying up wells, reducing river flow, decreasing water quality, subsiding in lands, diminishing fresh water supplies and increasing salts in water. Climate change and its associated problems appear to be a serious challenge in the regions as Khorasan Razavi that 32 of 34 desert plains were declared prohibited. Due to cold and dry weather conditions and use of aquifers and wells as the main sources of water, Mashhad relies on groundwater as well as underground resources. The water resources of this plain are affected and threatened by changes in groundwater levels, temperature, rainfall, and frequent droughts. This research aims to study the groundwater level decline in Mashhad plain which is located in Khorasan Razavi province, northeastern of Iran.
The study tries to conduct a comprehensive study through the analysis of monthly precipitations data acquired from 34 synoptic, climatology and rain-gauge stations, monthly discharge of 13 hydrometric stations, and 60 groundwater piezometric wells. It confirmed the normality of climatic data, using the Kolmogorov-Smirnov test. The research also used multivariable regression methods as well as HadCM3 climate model scenarios to model groundwater perspective of Mashhad’s plains and to predict future climate variables, respectively. To compare the output scale of the mentioned model with the required scale of climate change studies, the data of temperature and precipitation were downscaled for the two periods of 2015-2030 and 2046-2065, under three scenarios of A1B, A2 and B1 using the LARS-WG5.5 model. It later employs geographical weighted regression (GWR) to model the ground water level (as the dependent variable) with climatic parameters of temperature, precipitation, and evapotranspiration (as independent variables).
Mashhad’s prospect of precipitation and temperature revealed a rainfall reduction for the cold season, its increase in the warm season, and at least one-degree increase compared to the base year. Modelling of groundwater level changes (using the parameters of rainfall, temperature and evapotranspiration) showed decline in groundwater levels in the range of at least 0.2 meters and maximum 0.7 meters. The research provides an intelligent simulation for fluctuations of groundwater using multiple regression models and the historical data of rainfall, temperature and evapotranspiration, which helps to estimate underground water level in the coming years. The results indicate a drop in the water level of all studied piezometric wells, under three climate change scenarios for future periods. The study concluded that the most important reason of water loss in Mashhad’s plains is the human overuse of groundwater. Fluctuations in groundwater assessment have shown that the highest annual fluctuations of groundwater were occurred in the southern part of Mashhad plain. Fluctuation assessments of surface water showed negative fluctuation in upstream and positive in downstream. These issues reflect the role of human factors in increasing the groundwater level in downstream and reducing the level in upstream. Modeling of groundwater level fluctuations with climate parameters (temperature, precipitation and evapotranspiration), using GWR and OLS models, represents superior of geographical weighted regression model compared with ordinary least squares models. Fitting weighting matrices to GWR model showed high accuracy of Gauss method than the other studied methods. The important issue that was obtained from simulations of Mashhad station is reducing the variance of rainfall in A1B emission scenario, indicating the absence of stable conditions in the future. The results of this research showed that the most important reason for the drop in Mashhad plain is overuse of groundwater.