تحلیل روند تغییرات عمق و خشکسالی آب‌های زیرزمینی در آبخوان مشهد-چناران با تأکید بر تأثیر تغییرات کاربری اراضی و شاخص‌های اقلیمی

Abstract

Introduction:

Groundwater is a vital resource, particularly in arid and semi-arid regions, where surface water availability is limited. Sustainable groundwater management is crucial for agriculture, industry, and domestic consumption. However, excessive groundwater extraction, rapid urbanization, and climate variability have led to significant declines in groundwater levels worldwide. In Iran, one of the most concerning cases of groundwater depletion is observed in the Mashhad-Chenaran aquifer, which has experienced a persistent decrease in groundwater levels over the past decades.

Understanding the factors influencing groundwater depletion is critical for developing appropriate mitigation strategies. Various studies have investigated the role of climate variability and human activities in groundwater level fluctuations. While precipitation deficits and prolonged droughts can contribute to declining groundwater recharge, human-induced factors such as land use changes, agricultural expansion, and unregulated groundwater extraction often play a more dominant role.

Despite the importance of this issue, a comprehensive assessment combining groundwater trend analysis, drought evaluation, and land use change impacts remains limited in the Mashhad-Chenaran aquifer. Therefore, this study aims to analyze the long-term trend of groundwater level changes and groundwater drought conditions, with a particular emphasis on the impact of land use changes and climatic indices. By integrating hydrogeological, meteorological, and remote sensing data, this study provides a multifaceted approach to understanding groundwater depletion and its driving factors.

Methods:

In this study, the trend of groundwater depth changes in this Mashhad-Chenaran aquifer was analyzed using time-series data from 36 piezometers between 1989 and 2020. Modified Mann-Kendall and Sen's slope tests, along with the Pettitt test, were employed to examine groundwater depth trends and identify time-series breakpoints. The Mann-Kendall test is a non-parametric method widely used to identify monotonic trends in hydrological and environmental time series, while Sen’s slope provides an estimate of the rate of change over time. To further investigate the groundwater changes over the study period and evaluate groundwater drought conditions, the Standardized Groundwater Index (SGI) was calculated for each well. This index is particularly useful in distinguishing normal, moderately dry, severely dry, and extremely dry periods. By analyzing SGI over the 32-year study period, this research identifies critical drought periods and their spatial distribution across the aquifer. In addition, to assess the relationship between anthropogenic and meteorological droughts, Palmer Drought Index (PDSI) values in the location of each well were obtained from TerraClimate product and correlations between SGI and PDSI were calculated. To investigate the anthropogenic effects on groundwater depth changes, GLAD land cover maps from 2000 to 2020 were used. These maps, derived from Landsat satellite imagery, provide high-resolution information on urban expansion, agricultural growth, and natural land cover transformations. By comparing land use classifications at different time intervals, the study quantifies the extent of urbanization and agricultural intensification.



Results:

The trend analysis revealed that most piezometers exhibit an increasing trend in groundwater depth, with one-quarter of them experiencing an increase in depth with a slope exceeding one meter per year. Furthermore, the transfer of water resources to the city of Mashhad has led to some piezometers showing a decrease in depth and water table rise. The SGI index revealed that groundwater drought in a significant number of piezometers began between 2001 and 2006 and continues to intensify. Over 20 years, since 2000, urban land use within the aquifer region has doubled, reaching approximately 745 square kilometers, which has noticeably impacted some piezometers. Additionally, agricultural land use, the dominant land use type in this area, has increased by about 167 square kilometers. To further investigate the influence of land use and land cover types, the changes of urban and agricultural class were investigated in 5-year periods. While urban land use has been constantly increasing over the whole period, agricultural land use experienced significant growth between 1999 and 2004. This could be one of the reasons for the onset of the drought period and the presence of a breakpoint between 2001 and 2006 in many piezometers. To evaluate the impact of meteorological drought on groundwater drought, the correlation between the Standardized Groundwater Index (SGI) and the Palmer Drought Severity Index (PDSI) was calculated for each piezometer. The lack of correlation between these two indices in most piezometers suggests a significant role of human activities in groundwater level decline in the region.