Total Water Storage Changes in Iran's Main Basins According to the GRACE Gravimetry Mission

The GRACE gravimetry mission included two satellites measuring Earth's gravity field changes from 2002 to 2017. These changes are primarily due to variations in groundwater and surface water storage. Due to its high accuracy and extensive coverage, GRACE data enables more precise and comprehensive monitoring of water resources compared to classical hydrological monitoring methods. This mission has continued with the post-GRACE mission since 2018. The GRACE mission data has provided a 15-year snapshot of total water storage changes on global and regional scales. This long-term dataset is invaluable for understanding the dynamics of water resources, especially in regions experiencing significant water stress. The mission's data have been extensively used in research and policy-making to better manage water resources and address issues related to droughts, floods, and other water-related challenges. This study presents a comprehensive summary of these changes across the major watershed basins of Iran, including the Caspian Basin, Eastern Border Basin, Qareh Qum Basin, Central Plateau, Persian Gulf and Oman Sea Basin, and the Urmia Lake Basin, from May 2002 to September 2017. Utilizing conventional signal processing techniques, such as the Mann-Kendall test, least squares trend analysis, autocorrelation analysis, and discrete and continuous wavelet transforms, this research seeks to address the following questions: From the perspective of the GRACE mission, what patterns do long-term, short-term, and periodic trends and changes in total water storage exhibit across Iran’s main basins, and how can these be liked to climatic and management parameters?
The extracted linear and nonlinear trends from the time series confirm the reduction of total water storage in all main basins of Iran and the alarming water stresses in these areas. According to the results, mean annual reductions in total water storage of 19, 4, 5, 9, 9, and 13 millimeters were observed in the Caspian Sea, Eastern Border, Qara Qum, Central Plateau, Persian Gulf and Oman Sea, and Lake Urmia basins, respectively. Alongside the intensity and variability of components derived from wavelet transforms, these findings demonstrate that climate change and water resource management have significantly impacted water storage trends across Iran’s main basins. The annual components of water storage changes in the basins are recognized as the most important part extracted from the time series of water resource changes in these regions. These components reveal noticeable variation in the intensity of the annual component in drier basins, indicating the significant impact of seasonal variation on water availability. Although in many basins, the occurrence and persistence of droughts after 2007 have led to changes in water storage, the non-weakening of the annual component in most studied areas indicates the significant impact of poor water resource management, in addition to drought, in the basins.
In conclusion, the GRACE mission data has provided valuable insights into the changes in total water storage in Iran's main basins. The results highlight the importance of effective water resource management and the need for appropriate strategies to tackle the issues posed by climate change and water scarcity. The continuation of the mission through GRACE-FO ensures that we can continue to monitor and understand these vital changes, enabling more informed decision-making for sustainable water resource management.