Performance of the Gravity Recovery and Climate Experiment (GRACE) method in monitoring groundwater-level changes in local-scale study regions within Iran

The Gravity Recovery and Climate Experiment (GRACE) twin satellites introduced a new opportunity to monitor changes in groundwater level. However, the performance of the GRACE-derived Liquid Water Equivalent Thickness (GRACE-LWET) in estimating groundwater-level changes at a local scale requires evaluation. Thus, the main aim of this study is to evaluate the performance of the GRACE-derived estimation in monitoring groundwater-level changes in Iran, which is experiencing decreasing trends and subsequent impacts. Another aim is to investigate the time lag between the water levels derived from the GRACE estimation and direct measurements. Four regions in Iran were studied between the years 2002 and 2016. To evaluate the results of GRACE-LWET, groundwater levels in 144 piezometric wells were measured monthly. The changes of the earth’s mass due to surface-water changes were assessed using four datasets of the Global Land Data Assimilation System. Furthermore, the statistical trend of the groundwater-level changes acquired from the GRACE estimations and observational data was investigated using the Mann-Kendall test and Sen’s slope estimator at a significance level of 0.05. The results showed that the best performance of the GRACE estimations was acquired when considering a 2-month time lag. In this case, the average correlation coefficient of the GRACE estimations against the observational data for the entire study region was 0.57. Moreover, the GRACE-LWET showed a significant decreasing trend for the whole study area using both considered tests. Hence, GRACE-derived estimation of groundwater-level changes can be used in regions with insufficient observational well data with an acceptable accuracy.