ASSESSMENT OF THE CAUSES OF GROUNDWATER DEPLETION BASED ON GPS OBSERVATIONS

Abstract:

Nowadays, huge number of countries spread around the world are struggling with the problem of water availability. These are the effects of a warming climate causing the disappearance of surface water, which are major water sources. As a result, in many parts of the world, groundwater is essential for drinking water supply, agriculture, industry as well as many other important issues. Hence, the quality and quantity of groundwater resources must be regularly controlled. Currently, groundwater is monitored, among others by measuring groundwater tables, analyzing groundwater abstraction or by estimating their quality. However, due to the high cost and time-consuming nature of such measurements, information on groundwater monitoring is lacking in many areas. So, to assess the size and causes of variations in vertical deformation values caused by the groundwater mass loading, we used widely available Global Positioning System (GPS) observations. However, GPS position is strongly influenced by regional and local phenomena such as atmospheric and oceanic changes, volcanic and tectonic activities. So, it is difficult to interpret the magnitude of the hydrological effect in the GPS position times series. Therefore, to assess the reliability of deformation obtained from GPS, the deformation values from Gravity Recovery and Climate Experiment (GRACE) mission were also calculated. Vertical deformations from GRACE mission are obtained using monthly RL06 mascon solution available from Center for Space Research (CSR) in Texas, by subtracting surface water, soil moisture and snow water using hydrological models. We utilized data from Noah Global Land Data Assimilation System (GLDAS) and WaterGAP Global Hydrology Model (WGHM) models in 0.25ºx0.25º and 0.5ºx0.5º resolution, respectively. Since GPS-derived vertical displacements (Nevada Geodetic Laboratory (NGL) time series are used here, spanning from 7 to 15 years) evidence on a wide variety of phenomena, to make them comparable to deformations induced by GRACE groundwater changes, we remove non-tidal oceanic and atmospheric effects using models available from GeoForschungsZentrum (GFZ). We also removed deformations that originate from surface water, soil moisture and snow water, similarly to components subtracted from GRACE observations. Also, both GRACE and GPS vertical changes were compared to in situ observations provided by the Global Groundwater Monitoring Network (GGMN). Finally, to assess the causes of groundwater depletion, the results were compared with drought indices and global tropical rainfall data provided by Tropical Rainfall Measuring Mission (TRMM). Detailed analysis was performed for 53 GPS stations in 7 areas located in 4 continents (except Eurasia). Areas were selected on the basis of extreme changes in the modelled groundwater data provided by WGHM and in situ observations availability. In our study, we obtained correlations above 0.6 between the GPS and GRACE time series. We also obtained the high similarity with in situ data for seasonal and long-term changes for both GPS and GRACE changes. Finally, we noticed a strong dependence on changes in extreme vertical deformation from GPS with maximum drought indices and rainfall totals, confirming the possibility of using GNSS observations to appraise the effects of human impact and climate change for groundwater resources.