Estimation of the levels of Groundwater Aquifers under the Influence of Land-Use Changes by Using GRACE Satellite Data

Groundwater plays a vital role in water resources, ecosystems, and human life. The role of groundwater is more prominent especially in areas where it is the main source of people's needs including drinking and agriculture water. Annually, about 982 billion cubic meters of groundwater is extracted worldwide and 70% of it is used for agriculture. The land use is one of the most important events through which humans cause changes to the environment. One of the most important natural resources that are severely damaged by land-use changes is water resources and groundwater. Therefore, it is necessary to study the land use and its effects on the environment, especially groundwater, along with other issues, such as precipitation fluctuations. Today, due to the high cost of constructing piezometric wells for studying groundwater aquifers to become aware of the process of its changes, such studies are done through remote sensing technology. GRACE gravimetric satellite is a revolution in surveying and estimating groundwater aquifers. 

The northwest of the country with an area of ​​about 126420 km2 includes Ardabil, East Azerbaijan, West Azerbaijan, and Zanjan provinces. Khiavchai Basin located to the right of Ardabil is one of the sub-tributaries of Qarahsu River. The geographical coordinates of the study area located in the eastern side of Meshkinshahr are within the east longitude of 47◦ 38' 12" and 47◦ 48' 1" as well as the north latitude of 38◦ 12' 9" and 38◦ 24' 16". In the present study, the 3 products of GRACE satellite (CSR, GFZ, and JPL) were employed to extract fluctuations of the mentioned groundwater level by using Google Earth Engine (GEE) in the period of 2002-2016. Land-use classification of Xiao Region was done after examining the status of the northwestern aquifers and obtaining information from 8 piezometric wells in Xiao Region, along with the precipitation information obtained through TRMM satellite. The verification of this data was done by using the piezometric information of the station. To do this, 8 images of LandsatTM sensors and Landsat satellite related to the years of 2016 and 2002 were taken, respectively. The land-use maps were drawn according to different land uses in the region in the 7 classes of irrigated agriculture, rainfed agriculture, rangeland, residential areas, gardens, snow-covered lands, and irrigated lands. In eCognition software, they were extracted by using the object-oriented technique and their overall accuracy coefficient and kappa coefficient were obtained.

Similar independent results from the 3 GRACE satellite products showed that the groundwater changes in the northwestern region of the country had a downward trend and decreased by approximately 32 cm from 2002 to 2016. According to the studies on the land-use maps and information zoning of piezometric wells in Xiao Region, a decreasing trend of the aquifers in Khiao Region was observed. The loss of rangeland use and the increase in construction and residential areas and agriculture lands were the causes of lack of permeability and surface pressures of the groundwater aquifers and the ultimate reduction of their levels. Therefore, according to the research results, the land-use change was an important factor in reducing the levels of the aquifers in the region. It is noteworthy that the precipitation phenomenon, although insignificant from 2002 to 2016, showed a decrease of 51 mm through TRMM satellite data and a 25-mm decrease in the precipitation of the region by using the piezometric station data, which could not be ineffective in lowering the groundwater aquifers. 

Due to the importance of groundwater aquifers, their statuses were studied in the northwest of the country by using the data obtained from the 3 GRACE gravimeter satellite products of CSR, GFZ, and JPL in the GEE environment, which indicated a 32-cm drop in the groundwater aquifers in the basin. The results of the land-use changes and groundwater level in Khiavchai Basin in Meshkinshahr showed that the uses of rangeland and residential and barren lands had generally decreased and increased in a period of 14 years (2002-2016), respectively. The decrease of groundwater level caused by the mentioned uses had been intensified by these changes. In general, it could be concluded that there was a correlation between land use and groundwater level change in the study area. Also, the study of precipitation phenomenon by using TRMM satellite data and the synoptic station data demonstrated a decrease in precipitation, which could not be ignored in the intensification of the groundwater levels. Other results of this research included the capability of the GEE processing system in providing users with valuable information without the need for heavy processing operations.