Evaluation of groundwater recharge potential using GIS and comparison with the result of hydroclimatological balance method (case study: Kor river basin)

The world's growing population has caused irrecoverable variations on natural resources. Groundwater as one of these resources has an important role in the health and economy of countries. Nowadays, groundwater is the first source of drinking water in arid and semi-arid regions such as Iran. In order to manage the consumption of groundwater resources, knowledge of groundwater recharge in spatial and temporal scales before detailed exploration such as pumping test and geoelectrical studies is required. Hence, preparing and analyzing the factors affecting groundwater recharge in the GIS (Geographic information system) can be very helpful. The existence of a groundwater recharge potential (GRP) map is vital in regions where groundwater is the most important source of water supply. The aim of this study is to provide a GRP map for Kore river basin in the Fars province considering the interaction between the factors affecting groundwater recharge. Quantitative estimate of recharge due to precipitation using remote sensing (RS) and GIS is another aim of this study. Kore river basin is located in the south of Iran and north of Fars province, between geographic coordinates of 560000 to 740000 and 3250000 to 3450000 (zone 39) in the Mercator scale (UTM). This basin has 5 sub-basins including Khosro-Shirin, Bakan, Dozkord-Kamfirooz, Asopas and Marvdasht-kherameh with an area of 8284 km2. The annual rainfall of the study area varies from 300 mm in the southeast (plains) to more than 800 mm in the northwest (mountainous region). Geologically, this area includes two different carbonated and non-carbonated parts. Carbonated formations with fractures or karst systems cover 40 percent of the study area. These formations are Asmari, Sarvak, Ilam, Tarbur, Daryan in the mountainous regions while non-carbonated formations cover other parts of the study area (lowland regions).
Satellite data (Landsat images) of the study area were interpreted in the Erdas Imaging, Envi and Arc GIS software. In this study, thematic maps of 7 factors including lineament, and longitudinal density drainage, lithology, karst terrains, land use-land cover, land slope and rainfall for Kor River basin were prepared. Then, each of the factors was classified in 9 classes and their maps were designed. These classes are in the numerical range for lineaments, drainage, lithology, slope and rainfall while they are in the descriptive manner for karst domain and land use. The interactions between the factors were considered using a schematic pattern and the internal rate of each factor was extracted. Then this rate was multiplied by the specific weight of each factor. In this way, the effect of each factor on GRP was obtained in terms of percentage which is known as refereeing method. Then, by assigning an internal weight and considering the interaction of these parameters with each other in regard to GRP a weighted rating was calculated for every parameter in GIS software. By applying these weighted rates and using the final weight of the parameters a classified map of GRP in five classes from very high to very low grades was prepared. Taking into account more factors and their interaction is the advantage of this study compared to previous studies.
Based on the obtained recharge rates using RS and GIS, the areas of recharge zones and annual rainfall, the volume of recharged water was estimated. This estimate was equal to 1090.59 MCM which is about 20 percent of annual rainfall in the Kore river basin. In order to verify this estimated recharge, hydroclimatological balance was applied to recalculate the recharge volume. Considering the lack of hydrometery data in the Kore river basin, the SCS method was used to calculate the runoff volume regarding different land use. By these methods, the percentage of recharge from rainfall was estimated to be 18 percent. Accordingly, recharge volume was equal to 954.13 MCM. Finally, by comparing recharge estimation in this article with that of hydroclimatological balance method, it was revealed that there is about 14% difference in the estimated values of each of these methods. It should be mentioned that in this study, the effect of unsaturated zone properties on the GRP has not been considered. This methodology can be applied in similar basins in arid and semiarid regions in order to estimate the percentage of infiltration resulted from monthly excess water in the hydroclimatological balance method.