Spatial analysis of rural groundwater resources in order to identify critical zones (Case study: Mahidasht watershed in Kermanshah province, Iran)

Introduction According to the theoretical frameworks in the spatial analysis, every "structure" in space has a specific "function". To study phenomena and to identify the relationships between them, these two concepts are of particular importance, especially in the "structural-functional dynamism" approach. The basic premise of this approach is based on the existence of a link between the structure and function of phenomena, the output of which represents the capabilities of that system. If a watershed can be considered as a spatial structure, then the existing wells in the area would represent one of its functions in form of groundwater exploitation. During the last three decades, the rapid increase in the population of urban centers adjacent to the Mahidasht watershed along with the change of cultivation pattern has led to the uncontrolled expansion of irrigation cultivation pattern, which in turn has led to excessive use of groundwater. For this reason, in the last four decades, the number of wells drilled in the study area has increased 23 times compared to the four decades before, from about 70 wells to more than 1,593 wells. Excessive exploitation of groundwater resources has led to an increase in the depth of wells and this, in turn, has led to lowering the groundwater level in the study area. Nowadays, this process has created some kind of zones that can be called critical zones in terms of increasing the depth of wells and water abstraction. In the present study, in order to determine and draw the map of such zones, the two main bases of spatial analysis, namely "position" and "distance" were applied, and common techniques and tools of GIS were used to analyze data related to wells in the study area.   Methodology Study area: Mahidasht watershed with an area of 1506.64 square kilometers and a perimeter of 239.9 kilometers is located in Kermanshah province, west of Iran. The area contains 132 springs, 14 Qanats (aqueducts) and 1593 wells.   Data: The data applied in the present study include seven layers namely; Digital Elevation Model (DEM), wells, rivers, country divisions, catchment divisions, urban and rural settlements. They were obtained from the US Geological Survey (USGS), Kermanshah Regional Water Company and the Governorchr('39')s Office of Kermanshah, respectively. For spatial analysis based on the location and distance of wells, the following data were applied: year of drilling, depth, type of consumption, annual discharge rate, annual performance, discharge of each well (litters per second), location of settlements in the study area and their attributes. In terms of techniques, the following ones were applied: point in polygon, spatial join, buffering and spatial interpolation. Techniques: In the present study, a table containing the coordinative and descriptive features of 1593 wells drilled in the Mahidasht watershed was converted into a point layer in ArcGIS. This layer, in addition to showing the distribution of wells in the region, was the basis for all other following analysis in this study. Mosaic, Extract, Hydrology, Thiessen Polygon and Natural Neighbor techniques and tools have also been applied to show and draw the final map of critical zones.   Discussion and results The results of the present study indicated that in a chain process the following events have occurred in the study area: Rapid increase in the population of the region during the 1980s and 1990s led to the increasing use of various resources, including groundwater resources, The number of wells in the study area increased with unusual speed, Increasing the number of wells led to increasing the depth of wells over time, especially during the 2000s. Increasing the average depth of wells in Mahidasht watershed led to over-depletion of groundwater aquifers and lowering its level. All of the above have led to the creation of zones known as critical groundwater abstraction zones. The result of the present study is the calculating, determining and mapping these zones based on common techniques in spatial analysis. These critical zones are located in the lands of 61 villages and cover an area of 20,000 hectares. However; despite the increase in the number of wells, the population of villages in the region has decreased. It means that the increase occurred in income due to the change of cultivation pattern from rain-fed to irrigated has not been invested in the villages of Mahidasht but has been transferred outside the region. Therefore, it can spatially be concluded that increasing the number of wells and high consumption of groundwater not only did not cause economic growth and prosperity in the villages of the region, but also has disturbed the ecological balance of the area. The last point is that; if no developmental measures are considered to improve and repair the conditions governing the critical zones in the study area, according to the structural-functional dynamism approach, it should be expected that change in a part of the spatial system will alter and even destroy the functions, structures and living space of the area. Thus, it can be concluded that, this problem can take the region away from the desired (and even minimum) conditions for a rational life and cause the destruction of other components of the system.