Abstract: The impact of climate change and reduced rainfall on the groundwater resources of the Kerman plain with its arid and semi-arid climate has led to a severe decline in groundwater level over the years. The purpose of this research is to model the groundwater levels over 22 years (2002–2024) and forecast them for 10 years (2025–2034) using the MODFLOW code. According to the 10 years forecast, the maximum annual decline will occur in the eastern and southeastern parts of the aquifer at 2 meters per year, while the minimum decline of 0.78 meters per year is expected in the central, northeastern, and southern sections of the aquifer. Furthermore, the lack of natural wastewater drainage through absorption wells, combined with the implementation and transfer of Kerman city's sewage network outside the area, has led to the prediction that groundwater will reach the bedrock in approximately 12 years.
Keywords: Modeling, Groundwater Level, Kerman Aquifer, Bedrock
Introduction: In recent years, excessive extraction from the Kerman plain aquifer through both authorized and unauthorized wells for agricultural, industrial, domestic, and urban purposes has resulted in a severe decline in the groundwater level, land subsidence, and the intrusion of various pollutants into the aquifer. These problems highlight the critical state of groundwater reserves, necessitating comprehensive studies and appropriate policies to improve the aquifer's condition. Given the crucial role of groundwater in supplying water for various uses monitoring of the Kerman plain aquifer through groundwater level modeling is essential for developing appropriate management strategies, and planning for the future. The groundwater flow in the shallow aquifer system of the Mahanadi Delta (East Coast, India) was evaluated using the MODFLOW model (Kumar Behera et al., 2022). According to their results, the net outflow from groundwater to the Bay of Bengal ranged from 8.92 to 9.64 MCM. They simulated and analyzed the groundwater level fluctuations of the Shahdad aquifer in Kerman Province using MODFLOW model (Sohrabi et al. 2020). The investigations showed the groundwater level of the aquifer indicate a decline in water levels in the northern, eastern, southern, and parts of the central. In this study, the groundwater level of the Kerman plain has been modeled and forecasted. Furthermore, the effects of both recharging and not recharging the aquifer through natural drainage of wastewater on groundwater storage within the Kerman city area have been investigated.
Methodology: In this study, in order to model the groundwater level using Groundwater Modeling System (GMS) software and the MODFLOW numerical code, a conceptual model was first developed based on the available hydrological and physical data for the region, which had been prepared as information layers in the GIS environment. The conceptual model was discretized using a uniform grid with cell dimensions of 500 × 500 meters. The information layers produced in GIS were assigned to each cell of this grid so that, according to the governing equations, mathematical relationships could be computed for each cell. After verifying the accuracy of the simulated model, the model was run for 80% of the data under both steady-state and transient conditions. The geographic location and distribution of the observation and abstraction wells are shown in Figure 2. In addition, Figure 3 presents the map of groundwater inflow and outflow at the aquifer boundaries. To evaluate the agreement between observed and simulated data, validation was performed using 20% of the available data. The assessment and prediction of the effects of recharge or lack thereof due to natural drainage were carried out by calculating the drawdown gradient from the observed water level and the time required to reach the average bedrock level of the region.
Results and Discussion: The results presented during the calibration stage for both steady and transient state modeling periods were associated with errors within the acceptable range. The Mean Error (ME), Mean Absolute Error (MAE), and Mean Squared Error (MSE) were calculated for the steady-state, transient-state, and validation periods and were found to be within acceptable ranges. For instance, these Errors for the validation period are 0.61, 0.54, and 0.06, respectively, confirming the satisfactory performance of the model. The modeling results indicated that the hydraulic hydraulic conductivity ranged from a minimum value of 11.2 to a maximum value of 44.12 meters per day. Specifically, the northern, western, and a limited area in the eastern parts of the aquifer due to the coarse-grained soil texture and high permeability exhibit high hydraulic conductivity values (ranging from 30 to 44.12 meters per day). In contrast, the central, southern, and southeastern regions of the aquifer characterized by fine-grained sediments and low permeability display the lowest hydraulic conductivity values (ranging from 11.2 to 10 meters per day). The calibrated specific yield coefficient varies from 0.01 to 0.295. The zoning maps of hydraulic conductivity (HK) and specific yield (SY) illustrate the spatial distribution of these parameters at the final calibration step. Various charts were plotted to analyze the aquifer’s status during the 22-year modeling period, revealing that the maximum average annual drawdown occurred in the western and eastern regions of the aquifer at 1.23 meters per year, while the central and southern regions experienced the lowest average annual drawdown at 0.27 meters per year. The groundwater level forecasts for a ten-year period indicated that the maximum annual decline in groundwater level averaging 2 meters per year will occur in the eastern and southeastern parts of the study area, while the minimum annual decline, at 0.78 meters per year, will be observed in the central, northeastern, and southern regions of the aquifer. Figure 10 presents the spatial distribution of groundwater level drawdown for both the modeling period and the forecast period in the Kerman Plain.The results of the drawdown gradient calculation indicated a 1.5% monthly decline under natural drainage conditions and an 11% monthly decline under conditions with wastewater transfer. This wastewater transfer and increased gradient have been identified as the cause of the sudden groundwater level decline in the plain over the last seven years. Currently, considering the existing wastewater transfer conditions and the lack of aquifer recharge, the time to reach bedrock is projected to be within the next 12 years (by 2037).
Conclusion: Natural wastewater drainage through absorption wells is an effective factor in reducing the groundwater level drawdown gradient in the Kerman city area and other regions. This factor, under natural wastewater drainage conditions, reduces the groundwater level drawdown gradient by up to 7.4% monthly, consequently delaying the time for groundwater in Kerman city to reach bedrock for an extended period. Unfortunately, with the current implementation of the Kerman wastewater collection network and its transfer outside the area, which began in 2018, calculations indicate that well Nodhri (No. 14) in the northern part of the plain will reach bedrock within the next 12 years, exhibiting an 11% monthly drawdown gradient. Existing wells in the Kerman region and the northern areas of the aquifer are also projected to become dry. Thus, the drying of wells and the loss of this valuable resource could severely impact the region's agriculture.