Abstract
The over-extraction of groundwater from thin fresh groundwater lenses is a threat to the livelihood of farmers in the Lower Indus Basin (LIB). It is essential to monitor and regulate this pumping to sustain fresh groundwater lenses. In this study, we applied a modelling approach in combination with geostatistical analysis to identify the critical locations to monitor the groundwater levels for sustaining fresh groundwater in the LIB. Our approach included four steps: (i) simulating temporal heads using a calibrated hydrogeological model; (ii) sampling monitoring locations using a hexagonal pattern of sampling; (iii) applying principal component analysis (PCA) of the temporal head observations, and selecting high scoring locations from the PCA; and (iv) minimizing the observation points to represent the water level contours. The calibrated model was able to replicate the hydro-dynamic behavior of the study area, with a root mean square of 0.95 and an absolute residual mean of 0.74 m. The hexagonal pattern of spatial sampling resulted in a 195 point network, but PCA reduced this network to 135 points and contour classification reduced it even further to 59 points. The 195, 135, and 59 point networks represented the water levels with average standard errors of 0.098, 0.318, and 0.610 m, respectively. Long-term simulations with increased pumping showed that the water levels would best be assessed by 195 monitoring points, although 135 and 59 points would represent the depleting area but would not capture the water logging area.
Highlights
Groundwater in the Lower Indus Basin (LIB) originates from the river system, which has flowed through the valley since late Tertiary times
Observed werefor arranged for pre-monsoon post- and pre-monsoon for each until of points points were used theincalibration
The analysis proves the robustness of a hexagonal pattern of sampling in the case of continuous spatial functions
Summary
Groundwater in the Lower Indus Basin (LIB) originates from the river system, which has flowed through the valley since late Tertiary times. The fresh water occurs in lenses near the Indus River or near the major canals. These freshwater lenses are underlain by dense saline water. This is due to the hydrogeological conditions in the LIB, which cause rapid mineralization of the groundwater. The largest freshwater lens occurs along the Indus River, and its quality deteriorates away from the river [1]. Inequity in surface water distribution and increasing irrigation intensities have led to an increase in groundwater dependence on fresh groundwater lenses.
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