HYDROGEOLOGIC ASSESSMENT OF SPATIO-TEMPORAL VARIATION IN GROUNDWATER QUALITY AND ITS IMPACT ON AGRICULTURAL PRODUCTIVITY

Abstract

The utilization of groundwater resources in irrigation has significantly increased during the last two decades, due to limited surface water supplies. More than 70% of the farmers in the Punjab province of Pakistan depend directly or indirectly on groundwater to meet their crop water demands and hence, unchecked paradigm shift has resulted in aquifer depletion and deterioration consequently, affected the agriculture. The problem could become more severe in its nature and distribution if special attention is not given on improving groundwater quality and its future scenario. Therefore, a comprehensive research was carried at central Punjab regarding spatio-temporal variation in groundwater level and quality and consequently the impact of deteriorated groundwater quality on the agricultural productivity. For this purpose a detailed analysis of existing groundwater level and quality patterns from 2003 to 2012 were carried out using field inventory and GIS mapping. Processing MODFLOW for window (PMWIN) model was used for future prediction of groundwater level and quality up to the year 2030. The comprehensive data set of aquifer lithology (soil texture, hydraulic conductivity, specific storage, specific yield and porosity), canal network, groundwater level, groundwater salinity, evapotranspiration, groundwater abstraction, recharge etc. were used in PMWIN development. The model was thus, successfully calibrated and validated with respect to groundwater level for the periods of 2003 to 2007 and 2008 to 2012, respectively. For solute transport model (MT3D), advection and dispersion parameters were used. Two scenarios were developed such as Scenario-I (increase in groundwater pumping according to historical trend) and Scenario-II (adjusted canal water supplies and groundwater patterns). The model predicted results of scenario-I revealed that the groundwater would decline with the rate of 0.0131 to 1.68m/year during 2013 to 2030 and the maximum decline would be on the middle and lower sides of the study area. Similarly, the predicted TDS of the groundwater would increase from 6.88 to 69.88mg/L/year during 2013 to 2030 and the maximum increase would be on lower side. This lowering of groundwater level can cause increase in the tubewell installation and pumping cost. In scenario-I, the good quality would xviii reduce by 21.4%, while marginal and hazardous quality water increased by 19.28 and 2%, respectively. The results of scenario-II indicated that if the groundwater abstraction is increased by 35% and irrigation recharge is decreased by 35% in upper part of the study area while for lower part, abstraction is decreased by 35% and irrigation recharge is increased by 35%, there will be an overall decline in groundwater levels by 3-4m on the average for the upper part of the study area and a recovery of groundwater level about 2m on the average for the lower part of the study area. Similarly, there would be an overall improvement in groundwater quality in the lower part by 250mg/L and an average deterioration in groundwater quality upto 500mg/L in the upper part of the study area. The comparison of both scenarios up to year 2030 shows that in scenario-II, there was 6.32% more good quality area and 12.48% less hazardous quality. Thus, an alternative conjunctive surface and groundwater irrigation policy between the upper and lower part of study area can provide a mean for sustainable water management for the study area. The impact of groundwater quality on agricultural productivity and farmers’ income was observed using hydro-economic model (double log production function). The analysis of hydro-economic model results indicated that the farmers associated with good quality groundwater zone had 11.46 and 25.69% more benefits in term of income than marginal and hazardous quality zones, respectively. The regression analysis showed that the one percent increase in TDS (mg/L) resulted decrease in the gross value of crop (GVP) up to 0.081 and 0.526 percent for marginal and hazardous quality zones, respectively. So, it is recommended that integrated agronomic and engineering useful practices should be applied for meliorate management of groundwater. Comparatively, more canal water should be applied in low quality groundwater zones than the good quality groundwater zone.