Hydrologic benefits

Another potential benefit of groundwater development is the increase in net recharge in those aquifers that, under natural conditions, have the water level close to the land surface. The drawdown of the water table can result in a decrease in evapotranspiration, an increase in the recharge from precipitation that was rejected under natural conditions and an increase in indirect recharge from surface water bodies. Johnston (1997) analysed 11 American regional aquifers, showing that intensive groundwater development in 9 of these aquifers has resulted in significantly increased recharge.

Shah et al. (2003) studied seasonal recovery of groundwater levels for the whole of India after the monsoon rain. They concluded that the depletion of the water table due to groundwater abstraction increases significantly the precipitation recharge. This is quite in agreement with the general hydrogeological principles. There is ample evidence showing that extractions increase the recharge rates augmenting the sustainable use level.

A clear example of this situation is the increase in available resources for consumptive uses that followed intensive groundwater pumping in the Upper Guadiana basin in central Spain (see Bromley et al., 2001). It has been estimated that average renewable resources may have increased between one-third and one-half under disturbed conditions. Figure 13.7 illustrates these results. Prior to the 1970s, groundwater pumping in the Guadiana basin did not have significant impacts on the hydrologic cycle. Intensive pumping for irrigated agriculture started in the early 1970s and reached a peak in the late 1980s. As a result, wetlands that, under semi-natural conditions, had a total extension of about 25,000 ha, cover only 7000 ha today. In addition, some rivers and streams that were naturally fed by the aquifers have now become net losing rivers.

The results of the decline in the water table have been twofold. On one hand, a significant decrease in evapotranspiration from wetlands and the water table, from about 175 million cubic metres per year under quasi-natural conditions to less than 50 million cubic metres per year today. On the other hand, there has been a significant increase in induced recharge to the aquifers from rivers and other surface water bodies. Consequently, more water resources have become available for other uses, mainly irrigation, at the cost of negative impacts on dependent natural wetlands. These impacts are highlighted later.

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