Should groundwater yield be based solely on groundwater hydrology? Yilan Wei CIVE 633 Environmental Hydrology Fall 2010 October 27, 2010 The answer to this question is: Absolutely not. First and foremost, an assessment of groundwater potential should evaluate the effect of pumping on the neighboring surface waters and related groundwater-dependent ecosystems. The sustainable amount to pump is not related to the aquifer volume or to the aquifer recharge, but rather to the maintenance of baseflow (the fraction of streamflow that originates in ground water) in the local and regional vicinity. Therefore, baseline studies of baseflow should be a required component of an evaluation of sustainable groundwater potential. Professionals with knowledge in both groundwater hydrology and surface water hydrology are essentially needed (Ponce, 2007). Groundwater yield is an interdisciplinary study. This essay will be approached in three steps: groundwater as source of surface water, groundwater sustainability based on baseflow conservation, and groundwater yield regulation (ground water as a "common"). Groundwater as source of surface water According to personal experience, one can simply describe the water cycle as water that comes from precipitation, goes over and through the ground; some of it stays in the surface and spreads out in the river network; some of it goes into aquifers and reaches the groundwater table. Actually, groundwater and surface water are connected in ways more than this statement can convey. Groundwater discharges into the land surface supporting riparian, wetland, and other groundwater-dependent ecosystems, and baseflow of streams and rivers. Under natural conditions, aquifers are in a state of dynamic equilibrium, whatever amount flows in will flow out. The net recharge is zero. Development, a well or a dam been built, a portion of the ground been paved or a river been linen up, will change the local groundwater flow. Think about groundwater as borrowed surface water. If we pump ground water without limit, groundwater-dependent ecosystems will be affected, river baseflow will be depleted, and/or flood magnitude will become higher. Thus, a sustainability study is needed to determine the optimum or reasonable compromise between conflicting interests. Both isciplines of surface-water and groundwater hydrology have a bearing on the analysis of the sustainable yield. Groundwater sustainability Groundwater takes years or decades to refill if depleted. Some deep aquifers take hundreds of years or centuries to recover. In regions where surface water is limited, the use of groundwater is often extensive. Since every gallon of water pumped from a well comes from the capture of increased recharge and decreased natural discharge, heavy, long-lasting pumping can result in the sequestering of all natural discharge. Thus, we have to limit groundwater usage to a reasonable, sustainable amount. A first approximation is to limit groundwater pumping to an amount equal to the deep percolation, i.e., the fraction of precipitation (or recharge) that returns directly into the ocean, bypassing the surface waters. Deep percolation is globally estimated at 2% of precipitation (Ponce, 2006a). This limit may be increased in cases where detailed interdisciplinary studies warrant it. Groundwater as a common Groundwater is a common, i.e., a natural resource used jointly by many stakeholders. Freedom in the commons brings ruin to all, since every person is compelled to increase his/her individual benefit without limit (refer to Hardin's "Tragedy of the Commons"). Groundwater depletion is a logical consequence of a commons exploited in the absence of regulation or sustainable practices. Thus, sustainability of groundwater has to be fostered by enlightened management. In summary, groundwater yield is an interdisciplinary study. Professionals specializing in groundwater hydrology, surface-water hydrology, ecology, and institutional and legal practices should be involved. References Ponce, V. M. (2006). Groundwater utilization and sustainability. Ponce, V. M. (2007). Sustainable yield of groundwater.

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