Quantifying historical impact of groundwater irrigation practices to the ecological environment and the availability of the High Plains Aquifer
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The High Plains Aquifer (HPA), one of the largest groundwater systems in the world, has sustained the regional food production industry since the nineteenth century. It has provided a great number of agricultural products which were worth 35 billion US dollar in 2007, but at the same time, the aquifer has experienced severe depletion. A number of studies have pointed out that the overexploitation for agrarian purpose is responsible for the groundwater depletion by providing information of the regional historical trend of groundwater use. But it needs to be more studied, especially to figure out where, when the excessive consumption occurred and its impact on environment. In order to better conduct an impact analysis on spatial basis, this study first divided the entire HPA area into 27 ecoregions which have similar geo-, hydro- and ecological- features. Based on the spatial division, zonal groundwater replenishment and zonal baseflow to streams estimated from PCR-GLOBWB hydrologic model were obtained. Then the acquired values were compared to the irrigated area by the groundwater footprint method. The result of this steps indicates the historical trend of regional hydrologic situations and consequently how excessively irrigation practices have affected the groundwater dynamics and regional riparian ecosystems. As a result, the groundwater recharge rate in most ecoregions of the study area is enough to sustain streams and riverine ecosystems. However, the abstraction availability for the sustainable use, the difference between groundwater recharge and its outflow to streams, differs from 0 to 0.4 m for each ecoregion, and the abstraction history has been greater than the sustainable availability. Recently, groundwater usage in some ecoregions has frequently intercepted the amount that would have sustained the environmental flow requirements. This implies that a better resource management could be established when it was specified in a smaller areal unit level such as ecoregion or county.