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Abstract:
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"Competing demands on limited water resources have resulted in persistent water shortages for the North China Plain. Despite years of effort to mitigate the problem, ground-water levels continue to decline steadily. A quantitative analysis of hydrologic changes affecting Luancheng County, Hebei Province, from 1949 to 2000 identifies crop evapotranspiration as the root cause of water-table declines beneath this ground-water irrigated area. Potential evapotranspiration greatly exceeds precipitation in the North China Plain. For centuries, farmers raised 2-3 rainfed crops every two years. By not using irrigation, the quantity of water taken up by crops was limited by -- and could not exceed -- precipitation. It was not until the proliferation of mechanized wells in the 1960s that farmers could consistently produce 2 crops every year. From the 1970s to the present, actual evapotranspiration from irrigated cropland in Luancheng County has remained constant at about 66 cm/y, compared to an average of about 46 cm/y of rainfall. Ground-water mining provides the additional 20 cm/y to meet crop water requirements, but also depresses the water table about 1 m/y beneath Luancheng County. Attempts to make water use sustainable have centered on improving irrigation efficiency to reduce ground-water pumping. Indeed, pumping rates have decreased about 50 percent since the 1970s. However, ground-water recharge from excess irrigation has decreased correspondingly. Therefore, water-table declines have continued unabated. Because irrigated areas overlie the shallow aquifer, excess irrigation water always replenishes the water supply. In this physical configuration, irrigation efficiency improvements save no water. The only way to save water is to reduce evapotranspiration, which can be accomplished by reducing the cropped area. Government leaders are currently promoting rapid urbanization of the North China Plain as a means to stimulate economic growth, absorb the rural labor surplus, and reduce the income disparity between rural and urban workers. From a hydrologic standpoint, is urban land use a viable alternative to irrigated cropland? Like agricultural areas, most cities on the North China Plain rely on ground water. However, unlike irrigated fields, water that passes through industrial and domestic users does not necessarily recharge the underlying aquifer. Rather, most wastewater discharges to rural areas outside the cities. Likewise, rainfall that lands on impermeable urban surfaces quickly drains away from cities. Therefore, although evaporation from urban areas is less than from cropland, water-table declines beneath cities are even more severe than beneath cropland. The major hydrologic impacts of urbanization, then, are localized displacement – but not necessarily consumption – and contamination of water. By replacing some cropland with urban areas, regional evapotranspiration would likely decrease. Then the problem would reduce to prudent management and distribution of urban water resources. Municipal wastewater treatment could transform a disposal problem into a water-resource solution, to be utilized by downstream irrigators. There appears to be considerable scope for improving industrial water-use efficiency, which would reduce potential costs of both ground-water pumping and wastewater treatment. Capturing and artificially recharging storm runoff into underlying aquifers could further alleviate ground-water declines beneath urban and nearby areas. Thus, replacing rural with urban land use potentially could benefit the North China Plain hydrologically, provided urban runoff and wastewater quality improve."
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