Past land-use changes have greatly impacted global water resources, with often opposing effects on water quantity and quality. Increases in rainfed cropland (460%) and pastureland (560%) during the past 300 years from forest and grasslands decreased evapotranspiration and increased recharge (two orders of magnitude) and streamflow (one order of magnitude). However, increased water quantity degraded water quality by mobilization of salts, salinization caused by shallow water tables, and fertilizer leaching into underlying aquifers that discharge to streams. Since the 1950s irrigated agriculture has expanded globally by 174%, accounting for ~90% of global fresh-water consumption. Irrigation based on surface water reduced streamflow and raised water tables resulting in waterlogging in many areas (China, India, US). Marked increases in groundwater-fed irrigation in the last few decades in these areas has lowered water tables (=1 m/yr) and reduced streamflow. Degradation of water quality in irrigated areas has resulted from processes similar to those in rainfed agriculture: salt mobilization, salinization in waterlogged areas, and fertilizer leaching. Strategies for remediating water-resource problems related to agriculture often have opposing effects on water quantity and quality. Long time lags (decades to centuries) between land-use changes and system response (e.g., recharge, streamflow, water quality), particularly in semiarid regions, mean that the full impact of land-use changes has not been realized in many areas and remediation to reverse impacts will also take a long time. Future land-use changes should consider potential impacts on water resources, particularly tradeoffs between water, salt, and nutrient balances, to develop sustainable water resources to meet human and ecosystem needs.
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Global distribution of land cover based on MODIS (1 km) satellite data using International Geosphere Biosphere Program land-cover classes prepared by Boston University (Earth Observing System (EOS) Data Gateway, http://edcimswww.cr.usgs.gov/pub/imswelcome/).
*cropland (2%) refers to cropland/natural vegetation mosaic.
Temporal variation in global irrigated land area (FAOSTAT, 2006).
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| Irrigation water withdrawal (IWW) and consumption (IWC) relative to global water withdrawal (GWW) and consumption (GWC) (Shiklomanov, 2000). Water withdrawal refers to water diverted from surface water or groundwater; however, only a fraction of this water is consumed by evapotranspiration (ET) losses or by humans or livestock. |
Temporal variation in global cropland and pastureland (Klein Goldewijk, 2001). |
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