Global Synthesis of Groundwater Recharge in Semiarid and Arid Regions

Bridget Scanlon, principal investigator

Global synthesis of the findings from ~140 recharge study areas in semiarid and arid regions provides important information on recharge rates, controls, and processes, which are critical for sustainable water development. Water resource evaluation, dryland salinity assessment (Australia), and radioactive waste disposal (U.S.) are among the primary goals of many of these recharge studies. The chloride mass balance technique is widely used to estimate recharge. Average recharge rates estimated over large areas (40–374,000 km2) range from 0.2 to 35 mm/yr, representing 0.1 to 5% of long-term average annual precipitation. Extreme local variability in recharge, with rates up to ~720 m/yr, results from focused recharge beneath ephemeral streams and lakes and preferential flow mostly in fractured systems. System response to climate variability and land use/land cover (LU/LC) changes is archived in unsaturated zone tracer profiles and in groundwater level fluctuations. Interannual climate variability related to El Niño Southern Oscillation (ENSO) results in up to three times higher recharge in regions within the SW U.S. during periods of frequent El Niños (19771998) relative to periods dominated by La Niñas (19411957). Enhanced recharge related to ENSO is also documented in Argentina. Climate variability at decadal to century scales recorded in chloride profiles in Africa results in recharge rates of 30 mm/yr during the Sahel drought (1970–1986) to 150 mm/yr during nondrought periods. Variations in climate at millennial scales in the SW U.S. changed systems from recharge during the Pleistocene glacial period (=10,000 yr ago) to discharge during the Holocene semiarid period. LU/LC changes such as deforestation in Australia increased recharge up to about two orders of magnitude. Changes from natural grassland and shrublands to dryland (rain-fed) agriculture altered systems from discharge (evapotranspiration, ET) to recharge in the SW U.S. The impact of land use change was much greater than climate variability in Niger (Africa), where replacement of savanna by crops increased recharge by about an order of magnitude even during severe droughts. Sensitivity of recharge to LU/LC changes suggests that recharge may be controlled through management of land use. In irrigated areas, recharge varies from 10 to 485 mm/yr, representing 1 to 25% of irrigation + precipitation. However, irrigation pumpage in groundwater-fed irrigated areas greatly exceeds recharge rates, resulting in groundwater mining. Increased recharge related to cultivation has mobilized salts that accumulated in the unsaturated zone over millennia, resulting in widespread groundwater and surface water contamination, particularly in Australia. The synthesis of recharge rates provided in this study contains valuable information for developing sustainable groundwater resource programs within the context of climate variability and LU/LC change.

Click map to enlarge.

Global distribution of climatic zones (UNESCO 1979). Information on recharge studies is in Table 1. Location ID's for the SW U.S. were omitted owing to the high density of study locations.

March 2006