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 (US) are among the primary goals of many of these recharge studies. The chloride mass balance (CMB) 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 year−1, representing 0·1–5% of long-term average annual precipitation. Extreme local variability in recharge, with rates up to ∼720 m year−1, results from focussed 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. Inter-annual climate variability related to El Nino Southern Oscillation (ENSO) results in up to three times higher recharge in regions within the SW US during periods of frequent El Ninos (1977–1998) relative to periods dominated by La Ninas (1941–1957). 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 year−1 during the Sahel drought (1970–1986) to 150 mm year−1 during non-drought periods. Variations in climate at millennial scales in the SW US changed systems from recharge during the Pleistocene glacial period (≥10 000 years ago) to discharge during the Holocene semiarid period. LU/LC changes such as deforestation in Australia increased recharge up to about 2 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 US. The impact of LU 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 LU. In irrigated areas, recharge varies from 10 to 485 mm year−1, representing 1–25% of irrigation plus 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 programmes within the context of climate variability and LU/LC change. Copyright © 2006 John Wiley & Sons, Ltd.

Global synthesis of groundwater recharge in semiarid and arid regions

Since the mid-1980s, a relative explosion of groundwater-recharge studies has been reported in the literature. It is therefore relevant to assess what is now known and to offer further guidance to practitioners involved in water-resource development. The paper summarizes current understanding of recharge processes, identifies recurring recharge-evaluation problems, and reports on some recent advances in estimation techniques. Emphasis is accorded to (semi-)arid regions because the need for information is greatest in those areas – groundwater is often the only water source, is vulnerable to contamination, and is prone to depletion. Few studies deal explicitly with groundwater recharge in temperate and humid zones, because recharge is normally included in regional groundwater investigations as one component of the water balance. The resolution of regional water-balance studies in (semi-)arid areas is, in contrast, often too low to quantify the limited recharge component with sufficient precision.

Groundwater recharge: an overview of processes and challenges

This paper examines the question of land surface-atmosphere interactions in the West African Sahel and their role in the interannual variability of rainfall. In the Sahel, mean rainfall decreased by 25–40% between 1931–1960 and 1968–1997; every year in the 1950s was wet, and nearly every year since 1970 has been anomalously dry. Thus the intensity and multiyear persistence of drought conditions are unusual and perhaps unique features of Sahel climate. This article presents arguments for the role of land surface feedback in producing these features and reviews research relevant to land surface processes in the region, such as results from the 1992 Hydrologic Atmospheric Pilot Experiment (HAPEX)-Sahel experiment and recent studies on aerosols and on the issue of desertification in the region, a factor implicated by some as a cause of the changes in rainfall. Included also is a summary of evidence of feedback on meteorological processes, presented from both model results and observations. The reviewed studies demonstrate numerous ways in which the state of the land surface can influence interactions with the atmosphere. Surface hydrology essentially acts to delay and prolong the effects of meteorological drought. Each evaporative component of the surface water balance has its own timescale, with the presence of vegetation affecting the process both by delaying and prolonging the return of soil moisture to the atmosphere but at the same time accelerating the process through the evaporation of canopy-intercepted water. Hence the vegetation structure, including rooting depth, can modulate the land-atmosphere interaction. Such processes take on particular significance in the Sahel, where there is a high degree of recycling of atmospheric moisture and where the meteorological processes from the scale of boundary layer development to mesoscale disturbance generation are strongly influenced by moisture. Simple models of these feedback processes and their various timescales have demonstrated that the net feedback to the atmosphere is positive for both wet and dry surface anomalies. Hence the role of the surface is to reinforce meteorologically induced changes. Recovery from the dry state is slower than from the wet state, suggesting that dry conditions would tend to persist longer, as is actually observed in the Sahel. These simple models suggest that the surface hydrology locks the system into a drought mode that persists for several years, until the system randomly slips into a persistent wet mode. The hypothesis that desertification in the Sahel might likewise be responsible for the persistent drought is found to be untenable. Rather than a progressive encroachment of the desert onto the savanna, the vegetation cover responds dramatically to interannual fluctuations in rainfall. There is little evidence of large-scale denudation of soils, increase in surface albedo, or reduction of the productivity of the land, although degradation has probably occurred in some areas. There has, however, been a steady buildup of dust in the region over the last half a century. Significant radiative effects of the dust have been demonstrated; therefore the dust has probably influenced large-scale climate. The buildup is probably mainly a result of changes in the land surface that accompanied the shift to drier conditions, but it may have been exacerbated by anthropogenic factors. Complex general circulation models nearly universally underscore the importance of feedback processes in the region. Although it has not been unequivocally demonstrated that the rainfall regime of the Sahel is modulated by surface processes, there is recent observational evidence that this is case.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RG900014

Land surface processes and Sahel climate

increases aquifer recharge. At the local scale (2 km 2 ), a physically based, distributed hydrological model showed that land clearing increased runoff threefold, whereas the rainfall deficit decreased runoff by a factor of 2. At a larger scale (500 km 2 , 1950–1992 period), historical aerial photographs showed a 2.5-fold increase in the density of gullies, in response to an 80% decrease in perennial vegetation. At the scale of the entire study area (5000 km 2 ), analytical modeling of groundwater radioisotope data ( 3 H and 14 C) showed that the recharge rate prior to land clearing (1950s) was about 2 mm a � 1 ; postclearing recharge, estimated from groundwater level fluctuations and constrained by subsurface geophysical surveys, was estimated to be 25 ± 7 mm a � 1 . This order of magnitude increase in groundwater fluxes has also impacted groundwater quality near ponds, as shown by a rising trend in groundwater nitrate concentrations of natural origin (75% of d 15 N values in the range +4 to +8%). In this well-documented region of semiarid Africa, the indirect impacts of land use change on water quantity and quality are much greater than the direct influence of climate variability.

/pdf/land-clearing-climate-variability-and-water-resources-4rgsfug9db.pdf

Land clearing, climate variability, and water resources increase in semiarid southwest Niger: A review

As water moves into the ground it begins to record information on the history of its recharge source and properties, mainly from rainfall solutes as well as isotopic ratios of the water molecule. The subsurface accepts water at variable rates of movement through the soil, via the unsaturated zone, to the water table. At this stage the groundwater composition undergoes significant modification due to two major processes: an increase in the concentration of atmospheric solutes due to removal of water via plant uptake and evaporation; and reactions between water and rock, leading to the build-up of dissolved substances with different relative ion concentrations to the atmospheric input. The principal and distinctive characteristics of groundwater are mainly established in the unsaturated zone. In the saturated zone the geochemical evolution, though less intense than in the soil and unsaturated zones, follows progressive changes in water quality towards areas of discharge. These processes are time-dependent and the chemical changes as well as isotopic variations may be used to identify this evolution and provide information on water flow paths.

Inorganic Ions as Tracers

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_51-52/010015688.pdf

J. Sudlow

Papers

Estimation of rainfall inputs and direct recharge to the deep unsaturated zone of southern Niger using the chloride profile method