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Journal ArticleDOI

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

01 Feb 1997-Journal of Hydrology (Elsevier)-pp 139-154

AbstractAn estimate of direct groundwater recharge below a region of natural woodland (tiger bush) has been made in south-west Niger using the solute profile technique. Data has been collected from a 77 m deep well drug within the study area covered by HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment), an international large-scale energy, water and carbon balance experiment carried out during the summer of 1992. During well construction samples were taken from the unsaturated zone at the following intervals: every 25 cm from 0–10 m, every 50 cm from 10–62.5 m, then every metre to the bottom of the well. Pore water was extracted from each sample either by centrifugation or elutriation and analysed for chloride; moisture contents of samples were obtained gravimetrically. These data have been used to produce depth profiles of pore water chloride concentration and moisture content throughout the unsaturated zone. From these profiles it has been possible to derive an estimate of historic direct recharge at the site. The chloride concentration of rainfall, which is required to make the estimate, was determined from the analysis of 123 rainfall samples collected from five EPSAT (vers une Estimation des Precipitation par Satellite au sahel) rain gauges in 1992. A mean recharge rate of 13 mm year−1 (range 10–19 mm) is estimated for the upper 70m of the profile, with a total residence time of 790 years (range 520–990 years). This is considered to be a representative estimate of the magnitude of direct recharge taking place below tiger bush areas.

Topics: Groundwater recharge (57%), Water content (53%), Vadose zone (52%), Pore water pressure (50%)

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Citations
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Journal ArticleDOI
Abstract: 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.

823 citations


Cites background from "Estimation of rainfall inputs and d..."

  • ...…of preferential flow in the southern Kalahari (S. Africa) is indicated by higher recharge rates based on tritium distribution (13 mm year 1, Appendix I, Table I) relative to those based on chloride in the unsaturated zone (1Ð8 and 5 mm year 1; Appendices I, II; Table 1) (Butler and Verhagen, 2001)....

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Journal ArticleDOI
Abstract: 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.

709 citations


Cites background from "Estimation of rainfall inputs and d..."

  • ...In such cases, the hydrological system is in a state of dynamic evolution; examples are cited by Allison et al. (1994), Phillips (1994), and Bromley et al. (1997) ....

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Journal ArticleDOI
Abstract: 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.

317 citations


Journal ArticleDOI
Abstract: 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.

177 citations


Book ChapterDOI
01 Jan 2000
Abstract: 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.

159 citations


References
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Journal ArticleDOI
Abstract: A portion of the Gambier Plain underlain by an unconfined aquifer with readily definable hydrologic boundaries has been divided into a number of areas within which soil types have similar hydrologic properties. Mean annual recharge has been estimated for each area using both the tritium concentration and the chloride concentration of water within the soil profile. Good agreement was obtained between the two methods with local recharge varying between 50 and 250 mm year-1. Total mean annual recharge for the area has been estimated to be 2.4 ± 0.3 x 108 m3 year-1, and this compares favourably with an estimated discharge of 2.5 ± 0.3 x 108 m3 year-1.

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Journal ArticleDOI
Abstract: A more general use of the filter-paper method for measuring soil-water potential over a very wide range of values is advocated, both for in situ and laboratory situations. Using Whatman® No. 42, both HgCl2-treated and untreated, filter papers, the calibration curves measured on two different batches two years apart were almost identical and were in good agreement with a curve published previously by other authors. Routine use of the same papers could therefore be made without recalibrating. Examples using the method for constructing water-release curves are given, and other uses are suggested.

183 citations


Journal ArticleDOI
Abstract: Sandy soils of the Sahel area in West Africa, mainly cropped to millet (Pennisetum typhoides) are very sensitive to crust formation Crusts strongly reduce infiltration capacity In this area most fields are gently sloping (1–3%) and hence runoff is a widespread phenomenon; on the average 25% of the rain (mainly in the form of a few large storms during the rainy season) is lost by runoff The causes of crust formation and its effect on the infiltration rate are discussed On untilled soils the presence of a crust is a permanent feature Rainfall characteristics play a key role in crust formation Major rainfall characteristics of the Sahel differ significantly from those of other semi-arid areas

163 citations