Global synthesis of the findings from ¾140 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 km 2 ) range from 0Ð2 to 35 mm year 1 , representing 0Ð1-5% of long-term average annual precipitation. Extreme local variability in recharge, with rates up to ¾720 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 Niño Southern Oscillation (ENSO) results in up to three times higher recharge in regions within the SW US during periods of frequent El Niños relative to periods dominated by La Niñas (1941)(1942)(1943)(1944)(1945)(1946)(1947)(1948)(1949)(1950)(1951)(1952)(1953)(1954)(1955)(1956)(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)(1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(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 (½10 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 ...

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Pingtung Plain is formed by Quaternary alluvial fan material from the three main rivers: Kaoping, Tungkang and Linpien. Ground water is the major water supply source on the plain. This is principally extracted from two aquifers. The natural ground water source is derived mainly from direct rainfall percolation and infiltration from the three rivers, with their catchments lying partly outside the plain. Rainfall characteristics are therefore the main factors controlling water resources availability. Pingtung Plain is an important primary production area for southern Taiwan, the comparatively warm climate allowing a long growing season, diversified cropping and the rearing of aquacultural produces. Approximately 75 percent of irrigation and domestic water supplies are derived from ground water. A water balance for the entire plain indicates that ground water resources, under optimized management, are sufficient to meet the existing multi‐purpose uses. Development of a hydrogeological conceptual model is the first phase of a numerical ground water flow simulation. Preliminary results are encouraging, with the final simulations affording better insight to the hydraulic behavior of the aquifer system. Data input requirements for model operation fall into three categories: hydrological stresses, hydrogeological parameters and boundary conditions. After the model is built, the normal numerical modeling process requires significant calibration and sensitivity analyses for the hydrogeological parameters and stresses which are the most sensitive, but the least well defined. A well‐calibrated simulation model can lead to a reliable and realistic management model. With this in mind, the calibration processes detailed are presented, and these data are introduced as initial values in the calibration process.

Since the mid-1980s there has been a relative explosion of recharge studies for (semi-)arid regions reported in the scientific literature. It is therefore relevant to assess what we now know and to offer guidance to the water resources development practitioner. This paper summarizes recurring recharge estimation 'problems' and reviews some recent advances in (inter alia) estimation techniques and recharge time/space variability.The determination of recharge fluxes in (semi-)arid regions remains fraught with uncertainty; multiple tracer approaches appear to offer the best potential for reliable results in local studies which require 'at-point' information. It is also clear that sufficient advances have been made in recent years to show that the value of water balance and Darcian approaches should not be under-estimated. The frequently studied issues of localized recharge and spatial variability need not be a problem if concern is with regional estimates.The key for the practitioner is the project objective; this will dictate whether multiple 'atpoint' or area-based estimation methods are appropriate. For the latter, a combination of reliable local data, remote sensing, GIS and geostatistical techniques offers considerable promise for a better understanding and determination of recharge over extended areas.Groundwater use is of fundamental importance to meet the rapidly expanding urban, industrial and agricultural water requirements in (semi-)-arid areas. To quantify the current rate of groundwater recharge is thus a basic prerequisite for efficient groundwater resource management in these regions, where such resources are often the key to economic development. It is indeed unfortunate that of all the factors in the evaluation of groundwater resources, this rate of aquifer replenishment is one of the most difficult to derive with confidence.Attention in this discussion focuses on recharge of phreatic aquifers, often the most readily available and affordable source of water in (semi-)arid regions. These aquifers are also the most susceptible to contamination, with the recharge rate determining their level of vulnerability. Historical frameworkstudy based largely on empirical information and are country-specific; the international community requires guidelines for wider application.Since the mid-1980s there has been a relative explosion of recharge studies reported in the scientific literature. Recognition of the growing need for reliable recharge estimation has also been reflected in the active support by international agencies and non-government organizations (NGOs), and the publications which have emerged from various international meetings (e.g.

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