The concept of a groundwater regime is based on the fact that the occurrence and distribution of groundwater is not merely a product of chance, but the result of a combination of climatic, hydrologic, geologic, topographic and soil-forming factors that together form an integrated dynamic system. These factors are interrelated in such a way that each provides some insight into the functioning of the total system and thus serves as an indicator of local conditions of groundwater occurrence and distribution (PAEL, 1993).
In recent years, population growth and land use changes all over the world have increased the demand for freshwater and large-scale development of groundwater using improved technologies of drilling and abstraction; in addition, it is believed that climate change effects have further reduced the water availability especially in coastal areas [Ranjan et al., 2006]. According to Zohu et al. (2013), we can categorize all these aforementioned factors as in the following figure:
Factors influencing groundwater regime (Zohu et al., 2013).
First and most importantly among the preceding relationships are the physical characteristics of the framework (geological formations) in which the groundwater system occurs and flows. Two hydraulic parameters that should be taken into consideration as preliminary evaluation to the occurrence of groundwater reservoirs are porosity and hydraulic conductivity. Variation of these hydraulic values classified the geologic units into aquifer, aquitard, aquifuge and aquiclude from hydrogeologic point of view. Weathering, structures and solutions have affected most rocks to some degree and enhanced their hydraulic parameters (Fetter, 1994) to store and transmit water. The relation landscape-lithology-geologic structure defines the occurrence, extension and productivity of aquifers.
Climate variability and change influence groundwater systems, both directly through replenishment by recharge and indirectly through changes in groundwater use. For direct impacts, natural replenishment of ground water occurs from both diffuse rain-fed recharge and focused recharge via leakage from surface water (that is, ephemeral streams, wetlands or lakes) and is highly dependent on prevailing climate as well as on land cover and underlying geology. Spatial variability in modelled recharge is related primarily to the distribution of global precipitation (Doll et al., 2008; Wada et al, 2010). Climate and land cover largely determined precipitation and evapotranspiration, whereas the underlying soil and geology dictate whether a water surplus (precipitation minus evapotranspiration) can be transmitted and stored in the subsurface. At high latitudes and elevations, global warming changes the spatial and temporal distribution of snow and ice. The aggregate impact of these effects on recharge is not well solved, but preliminary evidence (Sultana & Coulibaly, 2010; Tague & Grant, 2009) indicates that changes in snowmelt regimes tend to reduce the seasonal duration and magnitude of recharge.
These impacts can be modified by human activity such as land-use change (LUC) and resulted in indirect impacts. Links between climate and groundwater in the modern era are complicated by LUC, which includes, most pervasively, the expansion of rain-fed and irrigated agriculture. Managed agro-ecosystems do not respond to changes in precipitation on the same manner as natural ecosystems. Indeed, LUC may exert a stronger influence on terrestrial hydrology than climate change. During multi-decadal droughts in the West African Sahel in the latter half of the twentieth century, groundwater recharge and storage rose rather than declined owing to a coincidental LUC from savannah to cropland that increased surface runoff through soil crusting and focused recharge via ephemeral ponds (Leblanc et al., 2008; Tayloe et al., 2012).
From the above brief summary, it is possible, to evaluate the general potential of an area for groundwater development by appraising as many of the factors listed above as practical and then by interpreting the local regime on the basis of known relationships among the factors and their effect on groundwater regime.
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