Ground Water Availability in Low Precipitation Areas (Highlighting Porto Santo Island Case Study - Portugal)

Abstract

In the water cycle, groundwater corresponds to that part of the water that enters the soil, is not evapotranspirated and flows deeper to a saturated zone. Usually, in low precipitation areas, surface water availability is too low or sparse in time, and groundwater may be a good (or unique) solution to water availability in those areas, if the hydrogeological conditions are appropriate. Some numerical models are available to estimate the amount of water that reaches the saturated area (recharge). The importance of these models is that it allows for the determination of recharge, as the exploitation of groundwater should preserve the balance between recharge and abstraction. Two methods are described: the daily sequential water balance and the solute profile. The numerical models are used to show how the land use, the soil conditions and the precipitation regime may affect recharge. The Porto Santo island, located in the Atlantic Ocean, is characterized by water scarcity. This island was studied in order to evaluate the ground and surface water availability. Examples of groundwater availability and its use in other different low precipitations areas of the world are also presented. Groundwater is abstracted using wells. Besides natural recharge, man practices may increase groundwater availability. Among these are artificial or induced groundwater recharge, rainfall harvesting followed by infiltration, construction of small ditches, or dams, that retain surface storm water and allow it to infiltrate, and, in the case of appropriate hydrogeological conditions, the construction of underground dams.