**2. Groundwater recharge**

Groundwater recharge can occur in different ways:


Local groundwater is in many places not sufficient to recharge the aquifers. There are many aquifers in semiarid areas that are overused. Examples are common in S and SE Asia. Megacities with groundwater supply have in several places salinity problems resulting from land subsidence like in Bangkok and Jakarta [3]. In view of the climate change foreseen, there will be increase in sea levels that will affect the balance between groundwater and seawater. Ericson et al. [4] have evaluated 40 deltas regarding the effective sea level rise (ESLR), which in their evaluation is a combined effect of sea level rise and subsidence of the deltas. The Bengal delta in Bangladesh and India is experiencing an ESLR rate as high as 25 mm/year, while the Mississippi delta in USA has a rate of around 10 mm/year. The sea level rise as such is in the order of 3.3 mm/year, and the rate is expected to increase in the coming decades [5]. Ericson et al. [4] estimate that by 2050, about 8.6 M people are at risk due to ESLR. Local conditions such as tectonic movements and land subsidence due to excessive groundwater extraction thus play a large role in the risk of saline intrusion into costal aquifers in coming years [3]. The shoreline displacements may have a variety of reasons such as neo-tectonic movements but also anthropogenic activities such as deforestation and increased erosion [6].

Fog water condensation is a rare mechanism of groundwater recharge, but it may play a limited role under specific conditions. An example is the Al-Qara Mountain behind the Salalah Plain in Oman [7]. The browsing of a large population of camels has caused the degradation of mountain forest and decrease in recharge. The local precipitation is 100 mm, and the fog collection now adds just 10 mm to the recharge.
