Ground Water Recharge

An important issue in any rainfall enhancement program is the determination of the potential benefiting sectors. These would include direct benefits to rain fed agriculture, the forestry sector, and possible social and ecosystem service benefits, with irrigated agriculture and municipal and industrial (M&I) water users being secondary benefiting sectors. In the UAE there is little to no surface water, which forces agriculture to be solely sustained by supplemental irrigation through groundwater withdrawals or desalinated water. An exception is the historic Falaja's, which are man-made 'springs' that take advantage of local topography and groundwater gradients to transport water to end-user points via underground viaducts. Therefore, if rainfall enhancement is shown to be viable in the UAE, it would likely benefit only the secondary sectors (irrigated agriculture and M&I water supplies) through enhanced groundwater recharge.

Ground Water Recharge

Primary wadi boundaries that contribute to groundwater
recharge of alluvial aquifers in the eastern portion of the UAE
(click for enlargement)

Groundwater recharge is the replenishment of an aquifer with water from the land surface. It is usually expressed as an average rate of millimeters of water per year, similar to precipitation. Thus, the volume of recharge is the rate times the land area under consideration, and is typically expressed in millions of cubic meters per year.

The quantity of recharge to an aquifer has been considered equivalent to the "safe yield" or quantity of water that could be removed from an aquifer on a sustainable basis. It is believed that the "sustainable yield" of an aquifer is almost always appreciably less than recharge. This is because sustainable yield must also allow for adequate provision of water to sustain Falaj's, oasis, and groundwater-dependent ecosystems. Nevertheless, a sustainable yield figure is derived from a recharge determination, and any sustainable yield study will usually involve the determination of recharge as a necessary first step. However, recharge is not well understood, so it is difficult to estimate aquifer sustainability unless the recharge-related processes are carefully studied. Since the majority of renewable freshwater resources are derived from the Oman Mountains, cooperation between the UAE and Oman will be necessary.

The eastern area of the Abu Dhabi Emirate, the region near the Oman Mountains, was once a much more active fluvial system. In many locations, aerial photographs reveal evidence of strong outflow through the wadi gaps onto the western plains. During this period, the region was climatically 'wet' with a number of perennially flowing surface wadis. The contributions from this paleo rainfall and paleo wadi flow likely played a major role in creating secondary permeability, flushing out soluble matter from the soil matrix, which allows for a relative increase in groundwater storage in certain locations west of the Oman Mountains. In general, present groundwater sources in this area are a combination of paleo and present day groundwater storages.

The eastern aquifers of the UAE are both shallow-alluvial and deeper confined hydrogeological units that are significantly controlled by subterranean geologic structures, and varying paleo groundwater traps that are also controlled by structures and topography (Bright and Silva, 1998). The shallow-alluvial aquifers are those that are most rapidly charged by precipitation events. These aquifers occur within three different hydro-geological regimes, two of which are localized and restricted to the region close to the Oman Mountains and a third that extends to the west coast of the Abu Dhabi Emirate. These structures are multi-layered with varying permeability, but all three structural units are being recharged by the underflow from the Oman Mountains (wadi interflow) and surface running wadi flow that occurs during more intense precipitation events.

Although difficult to estimate, it is likely that groundwater reservoirs in this region are filled with both trapped paleo water and reservoirs connected to present recharge regimes. The subterranean geologic state is complex, and the laterally extending near-surface groundwater systems are channelized with varying degrees of permeability, transmissivity, and aquifer thickness, making their overall yield difficult to estimate. The channelization depends on primary and secondary permeability contrasts caused by the original rock characteristics and groundwater related process, respectively. These aquifers receive present day recharge via underflow from the Oman Mountains, infiltration from surface wadi or gap flow during periods of more intense rainfall, direct surface recharge from rainfall (likely to be insignificant), and in some locations, agricultural return. Understanding the relationship between the rainfall amount and intensity, the surface hydrologic response, and aquifer recharge is of fundamental importance.

There are two seasonally different rainfall mechanisms that lead to groundwater recharge. The majority of recharge most likely occurs during winter frontal rainfall episodes on the synoptic scale. However, hygroscopic seeding is amenable to convective and not synoptic weather systems, so the focus of enhancement analysis has focused on the summer convective season. Some summer convective rainfall likely leads to recharge, but on a smaller scale in terms of both volume and lateral extent when compared to winter recharging events. Early evidence from the rainfall and cloud physics experiments conducted during the summer of 2001 and 2002 suggests that the southeastern region of the Oman Mountains, a region east to southeast of Al Ain, seems to be a triggering point for significant convective rainfall events. There appears to be a preference for convective storms to form over this region and migrate west to northwest across the Oman Mountains and onto the desert plains. Convective storms that formed over the Oman Mountains to the north of Al Ain tended to be smaller and shorter lived.

Ultimately, results from randomized rainfall enhancement experiments should be used to suggest potential costs and benefits associated with a long-term enhancement program. To do this the water resource infrastructure in the UAE must be analyzed. A Decision Support tool, such as the Water Evaluation and Planning Model (WEAP) can be used for performing such an analysis. The WEAP model has been updated as a part of this project to include dynamic hydrology within the simulation framework (Yates, 1996).