Assessment of surface water runoff and groundwater recharge using mathematical models
In arid and semi-arid regions, surface water resources are scarce and, in most cases, groundwater is the only natural resource of freshwater. Pumping of groundwater often exceeds natural recharge. Therefore, groundwater levels are declining and its quality is deteriorating. Sustainable management of groundwater is thus a key issue and requires implementation of appropriate technologies to augment groundwater resources. Artificial recharge augments the natural movement of surface water into the underground formations using some means of construction whereby surface water from streams or lakes is made to infiltrate into the ground.
The UAE is known by its arid conditions and limited renewable freshwater resources. Surface water in the UAE is very limited and of a little significance in the water budget of the country. Despite the construction of many desalination plants, groundwater represents a vital natural resource. Although it may not be suitable, in most cases, for drinking, it represents the main source for irrigation. About 85% of the total water consumption in UAE is from groundwater. The sustainability of this precious resource is of prime concern in the UAE. Many dams have been constructed during the last two decades across the main wadis to harvest surface water runoff and recharge groundwater.
The importance of this study evolves from the need to assess surface water and groundwater resources in the main wadis which are of vital role in the sustainable development of UAE, specifically, the agricultural development. The study aims at the simulation and quantitative assessment of surface water runoff and the associated groundwater discharge in Wadi Ham, UAE. HEC-HMS and MODFLOW models were used. Due to data limitation, HEC-HMS was applied for the period 1979 to 1989 and MODFLOW was applied for the period from January 1, 1990 to December, 1993. The study provides a methodology that can be followed in other sites of hydrological and hydrogeological conditions. All the data and facilities were provided through a project entitled “Assessment of the effectiveness of Al-Bih, Al-Tawiyaen and Ham Dams in groundwater recharge using numerical models”. The project was funded by Ministry of Agriculture and Fisheries.
The catchment data and drainage network were delineated based on the available topo sheets and remote sensing images using ArcView GIS and AutoCAD softwares. Different wadi tributaries and properties were identified. Comprehensive analyses were conducted to study the variations of rainfall, surface water flow and groundwater levels based on historic records. Several lithological cross sections were developed to assess the hydrogeology of the area and identify the aquifer geometry. A rainfall/runoff model (HEC-HMS) was used to study the surface runoff process and quantify the total runoff yields. A three-dimensional groundwater flow model (MODFLOW) was used to quantify groundwater recharge and study flow directions and the water balance.
The total catchment area to the Wadi Ham Dam is approximately 195 km2. This includes Wadi Ham itself and the catchment of the Wadi Al-Farfar system. Rainfall distribution is intermittent and highly scattered. The mean annual rainfall, estimated for 23 years, is 154 mm. Surface water flow is also variable reflecting the intermittent nature of rainfall. Wadi Al-Farfar system has major contribution to the total runoff and accounts for about 40% of the total runoff yield accumulated at the dam site. The Wadi Ham Dam has an effective role in groundwater recharge and its effect is clearly reflected by rise in groundwater levels. The recharge from the dam ranged from 32% to 43% of the dam storage. The flow of the seawater to the aquifer is reduced to very low levels during recharge events from the dam while water losses to sea appear to be very minor
The study suggests some recommendations including continuity of measurements and additional installations of flow gauges and observation wells. The groundwater models developed in this study can be enhanced and its capabilities can be expanded by conducting field inventory of pumping wells, new drillings and pumping tests of longer durations.