Cost/Benefit Analysis

The model has been developed to account for two different hydrologic realities. The first is the concept that in upstream watersheds within more complex topographic settings (e.g. those in mountainous regions with steep slopes and abrupt hills and valleys), the contribution to deep aquifers used for water resource exploitation is limited, therefore most deep percolation returns to the stream or as baseflow with some time lag. In the case of highly ephemeral wadis such as those found in the UAE, there is little to no surface water, but the wadi channels are comprised of coarse, incised alluvium and the baseflow component of the model can be used to represent this flow component. So although it is referred to as "wadi flow", it is really a sub-terraean flow that is a result of transmission losses along the entire length of the wadi. The second watershed "type" are those catchments located in flatter terrain, for example, the catchments outside the outflows of the Wadi's near the Oman Mountains, where outflow from the ephemeral wadi channels contributes to the underlying groundwater aquifer, and this aquifer interacts with the wadi.

The groundwater-surface water interacts, and as the groundwater is depleted, the wadi stream contributes to the groundwater aquifer recharge (losing stream), and when the aquifer recharge is significant then the stream gains water from the aquifer (gaining stream). An additional complicating factor is that irrigated agriculture can draw from this aquifer, which simultaneously both depletes the aquifer and provides recharge back to it. The Irrigated aquifer provides additional "opportunities" for water to be evaporated from the watershed. Irrigation water can also be imported from outside the watershed (e.g. piped desalinized water), which would also change the hydrologic response of the system since this would generally increase the groundwater recharge and enhance the baseflow.

Defining Water Demands for northeastern UAE

Major outflows from the surficial aquifer system include pumpage from wells and sub-surface ground-water outflow. It has been estimated that as much as 70,000 m3 per-day move downgradient, beyond the pumpage wells, into brackish-water areas becoming unusable as a water supply (Silva and Al Nuaimi, 2000). Estimates of groundwater pumpage and usage have been made by Kendy et al. 1996, who estimated that in 1995 about 22 and 9 million cubic meters were pumped from municipal and industrial wells and agricultural wells, respectively.

Groundwater levels have been observed to decline 1 to 9 meters around localized pumping wells during the period 1985 to 1995, and the saturated thickness of the 1200 km2 alluvial aquifer does not exceed 20 meters. For estimating storage changes in the aquifer, it was assumed that the aquifer was a continuous area comprising 1200 km2 with an average saturated thickness of 8 meters, and a storage coefficient of 0.1, leading to a total water volume of approximately 1,000 Mm3/yr at steady state. The downgradient flow to the deeper saline aquifer was given as a function of the storage volume of the aquifer and the flow rate was assumed to increase with increasing aquifer storage and decrease with declining storage.

Water Evaluation Simulation for northeastern UAE

The WEAP water resource model was used to simulate the hydrologic dynamics of this aquifer system for the period 1950 to 1999, to simulate aquifer interaction without any withdrawls, in order to assess steady station conditions of each wadi's discharge. A plot of rainfall observations for this period is shown in to the left, which includes the monthly totals, the 1-year moving total and the 10-year moving average. The early 1950's and the late 1970's and both the 1980's and 1990's were generally wet when compared with the late 1950's through the middle of the 1970's. Mean annual precipitation was about 150 mm, with the blue line in the figure showing the significant amount of variance in the year-to-year precipitation.

The hydrologic simulation of the rainfall-runoff process can be separately evaluated in WEAP. In all four of the wadi's, the maximum storage capacity of the upper soil zone was assumed to be 90 cm's, while the capacity of the lower storage zone is much more significant, at 500 cm's. The lower soil moisture zone exhibits much less hysterisis, gradually declining until the 1990's, and then increasing following the heavy rainfall of the winter of 1995 and 1996.

The WEAP simulation model was then run for the period 1982 to 2013 to simulate historic and estimates of future water withdrawals from the aquifer for all demands. This 30-year simulation included a hypothetical enhancement program which was "carried-out" for a 10-year period, starting in the summer of 2003 and ending in 2013. Because meteorological data are obviously not available for this period, a portion of the historic record (e.g. specifically the period October 1970 to September 1980) was used to represent the future period starting in October 2003 and ending in September of 2013. It was assumed that an enhancement program would strategically target the recharge area of the four aquifers of this study (Sumyani, Safwan, Khaab, and Masiq), but only for the summer months. The summer enhancement represents a total increase in precipitation of 13 percent for northeastern portions of the UAE.