Seasonal Floods: Harvesting and Contribution to Aquifers’ Recharge

Abstract

Rainfall records at eight meteorological stations around Al Ain City indicate that most rain falls during the period between February and March, In addition, there are four- to five-year cycles with above-average rainfall, and the median rainfall in Al Ain is 76.1 mm/year, above which flood peaks are expected and below which drought seasons are predicted.

Drainage-basin parameters and rainfall data were used along with the HEC-HMS model to estimate the water storage from rainstorms in the reservoirs of the Wadi Al Bih, Wadi Al Tawiyean and Wadi Ham basins, in northeastern UAE. A group of curves illustrating rainfall–runoff and reservoir storage was developed based on the intensity of rainstorms and their duration. These curves were used to predict surface runoff and water storage in the dam reservoirs of the three wadis under rainstorms of various magnitudes. Because the catchment area of wadi Al Bih is larger than Wadi Tawiyean and Wadi Ham, the volume of surface runoff in wadi Al Bih resulting from the same rainstorm was twice the surface runoff on Wadi Ham and Wadi Tawiyean.

Based on an annual average rainfall of 115 mm/year, a weighted average runoff coefficient (%) of 7.63, an average annual runoff depth of 8.77 mm/year and the overland flow and flash-flood risk were calculated for drainage basins in the eastern mountain ranges and Jabal Hafit area. Based on ranking of the calculated bifurcation ratios (R_b) and overland flow velocity, relative infiltration rates (Ir) and flood danger were assigned for each drainage basin.

The lowest water volume harvested by these dams reached 2.4 Mm³ during the 1999–2000 season, while the largest amount of water retained by these dams reached 20 Mm³ during the 1994–1995 season. The low water harvest during 2004 and 2005 seasons is related to the dry conditions prevailing during this period or incomplete data covering that period of time.