Rainfall – Runoff Analysis of Wadis Contributing to the Dead Sea: Wadi Og as a Case Study

The distinctive features of arid and semi-arid areas affect rainfall-runoff modeling on both a discrete basis and continuous basis. Extreme events such as flash flood, severe storms, and droughts are the main features characterizing such environment. However, these features are prevailing in Jordan River Valley, in Palestine, where periodic fluctuations in rainfall affect runoff generation and cause rapid response to ephemeral streams of the wadis that contributes to the Dead Sea in winter season. In this research; rainfall-runoff process for Og watershed that lies in the northwest shore of the Dead Sea was studied, the drainage watershed that contributes floods was characterized, and the surface runoff volume and flood peak was predicted. The Hydrologic Engineering Center’s –Hydrologic Modeling System (HEC-HMS) model was used to simulate single event and continuous hydrologic model for the watershed. Three scenarios were simulated for the continuous model ( wet rainfall hydrological year 2002-2003, average rainfall hydrological year 2004-2005, and dry rainfall hydrological year 2003-2004) while flash flood from 8th January,2013 storm was simulated for event model. The physical characteristics of the watershed was related to the model through developing the Digital Elevation Model, Watershed Delineation, and stream network using GIS capabilities. The dimensionless SCS unit hydrograph was used to transform the computed excess rainfall to direct runoff at the outlet of the watershed. The SCS-CN method was also used to simulate excess rainfall and losses. The Muskingum routing method was used to account for the transmission losses, while the meteorological model was developed for event and continuous basis. Statistical rainfall analysis was performed for yearly, monthly and daily rainfall data for three rainfall stations surrounding the Og watershed. The consistency of rainfall data was checked and the areal rainfall was calculated using the Isohyetal map of the region. The results were used as an input to the meteorological model. Surface runoff and peak discharges were predicted for each continuous scenario beside the event model. The model outputs reflect the aridity of the area where only 9 MCM of surface runoff was generated in average hydrological rainfall year, 19 MCM of surface runoff was generated in wet hydrological rainfall year and less than 1 MCM in dry hydrological rainfall year. Event model output depicts that only two to three peak discharges may take place through a rainy season with 73,000 m3 volume. Model calibration and validation is essential need for hydrological modeling developed for arid and semi-arid environment. However, as Og watershed is ungauged, a kind of verification was conducted to the event model output. Apparently, there is an urgent need to calibrate the model parameters in order to be applicable for other hydrological purposes. The lack of high quality data to support the modeling, the difficulty of observing the generally high spatial variability of rainfall inputs and flow outputs, as well as the limited available tools for the Palestinian hydrologists to work on the field are the main obstacles facing this research.