1. INTRODUCTION
The Jordan Rift Valley (JRV) refers to the depression below the sea level extending over the range of latitudes 30-33 N and longitudes 35-36 E, including Lake Tiberias to the north and the Dead Sea in the middle, surrounded by Jordanian Highlands and Judaean Mountains. Due to the arid environment as compared with the other parts of Jordan, as mentioned in Tarawneh and Kadıoğlu (2003), the management of irrigation water is vital in JRV (Al-Weshah 2000). However, available data of precipitation and streamflows in JRV are limited to monthly resolution, significantly hindering efficiency in modeling with widely used hydrological methodologies (Rumman et al. 2009). An innovative water harvesting (WH) system has been established for research purposes in the Lisan Peninsula of the Dead Sea (Unami et al. 2015). The geology of the Lisan Peninsula is detailed in Closson et al. (2007). A barren catchment area of = 1.12 km2, as shown in Figure 1, yields flash floods several times a year, and hydraulic structures have been constructed at the outlet to harvest them as designed in Sharifi et al. (2015). The water current is fully collected at a gutter cutting across a 16 m wide valley bottom and then guided to an open-air reservoir of 1,000 m3 capacity through a conveyance channel of 60 m long (Unami and Mohawesh 2018). The conveyance channel is equipped with a spillway to release excess backwater from the reservoir. As there is no authorized guideline in Jordan, an arbitrary flood discharge of 1,000 L/s is used in the hydraulic design of the spillway. A solar-driven desalination plant, having a blackish water reservoir of 300 m3 capacity in a modified greenhouse, treats the harvested water to be used for irrigation (Unami et al. 2020), and year-round farming is performed under optimal water management policy (Unami et al. 2019). The available lands somewhat constrain the capacities of the reservoirs, but the open-air reservoir is large enough to buffer the brackish water to be sent to the desalination plant after siltation. An observation system is operating since September 30, 2014, so that time series data of rainfall and runoff in flash flood events are minutely acquired (Figure 2). Due to the extremely arid environment, water current as the runoff from the catchment is ephemeral, and the flash flood events are clearly distinguishable from each other.
Figure 1. Photo of the gorge in the barren catchment area
Figure 2. Photo of the observation system including VAISALA WXT536 weather transmitter and SR50A acoustic distance sensor for measurement of runoff discharge
This Scientific Briefing presents a statistical analysis of flash flood events observed at the observation system, in order to provide basic knowledge of hydrology in JRV to be utilized for designing WH systems. The duration, the rainfall depth, and the runoff coefficient of each flash flood event are standard hydrological parameters, but the most critical parameters for designing WH systems are the spillway design discharge and the reservoir capacity (Walsh et al. 2014). The correlations among those parameters as random variables are firstly analyzed in terms of the classical methods of Pearson’s correlation (Pearson 1896) and Spearman’s rank correlation (Spearman 1910). Then, probability distribution fitting is performed for each variable, focusing on the lognormal distribution with three parameters (LN3) (Levy and Kroll 1976) and the generalized extreme value distribution (GEV) (Jenkinson 1955) in particular. Fundamentals and recent developments in estimating the probability of extreme events from independent observation are presented in Makkonen and Tikanmäki (2019). The results support the design of the WH system, which has been already constructed with arbitrary hydrological parameters, providing quantitative information for designing and operating WH systems in the future as well.