Assessment of transmission loss in a Mediterranean karstic watershed (Wadi Natuf, West Bank)

Run-off transmission loss into karstified consolidated aquifer bedrock below ephemeral streams (wadis) has rarely been described nor quantified. This study presents unique data of long-term high-resolution field measurements and field observations in a semiarid to subhumid Mediterranean carbonatic mountainshed. The catchment with a 103 km(2) surface area is subdivided into 5 subcatchments. Coupled run-off measurements were made in the different stream sections (reaches), and transmission loss calculated from differences in discharge. Rainfall and run-off observations from 9 automated precipitation gauging stations and 5 pressure transducers for automatic water level recording are complemented by manual measurements during 34 run-off events covering a total measurement period of 8 consecutive years. Runoff generation is strongly event based depending on rainfall intensities and depths. Both, run-off generation and transmission losses are related to spatial patterns of bedrock lithologies (and hydrostratigraphy). Transmission losses range between 62% and 80% of generated run-off, with most of the smaller events showing 100% transmission loss. Therefore, although event run-off coefficients in the mountains can reach up to 22%, only 0.11% of total annual precipitation leaves the catchment as run-off. Most run-off infiltrates directly into the regional karst aquifers (Upper Cretaceous carbonate series), with transmission loss intensities of up to 40 mm/h below the stream channels. The factors determining run-off-such as geology, pedology, vegetation cover and land use, relief and morphology, the semiarid to subhumid Mediterranean climate with a strong elevation gradient, and the patchiness of individual storm events distributed over the winter seasons-as well as the lithology and epikarst features of the bedrock are all characteristic for larger areas in the Mediterranean region. Therefore, we expect that our findings can be generalized to a large extent.