I. Shentsis(1), A. Ben-Zvi1, and E. Rosenthal(2)

1. Hydrological Service of Israel, Research Div., POB 6381, Jerusalem, Israel.
2. Department for Planetary Research and Geophysics, Tel Aviv University, Ramat Aviv, Israel.

A hydrological - lithostratigrafical model was developed for the assessment of transmission losses from runoff events in arid water courses, under conditions of scarce hydromteric data. The losses were subdivided into channel moistening which subsequently evaporates and deep percolation, which recharges groundwater. The model was applied to certain major tributaries of Nahal (wadi) Arava (N. Zin, N. Neqarot and N. Paran). Once in 50-100 years the recharge volume may be 5 to 7 times higher than the mean annual volume (2-4 MCM for a watershed), but, the recharge during most years is very small.

The model was validated against data from two experimental watersheds in Israel and Saudi-Arabia which were better equipped with measuring networks. The results confirmed the ability of the model to produce reasonable estimates of transmission losses. A more substantial support was providcd by use of independent groundwater data (water levels and pumping rates) measured in boreholes drilled into the Hazeva Formation aquifer of the Paran watershed. A groundwater balance model was developed as an integrated system including surface - and groundwater flow and basing both on the hydrometric data of boreholes and on the model estimations of the aquifer recharge. It appears that the recharge from runoff events is a significant component of the groundwater balance. Only by considering this component it was possible to minimize --to a satisfactory degree of accuracy - the annual imbalances for each subaquifer and to approximate the main hydrological processes occurring in them. Such processes include lateral exchange of water masses between depletion cones (created by groundwater extraction) and the surrounding aquifer and also possible interaquifer flow.

The model enables forecasting of changes in groundwater storage, of resulting fluctuations of groundwater levels and of changes in groundwater salinity. The dominant model predictor is, in all cases, the net extraction from the aquifers which equals the volume of pumpage minus the recharge from surface runoff.