Field-based groundwater recharge and leakage estimations in a semi-arid Eastern Mediterranean karst catchment, Wadi Natuf, West Bank
Authors: Messerschmid Clemens , Lange Jens , Sauter Martin 
Published: June 2018

Abstract. Groundwater recharge processes in semi-arid climates are highly sensitive to spatial and temporal variability (event precipitation). However, in previous research, the spatial distribu-tion of recharge has received relatively little attention. This study differentiates recharge ac-cording to the following spatial characteristics: (a) soil type and soil condition, (b) land forms such as relief, vegetation and land use, and (c) lithology and hydrogeological characteristics of the subsurface rock formation. For the first time, this paper analyses and quantifies the specific recharge in the different individual rock formations of the Western Aquifer Basin (WAB). The WAB is a large transboundary karst aquifer that stretches from sub-humid to semi-arid climates from the recharge area in the West Bank Mountains to the Mediterranean Coast. The assessment is based on actual field data, including soil moisture and spring dis-charge in Wadi Natuf, a 103 km2 large sub-catchment in the West Bank slopes and mountain region, i.e. the recharge area of the WAB. A parsimonious soil moisture balance model was set up to calculate daily recharge rates from daily precipitation and evapotranspiration records over an extended period of 7 years (2003/2004–2009/2010). Unlike in most comparable studies, the simple but solid parsimonious soil moisture and percolation model and the budget calculations were based on actual quantitative field-observations, in contrast's daily deep percolation rates were compared with soil moisture field-data and in addition, by comparing major event recharge depths with daily spring discharge response. This combination of modelling and intensive field measurements, comprising eight different soil moisture measurement stations in six different litho-facies formations allowed identifying and quantifying the recharge characteristics of each formation at high spatial resolution, which is a first in the Western Aquifer. We found that recharge varies widely at the spatial dimension, ranging between 0 % and almost 60 % of annual rainfall. The spatially variable long-term average recharge coefficients were applied to other outcropping formations by a ranking procedure taking into account each of the three above spatial characteristics (landform, soil and geology). In addition to the recharge analysis, special emphasis was paid to the examination of the role of perched leaky aquifers positioned between the main regional Upper and Lower Aquifers. The particular geometry of the local aquifers, i.e. hills with well-defined aquifers, leaky aqui-tard bases and therefore well-defined catchment areas, allowed the quantification of the flow budget. By measuring and budgeting spring group discharge of each leaky sub-aquifer, for the first time also leakage coefficients could be calculated empirically. The methodology of this study is also applicable to comparable catchments and aquifers out-side the region. The resulting mean annual recharge coefficients allow the prediction of the spatial distribution of recharge at any given sub-catchment or management cell size, also beyond Wadi Natuf (in a follow-up paper).