Abstract

The southern part of the coastal aquifer is one of the important natural water resources of
Israel. Understanding effects of land use over its recharging outcrops on quantity and
quality of the recharging water, is essential for improving water-resources management.
Therefore the following goals were set for this study: a) monitoring the vadose zone under
different land uses overlaying the aquifer, b) calibrating and validating flow and transport
models to the monitoring observations, and c) investigating recharge under the different
land-uses and evaluating the effects of climate and land-use change using the calibrated
models.
Vadose zone monitoring systems (VMS), which enable continuous measurements of the
vadose-zone water content and frequent sampling of the pore water from the entire width of
the vadose zone were installed under: (1) a natural sand dune, (2) a dry-land crop field, and
(3) an irrigated orchard, all in the vicinity of Ashdod. High chloride concentration (average
890 mg L-1) and relatively small masses of nitrate (average 75 mg L-1) are found under the
orchard reflecting high efficiency in water and fertilizer use. In contrast, under the crop
field the vadose-zone pore water was less saline (average 170 mgCl L-1) but contained
higher masses of nitrate (average 213 mg L-1). Under the sand dune solutes concentrations
were < 20 mg L-1.
HYDRUS-1D was used to calibrate flow and chloride transport models to transient water
contents and chloride concentrations obtained at the three sites. Good fits were attained for
most calibration and validation runs. It was concluded that one-dimensional classical
Richards’-ADE models can capture the major phenomenon revealed in the monitoring data
of the thick vadose zones that were investigated, hence can be used for scenario
simulations.
The mean annual calculated recharge (1987/8 – 2009/10) under the natural sand-dune was
341 mm yr-1, 66% of the annual surface-water inputs (precipitation), reflecting low
evapotranspiration and runoff. Simulations of the dry-land crop field resulted in a mean
recharge of 204 mm yr-1, 39% of the annual precipitation, whereas the lowest average
recharge of 135 mm yr-1 was calculated under the irrigated citrus orchard, which is only
10% of the annual surface-water inputs (Precipitation + irrigation). These results strengthen
the chloride results, reflecting the high efficiency of water use in the orchard relatively to
that of the crop field.
Climate change scenarios in which a 25% drop in annual precipitation was simulated
revealed a decrease of 54% in recharge fluxes under the crop field and 33% under the
natural sand dune. This emphasizes the differences between decreases in precipitation and
decrease in recharge rates under the different land uses that should be considered in water
resources management.
In order to evaluate the influences of land use change, the conversion of the natural sand
dune to commercial vegetable greenhouse was simulated. 18 years of intensive cultivations
resulted in significant impact on groundwater recharge (from 341 to 150 mm yr-1), 11 fold
increase in chloride fluxes to the aquifer and accumulation of salts in the vadose zone
overlying the aquifer.