Climate change and population growth will challenge water security in Israel. Adapting to future water security challenges requires policymakers to act today. The Water Futures and Solutions for Israel project (WFaS-Israel) aims to assess water scarcity in Israel under different plausible future scenarios. Each scenario combines different climatic trajectories with diverse socio-economic and spatial planning storylines.
The State of Israel has been experiencing increasing water scarcity since its establishment. In response, Israel has cultivated a diverse and modern national water management system. Water supply in Israel comes from a combination of fresh groundwater, surface water, desalinated seawater, brackish water, and treated wastewater.
The planned analysis in WFaS-Israel uses IIASA state-of-art Community Water Model (CWatM) to simulate the water cycle in selected Israel-based hydrological basins, using a daily time step and hyper spatial resolution less than one kilometre. The hydrological basin of the Ayalon stream will be used to showcase the model outcomes.
The Ayalon Hydrological Basin
The Ayalon hydrological basin drains an area of around 800 km2, originating from the lower western slopes of the Judea hills in the East. It ranges from approximately 15 to 870 meters above sea level and flows westwards into the most densely populated areas in Israel, where it merges with the Yarkon stream before meeting the Mediterranean Sea (see figure 1). The model is calibrated against discharge data over a five-year period, taken from a hydrometric station located in the main channel at the edge of the urban area (see figure 1).
Figure 2 shows the simulated and observed discharge data from both calibration (August 2001 – December 2005) and validation (January 2001 – December 2010) periods at daily (top plots) and monthly time steps (bottom plots). Overall, the model represents discharge relatively well (Kling-Gupta Efficiency of 0.77 and 0.67 for the validation and calibration periods respectively, at a daily time step). Most of the differences between simulated and observed data occur in data with higher-than-average discharge. Yet, in these events the model alternately over- and under-estimates the observed discharge, suggesting that uncertainty of input data, such as precipitation, may be the cause for such differences.
For many applications related to water security and basin management (e.g., constructing irrigation reservoirs, water demand management, etc.), a monthly water balance is sufficient. At a monthly time step the model performance increases with a KGE of 0.86 and 0.83 for the calibration and validation period, respectively.
CWatM yields various spatial and temporal explicit results, which collectively describe the hydrological cycle of the basin in question. A CWatM related development, the Water Circles, provides a simplified but thorough description of that cycle.
Figure 3 shows the water circle of the simulated hydrological cycle of the Ayalon basin over a course of approximately five years (August 1st, 2001, to December 31st, 2005). Water inputs (2,089 MCM, million cubic meters) split between precipitation (92.7%) and ‘Fossil water’ (~7%). The latter are extracted from a virtual non-renewable aquifer and stands for water shortage. Total outputs of 2,076 MCM are split between evapotranspiration from land and plants accounts for 76.7% (159.2 MCM), domestic and industrial water use (319.7 MCM, 15.4%), and downstream discharge (142.4 MCM; 6.9%). Transpiration accounts for approximately half of the outputs, of which 795.6 MCM (72.6%) originates from natural area and the rest from irrigation (299.7 MCM).
The water circle in Figure 2 demonstrates basin-scale water shortage. It is represented by the virtual 164.7 MCM of 'fossil water', implying a relatively small (~7%) water shortage. The outputs from the basin are 2,076 MCM of which Transpiration from less developed, rural environments and from natural areas (795.6 MCM) and irrigation (10 MCM) account for about 52% of the outputs.
Water inputs and outputs vary spatially, as does water scarcity. Figure 4 presents water scarcity as the share of the unmet from the total water demand. Higher water scarcity downstream is partly due to the high water demands from urban-dense areas, but also due to the low water availability in nearby grid cells.
Future research in the project is planned to estimate water scarcity for different hydrological basins in Israel, embedding the complex structure of the Israeli water system into CWatM. For that purpose, a few model extensions are planned, including water mix control, wastewater treatment and reuse, sea water desalination, and inter-basin water transfers. Additionally, solutions to reduce water scarcity will be introduced to the model, these include surface runoff/flood capturing, groundwater recharge basins/wells, urban water collection, crop selection, etc. Finally, climate change and socio-economic storylines will be developed and used to assess plausible Water Futures and Solutions for Israel.