Climate change and groundwater consumption scenarios

The model input of this study stems from two coherent dynamically downscaled high-resolution regional climate projections (daily, 3km, and 8km resolution) until the year 2070 over the region Israel and West Bank (Adinolfi et al., 2020; Hochman et al., 2018), assuming the IPCC RCP4.5 scenario. The coarser climate projection at 8km resolution (ISR8) derives from a COSMO-CLM simulation nested within a regional model of CORDEX-MENA (Coordinated Regional Downscaling Experiment Middle East North Africa). The finer resolved climate projection at 3km resolution (ISR3) also stems from a COSMO-CLM simulation nested into the coarser ISR8 model, where the increased resolution accommodates the spatial variability such as the orographic and urban influence on precipitation. Geographical factors firmly control the precipitation regime of the Eastern Mediterranean and lead to a pronounced spatial and temporal variability of precipitation (Goldreich,1994, 1995), emphasizing the need for finely resolved regional climate models.

The ISR3 climate model projection provides precipitation input to the applied models.

In contrast, the ISR8 model provides the input for the Penman-Monteith equations (Allenet al.,1998) to compute potential evapotranspiration, as it provides all relevant vari- ables (such as wind speeds and irradiance). The applied climate models assume greenhouse gas emissions according to the representative concentration pathway 4.5 (IPCC RCP4.5) scenario, expecting peak carbon emission by 2040 and radiative forcing of4:5 W m⁻² or a change of the average global temperature of 2:5°C until 2100 (Moss et al., 2010).

Warming will occur though beyond 2100 after this scenario. In summary, this scenario is considered an intermediate scenario of climate change. The climate model provides daily precipitation and temperature values, providing suitable input assessing the impact on groundwater recharge, which results from short-term processes since exposed epikarst provides pathways of fast infiltration.

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31.25

31.50

31.75

32.00

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32.50

Moderate (-1.0SPI>-1.5) 35.0035.25

Severe (-1.5SPI>-2.0) 35.0035.25

Extreme (SPI2.0) 201001020

Change of percentage of months (%)

(a) SPI-48 drought periods 35.0035.25

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Moderate (1.0SPI<1.5) 35.0035.25

Severe (1.5SPI<2.0) 35.0035.25

Extreme (SPI2.0) 201001020

Change of percentage of months (%)

(b) SPI-48 wet periods 198019902000201020202030204020502060 Date

2.50.02.5

SPI (-)

(c) SPI-48 of the recharge area Figure5.4:Changeofpercentageoftotalmonthswitha)droughtandb)wetconditionsintherechargeareameasuredby the48-monthSPIundertheclimatechangescenariountil2041-2070ascomparedtothereferenceperiod1981-2010,and(c) theSPI-48averagedovertheentirerechargearea(climatedatabasedonthe3kmregionalclimatemodelobtainedfromthe Euro-MediterraneanCenteronClimateChange,Adinolfietal.,2020).

Other regional climate change models also suggest an overall weakening of extratropical storm activity across the Mediterranean (Hochmanet al.,2018; Realeet al., 2021), corresponding to a decrease in precipitation. The ISR3 climate model employed in this study (Adinolfiet al., 2020) projects a decrease of the long-term average annual precipitation height to400 mm, corresponding to a decrease of30 %(see Fig. 5.3b). The precipitation height of an average wet day is expected to increase by 3 mm, indicating an increase in rainfall intensity (see Fig. 5.3c). However, infrequent heavy rainstorm events with rainfall above30 mmmainly drive the increase of precipitation intensity, while average diurnal rainfall below30 mm is expected to decrease according to the climate model. The projected maximum diurnal precipitation for 2041-2070 is 420 mm. In addition, the applied climate simulation indicates a shortening of the wet rain season (see Fig. 5.3a) with the maximum consecutive dry days (i.e., daily precipitation below1 mm) lengthening by about 70 days. A reduction of the precipitation height can be observed in all months, except for January (see Fig. 5.3a and 5.2b). While the annual potential evapotranspiration is projected to increase from 1970 mm to about 2070 mm, with the most notable change occurring from 2061 to 2070. Despite the applied climate model suggesting elevated rainfall intensity for the WMA, it should be noted that not all climate simulations indicate a statistically significant change of rainfall intensity for the Eastern Mediterranean region (e.g.,Alpert et al., 2002). However, it is a consensus that rainfall intensity will increase extensively across many areas of the Mediterranean region. Generally, we conclude that the entire recharge area exhibits an increase of months with moderate to extreme droughts and a decrease of moderately wet months until 2070 (see Fig. 5.4a,b). However, some sub-basins in the Be’er Sheva region and the West Bank may also experience an increase in extremely wet months. The 48-month SPI indicates that droughts will lengthen substantially, lasting over several years to decades (see Fig. 5.4c).

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1e9

Pumping rate (Historic) Pumping rate (RRI) Pumping rate (B)

Pumping rate (RNC)

Mean net infiltration Net infiltration (Historic) Net infiltration (RCP4.5)

Figure 5.5: Model input for the groundwater flow model of (1) net infiltration at the soil level according to the IPCC RCP4.5 climate scenario and (2) annual groundwater abstractions according to the RNC, B, and

RRI groundwater consumption scenarios.

We consider three different groundwater consumption scenarios, a Regional Nature Conservation (RNC), Baseline (B), and Regional Resources Intensive (RRI) scenario, corresponding to different abstraction rates from the Western Mountain Aquifer (see Fig.

1b). The RNC scenario assumes a political focus on preserving or recovering the local state of nature and its water resources. The abstraction of water from local freshwater resources is restricted. Discrepancies in demand and supply are compensated via the import of virtual water (e.g., import of food). The RRI scenario focuses on economic growth despite the extensive damage to local freshwater resources and nature. The abstraction of water from local freshwater resources is not restricted. Agricultural and industrial demand is prioritized, reducing virtual water imports. The B scenario is based on currently prevailing trends, plans and policies. The abstraction rates for the B scenario are defined based on the current management plan of the Israeli water authority that limits groundwater abstractions to the 5-year moving average of recharge, assuming that the joint abstraction by Israeli and the Palestinian authorities comply with this threshold. In contrast, the RNC and RRI scenarios assume respectively a steady 20 % decrease and increase until 2050 from the B scenario (see Fig. 5.5). After 2050 the deviation from scenario B remains constant. In summary, the scenarios B, RNC, and RRI correspond to a total of1:60×10¹⁰, 1:38×10¹⁰, and1:82×10¹⁰m³of pumped water from 2020 to 2070. We implemented the groundwater abstraction scenarios through the 506 presently operating wells (Abusaada, 2011). The spatial distribution of pumping rates is assumed to be proportional to the present distribution.