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4.2 Study case

4.2.2 Current climatic characteristics and predicted shifts in climate

South: it reaches an annual average of more than 1000 mm a−1 in the North near Lebanon and Syria and reduces to less than100 mm a−1 within the Negev (in the South) and at the Dead Sea (in the East, see Fig. 4.1i). The WMA is located between these two climatic extremes. For the period1970–2020, analyses of1301IMS (Israel Meteorological Service) climate stations show that precipitation rates of the WMA recharge area vary between 400–700 mm a−1 with long-term average precipitation of 580 mm a−1. Temporal variations in precipitation range between very wet (1991/92) and very dry (1998/99) years, with 1170 mm and320 mm annual precipitation, respectively (see Fig. 4.1).

The climate of Israel is semi-arid, with hot, dry summers and mild, wet winters with average daily maximum temperatures of circa 31°C and average daily minimum temperatures of circa 8°C, respectively. During the past 50 years, the recharge area of the WMA has already experienced a temperature increase of 0:9°C (see Tab. 4.1). The rainy season extends from November to April and contributes94 %of the annual precipitation within the recharge area of the WMA.

Shifts in climate over the past 50 years resulted in an increasing number of dry days (CDD) and decreasing number of wet days (CWD), while the number of days with extreme precipitation (>15 mm d−1) remain constant (see Tab. 4.1). These trends are responsible for a reduction in rainfall by 44 mm a−1. The decrease in precipitation and the number of wet days, but an unaltered number of days with high rainfall and increasing temperature, indicate the more erratic behavior of the hydrological variables. The trend was previously observed byZiv et al. (2014), who found a decrease in rainy days in spring by more than three days per decade between 1975–2010for Israel.

The entire Mediterranean region and the WMA are projected to experience a decrease in precipitation and an increase in temperature (Zittis et al., 2019), as is the case for the WMA (see Tab. 4.1). For instance, Zittiset al. (2019) predict a temperature

Table 4.1: Comparison of average climate parameters, i.e., consecutive dry days (CDD), consecutive wet days (CWD), extreme precipitation, annual precipitation, and temperature, in the WMA recharge area (data obtained

from the Israel Meteorological Service).

1970 – 1990 2000 – 2020

CDD with <1 mm(d) 298 305

CWD with>1 mm(d) 68.6 59.4

Extreme precipitation

days with>15 mm(d) 11.6 11.2 Average precipitation

(mm a−1) 593 549

Average temperature (°C) 18.7 19.6

increase of 1-5°C and a precipitation decrease of up to 40 % in 2081–2100 compared to 1986-2005. During summer months, the temperature increase can even reach up to 7°C. According to the projections of the CORDEX ensemble (Zittis et al., 2019), the Southern part of the Mediterranean is mainly affected by increasing temperatures. In Israel and the West Bank, Hochman et al. (2018b) present a dynamically downscaled high-resolution regional climate model (the ISR8 projection with a resolution of 0:0715° or 8 km) nested into the CORDEX-MENA long-term climate prediction assuming the RCP4.5 greenhouse gas emission scenario that exhibits a reduced bias for extreme precipitation by 13 %. Nevertheless, the model still exhibits a considerable bias for coastal- and mountainous rainfall (see Fig. 4.1). Furthermore, a climate model with a resolution of 0:0025° (i.e., 3 km, the ISR3 climate model) was obtained by an additional level dynamical downscaling from the coarser regional climate model ISR8. The models indicate a temperature increase during autumn and winter of up to 2:2°C for2041–2070 compared to 1981 – 2010. At the same time, the precipitation in autumn is expected to decrease by59 %(ISR3) to42 % (ISR8). Due to the higher spatial resolution, the 3 km model produces areas with lower winter precipitation West of Jerusalem. According to the3 kmmodel, the Eastern part of the recharge area will experience significantly higher maximum daily precipitation. However, the ISR8 model shows decreasing values for the entire recharge area. Here, the3 kmmodel predicts more extreme precipitation.

The precipitation decrease and temperature increase, described by climate models, will also effect natural evapotranspiration. The measured evapotranspiration rates from20 years of the NASA MODIS mission indicate high evapotranspiration rates in the wet winter and low evapotranspiration in the dry summer. The average annual actual evapotranspiration is shown in Figure 4.2. Evapotranspiration is higher at the coastline than at the exposed carbonate rocks of the mountain ranges along the Eastern Mediterranean. Within the WMA recharge area, evapotranspiration decreases from North to South but is exceptionally

high in the Jerusalem area, which exhibits denser vegetation. In the future, population growth and agricultural intensification will increase evapotranspiration in this area further.

Figure 4.2: Average annual actual evapotranspiration in the Eastern Mediterranean (left) and the WMA (right) derived from the global MODIS data set "MOD16A3GF" and averaged over 2000-2021. The black line indicates the recharge area of the WMA. The grey area indicates no data


Karst aquifers in Mediterranean to semi-arid climates may receive direct recharge from surface runoff that depends mainly on the characteristics of individual precipitation events rather than the seasonal sum of precipitation (Ries et al., 2017). The runoff coefficient characterizes the runoff–precipitation relationship and mainly depends on the geography and climate. It can vary significantly between different events, seasons, or years. Annual runoff coefficients generally decrease with increasing catchment area and decreasing topographic slopes (Yair &Raz-Yassif, 2004). They are commonly less than 3 %in semi-arid environments (Yitshaket al., 2002), with significant variations between individual years (Gunkel et al., 2015). For instance, Rieset al. (2017) demonstrate that the development of saturation excess predominantly controls runoff generation in three catchments in the Eastern Mountain Aquifer. Furthermore, a 10-day-long precipitation event in January 2013resulted in high event-based runoff coefficients ranging between2:9 and12:6 %, in contrast to a total runoff coefficient of 0:7 to2:7 %. Due to the hydraulic characteristics of karst aquifers, flash floods can develop rapidly only a few hours after the rainfall peak (Maréchalet al.,2008).

FollowingRies et al. (2017), we analyzed hydrometric records across the WMA recharge area. For example, an extreme precipitation event of up to 26 mmper hour was analyzed byRies et al. (2017) beginning of January 2013, when hydrometric stations of the WMA recharge area measured surface runoff of up to 35 m3s−1 (on 8/1/2013, see Fig.

4.5) resulting in a temporal runoff coefficient of more than20 % for the topographically

elevated area close to Jerusalem (see Tab. 4.2). In contrast, other larger catchment areas showed event runoff coefficients ranging between1 and13 %for this event. In December 2013, the first significant rainfall in the wet season was subject to lower runoff coefficients because the soil did not yet reach the saturation excess. Nevertheless, the total runoff coefficient is similar to the values for the EMA afterRieset al. (2017), with values ranging between0:4 and6 %, depending on the relief and length of the observed period.

Table 4.2: Precipitation and surface runoff measured at hydrometric stations for selected topographic sub-catchments in the WMA recharge


Time period Station ID:

14115 18103 17162 17155 17110 17123 Runoff coefficient (%) Entire active

period 6.09 7.7 2.6 1.9 0.42 1.05

Runoff coefficient (%) 2012/10-

2015/10 7.12 7.7 4.45 2.5 0.4 0.94

Precipitation depth

(mm) 2013/01/01-

2013/01/11 221 222 226 238 219 222

Event runoff coeffi-

cient (%) 2013/01 12.7 23.8 12.4 10.8 1.21 7.65

Precipitation depth

(mm) 2013/12/02-

2013/12/15 190 256 250 256 248 250

Event runoff coeffi-

cient (%) 2013/12 3.1 1.91 8.44 2.55 0.96 2.5