5.3 Results
5.3.2 Potential changes in the water budget
The simulations indicate a reduction of the long-term average recharge from 313×106 to 300×106m3 (i.e., circa−5 %). However, two extremely unusual wet rain seasons in the years 1991/92 and 2061/62, with precipitation heights above 1000 mm and an annual net infiltration above800×106m3, largely influence the averaged recharge rates within the two reference periods (see Fig. 5.3 and 5.7). Due to the limited observed records, frequency changes of events with return times greater than the climatological normal are difficult to predict. Also, they may exaggeratedly affect the averaged values of a climatological normal. The extremely wet hydrological year 2061/62 only takes place once in the entire simulation time of the climate model. Therefore, when excluding the two extreme wet years in the historical and future period, a decline of the long-term average recharge from280×106 to250×106m3 (i.e., circa−10 %) can be observed. In addition, it should be noted that an intermediate reference period from 2021 to 2060 also exhibits a reduction on average by 10 %.
In summary, the simulations suggest an overall decrease of annual recharge by about 5to 10 %, depending on the actual return time of extreme years with precipitation heights above1000 mm (see Fig. 5.3b). Changes to mean annual net infiltration are mostly driven by reducing precipitation and less by temperature changes, which can be explained by the more pronounced changes to precipitation regime and dual infiltration dynamics of karst. The months at the beginning and end of the rain season exhibit the largest reduction in precipitation and subsequently in net infiltration (see Fig. 5.3a and 5.7a)
However, it should be noted that not all sub-basins exhibit decreasing recharge heights. Some sub-basins in the Southern part of the recharge area are subject to slight increases in recharge quantities (see Fig. 5.8). It is in agreement withLetz et al. (2021), who concluded an increase in groundwater recharge for the Southern parts of the recharge area. The increased recharge rates are likely an effect of the higher frequency of severely wet months as indicated by the 48-month SPI (see Fig. 5.4b).
However, the frequency of relatively dry years with net infiltration below 200× 106m3 is projected to increase from 10 to 15 events, and the frequency of drought years with net infiltration below100×106m3 might increase from one to three events per 30-year reference period (see Fig. 5.7b). More frequent and longer dry periods can also be observed across the entire recharge area when considering meteorologic droughts (see Fig. 5.4a). In general, we conclude that average annual net infiltration is expected to decrease, though not to the same degree as precipitation rates, due to the high infiltration capacities of exposed karst. The recharge flux at the control plane groundwater table is obviously exhibiting equal changes to long-term average values but exhibits less inter-annual variation since the
1 3 5 7 9 11 Month
0 20 40 60 80
Net infiltration (mm)
(a)
1981-2010 2041-2070
0 500 1000 1500
Net infiltration (106 m³) 0
5 10
Number of events (-)
(b)
1981-2010 2041-2070
0 200 400 600
Recharge (106 m³) 0
5 10
Number of events (-)
(c)
1981-2010 2041-2070
Figure 5.7: Change of (a) mean monthly net infiltration at the soil level (the colored area indicates the 75 % confidence interval), (b) the distribution of annual net infiltration per hydraulic year (i.e., from Sep.
15th to Sep. 14th) and (c) the distribution of recharge per hydraulic year at the control plane groundwater table.
35.00 35.25
31.25 31.50 31.75 32.00 32.25 32.50
150 100 50 0 50 100 150
Change of annual r echar ge height (mm)
Figure 5.8: The average annual recharge height change until 2041-2070 compared to the reference period 1981-2010.
Table 5.1: Meteorological and hydrogeological fluxes within the recharge area under present and future climate. We define a wet day as a day with a precipitation height above1 mm, and a dry day, vice versa. The Values
are averaged over the entire recharge area.
Present (1981 - 2010)
Near future (2021 - 2060)
Future (2041 - 2070) Annual precipitation (mm):
Mean 583 404 400
25th percentile 487 317 296
50th percentile 539 371 338
75th percentile 651 487 486
Standard deviation 162 148 196
Rainfall on a wet day (mm):
Mean 8:6 11:4 11:7
25th percentile 2:2 2:8 2:8
50th percentile 4:7 6:6 6:2
75th percentile 11:2 15:3 14:5
Standard deviation 9:8 12:9 19:9
Consecutive dry days (d):
Maximum 201 294 295
Annual potential evapotranspiration (mm):
Mean 1971 2035 2075
Annual net infiltration (mm):
Mean 173 156 165
25th percentile 97 94 82
50th percentile 126 117 111
75th percentile 223 213 204
Standard deviation 128 99 161
Annual recharge (mm):
Mean 189 156 172
25th percentile 163 129 136
50th percentile 185 153 161
75th percentile 202 180 197
Standard deviation 39 36 53
thick vadose zone provides storage such that annual recharge rates below 200×106m3 only occurs once in the future period 2041-2070 (see Tbl. 5.1 and Fig. 5.7c).
2030 2040 2050 2060 2070 10
0 10
Head change (m)
(b) Northern Part
RNC B RRI
2030 2040 2050 2060 2070
0.0 0.5 1.0 1.5
Discharge (m³ s¹) (a) Taninim spring discharge
RNC B RRI
2030 2040 2050 2060 2070 (c) Central Part
RNC B RRI
2030 2040 2050 2060 2070 (d) Southern Part
RNC B RRI
Figure 5.9: Impact of the IPCC RCP4.5 climate change scenario and the RNC, B, and RRI groundwater consumption scenarios on (a) spring discharge at the Taninim spring and the hydraulic head in the (b) Northern, (c) Central, and (d) Southern Coastal Plain. The RRI simulation was
discontinued in 2042 due to dry pumping wells.
Lastly, the numerical model for distributed infiltration and groundwater flow was applied to measure the collective impact of groundwater abstraction and climate change on the water resources of the WMA (see Fig. 5.9). The simulations indicate that scenarios B and RRI bear the risk of groundwater depletion. For instance, the scenario RRI exhibits a rapid groundwater level decline by circa 7 meters, partly resulting in dry pumping wells.
Consequently, the RRI scenario can not be computed beyond the year 2042, as the intended pumping rates of the RRI scenario can not be maintained throughout the entire period.
Spring discharge at the Taninim spring promptly ceases according to the RRI scenario.
The B scenario also observes substantial declines in groundwater levels and drying up of the Taninim spring during more extended drought periods, where groundwater abstraction exceeds replenishment. Also, some of the pumping wells partly fall dry. However, the groundwater storage under the B scenario can recover in subsequent wet periods. Under the RNC scenario, groundwater levels may increase up to 9 meters. However, it should be mentioned that the results are not to be misunderstood as exact predictions in time (on a daily, monthly, and yearly time scale) since the climate model only provides a statistically-representative prediction of the weather.
Consequently, the climate and groundwater models can only be evaluated over a climate-representative 30-year period, i.e., a climatological normal. However, the forecast of groundwater resources may indicate the risk of specific management options and highlight
the necessity to adapt to climate change. In summary, the depleted groundwater resources under the pumping scenario B indicate that simply limiting groundwater abstraction to the 5-year moving average of groundwater recharge may not be sufficient to avoid over depletion of the available resources since the length and severity of meteorological droughts will increase.
