3.3 Results
3.3.4 Modeling of phreatic and vadose storage dynamics
Finally, we utilize the presented numerical model to assess the dissipation of the infiltration signal and quantify the relevance of the vadose zone for storage dynamics. The precipitation signal and consequently the recharge at the soil level observe pronounced impulses. Figure 3.10 illustrates the hydraulic response to an exceptionally intensive wet winter season in 1991/92 with an average annual precipitation depth of1169 mm(compared to the long-term average precipitation depth of 570 mm) in the recharge area. The thick vadose zone substantially dissipates the distinctive input signal, such that recharge at the groundwater table occurs dispersed throughout the entire year (see Fig. 3.10a,b) with an average minimum recharge flux of7:4 m3s−1 during dry summers (under the present average climatic conditions). The simulations indicate that the signal reaches the groundwater table on average after circa 55 days via preferential pathways within the vadose zone. However, the hydraulic signal at the control plane groundwater table exhibits a long-term recession for several months to years. The dissipation of the hydraulic signal depends on the intensity of the individual precipitation input (see Fig. 3.11a) and the antecedent saturation conditions of the vadose zone.
In Figure 3.11, we measure signal intensity and dissipation of the input signal by
1992 1994 1996 1998 2000 10
20
Head (masl.)
Hydraulic conductivity
0.05×
0.10×
0.50×
5.00×
10.00× 50.00×100.00×
1992 1994 1996 1998 2000
1020 30
Hydraulic conductivity
0.91 ×
5.00 × 10.00 ×50.00 × 100.00 ×
1992 1994 1996 1998 2000
10 20
Head (masl.)
Van-Genuchten alpha
0.80×
0.90× 1.10×1.20× 5.00×10.00×
1992 1994 1996 1998 2000
10 20
30 Van-Genuchten alpha
0.05 ×
0.10 × 0.50 ×0.80 × 0.90 ×1.10 ×
1992 1994 1996 1998 2000
10 20
Head (masl.)
Van-Genuchten beta
0.95×
0.98× 1.02×1.05× 1.10×1.70×
1992 1994 1996 1998 2000
10 20
Van-Genuchten beta
0.95 ×
0.98 × 1.05 ×1.38 × 1.38 ×1.75 ×
1992 1994 1996 1998 2000
15 20
Head (masl.)
Min. relative conductivity
0.01×
0.10× 10.00× 100.00×
1992 1994 1996 1998 2000
10 20
Min. relative conductivity
0.01 × 0.05 × 0.10 ×
0.50 × 5.00 × 10.00 ×
50.00 × 100.00 ×
1992 1994 1996 1998 2000
10 20
Head (masl.)
Specific storage
0.01×
0.10× 5.00×10.00× 100.00×
1992 1994 1996 1998 2000
10 20
Volumetric fraction
0.90 ×
1.10 × 1.20 ×2.00 × 3.00 ×
1992 1994 1996 1998 2000
10 20
Head (masl.)
Residual water saturation
0.01×
0.05× 0.10×0.20× 0.50×2.00×
1992 1994 1996 1998 2000
15 20
Interface conductivity
0.01 ×
0.10 × 0.91 ×1.10 × 1.70 ×10.00 ×
1992 1994 1996 1998 2000
0 25
Head (masl.)
Saturated water content
0.10×
0.90× 1.10×1.50× 2.00×10.00×
a) Matrix continuum: b) Conduit-fracture continuum:
observed calibrated
Figure 3.8: Hydraulic response in the observation well Kiryat Gat to variations in the hydraulic parameters of a) the matrix and b) the second
continuum. The color of the line indicates the parameter multiplier.
10 2 10 1 100 101 102 Parameter multiplier (-) 1.0
0.5 0.0 0.5 1.0
NNSE (-)
a) Matrix continuum
10 2 10 1 100 101 102
Parameter multiplier (-) b) Conduit-fracture continuum
Hydraulic conductivity Specific storage Residual water saturation Van-Genuchten alpha Van-Genuchten beta Min. relative conductivity Saturated water content Volumetric fraction Interface conductivity
Figure 3.9: Model performance of the observation well Kiryat Gat com- pared to variations of a) the matrix and b) conduit-fracture hydraulic parameters, using the normalized Nash–Sutcliffe model efficiency (NNSE)
coefficient.
0 500 1000
Flux (m³ s¹)
a) Net infiltration at the level of the zero-flux plane
101010101001231
Flux (m³ s¹)
b) Infiltration at various depths below surface
Groundwater table -200 m -100 m -40 m -20 m -10 m
1991 1992 1993 1994
1.9 2.0
Phreatic storage (m³)
1e10 c) Water storage in the individual flow compartments
1.350 1.375 1.400 1.425
Vadose storage (m³)
1e10
40 41 42 43
Vadose proportion (%)
Figure 3.10: Simulation results of a) net infiltration at the soil level, b) infiltration at various depths below the surface, and c) the change of water storage in the individual flow compartments due to recharge. The term vadose storage comprises the water in the vadose zone beneath the recharge area. The term phreatic storage refers to the dynamic phreatic water, defined here as the water above its natural outlet (i.e., the Taninim
spring at 4 masl.).
the standard deviation (ff). We determine a standard deviation value per hydrological year based on a resampled daily time series of a representative column in the recharge area of the Judean Mountains (i.e., near the Bar Giora site, in the district Jerusalem). This column was selected since this region with an average annual precipitation input of610 mm largely contributes to the overall recharge (i.e., it is relevant to the overall recharge dynamics) and at the same time exhibits a fairly thick vadose zone of about 335 m. We measure the annual signal standard deviation at all 66 unsaturated nodes of the vertical column.
The signal intensity of the recharge flux at the control plane groundwater table is largely controlled by the intensity of the precipitation input (see Fig. 3.11a). However, the thick vadose zone substantially contributes to the dissipation of input signal depending on the system state (see Fig. 3.11b), leading to high time-varying dynamics within the vadose zone. These results highlight the need for process-based modeling approaches since linear transfer functions are limited to time-invariant and linear responses.
As illustrated in Figure 3.10c, the vadose zone presently comprises circa 45 %of the dynamic storage volume, while under predevelopment conditions (i.e., before the 1950s when large-scale groundwater abstraction started), the vadose zone storage comprised circa 38 %of the dynamic storage. The increase in stored vadose water results from the substantial decline in groundwater levels since the 1950s and the consequent increase of the vadose zone depth. Dynamic storage refers to the stored water above the natural outlet (Taninim at 4 masl.), i.e., the water available to spring discharge. The dynamic storage is relevant for aquifer management since this water eventually discharges at its natural outlet and represents the available storage for groundwater abstraction. Figure 3.10c illustrates the storage dynamics after the exceptionally wet rainy season, with the vadose storage rapidly increasing and a long-term recession over roughly two years, despite the absolute vadose storage decreasing very quickly to levels before the wet year. This is a secondary effect of increasing water tables (i.e., mainly fed by pathways of fast infiltration) and a consequent decrease in the volume of the vadose zone. The results demonstrate the importance of accounting for vadose flow processes for managing semi-arid karst aquifers with thick vadose zones. Shifted water budgets because of climate change and increased anthropogenic consumption may further accentuate the relevance of the vadose zone for short-term storage and the dissipation of the hydraulic signal.
