Modeling Investigation of Water
Partitioning at a Semi-arid Hillslope
Huade Guan, John L. Wilson
Dept. of Earth and Environmental Science, NMT
Brent D. Newman
Earth and Environmental Sciences Division , LANL
Jirka Simunek
Department of Environmental Sciences, UCR
AGU Fall, 2003
Acknowledgements
• The analysis in this presentation was supported by
SAHRA–
the NSF Science and Technology Center
for Sustainability of semi-Arid Hydrology and Riparian Areas
• Site data was collected as part of the Los Alamos
Environmental Restoration Project
• Modifications to the numerical code were funded by NSF
grant, SAHRA, and Swedish Research Council
Motivation: Mountain Front Recharge
Is distributed mountain block recharge significant?
To allow distributed mountain block
recharge occur, you need water to
enter the bedrock at the hillslope
scale.
Hillslope
scale
Preliminary
(generic)
simulations
Precipitation
Two primary controls on percolation
1. Water availability @ the soil-bedrock interface
2. Bedrock permeability
Percolation  Min (water availability, Ksat_rock)
Soil
Bedrock
Field site:
ponderosa pine hillslope at a semi-arid area
Highly permeable volcanic bedrock.
Apparently little percolation reaches the bedrock (Wilcox et al., 1997).
Water availability controls percolation, not bedrock permeability. Why?
Macropore flow appears to occur in the low permeability soil horizon
(Newman et al., 2003).
1.0E-10
1.0E-08
1.0E-06
0
depth (cm)
20
40
60
80
100
120
Saturated K (m/sec)
Figures from Wilcox et al. (1997)
Objectives of this study
Use numerical modeling to synthesize the observations
and previous generic simulations
• Is the percolation into the bedrock really negligible?
It wasn't directly observed, just inferred.
• If it is negligible, why?
What impedes downward movement of water into the
highly permeable tuff?
• For what situations will percolation to bedrock
become significant for this climate?
…and with this permeable volcanic bedrock?
What we know and don’t know
We know
– Soil horizons and hydraulic parameters
– Root density profile
– Precipitation and other meteoric parameters
– Soil moisture
– Surface runoff and interflow
– Root-derived macropore flow
We don’t know
– ET
– Percolation
Modeling challenges
• Modeling ET
– System-dependent ET model
– Appropriate root-water-uptake model
• Modeling macropores
– Root-derived macropores
– Sub-parallel to the slope
• Numerical issues
– Highly non-linear, coupled processes
– Dual permeability
We used a modified version of HYDRUS-2D
Hillslope setting
Figure from Wilcox et al. (1997)
P, PE, PT
Root zone
A
Bw
Bt
CB
Seepage face
R
50cm
Free drainage
Moisture profiles at three seasons, 1993
ET modeling
• ET accounts for 95% of the annual water
budget (Brandes and Wilcox, 2000)
• ET modeling
PT
PE
E
T/PT
h2
h3
(wilting point)
hmin
Water potential
h1
Water potential
h4
Calibration of ET model
illustrated using measured moisture profiles for 4 of 19 sampled days
PE=50%, PT=50%, h4=-50m
Root density A+Bw=0.59, Bt=0.4
PE=70%, PT=30%, h4=-15m
Root density A+Bw=0.65, Bt=0.35
PT
T/PT
h2
PE=70%, PT=30%, h4=-50m
Root density A+Bw=0.65, Bt=0.35
h3
h1
Water potential
(wp)
h4
PE=70%, PT=30%, h4=-50m
Root density A+Bw=2.0, Bt=0.3
PE=70%, PT=30%, h4=-15m
Root density A+Bw=2.0, Bt=0.3
Representing root-derived macropores
1. Annular root
macropore aperture
3. Equivalent root dip angle
Root
D
b
x
β
2. Radial root distribution
z
Kxroot
g cos 

64 
4
j
j
Kzroot
g sin 

64 
j
j
2
j
x
θ
[C ( D  b ) n ]
1
[ 4 n ( D  b ) ]
j
j
4
[
C
(
D

b
)
 j j j nj ]
1
2
[

n
(
D

b
)
4 j j j ]
Conceptual models for macropore flow
• Control: Model without macropores
– Single continuum (sc)
• Models with macropores
– Single continuum
with anisotropic K
with three root dips (x1:1°, x2:15°, x3:30°)
– Composite continuum (cc)
– Dual permeability model (dp)
Simulated1994 water balance
cumulative annual flux (cm)
60
50
ET
Infiltration
runoff
interflow
percolation
40
30
20
10
0
sc
x1
x2
x3
conceptual models
cc
Simulated and observed runoff
No macropore (sc)
Composite continuum (cc)
94_wy_runoff (cm)
10
simulated
8
6
Observation
4
2
0
observed
sc
x1
x2
x3
conceptual models
Macropore, β=1° (x1)
Macropore, β=15° (x2)
Macropore, β=30°(x3)
cc
Results of best-fit simulation(x2)
Infiltration (cm)
48.5
ET
46.0
Runoff
3.0
Interflow
0
Percolation
0.38 (0.7%P)
What happens if root-zone directly
contacts the tuff?
simulated
observed
Infiltration (cm)
48.5 (x2: 48.5)
ET
39.3 (46.0)
Runoff
3.0 (3.0)
Interflow
0 (0)
Percolation
5.0, 10.0%P (0.38, 0.7%P)
Conclusions
• The simulated percolation across the soil-bedrock
interface at this site is less than 1% of annual precipitation,
in good agreement with previously inferred.
• The simulation results are consistence with Wilcox et al’s
(1997) alternative hypothesis that the CB horizon, without
roots, behaves as a barrier to downward movement of
water into the bedrock.
• The results also indicates that sub-horizontal root-derived
macropore flow increases the infiltration capacity and
decreases surface runoff at this site.
• In this climate, at a location with a shallower soil layer
where the root zone contacts the highly permeable tuff,
percolation can be as large as 10% of the annual
precipitation.
The End
Implication about the ET model
Feddes model overestimate ET loss based on the observed wilting point
(h4). S-shape model is better if the numerical instability can be avoided.
11
ET/PET
0.8
0.8
0.6
0.6
loam
0.4
0.4
Sandy loam
0.2
0.2
00
0.00001
0.00001
0.001
0.1
10
water tension(m)
1000
1000
100000
100000