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 To allow distributed mountain block recharge occur, you need water to enter the bedrock at the hillslope scale. Is distributed mountain block recharge significant?

Precipitation Soil water Bedrock Soil Hillslope scale Preliminary (generic) simulations 1.Water the soil-bedrock interface 2.Bedrock permeability Percolation  Min (water availability, Ksat_rock) Two primary controls on percolation

Field site: ponderosa pine hillslope at a semi-arid area Figures from Wilcox et al. (1997) 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).

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

Figure from Wilcox et al. (1997) Hillslope setting Moisture profiles at three seasons, 1993 A Bw Bt CB R P, PE, PT Free drainage Seepage face Root zone 50cm

ET modeling ET accounts for 95% of the annual water budget (Brandes and Wilcox, 2000) ET modeling Water potential T/PT h3h2 h1 h4 PT (wilting point) Water potential E h min PE

Calibration of ET model illustrated using measured moisture profiles for 4 of 19 sampled days PE=70%, PT=30%, h4=-50m Root density A+Bw=2.0, Bt=0.3 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=2.0, Bt=0.3 PE=70%, PT=30%, h4=-50m Root density A+Bw=0.65, Bt=0.35 PE=70%, PT=30%, h4=-15m Root density A+Bw=0.65, Bt=0.35 Water potential T/PT h3h2 h1 h4 PT (wp)

Representing root-derived macropores x β z θ x 1. Annular root macropore aperture 2. Radial root distribution 3. Equivalent root dip angle Root D b

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

Observation Simulated and observed runoff No macropore (sc)Composite continuum (cc) Macropore, β=1° (x1)Macropore, β=15° (x2) Macropore, β=30°(x3) simulated observed

Results of best-fit simulation(x2) Infiltration (cm) ET Runoff Interflow Percolation (0.7%P)

What happens if root-zone directly contacts the tuff? Infiltration (cm) ET 48.5 (x2: 48.5) 39.3 (46.0) Runoff Interflow Percolation 3.0 (3.0)0 (0) 5.0, 10.0%P (0.38, 0.7%P) simulated observed

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

loam Sandy loam 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.