New Mexico Geological Society Annual Spring Meeting — Abstracts

Variably saturated, steady state numerical simulations were conducted on 100-meter long conceptual hillslopes representing conditions typical of mountains bounding New Mexico's Rio Grande lift. Water partitions between interflow in the soil layer and percoiation across the soil-bedrock interface. The latter eventually leads to subsurface recharge of adjacent valleys, Simulations suggest that water percolation into bedrock is significant for bedrock with permeability greater than 10^-16 m² . This permeability threshold is below frequently reported fractured rock permeability. Given a hillslope with bedrock permeability above this threshold, the mechanism of water partitioning the soil-bedrock interface dynamically depends on the ratio of infiltration rate over saturated bedrock hydraulic conductivity (R). When R is about 3 or larger, interflow dominates, and the amount of percolation depends on the magnitude of bedrock pemleability. When R is about 1/3rd or lower, percolation is dominant, and the amount of percolation depends on infiltration rate. When R is about 1, both bedrock topography (slope and depression and soil charaeteristics affect water partitioning by affecting the water pressure distribution at the soil-bedrock interface. When the soil becomes saturated, the effects of bedrock surface topography and soil properties become smaller or insignificant. This suggests that water availability with respect to bedrock permeability and water pressure at the soil-bedrock interface are two critical parameters needed to estimate water partitioning on hillslopes.