Soil Moisture Sequestration in Pinyon – Juniper Woodland, Induced by Soil Water Repellency.

Encroachment of pinyon-juniper woodland into rangeland ecosystems is prevalent across the western US. Mechanisms associated with this successful encroachment are speculative, but likely in part, involve effective use of water resources.  We explored the ecohydrological characteristics of pinyon-juniper woodland on the Colorado Plateau in Utah, especially the role of soil water dynamics as affected by soil water repellency under the tree canopies. Using soil water content measurements along a 100m transect, an infiltration experiment, and 2-D soil moisture simulation we were able to demonstrate the impact of soil water repellency on soil moisture measured in this ecosystem.

We have discovered that a high level of natural soil-water-repellency / hydrophobicity exists under the canopies of both pinyon and juniper species. We found, following summer precipitation events that soil-water-repellency under trees concentrated soil water below the surface through finger-flow / bypass-infiltration and contrasted with piston flow, and much more uniform soil wetting in intercanopy locations. We propose that the trees ‘engineer’ their environment, creating water repellency as a way of providing an ecohydrological advantage, and reducing potential water uptake by shallow-rooted herbaceous species. As a result of the altered dominant soil moisture process from piston flow to bypass flow under the canopy patches, community and regional scale estimates of other linked processes such as evaporation and transpiration may be altered.