Extreme soil dryness during drought affects the distribution and availability of water to the ecosystem, and may strongly influence the fate of precipitation that comes with interruption or termination of the drought. Consequently, unsaturated zone processes are crucial to many of the water-storage and loss-minimization strategies that humans, plants, and animals rely on.
The unsaturated zone provides little-known storage mechanisms that can hold substantial water during long intervals between rainstorms, affecting the relative advantage of shallow-rooted vs. deep-rooted plants, and the effective functioning of drought-survival strategies.
Unsaturated zone processes play crucial roles in determining groundwater availability and quality during times of drought and recovery from drought. Flowpaths within the unsaturated zone strongly depend on the water content and input. Low soil moisture is known to strongly influence flow fast preferential flow channels, though the dominant processes and effects are largely unexplored. A major effect comes from soil hydrophobicity or water repellency, which becomes much greater in drier soil. The result can be an enhancement of preferential flow processes that partitions incoming water more strongly to long-range flow paths and less to storage in soil matrix material.
Highlights:
• Preferential flow, essential to certain recharge mechanisms and ecosystem functions, is commonly a major factor when water infiltrates into dry soil
Nimmo, J.R., 2012, Preferential Flow Occurs in Unsaturated Conditions: Hydrological Processes, v. 26, no. 5, p. 786-789.
(PDF).
• Our research on soil moisture influenced by forest restoration demonstrated that hydrophobicity (which increases with soil dryness) correlates with preferential
flow.
Perkins, K. S., Nimmo, J.R., and Medeiros, A.C., 2012, Effects of native forest restoration on soil hydraulic properties, Auwahi, Maui, Hawaiian Islands: Geophysical Research Letters, 39:L05405 doi:10.1029/2012GL051120. (PDF) Perkins, K.S., Nimmo, J.R., Medeiros, A.C., Szutu, D.J., and von Allmen, E., 2014, Assessing effects of native forest restoration on soils moisture dynamics and potential aquifer recharge, Auwahi, Maui, Ecohydrology, DOI:10.1002/eco1469. (PDF)
• Our investigation of flow in a fractured-rock unsaturated zone showed how a little-known fracture-facial storage mechanism retains substantial water that is released
gradually over a period of months, thereby maintaining water levels and streamflows in periods with no net infiltration
Nimmo, J.R., and Malek-Mohammadi, S., 2015, Quantifying Water Flow and Retention in an Unsaturated Fracture-Facial Domain, in Faybishenko, B., Benson, S.M., and Gale, J.E., eds., Dynamics of Fluids and Transport in Fractured Porous Systems, Geophysical Monograph 210, AGU (Wiley), p. 169-179.
(PDF).
• Spatial heterogeneity of the soil provides a means of soil water storage for use by plants during rainless intervals.
Nimmo, J.R., Perkins, K.S., Schmidt, K.M., Miller, D.M., Stock, J.D., and Singha, K., 2009, Hydrologic Characterization of Desert Soils with Varying Degrees of Pedogenesis -- I. Field Experiments Evaluating Plant-Relevant Soil-Water Behavior: Vadose Zone Journal, v. 8, no. 2, p. 480–495.
(PDF).
Continuing work:
Develop understanding and practical models to quantify the relative importance of soil conditions (e.g. pre-storm wetness) and storm characteristics (e.g. magnitude intensity, duration) in controlling the proportion of storm water that moves deep rapidly as preferential flow.
Continue studies of soil-water relations related to plant growth and water-use efficiency in water-limited ecosystems at sites in desert and semiarid locations.
Continue development of the EMR recharge method , as well as our research on preferential flow processes, including new test cases in drought-affected locations. As resources permit, apply this model with hypothetical meteorological inputs to investigate effects on groundwater of drought-altered frequency and magnitude of infiltration.
