The effect of vegetation on infiltration in shallow soils underlain by fissured bedrock

Abstract

Mean annual infiltration above the high-level waste repository proposed to be sited at Yucca Mountain, Nevada, has a large impact on assessments of repository performance. Ongoing investigations of infiltration processes have identified the relatively horizontal caprock environment above portions of the repository as a potentially large source of infiltrating waters, due to shallow, permeable soils above a moderately welded tuff with large soil-filled fissures. The combination of shallow soils and fissured bedrock allows rapid penetration of wetting pulses to below the rooting zone. Plant uptake can strongly reduce net infiltration in arid environments with high water storage capacity, and, despite the low water storage capacity, there is a relatively high vegetation density in this environment. The apparent discrepancy between high vegetation density and low water storage motivates the study of plant-hydrologic interactions in this semiarid environment. Field observations were coupled with plant- and landscape-scale models to provide insight into plant-hydrologic interactions. Several lines of evidence, including: (i) linear plant growth features observed on aerial photographs; (ii) comparisons of plant cover within the fissured environment and comparable environments lacking fissures; and (iii) direct excavations, all suggest that the widely spaced soil-filled fissures are conducive to plant growth even when fissures are buried at soil depths exceeding 30 cm. Results from a mechanistic simulation model for root growth into fissures suggest that the additional (sheltered) plant-available soil water within fissures provides a competitive advantage for plant establishment. Therefore, plants that germinate above a fissure are more likely to survive, in turn developing linear features above fissures. Having established that plants preferentially root within soil-filled fissures in the caprock environment, a set of simulations were performed to examine the hydrologic consequence of plant roots within fissures at the landscape-scale. The response to three rainfall amounts was simulated. For the largest storm, fluxes at the fissure bottom peaked at 1–4 weeks after the storm when plant uptake was not active, but were eliminated when fissures had active vegetation. When plants were active within a fissure, uptake eliminated net infiltration in the fissure regardless of the size of the storm. Two plant-related mechanisms reduced total flux through the plant-filled fissures: (i) transpiration during fissure flow, and (ii) wetting-pulse retardation due to drier fissures prior to rain. The first mechanism appears to be dominant in these simulations. Results suggest that transpiration may strongly limit net infiltration (i.e. total deep percolation flux escaping the plant root zone); significant infiltration can occur, however, when plants are dormant, so that most infiltration would be expected to occur during winter.