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

Mean annual infiltration above the high-level waste repository proposed to be sited at Yucca Mountain, Nevada, has a large impact on assessments of re pository 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 t o shallow, permeable soils above a moderately welded tuff with large soil-f illed fissures. The combination of shallow soils and fissured bedrock allow s rapid penetration of wetting pulses to below the rooting zone. Plant upta ke can strongly reduce net infiltration in arid environments with high wate r storage capacity, and, despite the low water storage capacity, there is a relatively high vegetation density in this environment. The apparent discr epancy 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, inc luding: (i) linear plant growth features observed on aerial photographs; (i i) comparisons of plant cover within the fissured environment and comparabl e environments lacking fissures; and (iii) direct excavations, all suggest that the widely spaced soil-filled fissures are conducive to plant growth e ven when fissures are buried at soil depths exceeding 30 cm. Results from a mechanistic simulation model for root growth into fissures suggest that th e additional (sheltered) plant-available soil water within fissures provide s a competitive advantage for plant establishment. Therefore, plants that g erminate above a fissure are more likely to survive, in turn developing lin ear features above fissures. Having established that plants preferentially root within soil-filled fissures in the caprock environment, a set of simul ations were performed to examine the hydrologic consequence of plant roots within fissures at the landscape-scale. The response to three rainfall amou nts was simulated. For the largest storm, fluxes at the fissure bottom peak ed 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 wit hin 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, a nd (ii) wetting-pulse retardation due to drier fissures prior to rain. The first mechanism appears to be dominant in these simulations. Results sugges t that transpiration may strongly limit net infiltration (i.e. total deep p ercolation flux escaping the plant root zone); significant infiltration can occur, however, when plants are dormant, so that most infiltration would b e expected to occur during winter. (C) 1999 Elsevier Science B.V. All right s reserved.