Hillslope hydrology under glass: confronting fundamental questions of soil-water-biota co-evolution at Biosphere 2

Recent studies have called for a new unifying hydrological theory at the hillslope and watershed scale, emphasizing the importance of coupled process understanding of the interactions between hydrology, ecology, pedology, geochemistry and geomorphology. The Biosphere 2 Hillslope Experiment is aimed at tackling this challenge and exploring how climate, soil and vegetation interact and drive the evolution of the hydrologic hillslope behavior. A set of three large-scale hillslopes (18 m by 33 m each) will be built in the climate-controlled experimental biome of the Biosphere 2 facility near Tucson, Arizona, USA. By minimizing the initial physical complexity of these hillslopes, the spontaneous formation of flow pathways, soil spatial heterogeneity, surface morphology and vegetation patterns can be observed over time. This paper documents the hydrologic design process for the Biosphere 2 Hillslope Experiment, which was based on design principles agreed upon among the Biosphere 2 science community. Main design principles were that the hillslopes should promote spatiotemporal variability of hydrological states and fluxes, facilitate transient lateral subsurface flow without inducing overland flow and be capable of supporting vegetation. Hydrologic modeling was used to identify a hillslope configuration (geometry, soil texture, soil depth) that meets the design objectives. The recommended design for the hillslopes consists of a zero-order basin shape with a 10 degree overall slope, a uniform soil depth of 1 m and a loamy sand soil texture. The sensitivity of the hydrologic response of this design to different semi-arid climate scenarios was subsequently tested. Our modeling showed that the timing of rainfall in relation to the timing of radiation input controls the spatiotemporal variability of moisture within the hillslope and the generation of lateral subsurface flow. The Biosphere 2 Hillslope Experiment will provide an excellent opportunity to test hypotheses, observe emergent patterns and advance the understanding of interactions.