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

Abstract

Abstract. 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 semiarid climate scenarios was subsequently tested. Our modeling showed that the timing of rainfall in relation to the timCorrespondence to: L. Hopp (luisa.hopp@oregonstate.edu) ing 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.

Incipient subsurface heterogeneity and its effect on overland flow generation – insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory

“…Experimental facilities that can tackle the evolution of structure and function in physical and biological systems, along with their emergent processes at scale, will be extremely useful for understanding future Earth system states and the significant deviation from stationarity seen in our current climate system. Unlike other artificial laboratories such as the Hydrohill in China (Kendall et al, 2001) and the Chicken Creek in Germany (Gerwin et al, 2009;Hofer et al, 2011), LEO was built with homogeneous soil and with a focus on evolving heterogeneity from a "time-zero" homogeneous condition through co-evolution of the soil-water-biota system over a timescale of years (Hopp et al, 2009;Dontsova et al, 2009). Development of catchment morphology and soil catena driven by hydrological processes through soil erosion and deposition may be one of the major causes that induce heterogeneity and that in turn exert strong feedbacks on hydrological processes (e.g., Beven et al, 1988;Sivapalan, 2005;McDonnell et al, 2007;Troch et al, 2009).…”

“…The infrastructure is designed to facilitate investigation of emergent structural heterogeneity that results from coupled Earth surface processes (Hopp et al, 2009). Feedbacks and interactions between different Earth surface processes are studied through iterations of experimental measurement and development of coupled, physically based numerical models (Huxman et al, 2009).…”

“…The Biosphere 2 LEO facility has been constructed after a period of community-based scientific planning (Hopp et al, 2009;Dontsova et al, 2009;Ivanov et al, 2010). The first hillslope of LEO (LEO-1) was commissioned at the end of 2012, while the second and third hillslopes were completed in the fall of 2013.…”

What makes Darwinian hydrology "Darwinian"? Asking a different kind of question about landscapes

“…Such models could help evaluate the "reachability" requirement of the Darwinian hypothesis and quantify the relative duration of different configurations of landscapes. There are great challenges to be overcome in finding the appropriate ways to represent the feedbacks between hydrology and other longer-term landscape processes and in the parameterization of these relationships from observations, but progress is being made (Hopp et al, 2009;Tucker and Hancock, 2010). For example, Pelletier et al (2013) recently circumvented these feedbacks by connecting effective process parameters to a higher-order variable (effective energy and mass transfer -EEMT) that captures water and energy constraints on landscape-forming processes and used this to explain variations in topography, soil thickness, land forms, and biomass across a climate gradient in southern Arizona.…”

The Hydrological Open Air Laboratory (HOAL) in Petzenkirchen: a hypothesis-driven observatory

“…Ideally, one would like to have at least three replicas of the same experiments. There are a few examples where this is already put into practice, such as the Biosphere 2 hillslope experiments performed in a green house (Hopp et al, 2009) but, usually, even dedicated largescale experiments such as the Chicken Creek artificial catchment (Holländer et al, 2009(Holländer et al, , 2014 do not involve multiple replicas.…”