Earth System Models (ESMs) are essential tools for understanding and predicting global change, but they cannot explicitly resolve hillslope-scale terrain structures that fundamentally organize water, energy, and biogeochemical stores and fluxes at subgrid scales. Here we bring together hydrologists, Critical Zone scientists, and ESM developers, to explore how hillslope structures may modulate ESM grid-level water, energy, and biogeochemical fluxes. In contrast to the one-dimensional (1-D), 2- to 3-m deep, and free-draining soil hydrology in most ESM land models, we hypothesize that 3-D, lateral ridge-to-valley flow through shallow and deep paths and insolation contrasts between sunny and shady slopes are the top two globally quantifiable organizers of water and energy (and vegetation) within an ESM grid cell. We hypothesize that these two processes are likely to impact ESM predictions where (and when) water and/or energy are limiting. We further hypothesize that, if implemented in ESM land models, these processes will increase simulated continental water storage and residence time, buffering terrestrial ecosystems against seasonal and interannual droughts. We explore efficient ways to capture these mechanisms in ESMs and identify critical knowledge gaps preventing us from scaling up hillslope to global processes. One such gap is our extremely limited knowledge of the subsurface, where water is stored (supporting vegetation) and released to stream baseflow (supporting aquatic ecosystems). We conclude with a set of organizing hypotheses and a call for global syntheses activities and model experiments to assess the impact of hillslope hydrology on global change predictions.
Plain Language Summary Hillslopes are key landscape features that organize water availability on land. Valley bottoms are wetter than hilltops, and sun-facing slopes are warmer and drier than shaded ones. This hydrologic organization leads to systematic differences in soil and vegetation between valleys and hilltops, and between sunny and shady slopes. Although these patterns are fundamental to understanding the structures and functions of water and terrestrial ecosystems, they are too fine grained to be represented in global-scale Earth System Models. Here we bring together Critical Zone scientists who study the interplay of vegetation, the porous upper layer of the continental crust from vegetation to bedrock, and moisture dynamics deep into the weathered bedrock underlying hillslopes and Earth System Model scientists who develop global models, to ask: Do hillslope-scale processes matter to predicting global change? The answers will help scientists understand where and why hillslopes matter, and to better predict how terrestrial ecosystems, including societies, may affect and be affected by our rapidly changing planet.
1.State Univ New Jersey Rutgers, Dept Earth & Planetary Sci, New Brunswick, NJ 08901 USA 2.Natl Ctr Atmospher Res, POB 3000, Boulder, CO 80307 USA 3.Univ Virginia, Dept Environm Sci, Clark Hall, Charlottesville, VA 22903 USA 4.Penn State Univ, Dept Geosci, Earth & Environm Syst Inst, University Pk, PA 16802 USA 5.Univ Utah, Dept Geol & Geophys, Salt Lake City, UT 84112 USA 6.Univ Calif Berkeley, Dept Earth & Planetary Sci, Berkeley, CA 94720 USA 7.Boise State Univ, Dept Geosci, Boise, ID 83725 USA 8.Oregon State Univ, Coll Earth Ocean & Atmospher Sci, Corvallis, OR 97331 USA 9.Swiss Fed Inst Technol, Dept Environm Syst Sci, Zurich, Switzerland 10.SUNY Buffalo, Dept Geog, Buffalo, NY USA 11.Univ Saskatchewan, Sch Environm & Sustainabil, Saskatoon, SK, Canada 12.US Geol Survey, Princeton, NJ USA 13.Univ Kansas, Dept Geog & Atmospher Sci, Lawrence, KS 66045 USA 14.Univ Calif Santa Barbara, Bren Sch Environm Sci & Management, Santa Barbara, CA 93106 USA 15.Univ Calif Riverside, Dept Environm Sci, Riverside, CA 92521 USA 16.Duke Univ, Dept Civil & Environm Engn, Durham, NC 27706 USA 17.Univ Freiburg, Dept Earth & Environm Sci, Freiburg, Germany 18.Univ Arizona, Dept Hydrol & Atmospher Sci, Tucson, AZ USA 19.Univ Arizona, Dept Geosci, Tucson, AZ 85721 USA 20.Univ Ghent, Lab Hydrol & Water Management, Ghent, Belgium 21.Univ Bristol, Dept Civil Engn, Bristol, Avon, England 22.Northeastern Univ, Dept Civil & Environm Engn, Boston, MA 02115 USA 23.Purdue Univ, Dept Earth Atmospher & Planetary Sci, W Lafayette, IN 47907 USA 24.Pacific Northwest Natl Lab, Atmospher Sci & Global Change Div, Richland, WA 99352 USA 25.Univ Colorado, Dept Civil Environm & Architectural Engn, Boulder, CO 80309 USA 26.Texas A&M Univ, Dept Biol & Agr Engn, College Stn, TX USA 27.Univ Washington, Dept Civil & Environm Engn, Seattle, WA 98195 USA 28.Univ Calif Merced, Sch Engn, Fresno, CA USA 29.US Forest Serv, USDA, PSW Res Stn, Fresno, CA USA 30.Penn State Univ, Dept Civil & Environm Engn, University Pk, PA 16802 USA 31.Deltares, Dept Catchment & Urban Hydrol, Delft, Netherlands 32.Univ Nevada, Dept Geol Sci & Engn, Reno, NV 89557 USA 33.Univ Tokyo, Inst Ind Sci, Tokyo, Japan
Recommended Citation:
Fan, Y.,Clark, M.,Lawrence, D. M.,et al. Hillslope Hydrology in Global Change Research and Earth System Modeling[J]. WATER RESOURCES RESEARCH,2019-01-01,55(2):1737-1772