Slope and aspect controls on soil climate: Field documentation and implications for large-scale simulation of critical zone processes

Authors: Seyfried, Mark; Flerchinger, Gerald; BRYDEN, SAGE - University Of Idaho; LINK, TIMOTHY - University Of Idaho; Marks, Daniel; MCNAMARA, JAMES - Boise State University

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2021
Publication Date: 9/28/2021
Citation: Seyfried, M.S., Flerchinger, G.N., Bryden, S., Link, T., Marks, D.G., McNamara, J. 2021. Slope and aspect controls on soil climate: Field documentation and implications for large-scale simulation of critical zone processes. Vadose Zone Journal. 20(6). Article e20158. https://doi.org/10.1002/vzj2.20158.
DOI: https://doi.org/10.1002/vzj2.20158

Interpretive Summary: Soil climate is largely described by the amount of water in the soil and the soil temperature. These two factors largely determine the rates of soil processes such as carbon sequestration, movement of chemicals through soil to groundwater or streams, breakdown of pollutants and uptake of water and nutrients by crops and native vegetation. It is therefore important to know and/or predict soil climate on the landscape. Soil climate distribution is strongly effected by the (atmospheric) climate, which can be predicted reasonably well across the landscape, but also by local topography, especially the slope and aspect. We measured soil climate on two different aspects, North- and South-facing, to determine: the magnitude and timing of slope/aspect effects on soil climate and test and evaluate a computer model accuracy. We found that slope/aspect has a very large impact on soil climate, roughly equivalent to 310 miles north-south, or 3000 ft elevation difference. We also found that, while differences in exposure to solar radiation ultimately drive slope/aspect effects, a number of local features modify soil climate and largely determine the seasonal timing of slope/aspect effects. These include snow cover, soil depth and vegetation pheonolgy. Our computer model captured most of those effects. It is clear that current large-scale models do not have sufficient resolution to capture slope-aspect effects and that this will result is substantial reduction in the accuracy of those models. The magnitude of slope\aspect effects can be effectively explored with current, mechanistic models of soil processes.

Technical Abstract: Soil climate, as quantified by soil temperature (TS) and water content ('), exerts important controls on critical zone processes. It may be strongly impacted by local slope and aspect (SA), but remains poorly quantified at the local scale and unresolved resolved in large-scale models. It may be possible to estimate SA effects on soil climate across multiple scales from the topographically modified incoming clear sky solar radiation (SR, CS, T). We established six paired automated soil climate monitoring stations on opposing north-facing (NF) and south-facing (SF) slopes (4 years) and collected spatial TS and ' data in the hectare surrounding four stations (2 years) to measure SA effects on soil climate. Results were compared to physically-based simulations and evaluated in the context of SR, CS, T. Spatial ' data were more variable than TS, and both were consistent with monitoring stations. On average, the SF TS was much greater (4.7 °C) and the annual summer drought longer (36 days), than on the adjacent NF aspect. Seasonal variation of Ts and ' were different from each other and different from SR, CS, T. Although SR, CS, T is the fundamental driver of SA effects, local conditions including snow cover, precipitation patterns, and soil properties, largely control seasonal variations of TS and '. We have shown that field monitoring and application of existing models can provide valuable information for determining the impacts of SA at those locations.