The Influence of the Soil Water Balance Within Catchment Hillslopes on Runoff Variability and Fluvial Incision

Abstract:

The variability of daily runoff has direct consequences on water availability, flood hazard, ecosystem function, and erosional processes. One component to predicting runoff variability is solid understanding of soil moisture dynamics and the runoff response to rainfall events. By using a point-scale soil water balance model that accounts for vegetation and soil properties, we test how daily rainfall statistics and vegetation response to water availability affect mean hydrologic partitioning and the runoff response during rare storms. Simulations span a transect in mean annual precipitation (MAP) from 200 to 1200 mm/year and are motivated by analysis of a long-term observational network of rainfall in the contiguous U.S. Whether driven by higher storm depths or increased frequency of storm arrivals, increases in MAP always lead to more fractional runoff generation due to the synchronous increase in mean antecedent soil moisture and probability of large rainfall events. Simulations also show that at any given MAP, there are important tradeoffs between increasing the probability of large rainfall events and associated decreases in the mean soil moisture state.

Results from this ecohydrologic analysis are then used to drive 1-D simulations of the longitudinal river profile that account for runoff variability and thresholds to incision. We show that in natural settings where long-term hydrologic observations are available and where millennial-scale erosion rates were measured, much of the observed variation between local relief and long-term erosion rates can be explained by combining these two relatively simple models. However, we also show that large differences in MAP often lead to comparatively small differences in the geomorphically effective climate. This partially explains why prior studies have struggled to isolate a climatic control on long-term erosion rates. Expectations for how vegetation influences landscape evolution are often based on the seminal work by Langbein and Schumm (1958) which argues that vegetation introduces negative feedbacks between climate and erosion by stabilizing and protecting the hillslopes. Alternatively, we hypothesize that the role of vegetation in altering hydrologic partitioning may be its most diagnostic imprint on fluvial landscape form and function.