Structural Hillslope Connectivity Is Driven by Tectonics More Than Climate and Modulates Hydrologic Extremes and Benefits

Abstract

Hillslopes are dynamic landscape features that connect ridges to valleys, linking terrestrial and aquatic ecosystems through water, sediment, and nutrient fluxes (Fan et al., 2019;Stieglitz et al., 2003). The degree to which a hillslope parcel is connected to a downstream waterbody is a function of the energy inputs into the parcel and the likelihood for disconnectivity during downstream transport (Najafi et al., 2021). The strength of this connection is a product of tectonic and climatic drivers (Montgomery & Dietrich, 1992; Whittaker, 2012). Recent large-scale analyses of rivers, which act as end-points to the hillslope continuum, disagree whether tectonic or climatic processes control longitudinal river structure (Chen et al., 2019;Seybold et al., 2021). However, similar large-scale analyses of hillslope connectivity are lacking despite being crucial for understanding how landscapes adapt to climate change and other anthropogenic stressors.Structural (or "static") connectivity sets the long-term propensity for downgradient transport whereas functional (or "dynamic") connectivity varies over the short-term and includes the interplay of spatial and temporal energy fluxes (Bracken et al., 2015;Mahoney et al., 2020aMahoney et al., , 2020b. For example, short-term meteorological conditions may characterize the functional hillslope response whereas long-term climatic conditions may set structural landscape evolution trends. To this end, structural connectivity is informed by landscape topographic properties, including elevation, slope, aspect, curvature, and roughness . Further, topography serves as a "tape recorder" of the history of a landscape as tectonic and climatic drivers leave behind features through uplift and downcutting that encode their historic influence on connectivity (Whittaker, 2012).