Watershed zonation through hillslope clustering for tractably quantifying above- and below-ground watershed heterogeneity and functions

Abstract

Abstract. In this study, we develop a watershed zonation approach for characterizing watershed organization and functions in a tractable manner by integrating multiple spatial data layers. We hypothesize that (1) a hillslope is an appropriate unit for capturing the watershed-scale heterogeneity of key bedrock-through-canopy properties and for quantifying the co-variability of these properties representing coupled ecohydrological and biogeochemical interactions, (2) remote sensing data layers and clustering methods can be used to identify watershed hillslope zones having the unique distributions of these properties relative to neighboring parcels, and (3) property suites associated with the identified zones can be used to understand zone-based functions, such as response to early snowmelt or drought and solute exports to the river. We demonstrate this concept using unsupervised clustering methods that synthesize airborne remote sensing data (lidar, hyperspectral, and electromagnetic surveys) along with satellite and streamflow data collected in the East River Watershed, Crested Butte, Colorado, USA. Results show that (1) we can define the scale of hillslopes at which the hillslope-averaged metrics can capture the majority of the overall variability in key properties (such as elevation, net potential annual radiation, and peak snow-water equivalent – SWE), (2) elevation and aspect are independent controls on plant and snow signatures, (3) near-surface bedrock electrical resistivity (top 20 m) and geological structures are significantly correlated with surface topography and plan species distribution, and (4) K-means, hierarchical clustering, and Gaussian mixture clustering methods generate similar zonation patterns across the watershed. Using independently collected data, we show that the identified zones provide information about zone-based watershed functions, including foresummer drought sensitivity and river nitrogen exports. The approach is expected to be applicable to other sites and generally useful for guiding the selection of hillslope-experiment locations and informing model parameterization.

A distributed temperature profiling system for vertically and laterally dense acquisition of soil and snow temperature

“…The DTP system has the potential to significantly improve the sampling of these properties and their variability along these gradients. Two collocated DTP systems in a standard plastic housing, one above and one below the ground surface at a mountainous headwater site in the East River watershed of the upper Colorado River basin (Hubbard et al, 2018;Tran et al, 2019;Wainwright et al, 2022), are used to illustrate the DTP data information content on the timing and amplitude of thermal and hydrological processes in the snow and soil columns (Fig. 5).…”

On the similarity of hillslope hydrologic function: a clustering approach based on groundwater changes

“…A threshold of flow accumulation was set to match the stream observations at major tributaries of the East River (Carroll et al, 2018). Because the hillslope delineation could be sensitive to the threshold of the drainage area, we tested different threshold values to find that the selected threshold value (810 000 m 2 ) represents the scale of hillslope at which the within-hillslope variability of key properties (such as elevation and aspect) is minimized and hillslope-averaged properties can account for the majority of watershed-scale variability (Wainwright et al, 2022).…”

“…These two key variables allow us to quantify water release ( P 1 ) and recharge ( P 2 ) within a hillslope; two key dynamics of the watershed hydrologic function (Sivapalan, 2006;Wagener et al, 2007, Wainwright et al, 2022. We note that these dynamics are also illustrated by the changes in measured groundwater levels as depicted in Appendix A.…”

“…Hillslopes are the scale at which hydrologic flow and transport processes can be tractably and frequently measured. It is also the scale at which flow and travel time are quantified and the instrumentation, conceptualization, and modeling of hydrologic processes occur (Fan et al, 2019, Wainwright et al, 2022. While advancements have been made in the general understanding of hillslope dynamics over the last several decades, there is yet to be a globally agreed-upon classification and/or clustering for this important scale of interest in hydrology (McDonnell and Woods, 2004).…”

“…One challenge in developing a similarity framework is the inherent heterogeneity of a given hillslope. For example, snow water equivalent (SWE), infiltration (I ), and actual evapotranspiration (ET) distributions can range over an order of magnitude within a single hillslope (Wainwright et al, 2022). Defining a single integrative measure that can capture this spatio-temporal variability is difficult.…”