Modifying the Jackson index to quantify the relationship between geology, landscape structure, and water transit time in steep wet headwaters

Abstract

The relationship between stream water mean transit time (MTT), catchment geology, and landscape structure is still poorly characterized. Here, we present a new simple index that builds on the Jackson, Bitew, and Du (2014) index that focuses specifically on permeability contrasts at the soil-bedrock interface and digital elevation modelbased physical flow path measurements to identify broad landscape trends of moisture redistribution in the subsurface of steep wet headwater catchments. We use this index to explore the relationship between geology, landscape structure, and water transit time through the lens of landscape anisotropy. We hypothesize that catchments with a greater tendency to shed water laterally will correlate with younger stream water MTT and catchments with a greater tendency to infiltrate water vertically will correlate with older stream water MTT. We tested the new index at eight geologically diverse Pacific Rim catchments in Oregon, Japan, and New Zealand. The new index explained 77% of the variability in measured stream water MTT across these varied sites. These findings suggest that critical zone anisotropy and catchment form are first-order controls on the time scales over which catchments store and release their water and that a simple index may usefully capture this relationship.anisotropy, bedrock permeability, critical zone, headwaters, hydrology, landscape structure, mean transit time, turnover time

review

“…Moreover, what is the extension of the riparian zone compared to the hillslopes in the study sites? 5) Have the authors considered their analysis of subsurface connectivity in light of recent findings by Klaus and Jackson (2018) and Gabrielli and McDonnell (2020)? Are there bedrock permeability data for the selected study sites?…”

Review

“…-Page 24, lines 461-464: I suggest to the authors to consider the recent findings by Klaus and Jackson (2018) and Gabrielli and McDonnell (2020), and, if possible, check whether they can really consider their riparian and hillslope sites connected to the stream. Are there bedrock permeability data for this agricultural catchment?…”

Response to Anonymous Referee #1

“…It is very likely that these heterogeneities allow a hillslope or at least the footslope to connect to the stream via interflow for a short period of time. Gabrielli and McDonnell (2020) built upon the DTD and developed a (gridded) Index which describes the general tendency of a catchment to either shed water laterally to the stream channel or infiltrate water to depth. They found high correlations between their Anisotropy Index (AI) and the assessed Mean Transit Time (MTT) for several catchments.…”