Watershed structural influences on the distributions of stream network water and solute travel times under baseflow conditions

Abstract

Watershed structure influences the timing, magnitude, and spatial location of water and solute entry to stream networks. In turn, stream reach transport velocities and stream network geometry (travel distances) further influence the timing of export from watersheds. Here, we examine how watershed and stream network organization can affect travel times of water from delivery to the stream network to arrival at the watershed outlet. We analysed watershed structure and network geometry and quantified the relationship between stream discharge and solute velocity across six study watersheds (11.4 to 62.8 km 2 ) located in the Sawtooth Mountains of central Idaho, USA. Based on these analyses, we developed stream network travel time functions for each watershed. We found that watershed structure, stream network geometry, and the variable magnitude of inputs across the network can have a pronounced affect on water travel distances and velocities within a stream network. Accordingly, a sample taken at the watershed outlet is composed of water and solutes sourced from across the watershed that experienced a range of travel times in the stream network. We suggest that understanding and quantifying stream network travel time distributions are valuable for deconvolving signals observed at watershed outlets into their spatial and temporal sources, and separating terrestrial and in-channel hydrological, biogeochemical, and ecological influences on in-stream observations. Figure 4. Progression (vertically) of the calculated travel time function for all six study watersheds (horizontally). The basis of the travel time function is the distribution of network distances (the width function) (a). Travel time can be calculated with a constant velocity (b) or a variable velocity (c). The travel times can be weighted by lateral inflows to the stream network (d). When all pieces are incorporated, the result is the inflow weighted variable velocity travel time function (e) presented as proportion of total discharge (Q) in the network at a given time 2677 STREAM NETWORK WATER AND SOLUTE TRAVEL TIMES a Mean and maximum inflow weighted travel time (IWTT) in hours and skewness of each watershed calculated using both and constant and variable velocity (CV and VV, respectively). 2678 A. BERGSTROM ET AL. DISCUSSIONWe present analysis of how watershed structure and stream network geometry can affect solute travel times and discuss implications for interpreting observed watershed outflow signatures. We developed an IW-VV-TTF that represents the probability distribution of conservative solute travel times, as a surrogate for the water molecules themselves, from entry into the stream network to a given watershed outlet. The IW-VV-TTF builds on the commonly used width function and combines direct measures of watershed structure, stream network geometry, and solute velocity to estimate stream network travel time distributions. Network geometry sets the distribution of travel distances for water and solutes from entry to the stream network to the water...

Saturated areas through the lens: 2. Spatio‐temporal variability of streamflow generation and its relationship with surface saturation

“…Catchment area showed to be the dominant topographic control on subcatchments' and reaches' discharge. The same relationship has been reported in several studies (e.g., Anderson & Burt, 1978;Bergstrom, McGlynn, Mallard, & Covino, 2016;Jencso et al, 2009;Payn et al, 2012). During dry and intermediate catchment wetness…”

How network structure can affect nitrogen removal by streams

“…Our synthetic findings agree with results from actual catchments where variation in network geometry and catchment shape explain travel time distributions of water (Bergstrom, McGlynn, Mallard, & Covino, 2016 (Hall et al, 2013;Mulholland et al, 2008;Wollheim, Peterson, Thomas, Hopkinson, & V€ or€ osmarty, 2008;Wollheim, V€ or€ osmarty, Peterson, Seitzinger, & Hopkinson, 2006). The stream network draining the narrow catchment had higher propensity for NO À 3 removal than in rectangular or square catchments.…”

“…This effect was likely due to longer flowpaths and more streambed area in the narrow network, thus increasing potential for NO À 3 removal. Our synthetic findings agree with results from actual catchments where variation in network geometry and catchment shape explain travel time distributions of water (Bergstrom, McGlynn, Mallard, & Covino, 2016). The narrow network had a larger fraction of its NO À 3 removal occur in larger streams than the other two networks, especially at intermediate loads.…”

Evaluating spatiotemporal variation in water chemistry of the upper Colorado River using longitudinal profiling

“…Stream water quality is traditionally measured using a Eulerian approach that tracks changes in water chemistry over time at fixed points in space. Characteristics measured at the sampling site represent the integration of delivery, transport, and biogeochemical processing across all flow paths within the upstream catchment (Bergstrom, McGlynn, Mallard, & Covino, ; McKnight & Bencala, 1990; Mulholland & Hill, ). The resulting time series have revealed temporal variation in response to both hydrologic events such as floods and droughts (Inamdar & Mitchell, ; Pellerin et al, ; Sebestyen et al, ) and biogeochemical processing at seasonal and diurnal cycles (Nimick, Gammons, & Parker, ; Cohen et al, 2013; Bernhardt et al, ).…”