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

Abstract

Stream water chemistry is traditionally measured as variation over time at fixed sites, with sparse sites providing a crude understanding of spatial heterogeneity. An alternative Lagrangian reference frame measures changes with respect to both space and time as water travels through a network. Here, we collected sensor-based measurements of water chemistry at high spatial resolution along nearly 500 km of the Upper Colorado River. Our objective was to understand sources of spatiotemporal heterogeneity across different solutes and determine whether longitudinal change manifests as smooth gradients as suggested by the River Continuum Concept (RCC) or as abrupt changes as suggested by the Serial Discontinuity Concept (SDC). Our results demonstrate that Lagrangian sampling integrates spatiotemporal variation, and profiles reflect processes that vary in both space and time and over different scales.Over each day of sampling, water temperature (T) and dissolved oxygen (DO) varied strongly in response to diel solar cycles, with most of the variation driven by sampling time rather than sampling location. Equilibration of T and DO with the atmosphere limited small scale spatial heterogeneity, with variation at the entire profile scale driven by regional climate gradients. As such, T and DO profiles more closely approximated the smooth gradients of the RCC (though including temporal sampling artefacts). Conversely, variation in specific conductance and nitrate (NO 3 -N) was largely driven by spatial patterns of lateral inflows such as tributaries and groundwater. This resulted in discrete shifts in the profiles at or downstream of discontinuities, appearing as the profiles expected with the SDC. The concatenation of spatiotemporal variation that produces observed Lagrangian profiles presents interpretive challenges but also augments our understanding of where, how, and critically why water chemistry changes in time and space as it moves through river networks.

Flowpath controls on high‐spatial‐resolution water‐chemistry profiles in headwater streams

“…The latter interpretation differs from previous spatial sampling studies in mesoscale catchments where in-stream processes played a major role (Dent andGrimm 1999, McGuire et al 2014) but is consistent with recent observation in the study region that in-stream processes produce more visible effects in high-order rivers than in headwaters (Casquin et al 2020). Although most variations in water-chemistry profiles were smooth, abrupt changes also existed at confluences, a farm point-source and a localized groundwater inflow point (Konrad 2006, Hensley et al 2020).…”

“…The latter interpretation differs from previous spatial sampling studies in mesoscale catchments where in‐stream processes played a major role (Dent and Grimm 1999, McGuire et al 2014) but is consistent with recent observation in the study region that in‐stream processes produce more visible effects in high‐order rivers than in headwaters (Casquin et al 2020). Although most variations in water‐chemistry profiles were smooth, abrupt changes also existed at confluences, a farm point‐source and a localized groundwater inflow point (Konrad 2006, Hensley et al 2020). We also observed erratic concentration variability in the most upstream section of two catchments (Dz3 and LdG1), arguably because their low water level and flow velocity make these locations more sensitive to local conditions, such as fractures in the bedrock or small pools in the stream, which are not represented in the regional maps used to characterize the landscape (Floriancic et al 2019, Hensley et al 2019).…”

Using large, open datasets to understand spatial and temporal patterns in lotic ecosystems: NEON case studies

“…While this preliminary analysis suggests patterns of water chemistry, community composition, and ecosystem functioning along the NEON D08 sites are consistent with predictions of the RCC, the nature of at‐a‐point sampling makes it difficult to identify whether changes occur discretely or along a continuum (Ensign et al, 2017; Hensley et al, 2020). It is worth noting that the BLWA and TOMB sites are located 70 and 140 km downstream of dams, distances potentially sufficient for any effects to have largely dissipated (Ellis & Jones, 2013; Ward & Stanford, 1983).…”

Sensor-Equipped Unmanned Surface Vehicle for High-Resolution Mapping of Water Quality in Low- to Mid-Order Streams

“…Specifically, some solute concentrations converge with increasing watershed area, , following the hydrological concept of Representative Elementary Area . Furthermore, the range in nitrate concentrations was larger in agriculturally influenced, low-order streams in Iowa than along a 500 km reach of the large Upper Colorado River (Colorado) . Understanding how water quality varies along the stream-to-river continuum and how that variation may be modified by land use and land cover changes has been an important area of research for many years, − but studies have primarily synthesized data from site-specific data sets with relatively low spatial resolution.…”

“…Water quality varies at multiple spatial scales along a river network. − Most spatially resolved surveys are carried out via manual synoptic sampling, but because sampling is time and labor intensive, there are tradeoffs in the spatial resolution versus spatial extent of measurements that can be achieved with this approach. As such, manual synoptic sampling often results in coarse spatial resolution, missing important water quality patterns that exist at smaller spatial scales, , or results in highly spatially resolved measurements that are collected at the expense of sampling longer reaches.…”