Abstract
Despite the importance of temporary streams for the provision of key ecosystem services, their experimental monitoring remains challenging because of the practical difficulties in performing accurate high-frequency surveys of the flowing portion of river networks. In this study, about 30 electrical resistance (ER) sensors were deployed in a high relief 2.6 km2 catchment of the Italian Alps to monitor the spatio-temporal dynamics of the active river network during the fall of 2019. The set-up of the ER sensors was personalized to make them more flexible for the deployment in the field and more accurate under low flow conditions. Available ER data were analyzed, compared to field based estimates of the nodes' persistency and then used to generate a sequence of maps representing the active reaches of the stream network with a sub-daily temporal resolution. This allowed a proper estimate of the joint variations of active river network length (L) and catchment discharge (Q) during the entire study period. Our analysis revealed a high cross-correlation between the statistics of individual ER signals and the flow persistencies of the cross sections where the sensors were placed. The observed spatial and temporal dynamics of the actively flowing channels also revealed the diversity of the hydrological behaviour of distinct zones of the study catchment, which was attributed to differences in the catchment geology and stream-bed composition. The more pronounced responsiveness of the total active length to small precipitation events as compared to the catchment discharge led to important hysteresis in the L vs. Q relationship, thereby impairing the performances of a power-law model frequently used in the literature to relate these two quantities. Consequently, in our study site the adoption of a unique power-law L-Q relationship to infer flowing length variability from observed discharges would underestimate the actual variations of L by 40%. Our work emphasizes the potential of ER sensors for analysing spatio-temporal dynamics of active channels in temporary streams, discussing the major limitations of this type of technology emerging from the specific application presented herein.
Highlights
20 Headwater streams - as well as rivers located in semiarid regions - are often characterized by the presence of reaches where water doesn’t flow permanently throughout the year
The major issues that were faced during the deployment 340 changed based on the location where the HOBOs were placed: in the portion of the network dominated by debris (Zone 2) electrical resistance (ER) sensors were prone to be covered by sediments or flushed away by the flow field; in other regions of the network where the substrate was made by a thin organic soil covered by grass (Zone 1), the main problems were represented by grazing of animals and the formation of local pools with standing water during the drying of the network that could not be distinguished from flowing stream in the ER time series
We have utilized a personalized version of the HOBO sensors previously proposed in the literature, which was modified to be suited for a deployment under different substrates and is deemed to be more accurate under unstable hydrodynamic conditions and during low flow conditions
Summary
20 Headwater streams - as well as rivers located in semiarid regions - are often characterized by the presence of reaches (or river segments) where water doesn’t flow permanently throughout the year. While the terminology might vary among different authors, these non-permanent rivers are typically referred to as temporary streams. Temporary streams are frequently classified into a number of different categories (e.g. intermittent, ephemeral, episodic, seasonal) depending on the underlying temporal patterns of flow persistency (Williamson et al, 2015; Skoulikidis et al, 2017; Costigan et al, 2016). Discussion started: 29 March 2021 c Author(s) 2021.
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