Diurnal Patterns in Solute Concentrations Measured with In Situ UV-Vis Sensors: Natural Fluctuations or Artefacts?

Abstract

In situ spectrophotometers measuring in the UV-visible spectrum are increasingly used to collect high-resolution data on stream water quality. This provides the opportunity to investigate short-term solute dynamics, including diurnal cycling. This study reports unusual changes in diurnal patterns observed when such sensors were deployed in four tropical headwater streams in Kenya. The analysis of a 5-year dataset revealed sensor-specific diurnal patterns in nitrate and dissolved organic carbon concentrations and different patterns measured by different sensors when installed at the same site. To verify these patterns, a second mobile sensor was installed at three sites for more than 3 weeks. Agreement between the measurements performed by these sensors was higher for dissolved organic carbon (r > 0.98) than for nitrate (r = 0.43–0.81) at all sites. Higher concentrations and larger amplitudes generally led to higher agreement between patterns measured by the two sensors. However, changing the position or level of shading of the mobile sensor resulted in inconsistent changes in the patterns. The results of this study show that diurnal patterns measured with UV-Vis spectrophotometers should be interpreted with caution. Further work is required to understand how these measurements are influenced by environmental conditions and sensor-specific properties.

A comparison of water quality sensor deployment designs in wadeable streams

“…The past decade has seen widespread deployment of in situ sensors for high‐frequency measurement of stream and river water quality (Kirchner et al 2004; Rode et al 2016). The location and manner sensors are deployed can potentially impact measurements (Siders et al 2017; Jacobs et al 2020). Within the literature, sensor make/model, principles of operation, and calibration procedures are often described in great detail, while descriptions of how they were actually deployed are often limited to “in a well‐mixed point” (Mulholland et al 2005), “in the center of the channel” (Pellerin et al 2009), or “in the advective zone” (Cohen et al 2013) with no mention of the additional supporting infrastructure required.…”

Assessment of a portable UV–Vis spectrophotometer's performance in remote areas: Stream water DOC, Fe content and spectral data

“…The development of continuously operating water quality sensors has led to a transition from studying long-term trends and seasonal patterns to the investigation of highly dynamic phenomena, such as storm events and diurnal patterns, using high-frequency in situ measurements [3] . With the currently available technology and decreasing costs, in situ sensors are more frequently used for monitoring, especially in remote areas [4] , [5] , [6] .…”

Assessment of a portable UV–Vis spectrophotometer's performance for stream water DOC and Fe content monitoring in remote areas

“…The development of continuously operating water quality sensors has led to a transition from studying long-term trends and seasonal patterns to the investigation of highly dynamic phenomena, such as storm events and diurnal patterns, using high-frequency in situ measurements (Jacobs et al 2020). With the currently available technology and decreasing costs, in situ sensors are more frequently used for monitoring, especially in remote areas (Langergraber et al 2003;Avagyan et al 2014;Rode et al 2016).…”