Abstract
Rainfall variability and extreme events can amplify the seasonality and storm pulses of stream water chemistry in mountainous watersheds under monsoon climates. To establish a monitoring program optimized for identifying potential risks to stream water quality arising from rainfall variability and extremes, we examined water chemistry data collected on different timescales. At a small forested watershed, bi-weekly sampling lasted over two years, in comparison to three other biweekly sampling sites. In addition, high-frequency continuous measurements of pH, electrical conductivity, and turbidity were conducted in tandem with automatic water sampling at 2 h intervals during eight rainfall events. Biweekly monitoring showed that during the summer monsoon period, electrical conductivity (EC), dissolved oxygen (DO), and dissolved ion concentrations generally decreased, but total suspended solids (TSS) slightly increased. A noticeable variation from the usual seasonal pattern was that DO levels substantially decreased during an extended drought. Bi-hourly storm event samplings exhibited large changes in the concentrations of TSS and particulate and dissolved organic carbon (POC; DOC) during intense rainfall events. However, extreme fluctuations in sediment export during discharge peaks could be detected only by turbidity measurements at 5 min intervals. Concomitant measurements during rainfall events established empirical relationships between turbidity and TSS or POC. These results suggest that routine monitoring based on weekly to monthly sampling is valid only in addressing general seasonal patterns or long-lasting phenomena such as drought effects. We propose an “adaptive” monitoring scheme that combines routine monitoring for general seasonal patterns and high-frequency instrumental measurements of water quality components exhibiting rapid responses pulsing during intense rainfall events.
Highlights
Headwater streams are a habitat for aquatic organisms, provide drinking water for downstream population centers, and contribute sources of organic matter, nutrients, and sediments to higher-order streams and rivers [1,2]
More frequent intense monsoon rainfall events, which might occur as a consequence of climate change [8], can have significant impacts on material transport and surface water quality in mountainous watersheds [9]
The comparisons of stream water chemistry data collected on different timescales suggest that conventional routine monitoring at weekly to monthly intervals can adequately describe seasonality and long-lasting or slowly changing patterns of the stream water chemistry, but its low-resolution data are limited in providing accurate estimates of chemical fluxes pulsing during storm events
Summary
Headwater streams are a habitat for aquatic organisms, provide drinking water for downstream population centers, and contribute sources of organic matter, nutrients, and sediments to higher-order streams and rivers [1,2]. Mountainous watersheds are important for providing water resources, as illustrated by the disproportionately high contribution of mountainous areas (32% of the world’s land area) to the total discharge in the world’s major river basins (63%) [3]. Instantaneous changes in discharge rates and stream chemistry in response to intense rainfalls cannot be adequately captured by conventional monitoring approaches and networks based on low frequency water sampling at weekly to monthly intervals. Novel monitoring approaches employing high frequency sampling and advanced sensor techniques have been proposed as tools to detect pulses of dissolved solutes, suspended sediments, or organic matter during important hydrologic events such as heavy rainfalls or snowmelts [10,11]. There have been few systematic assessments of sampling frequency and water quality components in the context of assessing climate-induced risks to water quality in mountainous headwater streams
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
