Multiple Time-Scale Monitoring to Address Dynamic Seasonality and Storm Pulses of Stream Water Quality in Mountainous Watersheds

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 OPEN ACCESSWater 2015, 7 6118 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.

Seasonal Variation in Flocculation Potential of River Water: Roles of the Organic Matter Pool

“…The turbidity of river water may vary seasonally and spatially. For example, a heavy storm can result in more turbid river water due to the production of cohesive sediments from land disturbance and soil runoff [1,2]. In contrast, biota-rich rivers tend to be less turbid because biota have the ability to trap cohesive sediments through flocculation, sedimentation, and filtration [3][4][5].…”

Land use and land cover changes in the Haean Basin of Korea: Impacts on soil erosion

“…The DO of the stream water was highly variable (CV = 1.81) ranging between 7.3 and 14.5 mg/L (mean = 10.9 mg/L) with a strong negative correlation with the water temperature (r = -0.905, p < 0.0001). The DO was at a seasonal low in summer, while it was at a seasonal high in winter because the oxygen solubility decreased with the increasing water temperature (Lee et al, 2015;Dick et al, 2016). Based only on the dissolved oxygen, the stream water was under very good conditions, irrespective of the turbidities.…”

“…6a). Coniferous forest composed of red pine, nut pine, and white birch trees accounts for ~26%, deciduous forest (oak, royal azalea, and dwarf small-leaf birch trees) account for 29%, and the remainder is mixed forest (Lee et al, 2015). The paddy field area did not significantly change during that period, but a slight decrease has been recorded recently due to the expansion of roads and buildings.…”

Temporal and spatial variability of stream water chemistry on Subantarctic Marion Island

“…The rainfall events resulted in the dilution of solute concentrations. Similarly, Lee et al (2015) found a response from solutes in the Haean Basin, between South Korea and North Korea, whereby dilution of stream ions varied relative to rainfall intensity, duration and antecedent moisture conditions. Furthermore, the uniform response of most solutes at different sites over the time series, in particular Na + and Cl − , indicates that variation in solute concentrations is predominantly controlled by rainfall inputs (Neal et al 2012).…”