River water temperatures are higher during lower flows after accounting for meteorological variability

Abstract

River water temperature is known to be important for water quality and ecosystem processes. We quantified the degree to which lower river flows are associated with warmer river water, after accounting for seasonality and meteorological variability.We applied a systematic methodology to analyse observed mean daily river water temperature and mean daily river flow from 47 sites draining mountain, hill, and lowland catchments across the Canterbury region of Aotearoa New Zealand. We fitted regression models to remove seasonal patterns from all variables, then removed correlations between water temperature and each of three meteorological variables (solar radiation, air temperature, and earth temperature) before quantifying water temperature-river flow relationships. Strong seasonal patterns were present in water temperature and each meteorological variable across all sites. Many sites also showed strong seasonal patterns in river flows. We demonstrated that seasonal patterns must be accounted for before day-to-day associations between water temperature and meteorological variables or river flow can be characterised. Higher water temperatures were associated with lower flows for 46 of 47 sites, even after having accounted for seasonality and associations with each meteorological variable.Increases in water temperature associated with a hypothetical reduction in river flow from the median to the fifth percentile varied with the site but were 0.5 C on average. This finding has important implications for river flow management because it indicates that increased river water temperatures would accompany reduced river flows regardless of site or catchment characteristics.

Drought impacts on river water temperature: A process‐based understanding from temperate climates

“…For instance, Folegot et al (2018) experimentally simulated drought conditions using outdoor flow-through flumes and reported those containing severely depleted water volumes exhibited a maximum warming of 3.3 C, $2 C higher than those conveying higher discharges. In New Zealand, Booker and Whitehead (2022) reported that declines in river flow from the median to the fifth percentile facilitated an average T w increased by 0.5 C, and similar findings have been reported elsewhere in Cfb climates (e.g., Van Vliet et al, 2011).…”

Modelling spatial and temporal variability of water temperature across six rivers in Western Canada

“…The cooler T w under higher Q could be partly explained in terms of snowmelt-driven high flows that bring cooler water to the rivers. On the other hand, the warmer T w under lower Q has been documented by previous studies (e.g., Booker & Whitehead, 2021;Woltemade & Hawkins, 2016) and attributed to greater temperature change per unit of heat exchanged with the reduced mass of water, which may be enhanced by heat accumulation due to reduced surface area and flow velocity (Booker & Whitehead, 2021).…”

Refined machine learning modeling of reservoir discharge water temperature

“…After the construction of a reservoir, thermal stratification can occur in the water body, causing the discharge water temperature (DWT) to deviate from the natural water temperature (Ren et al, 2019). Due to the crucial role of water temperature in water quality and aquatic ecosystem processes (Booker and Whitehead, 2022), DWT fluctuations may have adverse effects on downstream aquatic ecosystems (Lu et al, 2023). Therefore, DWT prediction and control is currently a topic of concern for reservoir managers.…”