Using equilibrium temperature to assess thermal disturbances in rivers

Abstract

Flow regulation is widely known to modify the thermal regime of rivers. Here, we examine the sensitivity of an empirical approach, the Equilibrium Temperature Concept (ETC), to detect both the effects of hydraulic infrastructures on the annual thermal cycle and the recovery of the thermal equilibrium with the atmosphere. Analysis was undertaken in a Pyrenean river (the Noguera Pallaresa, Ebro basin) affected by a series of reservoirs and hydropower plants. Equilibrium temperature (Te) is defined as the water temperature (Tw) at which the sum of all heat fluxes is zero. Based on the assumption of a linear relationship between Te and Tw, we identified changes in the Te–Tw regression slope, used as an indicator of a thermal alteration in river flow. We also assessed the magnitude of the alteration by examining the regression slope and its statistical significance. Variations in the regression parameters were used as indicators of the influence of factors other than atmospheric conditions on water temperature. Observed Tw showed a linear relationship with Te at all river stations. However, the slopes of the Te–Tw relationship appeared to be lower in the reaches downstream from hydraulic infrastructures, particularly below large dams. A seasonal analysis indicated that Te–Tw relationships had higher slopes and lower p‐values during autumn, while no significant differences were found at other seasons. Although thermal characteristics did not strongly depend on atmospheric conditions downstream of hydraulic infrastructures, the river recovered to pre‐alteration conditions with distance downstream, indicating the natural tendency of water to attain thermal equilibrium with the atmosphere. Accepting associated uncertainties, mostly because of the quality of the data and the lack of consideration of other factors influencing the thermal regime (e.g. discharge), ETC appears to be a simple and effective method to identify thermal alterations in regulated rivers. Copyright © 2015 John Wiley & Sons, Ltd.

Effects of Hydropeaking on Bed Mobility: Evidence from a Pyrenean River

“…As already reported by Buendia et al [28] the annual water yield of the whole catchment averages 1330 hm 3 /year (1 hm 3 = 1 × 10 6 m 3 ) with May and June concentrated most of it (180 hm 3 and 260 hm 3 , respectively), typically due to snowmelt. The flow regime of the river has been altered by dams that were built up along the 20th century for hydropower production.…”

“…Mean annual temperature ranges from 4 °C at the headwaters to 14 °C in the lowlands. Most of the catchment area (83%) is covered by forests and grassland, 9% is occupied by agriculture (located in lowlands and valley bottoms), while the remaining 8% belong to small villages and rocky outcrops [28,29].…”

Understanding the Thermal Regime of Rivers Influenced by Small and Medium Size Dams in Eastern Canada

“…Indeed, water temperature downstream of the two storage dams was less responsive to ambient meteorological conditions as demonstrated by the weak correlation between air and water temperature residuals (Table ). A similar decoupling between water temperature and atmospheric conditions was also observed downstream of dams in the Ebro basin, Spain (Buendía et al, ). Moreover, findings from the present study are consistent with the study by Steel and Lange () who also observed a reduction in water temperature variability at short temporal scales (1 to 8 days) downstream of hypolimnetic‐release dams in the Willamette basin, Oregon.…”

“…In the case of the St‐Jean River, the increase in net incoming solar radiation as a result of the small impoundment (reservoir area = 6 ha and storage capacity = 0.3 × 10 6 m 3 ; Table ) upstream of the dam was likely insufficient to significantly warm the regulated river. Recent studies suggest that the volume of the impoundment should be considered in conjunction with the size of the river when assessing thermal alteration by dams (Buendía et al, ; Maheu, ). For example, the ratio of reservoir maximum storage capacity (m 3 ) to median annual runoff (m 3 ), a useful indicator to assess the potential for hydrological and thermal alteration by dams (Batalla et al, ), was smaller than 0.1% for the run‐of‐river dam assessed in the present study (i.e.…”

“…Few studies have looked at the impacts of dams with shallow reservoirs and some of these systems have been associated with a warming effect (Fraley, ) while others showed a cooling influence on summer water temperatures (Crisp, ; Webb and Walling, ). Within the Ebro basin (Spain), Buendía et al () observed a wide range of thermal responses downstream of small hydropower dams (i.e. no impact or impact similar to large dams) which authors associated with differences in operational systems.…”