Climate-induced changes in river water temperature in North Iberian Peninsula

Theor Appl Climatol 

https://doi.org/10.1007/s00704-017-2183-9

This study evaluates the effects of climate change on the thermal regime of 12 rivers in the Northern Iberian Peninsula by using a non-linear regression model that employs air temperature as the only input variable. Prediction of future air temperature was obtained from five regional climate models (RCMs) under emission scenario Special Report on Emissions Scenarios A1B. Prior to simulation of water temperature, air temperature was bias-corrected (B-C) by means of variance scaling (VS) method. This procedure allows an improvement of fit between observed and estimated air temperature for all climate models. The simulation of water temperature for the period 1990–2100 shows an increasing trend, which is higher for the period of June-August (summer) and September-November (autumn) (0.0275 and 0.0281 °C/year) than that of winter (December-February) and spring (March-May) (0.0181 and 0.0218 °C/year). In the high air temperature range, daily water temperature is projected to increase on average by 2.2–3.1 °C for 2061–2090 relative to 1961–1990. During the coldest days, the increment of water temperature would range between 1.0 and 1.7 °C. In fact, employing the numbers of days that water temperature exceeded the upper incipient lethal temperature (UILT) for brown trout (24.7 °C) has been noted that this threshold is exceeded 14.5 days per year in 2061–2090 while in 1961–1990, this values was exceeded 2.6 days per year of mean and 3.6 days per year in observation period (2000–2014).

Assessment of Trends in Stream Temperatures in the North of the Iberian Peninsula Using a Nonlinear Regression Model for the Period 1950–2013

River Res. Applic. 32: 1355–1364 (2016)

DOI: 10.1002/rra.2971

A nonlinear regression model was used to estimate mean daily stream water temperature in 11 rivers of the North of the Iberian Peninsula employing as the only predictor variable the air temperature. The weighted mean value of air temperature of a variable number of preceding days was used as a predictor variable.

To obtain the weight of air temperature of each preceding day, initially, we calculate the weight of air temperature of current day. For this, we have included in the model the air temperature of current day and the water temperature of preceding day—representative of long-term effects of air temperature on current water temperature. Subsequently, the weight of remaining days was calculated by a negative exponential function.

The weight of air temperature of current day ranges between 0.28 and 0.10, and it was correlated to length of river (R2 = 0.69) and to time of concentration (R2 = 0.66). This fact implies that the number of preceding days required to obtain a good estimation differs across the rivers. The results show that the mean root mean square error (RMSE) between observed and estimated water temperatures was 1.23 °C (±0.30 °C), employing a number of days so that the sum of their weights was 0.65. For the validation period, RMSE was 1.20 °C (±0.18 °C).

For the period 1986–2013, estimated temperature of water was 0.6 °C higher than that estimated for the period 1950–1986. This increase value is slightly lower than that observed in the air temperature (0.8 °C). On the other hand, during the period 1986–2013, water temperature showed a rate of increase of 0.16 °C/decade, similar to the increase of air temperature (0.15 °C/decade).