Global river temperatures and sensitivity to atmospheric warming and changes in river flow

This study investigates the impact of both air temperature and river discharge changes on daily water temperatures for river stations globally. A nonlinear water temperature regression model was adapted to include discharge as a variable in addition to air temperature, and a time lag was incorporated to apply the model on a daily basis. The performance of the model was tested for a selection of study basin stations and 157 river temperature stations globally using historical series of daily river temperature, air temperature, and river discharge for the 1980-1999 period. For the study basin stations and for 87% of the global river stations, the performance of the model improved by including discharge as an input variable. Greatest improvements were found during heat wave and drought (low flow) conditions, when water temperatures are most sensitive to atmospheric influences and can reach critically high values. A sensitivity analysis showed increases in annual mean river temperatures of vertical bar 1.3 degrees C, vertical bar 2.6 degrees C, and vertical bar 3.8 degrees C under air temperature increases of +2 degrees C, +4 degrees C, and +6 degrees C, respectively. Discharge decreases of 20% and 40% exacerbated water temperature increases by vertical bar 0.3 degrees C and vertical bar 0.8 degrees C on average. For several stations, maximum water temperatures on a daily basis were higher under an air temperature increase of vertical bar 4 degrees C combined with a 40% discharge decrease compared to an air temperature increase of vertical bar 6 degrees C (without discharge changes). Impacts of river discharge on water temperatures should therefore be incorporated to provide more accurate estimations of river temperatures during historical and future projected dry and warm periods.