Net Evaporation Resulting from the Creation of a Boreal Hydroelectric Reservoir

English Abstract:

The creation of hydroelectric reservoirs results in a land-use change through the flooding of ecosystems and the environmental footprint of this change is of interest. In particular, there is increasing debate as to whether hydroelectric reservoirs are a significant consumer of water through evaporation. To date, most studies evaluating this change have used gross evaporation (indicating consumption). However, such studies neglect to account for the water loss that would have occurred through the pre-flooded ecosystem evapotranspiration (ET). Instead, the net ET is required to assess the net impact of the creation of a hydroelectric reservoir on the water consumption. We studied net ET loss in the Eastmain-1 hydroelectric reservoir located 800 km north of Montreal, QC from 2006-2012. The reservoir has a surface area of 600 km² and resulted from flooding of mature coniferous forest, peatlands, burned forest and aquatic systems. In contrast to the limited number of previous studies, we used eddy covariance (EC) towers to directly measure evaporation from the reservoir itself and ET from representative forests and wetlands at the same time. As expected, our results show that the ET rates were higher during the warmer months than during fall-winter time and that the gross ET and evaporation rates for the natural ecosystems and the reservoir were similar to those measured in other northern systems. Overall, we found the net evaporation [(reservoir – pre-flooded ecosystem ET) / kWh] to be on the order of 5-25% of gross. If the reductions in reservoir water level that normally occur through management are considered, this number reduces further indicating that northern reservoirs are small net consumers of water.