Application of stream heat budget modelling to assess thermal mitigation options – case study

Year: 2020
Presenter/s: David West
Symposium Session: 2020 - 01 Protecting critical flows
Topics covered: Canada, climate change, fish passage, floodplain, hydraulics, modeling, and stream


ABSTRACT

Stream temperatures have been rising in many streams of the Pacific Northwest due to land use and flow regime alterations and climate change. Fish species such as salmonids have narrow thermal tolerances that, when exceeded, can cause severe health impacts, prevent spawning, or cause mortality. Stream thermal regimes are governed by a variety of heat fluxes that are influenced by riparian vegetation characteristics, flow regime, channel morphology, and climate. Some of these fluxes can be modified by watershed managers through shade enhancement and flow regime modification. Determining the cost-benefit of modifying these processes requires high accuracy predictions of the expected outcomes. Additionally, trade-offs can exist between providing ideal flow rate and maintaining ideal temperature in lake-headed systems.
Stream heat budget models have been applied around the world to assess thermal management questions for decades; however, the available data and heat flux characterization methods have been continuously evolving, resulting in the ability to accurately predict the effects of management interventions at lower cost. The purpose of this talk will be to describe the modelling process required to assess thermal management approaches using a case study from the Okanagan region of British Columbia. Ecofish Research Ltd. and the BC Ministry of Forests, Lands, Natural Resources Operations and Rural Development (MFLNRORD) were awarded a grant by the Habitat Conservation Trust Fund (HCTF) to develop a model for a small regulated stream with multiple online lakes that supports resident Rainbow Trout and critical spawning habitat for Kokanee. The model was used to assess the potential thermal regime improvements that could be achieved with enhanced riparian vegetation, and to determine the maximum flow rate before temperatures exceed Kokanee spawning thresholds. The model was also used to predict the effects of climate change due to changes in air temperature and solar radiation.

The stream temperature model that was configured is an upgraded version of the heat budget model that has been developed over the past decade by Ecofish and various researchers at the University of British Columbia. The model applies an unsteady Lagrangian flow routing algorithm. Surface heat fluxes include direct and diffuse solar (shortwave) radiation, longwave radiation to and from the stream, sensible heat flux, and latent heat exchange (evaporation and condensation). The advective heat fluxes include inflows, hyporheic exchange, and streambed heat conduction. Each of the fluxes was measured in the field or calculated with public data sources.

The effects of climate change included maximum water temperature increases that approached 5oC for RCP4.5 and exceeded 8oC for RCP8.5 climate change scenarios. An ideal flow release during the Kokanee spawning period was identified that was slightly lower than the current standard release. Enhancing vegetation was also shown to provide strong benefits with a potential 4oC temperature reduction if all banks were planted with similar density to that of the top 50% of measured locations. This case study highlights the benefits of stream temperature models in quantifying the potential effects of flow and shade management interventions.