River temperature processes under contrasting riparian land cover: linking microclimate, heat exchange and water thermal dynamics

River temperature influences strongly growth and survival in salmonid fish, which are often the target of river management strategies. Temperature is controlled by transfers of heat and water to/ from the river system, with land and water management modifying exchanges and consequently thermal regime. In the UK, fisheries managers are promoting riparian forest planting as a climate change adaption measure to reduce water temperature extremes. However, scientific understanding lags behind management and policy needs. Specifically, there is an urgent requirement to determine planting strategies that maximise expected benefits of riparian forest in terms of reduction in maximum water temperature. Scientific knowledge is necessary to underpin conceptual and deterministic models to inform management. To address this research gap, this paper analyses high resolution (15 minute) hydrometeorological data collected over a calendar year in the western Scottish Highlands (Loch Ard) to understand the controls and processes determining river temperature dynamics under open moorland (control), semi-natural woodland and commercial forest. The research programme aims: (1) to characterise spatial and temporal variability in riparian microclimate and stream water temperature regime across forest treatments; (2) to identify the hydrological, climatological and site-specific factors affecting stream temperature; (3) to estimate the energy balance at sites representative of each forest treatment and, thus, yield physical process understanding about dominant heat exchanges driving thermal variability; and (4) to use 1-3 to predict stream temperature sensitivity under different forestry and hydroclimatological scenarios. Results indicated that inter-treatment differences in mean and maximum daily water column temperature were ordered open > semi-natural > commercial during summer, but semi-natural > commercial > open during winter. Minimum water temperature was ordered commercial > semi-natural > open; hence, the water temperature range was moderated substantially for the commercial site. Daily mean air temperature was ordered open > semi-natural > commercial; seasonality was less marked for the air than water column, although the range was larger for open and semi-natural than commercial site. Humidity was higher and wind speed markedly lower for the commercial than both the other sites. Net radiation was the dominant heat sink in autumn-winter and major heat source in spring-summer with the magnitude of this flux greater in summer and lower in winter for (in order) open, semi-natural and commercial reaches. Sensible heat was an energy source in autumn-winter and sink in spring-summer, with loss (gain) greater in summer (winter) for (in order) open, semi-natural and commercial reaches. Latent heat was predominantly a sink, with the magnitude and variability higher for open than both forested sites. These findings yield important information on: (1) dynamic heat exchange processes that drive stream temperature under different forest treatments, and (2) extent of influence of riparian land cover on stream thermal response. This research provides a basis to predict stream temperature impact given advocated changes to forest practice, and has potential to inform decision making by land/ water managers.