Estimating thermal responses using a heat budget model and detail monitoring of channel morphology in a headwater stream

Estimating thermal responses by heat budget is an effective approach for evaluating the changes in riparian forest conditions by forest management practices in headwater streams. In general, channel morphology and flow regimes in headwater streams are complex due to the availability of various channel substrate (i.e., cobbles and boulders). Hence, estimating channel dimension such as stream wetted width is a key parameter for estimating the rate of heating of solar radiation inputs to stream surface, while the channel dimension can be spatially and temporally varied by flow conditions and sediment movement. The recent advance of measurement of channel topography using structure from motion (SfM) can improve for measurement of channel dimension. This study aims to address the method of improving heat budget model using channel measurement by SfM. This study was conducted in 30-m headwater channel with 1.2 m wetted width and 7.3° mean gradient consisted of pool-riffle, and step-pool. Riparian forest consisted of 20 to 50-year-old of Japanese cedar with 16% canopy openness. Microclimate station above 1 m water surface was installed for measuring incoming and outgoing shortwave (between 0.3 and 3.0 μm) and longwave (between 4.5 and 45 μm) radiation, air temperature, relative humidity, and wind speed. Tidbit temperature loggers were installed at the upper and lower boundary for measuring stream temperature. DEMs with 5 mm resolution was developed by photographs taken by digital camera with the monocable system at the height of 2-3 m above the stream bed. By the day of 23.4°C of maximum air temperature (23 May 2019), maximum stream temperature occurred on the day was 14.9° and 15.0°C in upper and lower boundary, respectively. Based on the contemporary heat budget method, estimating maximum water temperature was 16.8°C, which became 1.8°C overestimating. We will present results how SfM data and accurate measurement of wetted width could provide information for improving the performance of heat budget temperature model.