AIMS Geosciences, 2018, 4(4): 192-214. doi: 10.3934/geosci.2018.4.192

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Environmental factors controlling stream water temperature in a forest catchment

Laboratory of Physical Hydrology, Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan

Heat budget of a stream, the Putoisaroma Stream, Hokkaido, Japan, in a forest catchment was estimated in order to investigate environmental factors controlling stream water temperature. In August 2008–October 2009, water temperature was measured at six sites along the stream channels, and the streambed temperature was measured at depths of 5 cm and 30 cm at one of the sites. In order to quantify incoming and outgoing radiations at the stream surface, hemispherical photographs were taken and the shading factors (ratio of the shade to the whole sky) were calculated at the observation sites over the summer. The shading factors, exhibiting seasonal and spatial variations, produced seasonal and spatial changes of shortwave and longwave radiations. The wind speed above stream surface was much smaller than in an open field, which produced turbulent heat fluxes one third as large as that in the open field. The shortwave and longwave radiations and the advective heat flux from upstream showed the major contribution to the stream heat budget, while the streambed heat conduction was secondary. The time series of stream water temperature were simulated well (RMSE = 0.771 ℃, NASH = 0.888) by applying the estimated heat budget. This evidences that the quantification of the shade above stream surface and the calculation of the heat budget are both reasonable. The sensitivity analysis for the simulation indicates that the shading factors along the stream channels control the stream water temperature.
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