Influences of forested watershed conditions on fluctuations in stream water temperature with special reference to watershed area and forest type

In order to gain insight into the effect of watershed conditions on fluctuations in stream water temperature, we statistically analyzed water temperature data for 1 year, using root mean square (Rms) and harmonic (A Amplitude, φ delay time) methods. The average values of delay time (days) between air and water temperatures (T _a and T _w) of small (< 0.5 ha), medium (0.5–100 ha) and large (> 100 ha) watersheds were 4.53 ± 0.82 days, 11.83 ± 3.88 days and 4.45 ± 1.52 days, respectively. Fluctuations in stream water temperature expressed by Rms (Rms T _w/Rms T _a) and harmonic methods (A −T _w/A −T _a) in the medium-sized watersheds with moderate slope gradients were 0.37 ± 0.09 and 0.56 ± 0.14, respectively. These values increased in the larger watersheds with low slope gradients, including five large rivers covered by various landscapes, with their averages of 0.53 ± 0.09 and 0.78 ± 0.09, respectively, indicating the influences of solar radiation and heat transfer processes. In the smaller watersheds with high slope gradients, these values were 0.73 ± 0.02 and 0.87 ± 0.03, respectively, suggesting that shorter passage time affected water temperatures. With respect to forest type, these values at badly managed hinoki forest watersheds (0.45 ± 0.04 and 0.73 ± 0.07) were larger than those at broadleaf forest (0.34 ± 0.04 and 0.51 ± 0.12) and well-managed hinoki forest (0.33 ± 0.04 and 0.51 ± 0.07) watersheds, indicating different proportions of flow paths.