Analysis of stream water temperature changes during rainfall events in forested watersheds

Despite continued interest in stream water temperature (Tw) analysis, there are few studies of Tw response to rainfall events at forested watersheds. We examined 61 sets of data on Tw for 21 rainfall events at 16 watersheds with various slope gradients (from 0.08 to 0.56) in four regions of Japan from June 2004 to December 2005. The investigation focused on the changes of specific discharge (Delta Qs) and Delta Tw at medium-sized watersheds (0.5-100 ha). The results clearly demonstrated different flow patterns expressed by Qs vs. Tw hysteretic loops. Those were clockwise in Period I (April-September) and counterclockwise in Period II (October-March), except for lower slope gradient at Aichi, where counterclockwise loops were observed in both periods. These differences in hysteretic loops could be explained by the differences in Tw and in response times to rainfall between surface/subsurface and groundwater flows. The response times were probably determined by the slope gradient and the vertical level of groundwater. We also found that the changes in air temperature (Delta Ta) influenced Delta Tw to a lesser degree than Qs. The average rainfall intensities in Period I and Period II (9.3 +/- A 1.7 and 5.4 +/- A 0.2 mm/h, respectively) affected the average values of Delta Qs and Delta Tw (6.62 +/- A 4.08 mm/h and 1.7 +/- A 0.4A degrees C; 0.85 +/- A 0.68 mm/h and 0.9 +/- A 0.3A degrees C, respectively). This indicates that slope gradient and Qs influenced Delta Tw by changing the relative proportions of flow paths. In addition, the water table changes influenced the percentage of groundwater flow to the stream.