PREDICTING THE EFFECTS OF SHADE ON WATER TEMPERATURE IN SMALL STREAMS

A computer model for stream water temperature was developed, and teste d in a small pasture stream near Hamilton, New Zealand. The model quan tifies shading by riparian vegetation, hillsides, and stream banks usi ng three coefficients: canopy angle, topography angle, and canopy shad e factor. Shade was measured directly and found to vary significantly along the channel. Using the maximum measured shade, a close match was achieved between observed and predicted daily maximum and minimum wat er and bed sediment temperature. Model predictions of incoming and out going long-wave radiation flux closely matched measurements, but there were unexplained discrepancies in short-wave radiation flux. Model pr edictions indicate that moderate shade levels (c. 70%) may be sufficie nt in temperate climates to restore headwater pasture stream temperatu res to 20 degrees C, an estimate of the thermal tolerance for sensitiv e invertebrates.