Modeling water temperature in small agricultural drainage watercourses

Dawn Joy Shepard

The historical practice of excavating accumulated sediment and vegetation from agricultural drainage watercourses has raised controversy between agricultural and environmental interests in King County. These low gradient channels play a critical role in draining flood waters from agricultural land and also serve as potential rearing habitat for ESA-listed salmonids. Protective policies require the application of Best Management Practices following drainage watercourse maintenance, but the effectiveness and efficiency of required practices is not well defined. Water temperature is a potentially important habitat limiting water quality parameter that may be impacted by drainage maintenance. Preferred temperatures for rearing salmonids are in the range of 12-14°C. Lethal temperature is approximately 26°C. One of the most manageable and potentially significant influences on water temperature is the shade of riparian vegetation. Riparian vegetation is typically partially cleared during drainage maintenance in order to remove accumulated sediment and reed-canarygrass. The removal of vegetation and change in channel morphology raises concern regarding the impact of drainage maintenance on watercourse temperature. Mitigation plantings are required following watercourse maintenance, yet the effectiveness of these plantings in protecting watercourse temperature is not known. Information on the relationship between water temperature, riparian vegetation and shade is critical for developing mitigation plans. The shade effectiveness of pre-maintenance existing vegetation versus mitigation plantings, and the impact of riparian shade on water temperature were evaluated with a reach scale study. Short reaches with uniform vegetation and uniform channel morphology were studied during summer base-flow conditions. Water temperature monitored above and below the experimental reach was used to develop and validate a physically based temperature model. Direct solar radiation was measured in and out of the shade of riparian vegetation to calibrate vegetation density for willows, Himalayan Blackberry and reed canarygrass. The developed model was found to be accurate to within 0.5 °C. Calibrated densities for buffers with complete uniform willow vegetation were 93% for mature willows, and 80% for willows approximately two years after planting. Reed canarygrass and Himalayan Blackberry growing uniformly at maturity were found to have approximately 100% calibrated densities. The difference in density between vegetation types was not found to have a significant impact on watercourse temperature. However, the presence of shade from riparian or topographic features was found to influence temperature. Narrow watercourses with vertical banks were found to be more sensitive to riparian vegetation characteristics, while wider watercourses with shallow side slopes were found to be more sensitive to air temperature. Temperature model results show that a steep incised bank increases the effectiveness of vegetation in providing riparian shade, and that shade cast by vegetation and the bank itself can decrease maximum seven day average temperatures by at least 0.7 °C.

Modeling water temperature in small agricultural drainage watercourses

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