Application of weather radar in estimation of bulk atmospheric deposition of total phosphorus over Lake Simcoe

Abstract

The decline of Lake Simcoe water quality has been attributed to high phosphorus inputs that result in excessive algae and macrophyte growth subsequently contributing to end-of-summer hypolimnetic dissolved oxygen depletion and loss of fish habitat. Out of the estimated 53 to 67 tonne/annum (1998 to 2004 water years) of phosphorus entering the lake, atmospheric deposition is believed to be responsible for 16 to 38 tonne/annum. Historical estimates for atmospheric deposition involved averaging rain gauge (rainfall depth) and rain quality (phosphorus concentration) station data. Through use of this procedure, any spatial variability in the data (quality and quantity) is lost as each gauge is given an equal weighting. This study proposes a methodology to use Next Generation Radar (NEXRAD) to spatially represent rainfall data and a method to correct radar-rainfall estimates to rainfall recorded by local rain gauges. From this analysis it was found that the radar generally represented localized rainfall well, with the majority of correlation coefficients (R 2) being over 0.90. Radar related issues that resulted in poor R 2 values included virga, overshooting beam, beam attenuation, range related issues and ground clutter. For large bulk atmospheric total phosphorus (TP) deposition events the dominant parameter in calculating TP loads was rainfall depth. Results from this analysis demonstrated a large (-88% to +44%) difference between historical and revised estimates of bulk atmospheric deposition of phosphorus over Lake Simcoe. © 2009 Canadian Water Resources Association.