VALIDATION OF A COMMERCIAL SYSTEM FOR REMOTE ESTIMATION OF WETNESS DURATION

To assess the accuracy of remote, real-time mathematical simulations o f wetness duration and air temperature, hourly measurements of wetness duration and air temperature at 18 sites in the United States and Can ada from May to September 1995 were compared with simulations for thes e sites provided by SkyBit, Inc. SkyBit simulations of mean, maximum, and minimum daily air temperatures varied from on-site measurements by less than 0.7 degrees C but underestimated the duration of wet period s by an average of 3.4 h/day. At five of six stations tested, SkyBit u nderestimates of wetness duration were significantly (P < 0.01) larger on days when no rain was measured than on rainy days, indicating that simulations of dew-period duration were much less accurate than simul ations of rain-period duration. The vast majority of hours SkyBit misc lassified as dry occurred either when entire wet periods were missed ( 59.3%) or when the onset of a wet period was detected late (28.4%). Th e results suggest that revision of SkyBit wetness-simulation models sh ould focus on reducing error rates during dew events. In simulations u sing two disease-warning models, TOM-CAST and Melcast, with mean value s of measured and SkyBit-simulated wetness duration, SkyBit-simulated values resulted in fewer and later fungicide spray advisories than did measured values. The magnitude of these impacts varied with the magni tude of the simulation errors and with differences in the models' deci sion rules.