A New Approach in Measuring Rainfall Interception by Urban Trees ...

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Rainfall Interception (a) (b) (c) Fig. 4. Meteorological and throughfall data for rainfall event three (western red cedar). 21
Asadian and Weiler Throughfall and interception loss. Interception loss was defined as the difference between the total gross and total net precipitation. Table 5 summarizes throughfall and interception losses for the selected trees and events. For events 1 and 2 there are no data available for one of the Douglas-fir trees (# 585) due to rain gauge failure (clogging). When evaluating the average interception losses, it is evident that two of the cedar trees (# 588 and 591) showed the highest interception losses compared with the other selected trees during the events in the fall and winter. Both western red cedar trees were codominant; however, one was of good health condition and the other, poor. Events 5, 6, and 7 occurred during the spring and summer, where both Douglas-fir and western red cedar had high interception losses. The highest interception loss calculated was in event 6 by a dominant Douglas-fir of a poor condition. Based on the results, compared with western red cedar, the Douglas-fir trees showed a wider range of interception losses during the seasons. Event 5 was the smallest precipitation event with relatively moderate levels of interception loss for all selected trees. Event 7 had the shortest duration in comparison with the other selected events. The highest interception loss was seen for event 6 with 26.3 mm of gross precipitation over 25 hours. The average temperature was recorded as 9.7ºC with a maximum rainfall intensity of 3.0 mm/hr. In general, rainfall type plays a role in determining interception loss. For instance, a low intensity, long-duration frontal rainfall generates a different interception loss than a high intensity short duration convectional storm (Xiao et al. 2000a, 2000b; Pypker et al. 2005; Deguchi et al. 2006). Discussion The measurement of throughfall using gauges under individual tree canopies has been successfully conducted to estimate interception loss. The methodology applied in this research is innovative and has not been applied in any other research within urban environments. The measuring system was easy to build and install, and the design minimized evaporation and splashing. These experimental units had the ability to collect spatially variable throughfall underneath the canopy. Net interception loss is determined as the difference between gross precipitation and the sum of throughfall and stemflow. In this study, we did not include the stemflow since it was assumed to be negligible. Based on the obtained throughfall data, the average percent I net for (a) (b) (c) Fig. 5. Meteorological and throughfall data for rainfall event three (Douglas-fir). 22
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