Evaluating Rainfall Interception in an Urban Forest

Abstract A rainfall interception methodology was implemented in a 15 year-old deciduous Ficus benjamina tree to evaluate the losses by interception of vegetation and pattern distribution in urban zones. Spatial pattern distribution of precipitation surrounding the tree was measured manually with 16 funnels. Losses by interception were calculated from Rutter's model under the analogy of the “big leaf” theory. Measurements of rainfall, throughfall and stemflow were made after each storm. Gross precipitation was recorded every 5 minutes. Air temperature and relative humidity, wind speed and direction, and atmospheric pressure were recorded every 20 minutes, while evaporation of reference readings were taken every 24 hours. Nineteen individual storms occurring between July to October, 2005, were analyzed. Total precipitation for the studied period was 150 mm representing 27% of mean annual precipitation (550 mm). As a result of rainfall interception, gross precipitation was partitioned as follows: 39 % throughfall, 2 % stemflow and 60 % losses by interception. Canopy saturation of Ficus benjamina was estimated from the throughfall and gross precipitation ratio and yielded a value of 2.50 mm. Time average for saturation was 18.40 minutes. Time lag between individual storms varied from 1 to 14 days, sufficient to dry out the surface of the tree leaves. Bivariate algorithm was used to explore spatial distribution patterns of rainfall and screen effect over and around the projected crown. The results indicated the presence of preferential flows for throughfall and runoff. Independently of the structure of canopy, it was observed that partitioning of precipitation depended on rainfall duration-intensity relationships. Key Words: Interception losses, water balance, urban vegetation