Rainfall Interception Loss by Forest Canopies

When rain falls onto a forest a proportion is intercepted by the canopy and evaporates back into the atmosphere, playing no further part in the terrestrial portion of the hydrologic cycle. This canopy interception loss, I c, can be appreciable (Table 20.1). The first published reports of I c date from the late nineteenth and early twentieth centuries (Hoppe 1896; Horton 1919; see also Gash and Shuttleworth 2007). Since then numerous studies have been conducted with I c being found to account for 10–50% of season-long or annual rainfall, P g, (Roth et al. 2007); varying with both forest characteristics and climate. Because I c is an important and sometimes dominant component of forest evaporation (David et al. 2005), several models have been developed ranging from simple linear regression (e.g., Helvey and Patric 1965) to physically based numerical (e.g., Rutter et al. 1971, 1975), analytical (e.g., Gash 1979) and stochastic (e.g., Calder 1986) models. Although I c has been extensively studied by hydrologists, key processes, such as those responsible for the relatively high during-precipitation evaporation rates from forest canopies, E, are still not fully understood (Dunkerley 2009).