water has typically infiltrated into the soil and moves
laterally in the shallow subsurface layer.
This relationship between the precipitation event
and associated discharge, specifically in the timing
and amount of runoff, is affected by climatic condi-
tions, geology, topography, soil characteristics, vege-
tation type, leaf area, and land use (Dunne and Leo-
pold 1978). The relative importance of these factors
varies with precipitation duration and intensity. Be-
cause the Monteverde area is within the headwaters
of two major drainage basins and contains many
primary watersheds, the lag time for peak flows is
relatively short compared to areas farther down-
stream. Even major streams such as the Rios Penas
Blancas and Guacimal peak and then recede quickly
in response to precipitation inputs (Fig. 2.11; Sec.
2.1.3).
2.6.1. Evapotranspiration
Vegetation plays a key role in the hydrology of
humid tropical forests (Douglas 1977, Shuttleworth
1989, Grace et al. 1996; Fig. 2.12). Because these for-
ests have relatively high leaf areas and canopies that
are essentially closed, virtually all water that reaches
the forest floor first comes in contact with the vegeta-
tion. This constant contact between precipitation and
vegetation also has implications for nutrient cycling
(see Chap. 9, Ecosystem Ecology).
Initially, cloud water and precipitation may be
stored on plant surfaces ("interception storage"). The
volume depends on the form, density, and surface
texture of the vegetation and the presence of epiphytes
and accumulated organic matter (Dunne and Leopold
1978, Nadkarni 1984, Veneklaas et al. 1990, Ingram
and Nadkarni 1993). For example, the canopy of an
epiphyte-laden leeward cloud forest stored approxi-
mately 8 mm of precipitation, while that of an adja-
cent second-growth forest (ca. 25 yr old) dominated
by Conostegia stored <2 mm of precipitation (Clark
1994, K. Clark and N. Nadkarni, unpubl. data). The
higher storage capacity of the former site is due to
greater leaf area, greater epiphyte mass, and greater
water storage on stems. Annual interception loss from
the epiphyte-laden leeward cloud forest site was es-
timated at approximately 1200 mm/yr, representing
about 38% of estimated total hydrologic inputs to the
canopy (Clark et al. 1998b).
Interception losses of-35% to 35% (negative val-
ues represent greater cloud water and mist inputs over
precipitation collected with a standard rain gauge)
have been reported from a range of tropical montane
forests (Bruijnzeel and Proctor 1993). The relatively
high interception loss at the leeward cloud forest siteFigure 2.11. The Rio Guacimal between the Monteverde Cloud Forest Preserve and the Monteverde
dairy plant. Photograph by Stephen Ingram.30 The Physical Environment