Figure 2.12. Flow of water through the vegetation
canopy and the soil in a tropical rain forest.in Monteverde is likely due to intermittent cloud
water and precipitation inputs, relatively high
windspeeds, high incident solar radiation, and the
large water storage capacity of the canopy. Intercep-
tion loss is likely responsible for a larger proportion
of ET for forests exposed to wind-driven cloud water
and mist than those lower on the Caribbean and Pa-
cific slopes.
At some precipitation intensity and duration, the
storage capacity of the vegetation is exceeded. Increas-
ingly greater amounts of cloud water and precipita-
tion are stored for shorter periods of time on vegeta-
tion and then fall to the ground as throughfall or flow
down stems as stemflow (Fig. 2.11). Some of the water
that falls on the forest infiltrates the litter layer and
soil. Roots take up water, which is subsequently tran-
spired through stomata on leaf surfaces. Estimates of
ET are lacking for Monteverde, but rates of ET of for-
est along the Continental Divide are probably rela-
tively lower because of low vapor pressure deficits of
the atmosphere, relatively low temperatures, and low
solar radiation inputs due to cloud cover (Gates 1969,
Bruijnzeel and Proctor 1993). Rates of ET of other
forests in the MCFP, particularly those on the Pacific
slope, are relatively high.
Evapotranspiration from forests alters the micro-
climate above and within the forest. For example, at
a leeward forest 0.5 km northeast of the MCFP, rela-
tive humidity was significantly higher within the for-
est than in a clearing (W. Calvert and A. Nelson, un-
publ. data). Estimating annual ET from Monteverde
forests is hampered by the lack of data on canopy
structure, the unknown role of epiphytes in the ET
process, the paucity of meteorological measurements
to drive ET models, and the poorly understood envi-
ronmental and physiological controls over stomatal
conductance in montane forest species (e.g., Roberts
et al. 1993). Montane forests that receive relatively
large wind-driven cloud water and precipitation in-
puts have calculated ET values of 570-695 mm/yr.
Montane forests that receive relatively little wind-
driven cloud water and precipitation have ET values
of 980-1265 mm/yr (Bruijnzeel and Proctor 1993).2.6.2. Subsurface Flow and Streamflow
Water that is not evaporated or transpired is stored
in the soil, or becomes streamflow or subsurface
flow. Monteverde undergoes a pronounced dry sea-
son from February through April, yet most streams
flow throughout the year. This baseflow is maintained
by water seeping out of banks and into streams by
means of two pathways. First, it is maintained by
throughflow (also termed shallow subsurface flow),
defined as water that has infiltrated the soil or parent
material and enters the stream channel via subsurface
flow paths. Second, streams may intercept the water
table. Water entering streams via these pathways typi-
cally moves slowly (up to several centimeters per hour
or per day) compared to surface water velocities (sev-
eral meters per second). During periods of high dis-
charge, these two subsurface pathways also contrib-
ute significantly to the total volume of runoff. Runoff
is also generated from precipitation that flows off the
land surface and from precipitation that falls directly
on saturated areas adjacent to the channel or onto the
surface of the stream (Dunne and Leopold 1978, Ward
1984).
Changes in vegetative cover can affect the timing
and peak discharge of flood events and the amount
of water that enters the channel as baseflow. In tem-
perate latitudes, tree removal by logging, fire, and
wind increases the runoff from the affected area. The
magnitude of the increase is roughly proportional to
the percentage reduction in tree cover (Dingman 1994)
but is also a function of precipitation intensity and31 The Physical Environment