duration. The same relationship appears to apply to
tropical areas after forests have been logged.
Wind-driven cloud water and precipitation are an
important input to the hydrologic cycle of montane
forests in the Monteverde area. This source of mois-
ture is significantly reduced when forested lands are
cleared, because forests are aerodynamically rough
surfaces that catch more of this water (Lovett and
Reiners 1986, Monteith and Unsworth 1990). There-
fore, the expected increase in runoff may be greater
than areas where "occult" precipitation is an impor-
tant input. More research on this subject in Monte-
verde is needed.
2.63. Erosional Processes
Under undisturbed conditions, the closed canopy and
litter layer of forests prevent virtually all rainsplash
erosion. When vegetation is removed, however, the
cumulative effect of raindrops hitting bare soil can
lead to serious erosion problems, especially in areas
with steep slopes (Dunne and Leopold 1978). High-
speed photography has shown that a raindrop gener-
ates a small explosion of soil and water upon impact
with the ground; in steep terrain, this material tends
to move downslope. When gravelly soils are exposed,
rainsplash removes the small particles and leaves the
pebbles sitting on top of columns. These are referred
to as "capstones" and are common along trails and
roads in the Monteverde area. In some cases, espe-
cially in pastures, large amounts of soil are removed,
leaving behind a surface covered with boulders. As
more soil is eroded, each boulder is increasingly ex-
posed. The process is detectable over a period of a few
years; local people refer to it as "growing rocks."
If precipitation is of high intensity or long dura-
tion, water flows and entrains (transports) soil par-
ticles. This process is referred to as sheetwash ero-
sion and may lead to the formation of rills and gullies
(Dunne and Leopold 1978). Sheetwash erosion can
increase the amount of sediment delivered to a river
channel and may have negative impacts. The river
channel is formed and maintained by the water and
sediment load it carries, but even in undisturbed
areas the channel is never large enough to carry all
flows without flooding (Dunne and Leopold 1978).
Excess sediment can lead to rapid channel adjust-
ments, primarily by causing the river to aggrade (fill
in), thus decreasing the cross-sectional area. The
river will then flood at lower discharges and/or erode
its banks to accommodate the flows delivered to it.
These problems are compounded if land-use prac-
tices such as deforestation have also increased the
volume of runoff.Because the Monteverde area is located in the
headward portions of its drainage basins, the effects
of accelerated erosion are primarily on-site, that is,
on the scale of a field or a hillslope. In other tropical
areas, these effects can be transferred downstream,
initiating a cascade of problems (Bruijnzeel 1990). Al-
though erosion problems in the Monteverde area have
not reached the point where large volumes of sedi-
ment are moved downstream, if serious on-site ero-
sion begins, it may take decades before reductions in
surface erosion in upland areas are manifest as a re-
duction in sedimentation downstream (Bruijnzeel
1990).
Water that infiltrates into the subsurface can also
have impacts. Many types of mass wasting are trig-
gered by changes in water pressure generated by the
movement of subsurface water. The rate of displace-
ment for slope failures may be slow (e.g., soil creep),
on the order of a few centimeters per year, to ex-
tremely rapid (e.g., landslides), with speeds in excess
of 300 km/hr (Keller 1996). Downslope movement
occurs when the gravitational forces acting along the
potential failure surface exceed the resistance (or
shear strength) of the geologic materials. Subsurface
water can increase the pressure in the pore spaces of
rock and soil on hillslopes, which in turn reduces
their strength or cohesion. Once the downslope force
(gravity) exceeds the shear resistance of the material,
the hillslope will fail (Dunne and Leopold 1978,
Keller 1996; Fig. 2.13). Often, slopes are moved closer
to failure by activities such as road-building and re-
moval of vegetation. An increase in water pressure
may then become "the straw that breaks the burro's
back."
In summary, the forest hydrologic cycle involves
complex interactions between the physical environ-
ment and the biosphere. The details of many of these
interactions, especially in humid tropic forests, are
poorly understood. With more land undergoing con-
version from forests to other uses, this information is
vital to understanding how these systems function
and predicting how they will respond to change. In
Costa Rica and elsewhere in the humid tropics, for-
ests represent a tremendous resource, but without
proper management they will become sources of ex-
cess sediment and increased flooding. The cost of
reclaiming devastated landscapes can be prohibitive.
Very few hydrologic data are available for the
Monteverde area. Many small watersheds in the vi-
cinity could be monitored to determine the relation-
ship between rainfall and runoff for various land uses.
The stream that runs near the Estacion Biologica is of
appropriate size. These studies require that a series
of rain gauges be deployed and a discharge-measur-32 The Physical Environment