Holwerda, F., 2005. Water and Energy Budgets of Rain Forests Along an Elevational Gradient Under Maritime Tropical Conditions. PhD Thesis, VU University, Amsterdam, The Netherlands.
Abstract:
From the hydrological point of view, mountains present somewhat of a paradox.
Although they provide the bulk of the Earth’s freshwater resources, knowledge of the
hydrological functioning of mountainous areas is generally much less extensive, reliable,
and precise than that of other, often more easily accessible physiographic regions. Indeed,
mountain regions have been referred to as ‘the blackest of black boxes in the hydrological
cycle’ (Bandyopadhyay et al., 1997). Data collection networks are more difficult to set up
and maintain in complex mountainous terrain, particularly in uninhabited forested
headwater areas without road access, and minimum recommended instrumental densities
are rarely met (Manley and Askew, 1993). Whilst the hydrological knowledge base on
mountains in general has increased considerably in the last few decades, most montane
research work has focused on determining catchment water and sediment outputs and their
distribution in time and space; snow cover and glacier dynamics; or flood frequencies
(Molnar, 1990; Lang and Musy, 1990; Bergmann et al., 1991; Young, 1992; Hofer, 1998),
as opposed to the underlying hydrological processes (cf. Bonell, 1993). Until very recently
(e.g. Motzer, 2003; Schellekens et al., 2004; Goller et al., 2005), the vast majority of this
work dealt with mountains in the temperate zone, with very little pertaining to forested
tropical mountains (see summaries of early research by Bruijnzeel and Proctor (1995) and
Bruijnzeel (2001)). Knowledge of such processes would serve as a basis for increased
understanding of how streamflows emanating from tropical mountains might change as a
result of changes in climate, including the lifting condensation level, frequency and density
of clouds and, by implication, water inputs and evaporative losses (Bruijnzeel, 2001).
The average cloud condensation level on tropical islands can be as low as 600-800
m (Malkus, 1955), although on larger mountains situated further inland this may be closer
to 2,000 m (Stadtmüller, 1987). Above this condensation level, the hydrology of the forest
changes profoundly because of contributions of cloud water (i.e. fog) deposited to the
forest canopy (Bruijnzeel, 2001). There is circumstantial evidence that complete
conversion of these ‘tropical montane cloud forests’ (TMCF) to pasture or vegetable
cropping may have an adverse effect on dry season flows or even on total water yield
because of strongly diminished fog interception after clearing (Ingwersen, 1985; Brown et
al., 1996). Similar effects may be expected when the average cloud condensation level is
raised because of warming of the atmosphere due to global climate change (Still et al.,
1999; Foster, 2001), or clearing of forest at lower elevations (Lawton et al., 2001; Van der
Molen, 2002).