hydrologic budgets

Water and energy budgets of rain forests along an elevation gradient under maritime tropical conditions

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).

Rainfall, Runoff and Elevation Relationships in the Luquillo Mountains of Puerto Rico

Garcia, A.R. Warner, G.S. Scatena, F. and Civco, D.L. 2002. Bisley Rainfall and
Throughfall Rainfall, Runoff and Elevation Relationships in the Luquillo
mountains of Puerto Rico. Caribbean Journal of Science. 2002 (In press).
152
Published as Scientific Contribution No. 1642 of the Storrs Agricultural
Experiment Station.

Abstract: 
Long-terrn rainfall and discharge data from the Luquillo Experimental Forest (LEF) were analysed to develop relationships between rainfall, stream-runoff and elevation. These relationships were then used with a Geographic Information System (GIS) to determine spatially-averaged, mean annual hydrologic budgets for watersheds and forest types within the study area. A significant relationship exists between 1) elevation and mean annual rainfall; 2) elevation and the average number of days per year without rainfall; 3) annual stream runoff and the weighted mean elevation of a watershed; and 4) annual stream runoff and the elevation of the gaging station. A comparison of rainfall patterns between a high and a low elevation station indicated that annual and seasonal variations in rainfall are similiar along the elevational gradient. However, the upper elevation station had greater annual mean rainfall (4436 mm/yr compared to 3524 mn/yr) while the lower station had a greater variation in daily, monthly, and annual totals. Model estimates indicate that a total of 3864 mm/yr (444 hm3) of rainfall falls on the forest in an average year. The Tabonuco, Colorado, Palm, and Dwarf forest types receive an estimated annual rainfall of 3537, 4191, 4167, and 4849 mm/yr, respectively. Of the average annual rainfall input, 65% (2526 mm/yr) is converted to runoff and the remainding 35% (1338 mm/yr) is lost from the system by evapotranspiration and other abstractions. In comparsion to other tropical forests, the LEF as a whole has more evapotranspiration than many tropical montane forests but less than many lowland tropical forests.
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