L.A. Bruijnzeel

Rainfall intensity–kinetic energy relationships: a critical literature appraisal

Abstract Knowledge of the relationship between rainfall intensity and kinetic energy and its variations in time and space is important for erosion prediction. However, between studies considerable variations exist in the reported shape and coefficients of this relationship. Some differences can be explained by methods of measurement and interpretation and sample size, range and bias, while part of the variability corresponds to actual differences in rainfall generating mechanisms. The present paper critically reviews published studies of rainfall intensity and kinetic energy with a view to derive a general predictive equation of an exponential form. The performance of this general equation is compared to that of existing equations using measured rainfall intensity and kinetic energy data for a site in southeastern Australia. It appeared that the energy of individual storms could only be predicted with limited accuracy because of natural variations in rainfall characteristics. By and large, the general equation produced energy estimates that were within 10% of predictions by a range of parameterisations of the exponential model fitted to specific data-sets. Re-calculation of rainfall erosivity factors as obtained by the older and revised USLE approaches does not seem warranted for most locations. However, in regions experiencing strong oceanic influence or at high elevations, overall rainfall energy appears to be considerably lower than predicted by the general or USLE equations. Conversely, data collected at semi-arid to sub-humid locations suggest that rainfall energy may be higher than expected under those conditions. Standardised measurements are needed to evaluate rainfall intensity–kinetic energy relationships for such areas.

CLIMATIC CONDITIONS AND TROPICAL MONTANE FOREST PRODUCTIVITY: THE FOG HAS NOT LIFTED YET

Tropical montane cloud forests (TMCF) differ from lowland moist forests in structure (low stature, small and tough leaves, low diversity) and functioning (low productivity, low nutrient-cycling rates). To explain these differences, a variety of hypoth- eses have been proposed, most of which are related directly or indirectly to climate, but none of these provides a satisfactory explanation for all typical TMCF traits. The single climatic factor shared by all TMCF, the frequent occurrence of low cloud, has multiple effects, but not all are well understood. In this paper we describe and analyze the climatic and soil-moisture conditions prevailing in TMCF as reported in the literature. TMCF evapo- transpiration is limited by both climatic conditions and canopy conductance. TMCF pro- ductivity is low, but our understanding of these forests carbon balance is incomplete. Leaf photosynthetic capacity is not particularly low, but canopy photosynthesis probably is, due to persistent cloudiness (low radiation) and a low leaf-area index (LAI). We suggest that the low LAI of TMCF is controlled by light climate and by leaf structure and longevity. TMCF productivity is probably further limited by a substantial investment of carbon in the growth and functioning of a relatively large root system, which is itself a consequence of unfavorable soil conditions.

Hydrology and Biogeochemistry of Tropical Montane Cloud Forests: What Do We Really Know?

Arguably, montane “cloud forests” (MCFs) are among the least understood of humid tropical forest ecosystems as far as their water and nutrient dynamics are concerned (Whitmore 1990). This is in spite of the fact that TMCF is often found in important headwater areas that, although scattered, together occupied about 500,000 km2 in the 1970s (Persson 1974). There is a growing recognition of the role of TMCF in supplying water to downstream areas during rainless periods (Zadroga 1981; Hamilton with King 1983; Stadtmuller and Agudelo 1990) and of their high degree of faunal and floristic endemism (La Bastille and Pool 1978; cf. Leo, this volume).

Modelling rainfall interception by vegetation of variable density using an adapted analytical model. Part 1. Model description.

The revised analytical model to predict rainfall interception by sparse canopies (Journal of Hydrology 170 (1995) 79) is further modified to improve the description of evaporation from wet vegetation whose canopy characteristics vary in time (e.g. agricultural crops, deciduous forest, fast-growing plantation forest, effects of storms, pests or logging, etc.). The main adjustments proposed are based on the following assumptions: (1) the canopy capacity is linearly related to leaf area index; (2) the evaporation rate from a saturated canopy can be expressed as an exponential function of leaf area index; and (3) evaporation from stems during the storms may be treated in a similar manner as that from the canopy. The comparative performance of the revised and the presently proposed version of the analytical model in predicting interception by a mixed cropping system in West Java, Indonesia is discussed in a companion paper (Part 2).

Modelling rainfall interception by vegetation of variable density using an adapted analytical model. Part 2. Model validation for a tropical upland mixed cropping system

Abstract To improve the description of rainfall partitioning by a vegetation canopy that changes in time a number of adaptations to the revised analytical model for rainfall interception by sparse canopies [J. Hydrol., 170 (1995) 79] was proposed in the first of two papers. The current paper presents an application of this adapted analytical model to simulate throughfall, stemflow and interception as measured in a mixed agricultural cropping system involving cassava, maize and rice during two seasons of growth and serial harvesting in upland West Java, Indonesia. Measured interception losses were 18 and 8% during the two measuring periods, while stemflow fractions were estimated at 2 and 4%, respectively. The main reasons for these discrepancies were differences in vegetation density and composition, as well as differences in the exposure of the two sites used in the two respective years. Functions describing the development of the leaf area index of each of the component crops in time were developed. Leaf area index (ranging between 0.7 and 3.8) was related to canopy cover fraction (0.41–0.94). Using average values and time series of the respective parameters, interception losses were modelled using both the revised analytical model and the presently adapted version. The results indicate that the proposed model adaptations substantially improve the performance of the analytical model and provide a more solid base for parameterisation of the analytical model in vegetation of variable density.

Tropical montane cloud forests : science for conservation and management

Part I. General Perspectives Part II. Regional Floristic and Animal Diversity Part III. Hydrometeorology of Tropical Montane Cloud Forest Part IV. Nutrient Dynamics in Tropical Montane Cloud Forests Part V. Cloud Forest Water Use, Photosynthesis, and Effects of Forest Conversion Part VI. Effects of Climate Variability and Climate Change Part VII. Cloud Forest Conservation, Restoration and Management Issues.

Evaporation from a tropical rain forest, Luquillo Experimental Forest, eastern Puerto Rico

Evaporation losses from a watertight 6.34 ha rain forest catchment under wet maritime tropical conditions in the Luquillo Experimental Forest, Puerto Rico, were determined using complementary hydrological and micrometeorological techniques during 1996 and 1997. At 6.6 mm d -1 for 1996 and 6.0 mm d -1 for 1997, the average evapotranspiration (ET) of the forest is exceptionally high. Rainfall interception (Ei), as evaluated from weekly throughfall measurements and an average stemflow fraction of 2.3%, accounted for much (62-74%) of the ET at 4.9 mm d -1 in 1996 and 3.7 mm d -1 in 1997. Average transpiration rates (Et) according to a combination of the temperature fluctuation method and the Penman-Monteith equation were modest at 2.2 mm d -1 and 2.4 mm d -1 in 1996 and 1997, respectively. Both estimates compared reasonably well with the water-budget-based estimates (ET - Ei) of 1.7 mm d -1 and 2.2 mm d -1. Inferred rates of wet canopy evaporation were roughly 4 to 5 times those predicted by the Penman- Monteith equation, with nighttime rates very similar to daytime rates, suggesting radiant energy is not the dominant controlling factor. A combination of advected energy from the nearby Atlantic Ocean, low aerodynamic resistance, plus frequent low-intensity rain is thought to be the most likely explanation of the observed discrepancy between measured and estimated Ei.

Spatial heterogeneity of element and litter turnover in a Bornean rain forest.

The spatial heterogeneity of element fluxes was quantified by meas- uring litterfall, throughfall and litter decomposition for 1 y in 30 randomly located sampling areas in a lowland dipterocarp rain forest. The idea tested was that turnover of elements is more variable than turnover of dry matter in a forest with extremely high tree species diversity. In spite of the low fertility of the soil (an ultisol), total litter production (leaves, trash, and wood <2 cm in diameter) was high (1105 g m -2 y -1 ) with inputs to the forest floor of carbon, nitrogen, phos- phorus, calcium, magnesium, potassium, manganese and iron of 550, 15.3, 0.47, 6.26, 2.49, 4.75, 0.95 and 0.14 g m -2 y -1 respectively. Throughfall was 81% of the annual rainfall and transferred 22.2, 1.37, 0.14, 1.07, 0.67, 0.39, 7.92, <0.06, and <0.06 g m -2 y -1 of organic carbon, nitrogen (all forms), phosphorus, sulphur, cal- cium, magnesium, potassium, manganese and iron, respectively. Average turnover rates of nutrients in litter were highest for potassium and decreased in the sequence calcium, magnesium, carbon, nitrogen and phosphorus. Concentrations of phosphorus, nitrogen and potassium in litterfall, litter mass and topsoil were closely correlated with each other. Concentrations of calcium and manganese were positively correlated with each other and with elevation. Variations in leaf chem- istry and total litterfall caused the spatial heterogeneity of element input to the forest floor to have a coefficient of variation of 30 - 70%, depending on the ele- ment. Due to the strong positive correlation between element fluxes and pools, the spatial variability of turnover rates (CV c. 20%) was lower than that of element input. Turnover rates for K varied by a factor of 4, and for Ca by a factor of 2.8 when the different sites were compared. The results strongly suggest that the most important factor determining spatial heterogeneity of organic matter and element dynamics on the forest floor is the site-specific amount of leaf fall, rather than spatially variable decomposition rates. 1 Corresponding author.

The hydrological and soil impacts of forestation in the tropics

In response to the continuing degradation and disappearance of the world’s tropical forests (Drigo, this volume) the establishment of plantation forest on degraded and previously forested sites as well as into (sub)tropical grasslands is becoming increasingly common (Evans, 1999). The hydrological effects of this practice and the potential of forestation to improve or restore the hydrological behaviour of degraded catchments constitute the prime focus of this chapter, expanding and updating an earlier review of the subject by Bruijnzeel (1997). Three aspects are highlighted in particular, namely: (i) the effects of tree plantations on annual and seasonal streamflow totals; (ii) the associated impacts on stormflow and sediment production; and (iii) concurrent changes in soil chemical characteristics (fertility). Because the hydrological changes associated with forest clearing and the establishment of a new vegetation cover during the first few years are discussed at length in the chapter by Grip, Fritsch and Bruijnzeel, much of what follows pertains to the post-canopy closure phase of plantations.

The biogeochemistry of phosphorus after the first century of soil development on Rakata Island, Krakatau, Indonesia

This study examined the accumulation of organic carbon (C) and fractions ofsoil phosphorus (P) in soils developing in volcanic ash deposited in the1883 eruption of Krakatau. Organic C has accumulated at rates of 45 to 127g/m2/yr during 110 years of soil development, resulting inprofiles with as much as 14 kgC/m2. Most soil P is found inthe HCl-extractable forms, representing apatite. A loss of HCl-extractableP from the surface horizons is associated with a marked accumulation ofNaOH-extractable organic P bound to Al. A bioassay with hill rice suggeststhat P is limiting to plant growth in these soils, perhaps as a result ofthe rapid accumulation of P in organic forms.