Evaporation from Pinus caribaea plantations on former grassland soils under maritime tropical conditions

Abstract

Abstract. Wet canopy and dry canopy evaporation from young and mature plantations of Pinus caribaea on former grassland soils under maritime tropical conditions in southwestern Viti Levu, Fiji, were determined using micrometeorological and hydrological techniques. Modeled annual evaporation totals (ET) of 1926 and 1717 mm were derived for the 6-and the 15-year-old stands, respectively. Transpiration made up 72% and 70% of annual ET, and modeled rainfall interception by the trees and litter layer was 20-22% and 8-9% in the young and the mature stands respectively. Monthly ET was related to forest leaf area index and was much higher than that for the kind of tall fire-climax Pennisetum polystachyon grassland replaced by the forests. Grassland reforestation resulted in a maximum decrease in annual water yield of 1180 mm on a plot basis, although it is argued that a reduction of (at least) 500-700 mm would be more realistic at the catchment scale. The impact of reforesting grassland on the water resources in southwest Viti Levu is enhanced by its location in a maritime, seasonal climate in the outer tropics, which favors a larger difference between annual forest and grassland evaporation totals than do equatorial regions. IntroductionOut of concern about the potentially adverse effects on soils, hydrology, and climate of the continuing conversion of the world's tropical rain forests to other, mostly agricultural, land uses, considerable effort has gone into the evaluation and prediction of such effects during the last two decades [Lal, 1987;Bruijnzeel, 1990Bruijnzeel, , 1996Bruijnzeel, , 1998Fritsch, 1992 [Bruijnzeel, 1990], the associated range reported for the initial increase in streamflow after total forest clearance is quite large (125-820 mm yr-•). This variation can be explained only partially by differences in rainfall between locations or years. Other factors affecting postclearing rates of growth and water uptake (e.g., soil fertility status, elevation), runoff disposal (catchment steepness, soil depth), or both (notably the degree of surface disturbance by machinery or fire) play a role as well [Bruijnzeel, 1996[Bruijnzeel, , 1998]. In addition, Shuttleworth [1989] suggested that compared to midcontinental sites, tropical deforestation is likely to have the most effect on river flow (though not necessarily on climate) at continental edge and island locations. In the absence of exper-

Modelling rainfall interception by a lowland tropical rain forest in northeastern Puerto Rico

“…Under more maritime tropical conditions, regression-based estimates of E w = R tend to be (much) higher than those based on the Penman-Monteith equation (cf. Bruijnzeel and Wiersum, 1987;Dykes, 1997;Waterloo et al, 1999). However, caution is needed when comparing regression-based values of E w = R and Penman-Monteith based values of E w : The cited maritime tropical studies all used mean rather than median rainfall intensities to represent R: In the present case, the use of an average rainfall intensity would yield an apparent E w of 1.52 vs. 0.94 mm h Ϫ1 for the median rainfall intensity.…”

“…Values of E w derived in this way generally tend to be (much) higher than those calculated with Eqs. (4) and (5) (Gash et al, 1980;Bruijnzeel and Wiersum, 1987;Waterloo et al, 1999).…”

“…; Bruijnzeel and Wiersum, 1987;Dykes, 1997;Waterloo et al, 1999) have also been inferred from rainfall and TF measurements at other maritime tropical locations but much lower values (typically about 0.2 mm h Ϫ1 or less; Hutjes et al, 1990;Asdak et al, 1998;Waterloo et al, 1999) are usually calculated from above-canopy climatic measurements using Eq. (4).…”

Negative trade-off between changes in vegetation water use and infiltration recovery after reforesting degraded pasture land in the Nepalese Lesser Himalaya

“…Further, a substantial fraction of the rainfall in the pine forest runs off as overland flow (see Results below), reducing E s even further. Waterloo et al (1999) determined E s in a similarly stocked stand of Pinus caribaea in Fiji at 9 % of the Penman open water evaporation, E 0 . Applying the same fraction and taking again an effective period of 4 rainy months yielded an estimated E s for the Dhulikhel pine forest of ca.…”

“…In other words, no positive effects of reforestation on soil water intake capacity could manifest and the observed reductions in water yield simply reflected the higher water use of the trees compared to the grasses and scrubs they replaced (cf. Trimble et al, 1987;Waterloo et al, 1999;Scott and Prinsloo, 2008).…”