evapotranspiration

Modelling the impact of recent land-cover changes on the stream flows in northeastern Puerto Rico

Wu W, Hall CAS, Scatena FN. Modelling the impact of recent
land-cover changes on the stream flows in northeastern Puerto
Rico. Hydrol Process 2007; 21: 2944-2956.

Abstract: 
We investigated the influence of recent and future land-cover changes on stream flow of a watershed northeastern Puerto Rico using hydrological modeling and simulation analysis. Monthly and average annual stream flows were compared between an agricultural period (1973–1980) and an urbanized/reforested period (1988–1995) using the revised Generalized Watershed Loading Function model. Our validated results show that a smaller proportion of rainfall became stream flows in the urbanized/forested period compared with the agricultural period, apparently because of reforestation. Sensitivity analysis of the model showed that evapotranspiration, precipitation, and curve number were the most significant factors influencing stream flow. Simulations of projected land-cover scenarios indicate that annual stream flows would increase by 9Ð6% in a total urbanization scenario, decrease by 3Ð6% in a total reforestation scenario, and decrease by 1Ð1% if both reforestation and urbanization continue at their current rates to 2020. An imposed hurricane event that was similar in scale to the largest recent event on the three land-cover scenarios would increase the daily stream flow by 62Ð1%, 68Ð4% and 67Ð1% respectively. Owing to the environmental setting of eastern Puerto Rico, where sea breezes caused by temperature differences between land surface and the ocean dominate the local climate, we suggest that managing local land-cover changes can have important consequences for water management. Copyright  2007 John Wiley & Sons, Ltd.

Spatial modelling of evapotranspiration in the Luquillo experimental forest of Puerto Rico using remotely-sensed data

Wu, Wei; Hall, Charles A.S.; Scatena, Frederick N.; Quackenbush, Lindi J. 2006. Spatial modelling of evapotranspiration in the Luquillo experimental forest of Puerto Rico using remotely-sensed data.. Journal of Hydrology 328, 733- 752.

Abstract: 
Actual evapotranspiration (aET) and related processes in tropical forests can explain 70% of the lateral global energy transport through latent heat, and therefore are very important in the redistribution of water on the Earth’s surface [Mauser, M., Scha¨dlich, S., 1998. Modelling the spatial distribution of evapotranspiration on different scales using remote sensing data. J. Hydrol. 212–213, 250–267]. Unfortunately, there are few spatial studies of these processes in tropical forests. This research integrates one Landsat Thematic Mapper (TM) image and three Moderate Resolution Imaging Spectroradiometer (MODIS) images with a hydrological model [Granger, R.J., Gray, D.M., 1989. Evaporation from natural nonsaturated surfaces. J. Hydrol. 111, 21–29] to estimate the spatial pattern of aET over the Luquillo Experimental Forest (LEF) – a tropical forest in northeastern Puerto Rico – for the month of January, the only month that these remotely sensed images were acquired. The derived aETs ranged from 0 to 7.22 mm/day with a mean of 3.08 ± 1.35 mm/day which were comparable to other estimates. Simulated aET was highest in the low elevation forest and decreased progressively toward higher elevations. Because of differences in solar radiation at different elevations, aspects and topographic positions, aET tended to be higher on south slopes and along ridges than on north slopes and in valleys. In addition, the Bowen ratio (the ratio of sensible heat to latent heat) varied across different vegetation types and increased with elevation, thus reflecting differences in the distribution of net solar radiation incident on the earth’s surface. Over a day, the highest simulated aET occurred at around noon. We also applied this model to simulate the average monthly aET over an entire year based on the cloud patterns derived from at least two MODIS images for each month. The highest simulated aET occurred in February and March and the lowest in May. These observations are consistent with long term data. The simulated values were compared with field measurements of the sap flow velocity of trees at different elevations and in different forest types. These comparisons had good agreement in the low elevation forest but only moderate agreement in the elfin forest at high elevations. ª 2006 Elsevier B.V. All rights reserved.

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

Schellekens, J., L. A. Bruijnzeel, F. N. Scatena, N. J. Bink, and F. Holwerda (2000), Evaporation from a tropical rain forest, Luquillo Experimental Forest, eastern Puerto Rico, Water Resour. Res., 36(8), 2183–2196, doi:10.1029/2000WR900074.

Abstract: 
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.

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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