Abstract
Impact of changes in land cover and land use on hydrological service of tropical watersheds is one of the focal research tropics in both hydrology and Land Cover Land Use Changes (LCLUC). Land fragmentation is an important feature of LCLUC, however, its impact on hydrological service of tropical watershed is unclear despite a few theoretical frameworks. In this paper, we described a simulation study of eight tropical watersheds in Puerto Rico using the Soil Water Assessment Tool. Annual average stream discharge was derived according to the simulations with the land cover maps in 1977, 1991, and 2000. Annual big stream discharge with risks of flooding and severe soil erosion was computed as the sum of daily discharge greater than 95th percentile. The impacts of changes in land cover and fragmentation represented by perimeter-to-area ratio of land patches on annual average and big discharges were further analyzed by means of the linear mixed-effects model. Most mountainous watersheds were characterized by reforestation in 1977–1991 but slight deforestation in 1991–2000. Forest perimeter-to-area ratio was significantly correlated with covers of forest (correlation coefficient of -0.97), pasture (0.94), and urban (0.95). Thus forest fragmentation was reduced by reforestation but increased by deforestation. The annual average and big discharges were significantly reduced by forest cover and forest perimeter-to-area ratio. The enhanced edge effect by forest fragmentation may have incurred more effective interception of the subsurface flow by forest root system, and promoted forest transpiration, thus reduced stream flows. Land cover change plays more important roles in regulating the big discharges than altering the annual average discharges. Due to the negative correlation between forest cover and fragmentation, the decreased forest fragmentation accompanied with reforestation offsets the impact of reforestation on lessening freshwater supply and flooding risk.
Highlights
Tropical and subtropical watersheds host the ecosystems with great carbon storage and carbon sequestration ability to offset the Green House Gas emission [1], and provide hydrological services for dense population [2]
The Rio Culebrinas watershed had the strongest reforestation with forest cover changing from 7% in 1977 to 41% in 1991
The average forest cover of all watersheds increased from 45% in 1977 to 58% in 1991 and slightly decreased to 56% in 2000
Summary
Tropical and subtropical watersheds host the ecosystems with great carbon storage and carbon sequestration ability to offset the Green House Gas emission [1], and provide hydrological services for dense population [2]. Recent projection of global warming and drying casts many concerns on the adaptation of tropical ecosystems and the sustainability of ecosystem services [3, 4]. The drying trend is projected for this century as one of the strongest in the globe [6] characterized by 0.7–4 ̊C increase in temperature, 25–50% reduction in rainfall [7], and increased chaotic extremes such as droughts and storms. The drying trend in the Caribbean threatens the tropical vegetation and the hydrological service to the dense population [8]. A moderate drought can lead to insufficient water supply [9]
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