Landscape Configuration and Composition Modulate Freshwater Supply and Flooding Risk of Tropical Watersheds

Abstract:

Impact of changes in land cover and land use (LCLUC) on hydrological service of tropical watersheds is important in both hydrology and LCLUC. Land fragmentation is an important feature of LCLUC, while its impact on hydrological service of tropical watershed is unclear despite a few theoretical frameworks. We described a simulation study of 8 tropical watersheds in Puerto Rico using Soil Water Assessment Tool. Annual stream discharge is computed based on simulations with LC maps of 1977, 1991, and 2000. Annual big stream discharge with risks of flooding and severe soil erosion is computed as sum of daily discharge greater than 95 percentile. Average annual discharge and big discharge are analyzed against changes in LC composition and fragmentation. Most of the mountainous watersheds are characterized by reforestation in 1977-1991 and slightly deforestation in 1991-2000. Forest cover is strongly correlated with pasture cover (r = -0.88), and forest fragmentation is significantly correlated with covers of forest (-0.97), pasture (0.94), and urban (0.95). Hence reforestation in the central mountains is mostly from pasture and to a less extent from agriculture. Forest fragmentation is reduced by reforestation but increased by deforestation for pasture, urban, and agriculture. The average annual discharge is significantly increased by agriculture but reduced by forest cover for the simulations with 1977 and 1991 maps. For the simulations with 1991 and 2000 maps, the average annual discharge and the average annual big discharge are significantly increased by forest fragmentation, and the former also reduced by fragmentation of pasture and agriculture. Land cover change plays more important roles in regulating the big discharges than altering the annual discharges. Considering the strong negative correlation between forest cover and fragmentation, we found the decreased forest fragmentation accompanied with reforestation augments the impact of reforestation on lessening freshwater supply and delivery. On the other hand, the decreased fragmentation with reforestation can help reduce the big stream flows to lower flooding risks and severe erosion. Our findings about forest fragmentation support the hypothesis that land fragmentation may enhance resources delivery and supply.