FOREST REGENERATION AND STREAMFLOW IN PUERTO RICO: A MULTIPLE-CATCHMENT ANALYSIS

Little is known of the effect on streamflow (Q) of regrowing tropical forest. Puerto Rico has experienced widespread abandonment of pasture and croplands, followed by natural forest regrowth since the 1950s. This paper examines the impacts of forest regrowth on several streamflow metrics for 12 meso-scale catchments (23-346 km²; mean annual rainfall 1720-3422 mm) in Puerto Rico with long (33-51 yr) and simultaneous records for Q, precipitation (P), potential evapotranspiration (PET), and land cover. A simple spatially-lumped, conceptual rainfall-runoff model using daily P and PET time series as inputs (HBV-light) was used to simulate Q for each catchment. Annual time series of observed and simulated values of nine Q metrics were calculated; four metrics related to hydrological processes and five to the annual frequency distribution of Q. A least-squares trend was fitted through annual time series of the residual difference between observed and simulated time series of each metric. From this the total change  was calculated, representing the change in each metric after controlling for climate variability and carry-over storage effects between individual years. Negative values of  were found for most catchments and Q metrics, suggesting enhanced actual evapotranspiration following forest regrowth. However, correlations between changes in urban or forest area and values of  were insignificant (p ≥ 0.091) for all catchments and Q metrics. This suggests there is no convincing evidence that changes in the chosen Q metrics in these Puerto Rican catchments can be ascribed to changes in urban or forest area. These results are in line with previous studies of meso- and large-scale (sub-)tropical catchments, which generally found no significant change in Q that can be attributed to changes in forest cover. Possible explanations for the apparent lack of a clear signal include: errors in land-cover, climate, and Q data; changes in forest area occurring mainly in the less rainy lowlands; and heterogeneity in the hydrological response of individual catchments. Different results were obtained for different catchments, and using a smaller subset of catchments could have led to different conclusions. These findings highlight the importance of including multiple catchments in land-cover impact analysis at the landscape scale.