DEVELOPMENT OF SITE SPECIFIC CLIMATE SCENARIOS FOR RIVER AND SEDIMENT DISCHARGE USING MACROPHYSICAL CLIMATE MODELS: AN EXAMPLE FROM PUERTO RICO

Climate change and land use change are the primary drivers of changes in ecosystem services globally. Global climate models suggest that in the future Puerto Rico and other small islands in the Caribbean will experience changes in rainfall seasonality. It is anticipated that water demands during low-rainfall periods may not be met, and an overall increase in river discharge due to greater amounts of rainfall during wet periods will cause increased damage from flooding and landslides. Land derived sources of sediment and nutrients carried to the marine environment by streams may also increase significantly. Changes in stream flow due to climate change may have profound effects on the downstream production of ecosystem services provided by the marine environment. Global climate models cannot predict the site specific changes in rainfall. However, site specific rainfall data is precisely what is needed in order to assess the potential localized impacts of increased stream flow on reefs and other benthic communities providing essential ecosystem services to island populations. The Bryson Macrophysical Climate Model (MCM) was used to model average monthly temperature, rainfall and its seasonality, and river and sediment discharge for nine USGS gauging stations in Puerto Rico for the past 14,000 years. Temperature and rainfall was calibrated to NOAA HCDN 1961-1990 climate normals. River and sediment discharge was calibrated to USGS gauging station data downloaded from their website. We provide an example from one of our modeled river basins, of how site specific climate scenarios may be developed from MCM output.

Hydrologic models such as SWAT, SEDMOD/RUSLE, or AnnAGNPS use daily rainfall data as model input. We chose the wettest and driest years (highest and lowest rainfall years) from the MCM results for the Guanajibo River near Hormigueros. As these data are monthly averages, we used the ratio of average daily rainfall to total monthly rainfall for the 30 year climate normals to derive daily rainfall datasets for use in hydrologic models. Stream and sediment discharge for the basin was modeled using modern rainfall data and compared to model runs using the derived rainfall data (wet and dry scenarios).