Changes in terrestrial ecosystem have cascading impacts on aquatic systems and the ecosystem services they provide. Ecosystems regulate the quantity, location, timing, and quality of water through a suite of ecosystem processes including rainfall interception, evapotranspiration, and soil processes that affect groundwater infiltration and flow. The mechanisms by which ecosystems regulate water are reasonably well understood. However, predicting the impact of ecosystem change on water regulation remains difficult because the relative dominance of the different processes, mediated by climate, geography, and ecosystem management, are not yet well quantified. Moreover, while land management decisions that affect aquatic systems are often made at local scales, there is growing demand for information about the status, trends, and potential for hydrologic service regulation by terrestrial ecosystems at the global scale.
A large and growing body of experimental studies of the water impacts of land use change clearly demonstrate that at small scales ecosystem change affects water distribution. Attempts to synthesize data for large watersheds and watersheds with mixed land cover show small effects, however. Based on work being done for the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) and the Group on Earth Observations Biodiversity Observation Network (GEOBON), I evaluate how effectively existing global data sets of hydrologic functions such as evapotranspiration can be used to evaluate hydrologic ecosystem services globally as well as locally.