Tracking raindrops in tropical catchments: Opportunities and challenges

Abstract:

Headwater catchments are the building blocks of tropical drainage basins. While tropical headwaters have been studied intensively since the pioneering work of Mike Bonell and others, only more recently has it been recognized that the mean transit time for water through these catchments can exceed the time scale of hydrologic response by orders of magnitude. Of course, differences between the flow velocities in the system (that control the tracer response) and the celerities (or speed with which perturbations are transmitted, which control the hydrograph) are to be expected since they are controlled by different mechanisms. But such mechanisms in the humid tropics remain poorly understood. Here we present new empirical data from sites in Veracruz (Mexico), Luquillo (Puerto Rico) and the Biosphere-2 Tropical Rainforest Biome that includes the joint collection and use of hydrographs and stable isotope tracers. We use these to make inferences about both velocities and celerities and explore scaling relations and their controls, how forest transpiration affects subsurface mixing and how these responses control the storage and release of water to the stream. We outline the opportunities and challenges in this research area and its link to fundamental issues in tropical ecohydrology.