Hydrological research is expensive and time consuming. If Payments for Environmental Services (PES) systems are really to integrate forest conservation and sustainable development in the world's poorest and most biodiverse watersheds, the forestry research community needs to find a quick and easy way of undertaking a sufficient level of hydrological analysis that while providing data of sufficient accuracy to guide the development of PES systems, can be collected cheaply and rapidly by local institutions. Currently, most PES schemes are based on the untested assumption of a direct relationship between forest cover and water.