Do changes in global biomass have a detectable impact on the water balance?

Recent studies have shown a wide range of decadal scale global biomass changes, with many boreal forests increasing in biomass, and tropical areas continuing to decline. A critical question that follows from this is whether or not these changes are having a measurable impact on the water balance. Here, we use a Budyko approach in combination with global datasets of precipitation (P), potential evaporation (PET), discharge (Q), and actual evapotranspiration (ET) to assess whether detectable trends in surface runoff exist, and whether this is consistent with the availability predicted by the Budyko water and energy balance. These trends are further complicated by the scaling of hydrological change with climate (PET/P), and the wide variation in climate trends, particularly P. Nonetheless, the detection of hydrological responses to vegetation change at the global scale is a large gap in our understanding of human impacts on the hydrological cycle, and this work seeks to clarify when and where such impacts may be present. This has important implications for future water resource availability, since biomass (or land cover) changes are not considered to the same extent as the potential impact of greenhouse gas emissions, and yet vegetation changes may have a far more direct and influential role in the hydrological cycle.