Global estimates of the storage and transit time of water through vegetation

Abstract

The time it takes for water to transit from the ground back to the atmosphere affects weather, climate, biogeochemistry and ecosystem function. The transit time of water through vegetation, defined as the age of water transpiring from vegetation since time of entry, is a particularly understudied aspect of the terrestrial hydrologic cycle. Here we use a synergy of satellite remote sensing measurements over a five-year period to estimate global aboveground vegetation water storage to be on average 484 km3, roughly half of which is stored in Earth’s water-limited savannah, grassland and shrubland ecosystems. We then combine these storage estimates with remotely sensed data for transpiration and find that mean transit times of water through aboveground vegetation vary from ~5 days in croplands to ~18 days in evergreen needleleaf forests, with a global median of 8.1 days. In herbaceous-dominated land-cover types with comparatively low water storage and high seasonal water use, such as grasslands, the water stored in biomass may be frequently transiting in less than one day. Our estimates contribute to resolving the role of vegetation in the terrestrial hydrologic cycle; plants store little water compared to other pools, and the time it takes to return that water to the atmosphere is among the fastest components of the hydrologic cycle. Using satellite data, this study presents global estimates of transit times of water through vegetation across ecosystems, highlighting the dynamic role of plants in the hydrologic cycle.