Ecohydrologic separation of water between trees and streams in a Mediterranean climate

Water movement in upland humid watersheds from the soil surface to the stream is often described using the concept of translatory flow1,2, which assumes that water entering the soil as precipitation displaces the water that was present previously, pushing it deeper into the soil and eventually into the stream2. Within this framework, water at any soil depth is well mixed and plants extract the same water that eventually enters the stream. Here we present water-isotope data from various pools throughout a small watershed in the Cascade Mountains, Oregon, USA. Our data imply that a pool of tightly bound water that is retained in the soil and used by trees does not participate in translatory flow, mix with mobile water or enter the stream. Instead, water from initial rainfall events after rainless summers is locked into small pores with low matric potential until transpiration empties these pores during following dry summers. Winter rainfall does not displace this tightly bound water. As transpiration and stormflow are out of phase in the Mediterranean climate of our study site, two separate sets of water bodies with different isotopic characteristics exist in trees and streams. We conclude that complete mixing of water within the soil cannot be assumed for similar hydroclimatic regimes as has been done in the past3,4. Links between plant water-use (transpiration) and hydrology have been examined quantitatively since the paired-watershed studies in 1921 (ref. 5). These watershed-scale experiments clearly demonstrated links between vegetation and streamflow. However, the paired-watershed approach can only infer the mechanisms behind these vegetation–streamflow interactions6–8. Central to these inferred mechanisms is translatory flow downslope to the stream, and mixing of water within the soil profile1,2. Complete mixing of water in the subsurface is the central tenant of most watershed hydrology models today9,10. These concepts influenced ecology, leading to the idea that roots take up water from the same pool that is moving to the stream. However, is this really so? Using stable isotopes, Dawson and Ehleringer11 demonstrated complex interactions between plant water and hydrological pools, showing that some streamside trees used deeper groundwater instead of streamwater. Nevertheless, diel fluctuations in baseflow at watersheds around the world demonstrate clear interactions between transpiration and streamflow12. Here, we directly explore links between hydrology and transpiration at the small watershed scale in a seasonally dry climate. Our central questions were: to what extent do trees and streams return the same water pool to the hydrosphere and how does this vary spatially within a watershed? These questions are fundamental to testing watershed hydrology models3,13 and coupled ecology– biogeochemical–hydrologymodels, which assume completemixing of water moving through the soil towards the stream. Little if any