Instructive Surprises in the Hydrological Functioning of Landscapes

Landscapes receive water from precipitation and then transport, store, mix, and release it, both downward to streams and upward to vegetation. How they do this shapes floods, droughts, biogeochemical cycles, contaminant transport, and the health of terrestrial and aquatic ecosystems. Because many of the key processes occur invisibly in the subsurface, our conceptualization of them has often relied heavily on physical intuition. In recent decades, however, much of this intuition has been overthrown by field observations and emerging measurement methods, particularly involving isotopic tracers. Here we summarize key surprises that have transformed our understanding of hydrological processes at the scale of hillslopes and drainage basins. These surprises have forced a shift in perspective from process conceptualizations that are relatively static, homogeneous, linear, and stationary to ones that are predominantly dynamic, heterogeneous, nonlinear, and nonstationary. ▪ Surprising observations and novel measurements are transforming our understanding of the hydrological functioning of landscapes. ▪ Even during storm peaks, streamflow is composed mostly of water that has been stored in the landscape for weeks, months, or years. ▪ Streamflow and tree water uptake often originate from different subsurface storages and from different seasons’ precipitation. ▪ Stream networks dynamically extend and retract as the landscape wets and dries, and many stream reaches lose flow into underlying aquifers. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.