Systems for management of water throughout the developed world have been designed and operated under the assumption of stationarity. Stationarity-the idea that natural systems fluctuate within an unchanging envelope of variability-is a foundational concept that permeates trainingand practice in water-resource engineering. It implies that any variable (e.g., annual streamflow or annual flood peak) has a time-invariant(or 1-year-periodic) probability density function (pdf), whose properties can be estimated from the instrument record. Under stationarity, pdf estimation errors are acknowledged, but have been assumed to be reducible by additional observations, more efficient estimators, or regional or paleohydrologic data. The pdfs, in turn, are used to evaluateand manage risks to water supplies, waterworks, and floodplains; annual global investment in water infrastructure exceeds U.S.$500 billion.

Water managers face tough challenges in sustaining the health
and availability of rivers while meeting increasing demands for
their use. One tool that can give hydro project owners guidance
is a six-step framework for ecologically sustainable water management
developed by The Nature Conservancy.