Whither hydrology? Looking back to move ahead

Power laws pervade hydrology, partly because of the field’s historical ties to engineering and partly because many natural systems exhibit fractal organization across scales. In 1991, the National Research Council’s Opportunities in the Hydrologic Sciences (“the Eagleson Report”) called for renewed emphasis on hydrologic theory, especially in the areas of scaling and ecological optimality. At the center of this challenge lies the water balance, one of the most fundamental and difficult problems in hydrology. Solving the water balance at subannual time scales requires understanding the coupled physical dynamics of catchments and plant–soil–atmosphere interactions. At longer time scales, the challenge shifts to explaining storage trends and evapotranspiration fluxes in terms of climatic drivers such as precipitation and potential evapotranspiration, while also accounting for ecological controls. This presentation argues that percolation theory is centrally relevant to this “central problem of hydrology.” Its relevance is examined through the lenses of scaling, the two-way interaction between water and life, plant diversity, ecological optimality, and the thermodynamic constraints that shape living systems.