The physics and the biology of the water balance: A personal journey through the critical zone into the water balance

The essay describes how a combination of scaling theory from percolation, that relates pore scale flow and transport through catchment scales to global scales (bottom‐up), as well as water fluxes to soil formation and vegetation growth, can be used to support an accurate ecological optimization that (top‐down): solves the central problem of hydrology, that is., “the water balance,” and generates critically important derived quantities, namely streamflow response to climate change, net primary productivity, and plant species richness. Moreover, the essay describes how this particular theoretical approach came to be designed and how it, in retrospect, fits in with the vision of the Committee on Opportunities in the Hydrologic Sciences which met 34 years ago to formulate a research, teaching, and infrastructure guide for the community, and “rebrand our science as a geoscience.” Finally, it demonstrates how the research satisfies the present desires of the community to unite Darwinian and Newtonian scientific methods in the solution of this central problem and how it relates to present research directions in the fields of hydrologic sciences and ecology. The essay addresses a unification of concepts and results around the central problem of hydrology, the water balance, from quantifying the dominant flow and transport paths using percolation theory and optimization of these fluxes to maximize net primary productivity. The results are discussed in relation to the position of the water balance in hydrology, to the history of the author, and the structure of water balance research.