A simple framework for relating variations in runoff to variations in climatic conditions and catchment properties

We use the Budyko framework to calculate catchment‐scale evapotranspiration ( E ) and runoff ( Q ) as a function of two climatic factors, precipitation ( P ) and evaporative demand ( E o = 0.75 times the pan evaporation rate), and a third parameter that encodes the catchment properties ( n ) and modifies how P is partitioned between E and Q . This simple theory accurately predicted the long‐term evapotranspiration ( E ) and runoff ( Q ) for the Murray‐Darling Basin (MDB) in southeast Australia. We extend the theory by developing a simple and novel analytical expression for the effects on E and Q of small perturbations in P , E o , and n . The theory predicts that a 10% change in P , with all else constant, would result in a 26% change in Q in the MDB. Future climate scenarios (2070–2099) derived using Intergovernmental Panel on Climate Change AR4 climate model output highlight the diversity of projections for P (±30%) with a correspondingly large range in projections for Q (±80%) in the MDB. We conclude with a qualitative description about the impact of changes in catchment properties on water availability and focus on the interaction between vegetation change, increasing atmospheric [CO 2 ], and fire frequency. We conclude that the modern version of the Budyko framework is a useful tool for making simple and transparent estimates of changes in water availability.