A predictive understanding of the response of catchment water balances to global change is becoming increasingly important for ensuring that water supplies are sustained under an uncertain future. The North American network of experimental headwater catchments is unique in that it provides the longest existing paired records of climate and hydrology, providing an opportunity to explore the effects of climate on water yields. The Budyko framework describes the theoretical energy and water limits on the catchment water balance and illustrates how precipitation (P) is partitioned into evapotranspiration (ET) and discharge (Q). The theoretical Budyko curve provides a “business as usual” baseline for the water balance; if we assume it depicts the expected partitioning of P into ET and Q we can begin to account for the reasons why sites depart from the baseline.
We define responsivity as the degree to which P and Q are synchronized, measured by the maximum interannual range in the Evaporative Index or AET/P (the dependent variable of the Budyko curve) during pre-disturbance conditions. We define elasticity as the ratio of the maximum interannual range in the Dryness Index or PET/P (the independent variable of the Budyko curve) to responsivity during pre-disturbance conditions. We ask the question: do responsivity and elasticity provide insights into the sustainability of catchment water yields under changing environmental conditions.
Results/Conclusions
We found that year-to-year deviations from the Budyko curve helped in the identification of sensitive catchments that are undergoing fundamental changes in response to global change. For example, catchments with low responsivity (i.e., < 0.5) stayed on the theoretical curve, while catchments with high responsivity (i.e., > 0.5) underwent substantial changes in the partitioning of water from higher ET (low Q) to lower ET (high Q) or vice versa in response to shifts in climatic conditions. Furthermore, catchments with low elasticity (e.g., < 1.0) took several decades or longer for water balances to return to the Budyko curve following a major disturbance (e.g., clear cut) while catchments with high elasticity (i.e., > 1.0) showed no effect or took minimal time for water balances to return to the Budyko curve.
Our deconstruction of the Budyko framework provided important insights into the nature and causes of year-to-year variability and the sensitivity of catchment hydrological functioning to environmental change. Future work will focus on how forecasted environmental changes may impact catchment water yields.