A New Hydrologic Sensitivity Framework for Unsteady‐State Responses to Climate Change and Its Application to Catchments With Croplands in Illinois

Climate change has been concerned as a trigger of hydrologic changes around the world. Budyko framework was widely used in hydrologic sensitivity analyses on the steady‐state water balance. The unsteady‐state responses, for example the inter‐annual hydrologic sensitivity, were not well investigated, especially for catchments influenced by croplands as that exist in Illinois in the United States. We propose a new hydrologic sensitivity framework for the unsteady‐state condition, together with a new hydrological model, to investigate the impact of croplands on the hydrologic sensitivity. In addition to the Budyko‐type relationship between the annual evapotranspiration ratio (E/P) and the aridity index (Ep/P), this framework incorporates the empirical relationship between the annual storage change ratio (ΔS/P) and Ep/P into sensitivity coefficients. The required data of annual E/P and ΔS/P could be obtained from observation and hydrological models. In particular, the bcd‐αK model is newly developed from the existing bcd model in considering croplands in Illinois, which includes only five parameters while capturing major hydrological processes for catchments with croplands. Results show that the annual E/P, ΔS/P and Q/P in 12 studied catchments have significant linear relationships with Ep/P. The annual runoff is much more sensitive to the change in precipitation than to the change in potential evapotranspiration. The traditional steady‐state analysis overestimates the sensitivity of the evapotranspiration to the change in the precipitation for the unsteady‐state condition and underestimates the variability of sensitivity coefficients among catchments. Plain Language Summary With the changing climate, frequent disasters of floods and droughts, shortage of water resources, and water quality deterioration, the importance of studying water cycle is increasingly recognized. Moreover, human activities have seriously changed the processes of hydrological cycle. Therefore, it makes no sense to only study the natural water cycle in which intensive and extensive human activities occur, such as agricultural development. In this study, we develop a new method for hydrologic sensitivity analysis on catchments bearing the changes in climate and land use. Case studies are undertaken for 12 catchments in Illinois, USA. Results show that the annual runoff is much more sensitive to the change in the precipitation than to the change in the potential evapotranspiration. Key Poin