Towards a novel water budget partitioning framework to better characterize the impact of climate and storage change on water fluxes.

The Budyko hypothesis provides a useful framework for comprehending the behaviour of long-term water balance for a natural and closed catchment. It is widely used for partitioning precipitation into other water-cycle components, characterising hydrological response, and assessing long-term water availability.

Since, Budyko framework was developed for natural catchments assuming no storage change, its suitability for studying catchments with human-intervention in a changing climate is debated. This study aims to contribute to this debate by assessing appropriateness of Budyko framework for studying and predicting long-term water cycle changes stemming from climate change and human-driven storage changes. We simulate various climate and anthropogenic scenarios in a closed-loop environment using SWAT (Soil and Water Assessment Tool) model across three climate zones (humid, semi-arid, and arid) for more than 70 years. The scenarios reflect secular changes in climatic variables such as precipitation and temperature, and anthropogenic changes such as change in storage. The long-term time-series data for climatic variables were synthetically generated by obtaining the best-fit probability distribution, which were used to create various scenarios by trend-injection method. The model outputs were used for both steady and unsteady-state conditions to get the Budyko plots and to understand the deviation from the Budyko curve for various climate and storage change scenarios from the reference scenario.

We found that for small changes in precipitation and temperature, catchments translate along the reference Budyko curve but deviates away from the curve for a large climatic change and even a small storage change. The Budyko framework is more sensitive to precipitation change compared to temperature change. For realistic long-term storage change, particularly in arid or semi-arid regions, Budyko points in the traditional framework breach water limit. We observed that when the storage change is considered in the Budyko framework, the points again constrain itself in the Budyko space. Therefore, we developed a generalised Budyko framework by incorporating storage change in a mathematical equation considering water and energy balance. The traditional Budyko framework is a special case within it. The novel generalised Budyko framework proposed here could prove to be an indispensable tool for effective water resources management and studying catchment response to various climate change projections.