Abstract: The causes of runoff changes in the upper reaches of Hutuo River were analyzed to clarify the driving force of runoff evolution and furthermore to promote the sustainable utilization of water resources.Based on the one-parameter Budyko-type equation,the influencing factors of runoff are divided into two categories:aridity index and catchment properties.First,the time-varying Budyko-type equations were derived to describe the temporal variations in catchment properties via the time-varying parameters in the Budyko-type equations.In subsequence,the long-term trends in climatic variables were modeled using time as an explanatory variable.Finally,the impacts of aridity index and catchment properties on runoff were decomposed at annual scale and multi-year scale.The research results demonstrated that:(1) Time-varying Budyko-type equations improve remarkably the fitting capability for the upper reaches of Hutuo River basin.(2) In terms of quantifying the impacts between two periods of multiple years,the results based on the time-varying Budyko-type equations are similar to the Budyko-type equations with fixed parameter,implying that the time-varying Budyko-type equations proposed in this paper is reliable.(3) The contribution rate of aridity index decreased from 86.79% to 71.01%,while the contribution rate of catchment properties increased from 13.21% to 28.09%.Therefore,aridity index is the dominant factor of runoff reduction in the upper reaches of Hutuo River basin.(4) The apparent weakening of the influence of aridity index on annual runoff constitutes the major cause of increment in the contribution of catchment properties to the annual runoff change.In conclusion,the significant reduction of runoff in the upper reaches of Hutuo River is mainly caused by the change of aridity index,i.e.,precipitation and potential evapotranspiration.

Key words: runoff change, aridity index, catchment properties, Budyko, Hutuo River