On the lower bound of Budyko curve: The influence of precipitation seasonality

As a boundary condition, the lower bound is a prerequisite for constructing the Budyko framework. However, the traditional lower bound i.e., the horizontal axis is an analytical boundary without considering realistic conditions. In fact, under real-word condition, the lower bound of Budyko curves, representing the maximum value of water yield, should be located above the horizontal axis due to complicated constraints from basin characteristics, especially the precipitation seasonality, a key driver of runoff yield and a dominant factor in constructing Budyko framework. Thus, this study focused on exploring the reasonable location of the lower bound by adequately analyzing potential influence of precipitation seasonality. To do that, multiple hydrological simulations in basins under different climate and topography conditions were developed based on various precipitation scenarios constructed by using Markov chain with full range of precipitation seasonality. To fully understand the influence from precipitation seasonality on the lower bound, the water-energy supply and the basin characteristics were fixed by aridity index and hydrological model parameters, respectively. The results reveal a positive correlation between runoff coefficient and precipitation seasonality with an obvious higher location above the horizontal axis, indicating the horizontal axis is imprecise to describe the upper bound of runoff yield under real-word condition. Furthermore, a potential location of the lower bound above the horizontal axis in Budyko space was found based on runoff yield mechanism under real basin condition, representing a more appropriate boundary condition of Budyko framework.