Seasonal patterns and hydrological regulations of root zone storage capacity across United States

Abstract

Root zone storage capacity (S_r) represents the maximum subsurface storage accessible to plant roots. It is primarily influenced by water availability and water demand, thus exhibiting temporal change in response to climate variations. Previous studies have primarily focused on the spatial patterns of S_r across local to global scales; however, there remains a limited understanding of its temporal patterns, particularly in relation to seasonal changes. This work explores the seasonal behavior of S_r for wet and dry seasons and the hydrological regulation of seasonal S_r. We propose a seasonal modeling framework based on apportionment entropy, which considers the phase difference between water and energy. Within this framework, the PDM-FLEX hydrological model, an integration of the probability distributed model (PDM) with the FLEX lumped model, was employed to calculate catchment-scale S_r for each season across 671 catchments in the contiguous United States. Results show that: i) this framework can effectively capture seasonal S_r, with wet season S_r (an average of 564 mm) generally being smaller than dry season S_r (an average of 820 mm) for most catchments. In the west, plants routinely access deep water, leading to comparable S_r for both wet and dry seasons. Incorporating seasonal S_r into the hydrological model can improve simulation performance across time scales; ii) dry season S_r is more responsive to hydroclimatic control compared to wet season S_r, as plants in arid climates are more sensitive to water accessibility; and iii) during the wet season, low S_r relative to precipitation leads to an unresponsive hydrological reaction. However, during the dry season, a routine correlation between S_r and precipitation produces responsive hydrological behavior. These findings indicate that plants seasonally adapt their root systems and that these seasonal variations in S_r would have significant hydrological implications.