The effect of seasonal variation in precipitation and evapotranspiration on the transient travel time distributions

Abstract

Precipitation (P), plant water use, and evaporation from the soil surface control the travel time of streamflow (Q) and evapotranspiration (ET) in a complex way. However, the impact of soil moisture and energy availability on the travel time distribution (TTD) of evaporated and transpired waters are yet less understood. In this study, we investigate how the seasonal variability of P and ET in terms of phase shift and rate influences the temporal dynamics of TTDs. To this end, we choose four contrasting climate types described as in-phase P and ET, out-of-phase P and ET, year-round constant P with seasonal ET, and year-round constant ET with seasonal P. We use a physically-based hydrological model to simulate dominant processes in the water and energy cycles as well as subsurface flow velocity, which are subsequently used in a Lagrangian particle-tracking model to characterize the age distribution of water particles in space and time. The results prove that the soil moisture availability than the ET rate imposes a stronger control on the TTD of transpired water in all climate scenarios. Plants indicate an overall tendency towards younger water particles unless in periods with a pronounced dry condition when old waters are more available in the deeper soil layers. In particular, the climate scenario with out-of-phase P and ET yields the highest percentage of old particles taken up by plants. Furthermore, ET age sampling mechanism shows hysteresis against the ET rate, with an opposite direction in climates with in-phase and out-of-phase P and ET. Our results also suggest that the ratio between the median travel time of Q and the median age of water storage has a dual relationship with the Q rate, indicating the existence of a threshold behavior that distinguishes the direct and inverse storage effects based on the storage volume.