Physics-based hydrological modelling of temporary streams

Understanding the spatio-temporal dynamics of runoff generation in headwater catchments is challenging, due to the intermittent and fragmented nature of surface flows. Non-perennial river catchments are characterized by an ever-changing spatial configuration of their flowing streams with their active network that contracts and expands, with a dynamic behavior that depends on the complex interplay among climate, topography, and geology. In this work, CATHY (CATchment HYdrology), an integrated surface-subsurface hydrological model (ISSHM), is used to simulate the stream network dynamics of two virtual catchments with the same, subsurface characteristics (hydraulic conductivity, porosity, water retention curves) but different morphology. First, a set of simulations reproducing a sequence of steady-states at different catchment wetness levels is run to identify the maximum spatial extension of the stream network for the two catchments and the joint variations of the stream length (L) and discharge at the outlet (Q) are computed. Then, the suitability of the topographic wetness index (TWI) and the contributing area (Ac) to identify the spatial configuration of the maximum stream length in the two catchments is investigated. These two morphometric parameters provide a good estimate of the spatial distribution of the maximum flowing network in both the simplified study catchments. Next, CATHY is used to investigate the impact of saturation-excess (Dunnian) and infiltration-excess (Hortonian) runoff generation on the stream network dynamics of the two virtual catchments. The numerical simulations show that when surface runoff is triggered by saturation-excess mechanisms, the subsurface domain is slowly saturated, and the stream network gradually expands upstream from the outlet towards the catchment divides. In these conditions, the specific inflow per unit contributing area is relatively uniform along the network, thereby implying that L and Q display a monotonically increasing one-to-one relationship. On the other hand, infiltration-excess mechanisms lead to more heterogeneous saturation patterns in the subsurface domain. In particular, during the wetting phase, Hortonian processes originate highly transient conditions and a non-uniform spatial distribution of the specific inflow along the stream network. This is reflected by a hysteretic L(Q) relation and a marked asymmetry between the wetting and drying phases of the event. Finally, the effects on the stream network dynamics of the non-homogeneous distribution of the hydraulic conductivity along the soil profile of the catchments and different boundary conditions at the bottom are considered. The comparison with the corresponding homogeneous configurations of the study sites evidences the different timing of saturation/desaturation of the surface and wetting/drying of the active stream networks. Nonetheless, the corresponding homogeneous and non-homogeneous L(Q) curves follow similar patterns. Overall, the numerical simulations indicate that ISSHMs have the potential to accurately describe the spatio-temporal variations of the stream networks and the processes driving such dynamic behavior and that they can be useful tools to gain insights into the main physical drivers of non-perennial streams both in simplified and more realistic conditions.