D8 Hydrological and Geomorphological Modeling

Topic

Hydrological and geomorphological modeling integrates the field measurements previously described in this series of documents into mathematical representations of the entire system, aiming to simulate how that system behaved in the past, how it behaves in the present, and how it might behave in the future.

Catchment models—whether conceptual or semi-distributed—divide the entire catchment into smaller hydrological response units; within each unit, they simulate the partitioning of precipitation into evapotranspiration, subsurface flow, and runoff, utilizing reservoirs that drain linearly or non-linearly, depending on the specific case.

Calibrating these catchment models involves adjusting internal parameters to minimize the difference between the simulated discharge and the discharge actually observed in the river; this goodness-of-fit is summarized by metrics such as the Nash-Sutcliffe coefficient or the percentage volume bias, with the magnitude of the bias indicating whether the model systematically tends to overestimate or underestimate actual discharge.

Hydraulic models, meanwhile, solve the Saint-Venant equations—previously described in the context of floods—in one or two dimensions to simulate how a flood wave propagates throughout the river network and across the surrounding floodplains.

Erosion models calculate both erosion rates and sediment transport rates at every point in the catchment, employing process-based equations that are far more detailed than the standard soil loss equation mentioned earlier.

Finally, hillslope models couple the one-dimensional Richards equation with the slope stability analysis previously described in the context of landslides, in order to predict where on a slope—and at what point during a storm—a rainfall-induced landslide is most likely to occur. Thus, from the integration of field measurements—with which this document begins—to the probability of landslides—with which it concludes—and encompassing catchment models, their calibration, hydraulic models, and erosion models, hydrological and geomorphological modeling enables the various elements measured separately in the preceding documents of this series—discharge, soil moisture, sediment, and landslides—to finally be combined into a single mathematical representation capable of simulating the entire system.

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