Intercontinental predictions of river hydraulic geometry from catchment physical characteristics

Hydraulic geometry is used to describe spatio-temporal variations in wetted width, mean depth and mean velocity with discharge in stream reaches. Models describing downstream (spatial variations) and at-a-station (temporal variations) hydraulic geometry are required for efficient and effective management of flow regimes, sediment and nutrient transport, and physical habitats at catchment to global scales. Data describing observed hydraulic conditions at 1327 stream reaches in France and New Zealand were compiled alongside reach descriptors and upstream catchments extracted from available GIS databases with nationwide coverage. Three contrasting empirical methods were developed to predict hydraulic geometry at unvisited sites. The first method applied basic models in which downstream hydraulic geometry parameters depend on discharge only. The second method applied separate country-specific models whose predictors were selected from the available nationwide GIS data from country. The third method applied an intercontinental model in which all observed hydraulic data were lumped together and predictors were selected from those available across both countries. Flexible regression models that account for interactions were used to parameterize second and third methods. Cross-validation tests showed that downstream parameters were well predicted for independent sites regardless of country. Model performance was higher for width than for depth. At-a-station parameters were less well predicted than downstream parameters, but the models provided satisfactory predictions of observed reach hydraulics. Importantly, the lumped intercontinental model performance was comparable to that of the country-specific models. This suggests that our approach can be applied in other regions where similar explanatory environmental variables are available.