Numerical Modeling Prediction of Vegetation Trajectories Under Different Flow Regimes in New Zealand Braided Rivers

Abstract

We use two‐dimensional physics‐based numerical modeling to study multi‐decadal evolution of vegetation and morphology under different flow regimes in real‐world gravel‐bed braided rivers. To assess model realism, we focus on two rivers in Canterbury (New Zealand) that, despite having been subjected to the introduction of similar invasive vegetation species in the last ∼100 years, show very different trajectories of vegetation presence due to their different flow regimes: the Lower Waitaki River and the Waimakariri River. The former, featuring a naturally damped flow regime—and having experienced further artificial flow damping due to hydropower generation from the 1930s, experienced vegetation encroachment; while the latter, featuring a flashy flow regime, retains an unvegetated braided planform. We propose an innovative calibration and validation procedure to determine an optimal setup of vegetation parameters that allows the model to robustly reproduce the trajectories of both rivers, thereby proving that the model responds sensibly to different hydrological conditions. Then, we isolate the impact of hydropower‐related flow regime modifications on vegetation encroachment in the Lower Waitaki by running the calibrated model with a natural flow regime that does not feature the effect of hydropower generation, and find that vegetation encroachment would have happened even without flow alteration, albeit to a milder degree. Finally, we apply to simulation results a conceptual framework based on a synthetic parameter that compares the relative strength of hydrological and vegetation controls, and discuss the use of this parameter as a predictor of vegetation presence across flow regimes.