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.

中文翻译：

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新西兰辫状河不同流态下植被轨迹的数值模拟预测

我们使用基于物理的二维数值模型来研究现实世界砾石床辫状河中不同流态下植被和形态的多年演化。为了评估模型的真实性，我们关注坎特伯雷（新西兰）的两条河流，尽管在过去 100 年里经历了类似的入侵植被物种的引入，但由于其不同的流态，它们显示出非常不同的植被存在轨迹：怀塔基河下游和怀马卡里里河。前者具有自然阻尼流态，并且自 20 世纪 30 年代以来由于水力发电而经历了进一步的人工阻尼流，并经历了植被侵蚀；而后者则以华丽的流态为特色，保留了无植被的编织平面形状。我们提出了一种创新的校准和验证程序，以确定植被参数的最佳设置，使模型能够稳健地再现两条河流的轨迹，从而证明该模型对不同的水文条件做出了明智的响应。然后，我们通过使用不具有水力发电影响的自然流态运行校准模型，分离出与水力发电相关的流态变化对下怀塔基植被侵占的影响，并发现即使没有水力发电的影响，植被侵占也会发生流量改变，尽管程度较轻。最后，我们将基于综合参数的概念框架应用于模拟结果，该综合参数比较水文和植被控制的相对强度，并讨论使用该参数作为跨流态植被存在的预测因子。