Nonstationarity in maximum annual daily streamflow series from Southern Brazil

ABSTRACT Extreme streamflow values estimates are important for flood risk assessment and also for the design and operation of hydraulic structures. The behavior of this hydrological variable is under climate and land use changes effects and river’s course modifications caused by construction and operation of large reservoirs. The assumption of stationarity commonly adopted in flood frequency and magnitude analysis studies is questionable under such circumstances. In this work we identified nonstationary annual maximum streamflow series corresponding to fluviometric gauges located in Southern Brazil. A nonstationary frequency model was applied to those series and the results were compared with those of a stationary model. We also evaluated the presence of abrupt changes in the series. The results indicate that 75 of 157 series of Southern Brazil may be considered nonstationary, most of which are in the Iguazu, the Paranapanema and the Uruguay basins. For a planning horizon equal to 10 years, the return period of the present 100 years flood changes to 48-75 years when considering the nonstationary model, respectively. Abrupt changes were identified mainly as occurring in the 70’s.

【摘要】极端径流值估算对于洪水风险评估以及水工建筑物的设计与运行均具有重要意义。该水文变量的变化特征受气候变化、土地利用变化，以及大型水库建设与运行引发的河道变迁影响。在此类背景下，洪水频率与量级分析研究中普遍采用的平稳性假设，其合理性已受到质疑。本研究针对巴西南部地区的水文测站，识别出了非平稳年最大径流序列。研究采用非平稳频率模型对上述序列进行分析，并将结果与平稳模型的分析结果进行对比，同时还对序列中是否存在突变点进行了评估。研究结果显示，巴西南部157条径流序列中有75条可被判定为非平稳序列，其中大部分分布于伊瓜苏（Iguazu）、巴拉那帕内马（Paranapanema）以及乌拉圭（Uruguay）流域。当规划基准期为10年时，采用非平稳模型分析后，当前重现期为100年的洪水，其重现期将变为48至75年。经识别，序列中的突变点主要集中发生于20世纪70年代。