Abstract
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.
Highlights
A suitable estimate of the expected extreme high flows of a river is a basic step for flood risk assessment and design, operation and management of hydraulic structures (PROSDOCIMI; KJELDSEN; SVENSSON, 2014)
Traditional methods developed for that purpose are based on the assumption of stationarity, which implies that the variable under analysis has a time invariant probability density function with fixed parameters (PETROW; MERZ, 2009; READ; VOGEL, 2015, 2016; SRAJ et al, 2016; VOGEL et al, 2015; VOGEL; YAINDL; WALTER, 2011)
There were 157 fluviometric series that attended all the minimum 30-year length and the missing value percentage criteria, Figure 2 shows the coverage period for each of these streamflow series, while the location of gauges are in Figure 3, which shows the gauges whose series had the linear trend model slope coefficient significantly bigger than zero
Summary
A suitable estimate of the expected extreme high flows of a river is a basic step for flood risk assessment and design, operation and management of hydraulic structures (PROSDOCIMI; KJELDSEN; SVENSSON, 2014). The stationarity hypothesis might be rendered invalid due to several factors that influence streamflow, e.g., hydroclimatic changes (MILLY et al, 2008), urbanization (VOGEL; YAINDL; WALTER, 2011), agricultural management practices (FOUFOULAGEORGIOU et al, 2015) operation of hydropower plants (RÄSÄNEN et al, 2017) and reservoirs (ZAJAC et al, 2017). The stationary approach is not appropriate to model hydrologic process in basins with large scale changes resulting from anthropogenic influences (KOUTSOYIANNIS, 2006). Et al (2008) suggested that “stationarity is dead” in face of hydroclimatic changes and that it is necessary to find ways to identify nonstationary probability models and use them in water resources risk assessment and planning (MILLY et al, 2015). Montanari and Koutsoyiannis (2014) advise caution in the use of nonstationary models and the expression “stationarity is dead”. The selection of a poor nonstationary model might turn out to be less efficient and less robust than a stationary one (MONTANARI; KOUTSOYIANNIS, 2014)
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