Trends and shifts in mean annual inflow time series of hydropower plants in Brazil

Tendências e mudanças de nível em séries temporais de afluência média anual de usinas hidrelétricas no Brasil

ABSTRACT

Previous Brazilian streamflow trend detection studies didn´t fully covered the notable recent period of multiple drought episodes over the country. Most studies focused on detecting monotonic trends and few on abrupt changes of level (shifts). We provide an updated study on trends and shifts of long-term mean annual flow (LTMAF) based on the simplistic linear trend and on an S-shaped shift with flexible duration. We used a set of selected 52 time series of naturalized monthly inflows for Brazilian hydropower plants (HPPs) updated until DEC 2023. The selection considered only HPPs with unregulated headwaters and discarded early years of the record with plenty filled data. We employed t-test statistics for usual calculations of statistical significance levels. Regression slopes magnitudes of were interpreted qualitatively in terms of “practical significances”. The application of the S-shaped shift model found only long-lasting shift cases yielding essentially the same conclusions as the simpler linear trend model. Moderate and large linear trend rates were detected in 47 series, 33 cases with negative trend rates and 14 cases with positive trend rates. The discarding of early years of the historical records resulted in more dispersed and reduced streamflow trend rates, increasing the number of negative trend cases.

Keywords:
Hydropower; Streamflow; Trends and shifts; Fluviometric records; Statistical tests; Detection studies