Articles | Volume 369
Proc. IAHS, 369, 87–95, 2015
Proc. IAHS, 369, 87–95, 2015

  11 Jun 2015

11 Jun 2015

Evolution of low flows in Czechia revisited

O. Ledvinka1,2 O. Ledvinka
  • 1Hydrological Database & Water Balance, Czech Hydrometeorological Institute, Na Sabatce 2050/17, 143 06 Prague 412, Czech Republic
  • 2Institute of Applied Mathematics and Information Technologies, Faculty of Science, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic

Abstract. Although a nationwide study focusing on the evolution of low flows in Czechia was conducted in the past, a need for the revision of the results has arisen. By means of the trend analysis, which specifically considers the presence of significant serial correlation at the first lag, the former study highlighted areas where 7-day low flows increase or decrease. However, taking into account only the lag-one autoregressive process might still have led to the detection of so-called pseudo-trends because, besides short-term persistence, also long-term persistence may adversely influence the variance of the test statistic when the independence among data is required. Therefore, one should carefully investigate the presence of persistence in time series. Before the trend analysis itself, the authors' previous studies aimed at the discrimination between short memory processes and long memory processes employing jointly the Phillips–Perron test and the Kwiatkowski–Phillips–Schmidt–Shin test. This analysis was accompanied by the Hurst exponent estimation. Here, the subsequent identification of trends is carried out using three modifications of the Mann–Kendall test that allow different kinds of persistence. These include the Bayley–Hammersley–Matalas–Langbein–Lettenmaier equivalent sample size approach, the trend-free pre-whitening approach and a block bootstrap with automatic selection of the block length, which was applied for the first time in hydrology. The general results are similar to those presented in the former study on trends. Nevertheless, the divergent minimum discharges evolution in the western part of Czechia is now much clear. Moreover, no significant increasing trend in series incorporating Julian days was found.

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Short summary
Selected series of drought-related characteristics derived from mean daily discharges measured in Czechia were tested for the presence of significant trends. Three modifications of the Mann-Kendall test were employed that account for short-term persistence and long-term persistence. One of them was utilised for the first time in hydrology. Unlike previous studies, the present study highlighted the differences among trend in deficit volumes (decrease in the west and increase in the east).