Frequency Analysis of Low and High Flows in Trend

Hydrological variables are under the effect of the climate change impacts, and therefore, this study concentrates on the frequency analyses of low and high flows depending on the duration, leading to a new approach. The durations are selected as month, season, 6‐month, and year coupled with the return periods 2‐, 5‐, 10‐, 20‐, 50‐, 100‐, 200‐, and 500‐years. The frequencies are equivalent to the reverse of these return periods. Increasing and decreasing trends are identified within the low and high flows, and the necessary graphs are presented. Similar to intensity‐duration‐frequency (IDF) curves, the proposed frequency‐flow‐duration curves are obtained instead of flow‐duration‐frequency (QDF) curves. The curves present important inputs for many hydrological events including flood forecasting, dam reservoir calculations, urban drainage design evaluations, transportation network planning, availability of agricultural land, groundwater recharge, etc. Furthermore, the flow values that may appear over n‐year frequency on the curves are calculated by taking into consideration the Gamma cumulative probability distribution function. For the study area with applications Görele River, Turkey, flow measurements are used and the necessary conclusions are derived. As a result, the comparison for high and low values separately present significant information so that monotonically decreasing trend are generally observed for high values (wet periods) and non‐monotonically increasing trend for low values (drought periods). The frequency‐discharge(flow)‐duration (FQD) curves approach indicates frequency values continuously for specific durations and has a perfect fit to scatter points. It is also easy to read and understand the graphs and formulations. In particular, FQD curves indicate the changes in discharge on the basis of time in a clear way. The exponential and power functions written on the graphs provide any frequency value corresponding to a given discharge value.