Estimates of the rainfall design values with a high frequency of occurrence

Short-term rainfall characteristics, especially their intensity and design values, are of great importance in engineering (e.g. sewerage systems) and hydrological practice. However, the measurement and data processing of these characteristics is rather complicated. The design values of short-term rainfall that are still widely used in Czechia are based on studies that are more than 60 years old.

Therefore, new Intensity-Duration-Frequency (IDF) and Depth-Duration-Frequency (DDF) curves have been prepared and published in a public database within the project “Prediction, Evaluation and Research for Understanding National sensitivity and impacts of drought and climate change for Czechia” (PERUN). These IDF and DDF curves have been based on 2-year to 100-year design values of 5-min–3-day rainfall totals estimated from rainfall intensity measurement at selected stations of the Czech Hydrometeorological Institute. More than 170 stations with sufficient length (mostly more than 25 years) of joined digitalized pluviograph records and the automatic rain gauge measurement series from 1951–2022 were processed. The design values were estimated using the General Extreme Value (GEV) distribution fitted to annual maxima series. The parameters of the distribution were estimated using L-moments and adjusted by the region-of-influence (ROI) method. However, design values with an average frequency of occurrence greater than 2 years (e.g. twice or once a year) are also highly required in engineering practice and therefore our database should be extended.

In our contribution, we focus on estimation of rainfall design values with a frequency of occurrence higher than 2 years. As the previously published estimates of 2-year to 100-year design values are based only on annual maxima series, a different methodology is needed in this case. Due to the high frequency of occurrence of additionally estimated design values and the use of relatively long station measurement series (20-70 years), the method of quantiles calculated from sets of rainfall totals selected above the threshold was suggested. The results and the possibility of using them to extend the already published estimates of 5-years to 100-year design values are analyzed and discussed.