1. Introduction
The extreme natural phenomena, which intensify over the last decades, such as sudden or long-lasting rainfalls, often accompanied by floods or outflows from the sewerage, cause significant economic losses. This should force us to continuously improve the rules of sewerage dimensioning, based on continuous precipitation measurements in order to identify possible climate changes patterns. Modern investigation methods used in hydrology (including precipitation monitoring) in connection with the knowledge of statistics, calculus of probability, and mathematical modeling now become necessary tools applied in engineering practice.
The designing of storm water or combined sewerage systems with facilities, such as storm overflows, separators, storage reservoirs, or wastewater treatment plants, encounters a primary difficulty in Poland, resulting from the lack of a reliable method of an authoritative determination of rainfall intensity for dimensioning or verification of a sewerage flow capacity. Namely, the precipitation model by Błaszczyk (1954), recommended for dimensioning of drains and system facilities in Poland, significantly underestimates measurement results of the calculated rain streams, which was shown in a number of comparative analyses (Kotowski 2009; Kotowski and Kaźmierczak 2009; Kotowski et al. 2010). The model developed by Błaszczyk is based on a statistical analysis of only 79 intense rainfalls (average height h in millimeters of the duration t in minutes: h > t0.5) registered in Warsaw in the years 1837–91 and 1914–25. The application of a model developed on the basis of precipitation measured about 100 yr ago, especially in the context of recently observed climate anomalies (De Toffol et al. 2009; Larsen et al. 2009; Leonard et al. 2008, Olsson et al. 2009; Schaarup-Jensen et al. 2009; Willems et al. 2012), affects in a negative manner the dimensioning of drainage areas in Poland according to recommendations of the EU standard PN-EN 752 (EU 2008), directly influencing a higher frequency of combined sewerage discharge resulting from the impossibility of rainwater collection. The standard restricts the occurrence frequency of such unfavorable phenomena to a rare “socially acceptable” repeatability: once per 10 years in case of rural areas and once per 20 to 50 years for urban areas, respective to the spatial development type. Because of the uncertainty of current projections of future rainfall, it is proposed to check the capacity of urban sewerage systems in terms of extreme rainfall with the frequency of once per 100 years (BLFU 2009; Siekmann and Pinnekamp 2011; Staufer et al. 2010; Willems 2011). The philosophy puts forward a new challenge of satisfying the recommendations by sewerage systems designers. Therefore, systematic research in precipitation patterns and statistical determination of the frequency of occurrence of maximum precipitation amounts are becoming so important nowadays, especially for such rare rainfall repeatability, in order to meet the rigorous requirements of the above-mentioned standard in the future as well.
2. Pluviographic material and research methods
Archival pluviograms from the Institute of Meteorology and Water Management (IMGW) Wrocław–Strachowice station from the years 1960–2009 constituted the research material. Precipitation was recorded by means of a float pluviograph until 2006, whereas the automatic rain gauge RG-50 SEBA Hydrometrie GmbH with electronic recording was used from 2007. Precipitation amounts were determined for 5-min-long intervals for the needs of the paper. Such accuracy is currently required to develop model precipitation (of Euler type) or series of torrential rainfalls (Bröker 2006; Schmitt 2000) or randomly generated synthetic rainfalls (Licznar et al. 2011a,b; Mehrotra and Sharma 2007a,b; Rupp et al. 2012) essential for hydrodynamic modeling of area drainage systems. Precipitation amounts were determined for the following 16 intervals of duration: 5, 10, 15, 30, 45, 60, 90, and 120 min and 3, 6, 12, 18, 24, 36, 48, and 72 h, totaled by means of the moving sum method. To isolate intensive rainfalls for statistical analyses, a precipitation amount criterion h ≥ 0,75t0.5 was assumed. The assumed criterion allowed for isolation of a number of the most intensive rainfalls in each year. A total of 514 synthetic rainfall instances were selected for detailed statistical analysis from the period of 50 yr of observation.






At first, interval precipitation amounts were arranged in a non-decreasing order (with durations ranging from 5 min to 72 h) with N = 50 yr of observation. Points xm, p(m, N) marked on the coordinate system (hmax, p) allow for conclusions about the form of the function of the probability distribution of the random variable X. Empirical cumulative distribution functions of the highest precipitation amounts from the 50-yr measurement period are presented in Fig. 1.


Example empirical cumulative distribution functions of the highest precipitation amounts from the 50-yr observation period in Wrocław–Strachowice: (top left) t = 5, 45, 180, and 1440 min and (top right) t = 10, 60, 360, and 2160 min; (bottom left) t = 15, 90, 720, and 2880 min and (bottom right) t = 30, 120, 1080, and 4320 min.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1

Example empirical cumulative distribution functions of the highest precipitation amounts from the 50-yr observation period in Wrocław–Strachowice: (top left) t = 5, 45, 180, and 1440 min and (top right) t = 10, 60, 360, and 2160 min; (bottom left) t = 15, 90, 720, and 2880 min and (bottom right) t = 30, 120, 1080, and 4320 min.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
Example empirical cumulative distribution functions of the highest precipitation amounts from the 50-yr observation period in Wrocław–Strachowice: (top left) t = 5, 45, 180, and 1440 min and (top right) t = 10, 60, 360, and 2160 min; (bottom left) t = 15, 90, 720, and 2880 min and (bottom right) t = 30, 120, 1080, and 4320 min.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
The determination of the theoretical probability distribution function that is best adapted to the phenomenon described in the paper is not a simple process. In the majority of cases, particularly in relation to continuous variables, we do not possess theoretical premises allowing for an unambiguous determination of the distribution type, appropriate for the variable describing the phenomenon in question. On the basis of literature data for description of precipitation phenomena, the following distributions are used (Alila 1999; Di Baldassarre et al. 2006; Brath et al. 2003; Ben-Zvi 2009; Bogdanowicz and Stachý 1998; Kottegoda et al. 2000; Overeem et al. 2008; Schaefer 1990): Fisher–Tippett type Imax; Fisher–Tippett type IIImin; lognormal; and Pearson type III.


















3. The characteristics of selected probability distributions












Calculated parameter values α, β, and ɛ for the Fisher–Tippett type IIImin distribution.



Parameter calculation results of the Fisher–Tippett type IIImin distribution for the highest precipitation amounts in Wrocław in the period of 1960–2009 and durations t ∈ [5; 4320] min are shown in Table 2.
Calculated parameter values λ, α, and ɛ for the Pearson type III distribution.






















4. Selection criteria for probabilistic precipitation models




At the assigned significance level α, the critical value λkr of Kolmogorov’s statistic is established from the statistic tables. For example, for the value of 1 − α = 0.95 the quantile value λkr = 1.36. The H0 hypothesis should be rejected, when λ ≥ λkr. In the opposite case, the analyzed sample does not negate the hypothesis verified on the assumed significance level α.
At the assumed significance level of α = 0.05, for the lognormal distribution the calculated value of the λ Kolmogorov test statistic was higher than λkr = 1.36 (for t = 2880 min). For the remaining three distributions, the calculated statistic values were significantly lower than λkr = 1.36 (for t from 5 to 4320 min). This means that the lognormal distribution is not applicable for the description of the analyzed maximum precipitation in Wrocław at the assumed significance level. It was therefore assumed that the maximum precipitation can be described by means of Fisher–Tippett type Imax, Fisher–Tippett type IIImin, and Pearson type III distributions on the significance level of α = 0.05.



The BIC criterion consists of two parts: the first describes a model adjustment measure, whereas the second describes its possible simplicity. In general, information criteria allow for the choice of the well-adjusted and simplest possible models that are, at the same time, not overtaught. To compare the analyzed models, the values of the BIC criterion were calculated and shown in Table 3. The two lowest values for each analyzed time are given in bold for clarity. The lognormal distribution was not taken into account, since the λ Kolmogorov statistic criterion was not fulfilled.
Values of BIC information criterion for analyzed probability distributions for the description of precipitations in Wrocław. The two lowest values for each analyzed time are in bold for clarity.



The BIC criterion does not unambiguously indicate the best model (the differences between the values of BIC of the analyzed models are small); however, it shows clearly that the Fisher–Tippett type Imax distribution diverges qualitatively from the two remaining ones. Thus, only two distributions (models), Fisher–Tippett type IIImin and Pearson type III, were further analyzed as better in terms of their quality.
5. The precipitation model based on the Fisher–Tippett type IIImin distribution










The graphical interpretation of maximum precipitation amounts hmax in Wrocław for the period of 1960–2009, calculated from the probabilistic model in the form (22), was shown in Fig. 2. This is the family of depth–duration–frequency (DDF) type curves: repeatable precipitation amounts with the occurrence probability p ∈ [1; 0,01] (with the occurrence frequency of C ∈ [1; 100] yr) and the duration of t ∈ [5; 4320] min.


Precipitation amount curves (DDF type) (left) measured in Wrocław and (right) calculated from the probabilistic model I in (22).
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1

Precipitation amount curves (DDF type) (left) measured in Wrocław and (right) calculated from the probabilistic model I in (22).
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
Precipitation amount curves (DDF type) (left) measured in Wrocław and (right) calculated from the probabilistic model I in (22).
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
6. The precipitation model based on the Pearson type III distribution









7. The quantitative evaluation of the probabilistic precipitation models



In the case of the maximum precipitation model (22), based on the Fisher–Tippett type IIImin distribution, the value of rRMSE = 7.10%. Figure 3 shows the graphical interpretation of the partial residuals of the analyzed model.


Matching and partial residuals diagrams for the model based on the distribution of the Fisher–Tippett type IIImin: (left) calculated vs measured h and (right) residual of h vs h.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1

Matching and partial residuals diagrams for the model based on the distribution of the Fisher–Tippett type IIImin: (left) calculated vs measured h and (right) residual of h vs h.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
Matching and partial residuals diagrams for the model based on the distribution of the Fisher–Tippett type IIImin: (left) calculated vs measured h and (right) residual of h vs h.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
In the case of the maximum precipitation model (25), based on the Pearson type III distribution, the value of rRMSE = 7.99%. Figure 4 shows a graphical interpretation of the partial residuals of this model.


As in Fig. 3, but for the Pearson type III distribution.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1

As in Fig. 3, but for the Pearson type III distribution.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
As in Fig. 3, but for the Pearson type III distribution.
Citation: Journal of Hydrometeorology 14, 6; 10.1175/JHM-D-13-01.1
On account of a lower value of rRMSE, model (22) based on the Fisher–Tippett type IIImin distribution was recognized as more precise, especially in the practical range (C ≤ 10 yr and t ≤ 3 h) for the sewerage designing and it was recommended to designing sewerage systems in Wrocław (according to EU 2008). However, model (25) is recommended for the verification of the excessive accumulation frequency and combined sewerage discharges (for the range C > 10 yr and t > 3 h).
8. Final remarks
The conducted investigation and research allow us to reach the following conclusions:
To obtain the comparability of precipitation models, created for different meteorological stations, measurement results of rainfall amounts in time should be described and generalized by means of one methodology that is proposed in this paper.
For designing and especially for the verification of probability of the occurrence frequency of sewerage outflows (for p ∈ [1; 0,01]) by hydrodynamic modeling, it is recommended to use the reliable local precipitation models [as in the case of Wrocław: the probabilistic models (22) or (25), respectively].
The pluviographic material from every meteorological station should be continuously updated; consequently, the mathematical form of the developed models should be periodically verified in order to increase their accuracy, especially for low occurrence probability values p < 0.1 (i.e., for C > 10 yr), and in order to consider nonstationarity of the precipitation in time.
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