As a component of a water supply planning assessment, three models for multiple site daily rainfall generation were tested with 12 sets of input data from an important water supply region in southeast Queensland. The three models were a two-part model, a pattern storage model, and a pattern storage model with adjustments of means and standard deviations of monthly and annual generated rainfalls. The main purpose of this study was to evaluate multisite rainfalls over catchments that form a major water supply source. The intended application was examination of the reliability of medium to long-term water supply for a project with supply from a range of sources and locations, and so emphasis was given to evaluation of monthly, annual and multiple-year results for the rainfall data. There were good results from all three models in reproducing the statistics of multiple year rainfalls. The two-part model did not preserve monthly or annual cross correlations among the data sets. The pattern storage model with adjustment of means and standard deviations was the most reliable in reproducing means, standard deviations and cross correlations.
The daily to hourly rainfall disaggregation model disaggregates stochastically generated 9 am to 9 am daily rainfalls into clock hour temporal patterns. The model uses a single parameter based on the ratio R of the amount in the hour of maximum rainfall to the daily total. Data from 394 pluviometer stations covering most of Australia were analysed to provide a map of the single parameter {15}R{mean} which is the mean of R for days of rainfall > 15 mm. Allowance is made for seasonal variation in R and for increasing uniformity of rainfall as the daily total increases. The model first generates daily rainfalls. A value of R is determined for each day based on the amount of rainfall and on mapped {15}R{mean} for the location and this determines the amount in the hour of maximum rainfall. The other 23 hourly values depend on this maximum value and on clustering rules to maintain the statistics of the 2-hour, 3-hour etc rainfalls. Test results are given for several stations to demonstrate the accuracy of reproducing hourly rainfall statistics.
6-minute rainfall data were used to calculate average adjustment factors for rainfall durations 1, 2, 3, 6, 12 and 24 hours at seven locations (Adelaide, Brisbane, Darwin, Hobart, Melbourne, Perth and Sydney). When combined with results from published studies, average values of 1.14 for 24-hour annual maximum rainfalls and 1.16 for sub-daily rainfalls were derived. Considering the large variability in adjustment factors among the various locations and studies, it is recommended that an average value of 1.15 be used for rainfall durations from 1 to 24 hours. There is some indication that adjustment factors for 24-hours duration decrease as the magnitude of the rainfall increases, and reduced factors for large daily rainfalls seem appropriate, but this reduction for large rainfalls was not evident in the sub-daily rainfalls. There is evidence from UK studies that the adjustment factor should be reduced for multiple day storm totals.
ABSTRACT The flood frequency characteristics of 18 watersheds in southeastern Arizona were studied using the log‐Boughton and the log‐Pearson Type 3 distribution. From the flood frequency study, a generalized envelope for Q 100 for watersheds 0.01 to 4000 mi 2 in area has been produced for southeastern Arizona. The generalized envelope allows comparisons to be made among the relative flood characteristics of the watersheds used in the study and provides a conservative estimate of Q 100 for ungaged watersheds in the region.
The effect of the October 1983 floods in southeastern Arizona, on a previously established generalized envelope for floods expected once in 100 years (Q100), is studied. The design envelope is found to produce more conservative estimates of Q100 than individual data sets find. The design envelope for Q100 is revised to correct for some longer periods of record now available, and to be consistent with floods on a wider range of drainage area than previously considered. Additional design envelopes for floods expected once in 2 years (Q2) and once in 10 years (Q10) are prepared, and the three envelopes are used to provide conservative estimates of flood frequencies on ungaged watersheds in southeastern Arizona with drainage areas between 0.01km2 and 10,000km2. A procedure is presented for developing regional flood frequency estimates that could be used in geographically and climatically homogeneous areas.
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