Daily rainfall disaggregation for Tocantins State, Brazil

Abstract

In order to design effective Brazilian hydraulic structures, it is necessary to obtain data relating to short-duration intense rainfall from historical series of daily rainfall. This recurring need can be fulfilled by rainfall disaggregation methodology. The objective of this study was to determine the intense rainfall disaggregation constants for the State of Tocantins and to compare these constants with those obtained for other regions of Brazil. For the modeling of the frequency of intense rainfall of different durations of less than 24 hours, the Gumbel probability distribution (GPD) was employed using rainfall series from 10 locations in Tocantins state. The results showed that the GPD was adequate by the Kolmogorov-Smirnov and Chi-square tests. The disaggregation constants presented low variability values for different return periods (from 10 to 100 years); the values for Tocantins state are: h12h/h24h=0.93, h6h/h24h=0.86, h4h/h24h=0.82, h3h/h24h=0.78, h2h/h24h=0.72, h1h/h24h=0.61, h50min/h1h=0.92, h40min/h1h=0.83, h30min/h1h=0.68, h20min/h30min=0.76 e h10min/h30min=0.46. The comparison of the results with those from studies developed for other Brazilian regions showed variations of up to -62.30%, allowing us to conclude that the use of local constants is important in the process of rainfall disaggregation.

Daily Rainfall Disaggregation Coefficients for the Doce River Basin, Brazil: Regional Applicability and the Return Period Influence

“…Greater distortions can be noticed at the beginning of the curves for the 2 years return period. Similar behavior was observed in the study of Silva Neto et al (2017) for 10 pluviograph gauge stations in the Tocantins state. The authors found small curvatures for small return periods while the values of the disaggregation coefficients remained almost constant for more than 10 years return periods.…”

“…In general, the lower values of standard deviation, coefficient of variation (CV) and amplitude show that the coefficients did not vary significantly. Silva Neto et al (2017) obtained a similar result for the Tocantins state, where CV for the disaggregation coefficients were obtained below 4.04%, indicating small variation among the estimated coefficients. For the p-value results obtained by comparing the average coefficients representing the entire basin with the data of each pluviograph gauge station (Table 3), only for the rainfall ratios of 2h/24h and 1h/24h all the coefficients were statistically equal to the disaggregation coefficients of the entire basin.…”

“…The rainfall disaggregation method assumes that the coefficients are independent from the return period. This fact was evaluated by Silva Neto et al (2017) who presented the disaggregation coefficient curves as a function of 10 to 100 years return periods for 10 stations in the Tocantins state and noted that the disaggregation coefficients remained practically constant in relation to the return period. However, there were small curvatures, especially in the early return periods, which may compromise the quality of the intense rainfall estimation by the rainfall disaggregation method.…”

Evaluation of generalized extreme value and Gumbel distributions for estimating maximum daily rainfall

“…However, in applications, we usually adjusted distribution parameters based on the series to be tested. Even with these limitations, the KS test is widely used often as the only adhesion test (Al-Suhili and Khanbilvardi, 2014;Pereira et al, 2017;Silva Neto et al, 2017;Ottero et al, 2018) and also commonly used as criterion for selecting the best distribution (Caldeira et al, 2015;Back and Cadorin, 2020). AD and Rf tests proved to be more rigorous and should be favored.…”