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

Abstract

Alternative methods to the intensity-duration-frequency equations have been used for intense rainfall study in places with no pluviographic data. A pioneering study, by CETESB (1986), established rainfall disaggregation coefficients for Brazil; however, the use of these coefficients take into account neither the rainfall characteristics of the interest locality nor the return period influence in obtaining the precipitation intensities. This study aimed to determine rainfall disaggregation coefficients for Doce river basin; to verify the validity of the regional coefficients use for the entire basin; and to evaluate the return period influence on these coefficients. Data from 19 gauges located in the basin, with rainfall associated with durations of 10 to 1440 min and return periods of 2, 5, 10, 20, 50 and 100 years, were used to obtain the coefficients. Student's t-test was used to assess the regional applicability of these coefficients and the return period influence. The coefficients do not provide regional validity for the entire basin; therefore, regional coefficients were estimated for five hydrologically homogeneous regions, confirming the validity of these coefficients use and the influence of the return period. We conclude that the disaggregation coefficients must be specific for the homogeneous regions and for each return period.

Daily Rainfall Disaggregation to Estimate the Intensity-Duration-Frequency Relationship in Minas Gerais State, Brazil

“…Spatialization techniques were efficient to determine these differences in the state of Minas Gerais. Alternatively, Passos and coauthors, (2021) [20] use hydrologically homogeneous regions in Doce basin river for coefficients use since the coefficients do not provide regional validity for the entire basin. This fact is essential in improving the process of rainfall disintegration, especially for the CD24h/day.…”

“…CETESB's disaggregation coefficients are general for the entire Brazilian territory (the same disaggregation coefficient is used throughout Brazil) as they constitute an average of the sub-daily extreme rainfall ratios of 98 Brazilian localities, from old and short series [19]. These disaggregation coefficients may not reflect the rainfall's regional or local characteristics due to the great rainfall variability in Brazil, promoting uncertainties in the determination of IDF relationships [20].…”

“…Recent studies [20,21] have shown that the generalization of the CD can lead to errors in the rainfall disaggregation process. This is due to the considerable spatial variability of the occurrence of intense rainfall [14,39], in most situations involving convective processes.…”

“…In Brazil, the second case is more common since disaggregation coefficients are established from a national average obtained by the Environmental Company of the State of São Paulo -CETESB [18], from rainfall series from 98 locations in Brazil, with relatively old and short series, between 5 and 10 years [19]. For this reason, these CETESB disaggregation coefficients are generalist, and may not reflect regional or local characteristics [20], promoting uncertainties in the determination of IDF relationships. Despite this, these coefficients are widely used in Brazil due to their simplicity.…”

“…The RRDD method was used in several regions of Brazil [21] and, in general, the authors attribute a great performance of the method. However, a national average of disaggregation coefficients may not adequately represent local or regional precipitation characteristics [15,20], and the evidence to support the equivalence and applicability of the RRDD method comparing this with IDF sub-daily resolution rain gauge data is restricted to studies in a single station in Pelotas, the Southern region of Brazil [12,13]. Besides, the equivalence was verified by linear regression between disaggregated data and sub-daily resolution rain gauge data for each duration and return period (RP), thus presenting contestable results, since it segments the IDF relationship (for each duration and RP).…”