Regionalization of Precipitation and Estimation of Flash Flood Flows for Ungauged High Andean Basins

The uncertainties related to the rainfall regime in a basin significantly impact the estimation of flash floods. Even more so in non-gauged basins, the hydraulic response to different precipitation conditions is not adequately investigated due to the lack of proven methodological tools. Most high Andean basins with hydric interest are not properly gauged. So, flash flood estimations may not be close to real values. In this investigation, we generate a precipitation regionalization through a regionalization vector to represent the rainfall regime and analyze the hydrological response of the basin to estimate flash floods. We consider the basin of the Machángara River as the study zone, located between longitude W 78° 58′ 30″ and 79° 90′ 00″ and latitude S 02° 48′ 00″ and 02° 36′ 00″. The basin has an area of 325 km², a mean slope of 0.0388 m/m, and a temperate climate. We used the regionalization vector of type mean species proposed by Y. Brunet Moret, which calculates the extended average and the indices of the regional vector by a method of least squares, minimizing the deviations between the indices of the stations and the vector. For this analysis, the historical maximum precipitation values of the nearest meteorological stations were used. In addition, to reduce the margin of uncertainties, a map of land cover and use has been prepared. In order to include drainage conditions and permeability of soils, processed satellite images have been used, and the information was obtained from the National Information System (SNI). Flash flooding was estimated using concentrated hydrological model simulations; the flows obtained were close to the few reported through gauging. Additionally, inundation of floodplains was evaluated by entering discharges into the hydraulic model HEC-RAS. The maximum width range of the floodplains was 148.68 m for a return period of 500 years; the range of return periods producing the maximum width values were 50, 200, 500, and 1000 years. The maximum reported flood was 151.4 m³/s, corresponding to a return period of 1000 years. We can conclude that the precipitation regionalization appropriately represents the rainfall regime, so the flash flood estimates are assertive.