The objective of this research was to determine the minimum number of years with observations needed to obtain the medium number of total monthly consistent precipitation values and the area of coverage of the pluviometric station located in the Vacacaí river watershed, in the Rio Grande do Sul state, Brazil. It were used the historic series of precipitation data of 13 pluviometric stations located in the area and around the watershed. The minimum number of years was calculated from the relation between the number of years with observations and the stability of coefficients of determination. The station's area of coverage was determined from the relation between the coefficients of determination of the total monthly precipitation between each station and the base station (Santa Maria), and the linear distances between the base station and each station. From those equations, the distance between stations in which 80%, 85%, 90%, 95% e 99% of the values has concordance was determined. It was found that, to have a reasonable precision in the total monthly precipitation in the Vacacaí river watershed, it is necessary between 30 and 40 years of observation and, in December, January and February months the coverage of the pluviometric stations is less than 1.0km and on the other months to achieve a precision of 99% distances between 1.3 and 2.2km, 95% distances between 6.3 and 11.1km and 90% for distances between 12.5 and 22.5km.
historical series; rain; consistence
