Trend of the Seasonal Water Index of Rio Grande do Sul State and its Relationship with El Niño and La Niña

Considering the trends observed in the original meteorological variables, mainly precipitation, sunshine and temperature, in the last decades, in Rio Grande do Sul State, this work had the objective of analyzing the trend of the seasonal water index (WI), derived from the serial hydric balance and to relate it to El Niño and La Niña, as well as to quantify the frequency of WI considered critical in relation to water availability (WI ≤0.6) from the 1979-2009 period. In the average of the State, there was a linear increase in WI, statistically significant, in Spring, associated with higher precipitation during this season of the year. In the Spring, 86% of the meteorological stations showed an increase in WI, with 58% of these with statistically significant signs. El Niño events determined higher WI and La Niña events, lower WI, in Summer and Spring, especially El Niño, with greater WI in 86% of cases in Spring and 77% of cases in Summer. In the southern half of the State there was a high frequency of occurrences of WI ≤0.6, for two or more consecutive months, mainly in December and January. These results confirm the water limitations of the southern half of the State for non-irrigated Spring-Summer crops and serve as a subsidy for the management of the agricultural calendar, when El Niño or La Niña is forecast.