Reference Evapotranspiration in the State of Paraíba, Brazil: Climatic Trends and Influencing Factors

Reference evapotranspiration (ET 0 ) plays an important role in the planning of irrigation and the water demand of crops, as well as in the planning of water uses in watershes. The identification of trends in ET 0 under climate changes is important to understand its effects on agriculture, water balance and water resources planning. The goal of this work was verify whether there was or there was not an increasing/decreasing trend on ET 0 in dry and rainy periods, as well as in annual period, identifying the influencing climate variables on ET 0 in the state of Paraiba, Brazil. In order to determine the ET 0 trend in the series the Mann-Kendall nonparametric test was used, whereas the series slope was determined by applying Sen’s estimator. To identify the most influencing clilmatic variable it was performed a multiple regression of normalized data. The data were obtained of six stations with monthly data from January 1961 to December 2015, a total period of 55 years. Results showed increasing on ET 0 in almost all stations during the study period, except for a single of them in which there was a decreasing. The most dominant variables influencing the trend on ET 0 was air temperature during annual time scale, while during the rainy and dry seasons there was an alternation between the most influencing variables (air temperature and sunshine duration).