Water and energy flux measurements in rainfed cowpea cultivated in Northeast Brazil

Abstract

In the areas of rainfed agriculture it is very important to be able to quantify losses of water by evapotranspiration (ET). The water (WB) and energy (EB) balance methods are commonly used for measuring ET due to their simplicity, robustness and low cost. Therefore, the objective of this study was to assess the WB and EB components in the soil cultivated with cowpea in an area of 4 ha in the municipality of Areia, PB (6o 58’ 12” S e 35o 42’ 15” W, 620 m), besides comparing the methods used to determine the cowpea ET. To determine the EB, the area was instrumented with a rain gauge, a pyranometer, a net radiometer and sensors for measuring air temperature and relative humidity at two levels. To determine the WB, three experimental plots, equipped with a one-meter access tube for neutron probe measurements, and 8 mercury manometer tensiometers were installed. The mean value of the net radiation was 76% of the global solar radiation, in which 73% appeared as latent heat flux, 18% as sensible heat flux and 9% as soil heat flux. This study showed that both EB and WB methods could be used to determine the ET. The mean value for actual evapotranspiration, obtained by using the WB and EB methods, was 4.20 and 4.28 mm day^-1, respectively.