Energy balance and evapotranspiration in cowpea under rainfed conditions

Abstract

The objective of this study was to assess the energy balance components (net radiation, latent heat flux, sensible heat flux and soil heat flux), evapotranspiration (ET) and water use efficiency (WUE) of cowpea grown under rainfed conditions. The experiment was conducted in a 4 ha area in the CCA, UFPB, belonging to the Municipality of Areia, PB (6^o58^’ S, 35^o41^’ W and 620 m) during the period from July, 22 2003 to October, 17 2003. To determine the energy balance components and ET, it was used the Bowen ratio method, and for that, the area was instrumented with a micrometeorological tower containing sensors for measuring air temperature and relative humidity at two levels above the canopy of cowpea. Measurements of net and global radiation, soil heat flux and rainfall were also made. The average value of the net radiation was 79% of the global solar radiation (Rn/Rg), in which 65% appeared as latent heat flux (LE/Rn), 23% as sensible heat flux (H/Rn) and 12% as soil heat flux (G/Rn). The total and average values for actual evapotranspiration were 330.7 mm and 3.8 mm day^-1, respectively. The production of cowpea was only 206.0 kg ha^-1 with low WUE (0.06 kg m^-3), which occurred probably due to water deficit at reproductive stage.