Analysis of energy fluxes and vegetation-atmosphere parameters in irrigated and natural ecosystems of semi-arid Brazil

Knowledge on evapotranspiration is essential in quantifying water use depletion and to allocate scarce water resources to competing uses. Despite that an extensive literature describes the theoretical mechanisms of turbulent water vapour transport above and within crop canopies fewer studies have examined land surface parameters within composite landscapes of irrigated crops and semi-arid natural vegetation. Aiming to improve parameterizations of the radiation and energy balance in irrigated crops and natural vegetation, micro-climatic measurements were carried out on irrigated land (vineyards and mango orchard) and natural vegetation (caatinga) in the semi-arid zone of the São Francisco River basin (Brazil) from 2002 to 2005. The fractions of 24 h incident solar radiation available for net radiation were 46%, 55%, 51% and 53%, for wine grape, table grape, mango orchard and caatinga, respectively. Daily evaporative fractions of the net available energy used as latent heat flux (λE) were 0.80, 0.88, 0.75 and 0.33 respectively. The daylight values of bulk surface resistances (r_s) averaged 128 s m^-1, 73 s m^-1, 133 s m^-1 and 1940 s m^-1 for wine grape, table grape, mango orchard and caatinga, respectively. Simplified parameterizations on roughness and evaporation resistances were performed. It could be concluded that net radiation can be estimated by means of a linear expression with incident global solar radiation depending on the type of vegetation. The variability of aerodynamic resistance (r_a) could be mainly explained by the friction velocity (u_*) which on turn depends on the surface roughness length for momentum transport (z_0m). The experimental data showed that for sparse canopies z_0m being 9% of the mean vegetation height is a doable operational rule for the semi-arid region of São Francisco River basin. The seasonal values of r_s for irrigated crops were highly correlated with water vapour pressure deficit. The availability of analytical methods to assess r_a and r_s makes the one-step Penman-Monteith equation suitable for the computation of actual evapotranspiration and water productivity analyses.