Effects of rainfall events on the evapotranspiration retrieved by an energy balance model

摘要

An alternative way to map the actual evapotranspiration (ET) spatial distribution at daily scale is the application of residual surface energy balance models to satellite images that are characterised by high temporal frequency and moderate spatial resolution, like those acquired by the MODIS sensors on board of TERRA and AQUA platforms. Within this research the well-known SEBAL model has been applied on an area located in the southern part of Sicily (Imera Meridionale catchment) using four images acquired between the 27th of March and the 11th of April 2007. The catchment extends for about 2000 km2 and includes both mountains and hill areas: the first are located in the northern part (the Madonie Mountains), while the latter characterise the southern area. The altitude ranges between 0 and 1900 m a.s.l., with an average altitude of 500 m a.s.L In order to validate the outputs of the energy balance model, a flux tower has been installed within a homogeneous field of cereal located in the valley part of the basin (close to Licata town within the Quignones Farm), characterised by semi-arid conditions. The selected images have been acquired before and after a three-rainfall events period (5 mm during the night between the 2nd and the 3rd of April, 17 mm on the 3rd evening, 8 mm on the 4th thoroughly all the day). The study area is mostly cultivated with cereals that, during the study period, were characterised by flag and bolting phenological phases. In the first, the leaf at the stem apex assumes a flag shape, while during the latter, the terminal part of the stem a barrel shape enlargement containing the ear appears. The study aim is the evaluation of the energy partition between sensible and latent heat fluxes, due to the increased water availability in a period characterised by a significant vegetation growth. The comparison between model results and flux tower measurements shows that both the latent heat flux and the other energy balance components are modelled with high accuracy. Moreover, the model well represents the variation in time of energy partition, which increases from 0.5 up to 0.65 in terms of evaporative fraction (A). Therefore a greater percent of energy is used to evapotranspiration after the rainfall events.