Location: Hydrology and Remote Sensing Laboratory
Title: Estimating surface energy fluxes over an Andalusian Dehesa ecosystem using a thermal-based two-source energy balance model and validation with flux tower measurements Authors
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: July 15, 2013
Publication Date: September 23, 2013
Citation: Andreu, A.,Gonzalez-Dugo, M.P., Kustas, W.P., Polo, M.J., Anderson, M.C. 2013. Modelling surface energy fluxes over an Andalusian Dehesa ecosystem using a two-source energy balance model and medium resolution satellite data [abstract]. Proceedings of the Society of Photographic Instrumentation Engineers SPIE 8887, Remote Sensing for Agriculture, Ecosystems and Hydrology XV 888717. DOI: 10.1117/12.2029235. Technical Abstract: The Dehesa, the most widespread agroforestry land-use system in Europe (˜ 3 million ha), is recognized as an example of sustainable land use and for its importance in the rural economy (Diaz et al., 1997; Plieninger and Wilbrand, 2001). It consists of widely-spaced oak forest (mostly Quercus Ilex L.), combined with crops, pasture and shrubs. It has a Mediterranean climate with severe periodic droughts. Understanding the hydrological, atmospheric and physicological processes affecting the ecosystem will improve its management and conservation. In particular, monitoring of the evapotranspiration (ET) is critical for assessing ecosystem condition or health, especially in this water-limited environment. Remote sensing from satellites can provide spatial information required for the ET analysis from local to regional scales. In such semi-arid sparse canopy-cover landscapes, thermal-based energy balance techniques which distinguish soil/substrate and vegetation contributions to the radiative temperature and radiation/turbulent fluxes have proven to be highly reliable. In particular, the two-source energy balance (TSEB) model by Norman et al. (1995) and Kustas and Norman (1999), has shown to be robust for a wide range of partially-vegetated landscapes. Its application in the Dehesa ecosystem is challenging due to the complexity of the canopy structure, having sparse tree cover, with large areas of grass and bare soil substrate strongly influencing the radiative and turbulent exchanges. For the evaluation of TSEB model over this environment, an energy flux measurement system has been installed in Andalucian Dehesa (Santa Clotilde, 39°56' N; 5°46' W, 260 m a.s.l) with 1 km homogeneous fetch in all directions. To ensure that satellite spatial resolution is similar to local fetch, a footprint analysis using two models (Schuepp et al., 1999; Kormann and Meixer, 2001) was conducted, indicating that peak source-area contribution is typically within 500 m and over 75% of the source-area is within 1 km of the tower. The fully corrected turbulent fluxes have a daily energy-balance closure with the available energy of 86% on average which is similar to other eddy covariance measurements over a variety of landscapes (Li et al., 2004; Leuning et al., 2005; Foken, 2008; Hendricks Franssen et al., 2010). Accounting for the storage terms as usually performed for forested ecosystems (e.g. Foken et al., 2006) had a minor affect on closure, probably due to the low fractional cover of the tall canopy (fc=0.19). The TSEB model was evaluated in the study area during 2012 summer season, using as input Landsat 7 TM images and meteorological data from a local station. The half-hourly flux estimates were compared with the flux tower measurements, and found to be within the uncertainty in the eddy covariance measurements when considering closure. This area is typical of semiarid ecosystems, with high sensible heat flux (H) reaching the maximum in the middle of the afternoon and low latent heat values (LE), increasing over the day. Soil heat flux (G) is not negligible even on a daily time scale, being at least 10% of net radiation (Rn). This is consistent with the climatic conditions, with low rates of soil water and high temperatures, with the humidity controlling the partitioning of the available energy (Rn-G) between H and LE (Entekhabi and Rodriguez-Iturbe, 1994).