Seasonal evapotranspiration, energy fluxes and turbulence variance characteristics of a Mediterranean coastal grassland

Agricultural and Forest Meteorology
Volumes 226–227, 2016

Trepekli A, Loupa G., Rapsomanikis S.

Abstract. A hydrological year of evapotranspiration (ET) and energy exchanges, for a coastal grassland in the Mediterranean-climate of Greece, are reported. Throughout the growing season, available energy (Rnet-G) was transformed to latent heat flux by 67% while during the senescence period the turbulent exchange was dominated by sensible heat flux accounting for 80.42% of (Rnet-G). Evapotranspiration reached maxima of 8.2 mm d−1 in July while it was negligible during winter. The annual hydrological excess equalled 200 mm y−1. Variation in water exchange depended primarily on (Rnet-G) and vapor pressure deficit (VPD). A significant positive linear correlation was found between values of ET and Rnet-G on a monthly timescale. The monthly mean ET had also a positive correlation with the mean Normalized Difference Vegetation Index (NDVI). Additionally, the bulk characteristics of the canopy, decoupling coefficient (Ω) and the daytime mean Priestley–Taylor (α) coefficient exhibited seasonal variations with higher values occurring in the rapid emergent period. Annual Ω was nearly 0.50, indicating that the surface was partially decoupled with the atmosphere. Given that this ecosystem is characterized by occasional flooding, the annual mean value of α was 0.52 implying seasonal growth limitation to transpiration. The impact of different wind origins on evapotranspiration was observed. ET was smaller than expected for winds of marine origin because of its response to small VPDs and the presence of sea salt aerosol. The results of the present study effected the explanation of the temporal patterns of ET in relation to the landscape of a Mediterranean coastal grassland and its meteorological and physiological characteristics.