Applicability of a two-stage evaporation approximate in a semi-arid environment

[Departement_IRSTEA]MA [TR1_IRSTEA]RIE / TRANSFEAU For a regional assessment of water consumption in semi-arid agricultural zones, one needs robust and simple tools that provide space-time estimates of evaporation losses, esp. for flood irrigation. Most of the simplest and operational evaporation estimates rely on empirical relationships such as the "beta function" (e.g. the FAO56 method)that are not well suited for all climate, soil, irrigation methods and vegetation conditions. Some of them for instance overlook the first to second stage evaporation processes, or do not represent separately the bare soil evaporation component. Several authors have proposed physically-based simple expressions, such as the desorptive approach, which gives accurate integrated capillary flows under constant boundary conditions. We propose here a new analytical formulation that uses the desorptive approach and reduces as much as possible the number of empirical relationships. It is tested for a wide range of soil and vegetation conditions against 1) previous expressions available in the litterature, 2) a physically based complex SVAT scheme SiSPAT that uses the Richards equations, and eventually against 3) experimental data acquired over a winter wheat site during the 2003 SUDMED/IRRIMED field experiment in the semi-arid Haouz (mostly flood-irrigated) agricultural area around Marrakech, Morocco. This expression gives accurate predictions of first to second-stage evaporation time series for the bare soil and fully vegetated cover conditions, while computing as expected a sharper time of switching than SiSPAT. An evaluation of its performance for sparse vegetation is provided according to SiSPAT outputs.