The aim of this study was to estimate soil evaporation (Es) in an intensive olive orchard. Measurements of Es were performed for 19 days using microlysimeters, during summers 2010, 2011 and 2012 in southeast Portugal. In order to relate each area type to radiation transmissivity, ground cover measurements were performed over the years. These data were used to calibrate and validate an empirical model for Es estimation. Measured daily average Es was 0.55 ± 0.14 mm; the model estimated 0.53 ± 0.18 mm for the same days, with a determination coefficient of 0.94. This corresponds to 9% of the reference evapotranspiration, representing well the overall values estimated for the summer, except for days after rain. Regarding the wet area, measured Es for the validation data set was 2.42 L/(m 2 of wet area), the estimated was 2.49 L/(m 2 of wet area). Measured average Es in dry area (validation data set) was 0.42 L/(m 2 of dry area), estimated Es was 0.43 L/(m 2 of dry area). The large exposed dry area had a significant contribution to evaporation. On average, estimated Es during a typical Mediterranean summer was 10% of reference evapotranspiration, representing 30% of transpiration and 23% of evapotranspiration.2 of 15 techniques, result in more water-conservative agriculture. The evaporation estimation is particularly critical in any crop with a large ground area exposed to radiation and turbulence.Studies about soil evaporation (Es) suggest that total evaporation from wet soil, in the first phase, is determined primarily by the solar energy reaching the soil surface [4-6] and could, therefore, be reduced by shade [7]. In fact, for a wet soil, Es is driven and limited by the energy available at the soil surface, at least until cumulative Es attains the threshold between phase 1 and 2 (U, upper limit of cumulative Es during the first stage, using nomenclature and description in the original publication [5]). After this energy-limited stage, Es switches to a soil-limited rate (or falling rate stage), usually expressed as an empirical function of time [5]. In this second phase, the soil evaporation rate is determined by soil hydraulic properties, being not completely independent from meteorological variables [4,6,8,9].Soil evaporation can be measured (ground truth data) by applying water balance methods including lysimeters (e.g. [10-12]), chamber methods and also from micro-meteorological methods if applied in a large continuous surface. These methods are expensive and some are very labor and time consuming [10,13,14]. Modelling is a common approach to overcome these limitations, by measuring soil evaporation over a short period of time and subsequently using the data to calibrate and validate a model to estimate Es over a larger time frame. There are mechanistic models for which detailed information about system parameters and variables are necessary, for example the Penman-Monteith equation with appropriate conductance of soil surface [15] and its modifications (e.g. [16]) or semi-empirical (or empirical) models [5,8,9]...
