Field experiments to study evaporation from a saturated bare soil

A field experiment to measure evaporation from a bare sandy soil, starting from saturation, has been carried out at the Lido beach of Venice during su mmer-autumn 1997. A large tank embedded in the sand of the Lido beach, was filled with the same sand. Time Domain Reflectometry (TDR) probes were buri ed at different depths in the tank to measure the soil water content. Water was then poured in the tank to reach saturation. The measurements were tak en from June 26 up to the end of November. Water content was automatically measured at 5, 10, 15, 20, 25, 35, 45 cm depths at two hours time steps. Th e temporal behaviour of the water content is characterised by successive dr ying intervals separated by precipitation events that sometimes were able t o restore the water content almost to the initial (saturated) value. Furthe rmore a very small diurnal cycle is recognisable in the water content behav iour at all depths. All the levels show to be involved in evaporation, redu cing their water content as the experiment goes on in spite of precipitatio n and decrease in solar radiation. Using soil water content profiles and pr ecipitation, the water budget, that is the total water evaporated, is evalu ated. The cumulative evaporation, computed on the whole soil column depth, shows a time behaviour that can be separated in two different stages. The f irst stage is well described by a linear function of time and corresponds t o a daily mean evaporation of 4.6 mm. The second stage results are approxim ated by a square root of time. The analytical solution of the diffusion equ ation, found for the second drying stage using an initial condition derived from the experimental data, allows an estimation of the mean hydraulic dif fusivity of the sand. (C) 1999 Elsevier Science Ltd. All rights reserved.