On the use of meteorological data to assess the evaporation from a bare soil

The main goal of this study was to evaluate different methods for the estimate of the evaporation from a bare soil. The starting point was the experimental data set of a field survey carried out for a period of 16 months in the area of Parco Lambro, in Milano (Italy), where meteorological and soil measurements were collected to estimate the local recharge to the phreatic aquifer. The evaporation from the nearly bare soil was assessed with two different approaches. A first, indirect approach was based on the evaluation of the actual evaporation as a fraction of the potential evaporation, which was computed with both Penman’s and Hamon’s equations. As a second approach, the Bowen ratio method, that directly estimates the actual evaporation from atmospheric data, was applied. Attention was mainly focused to the effects of the configuration of the measurement instruments (particularly the measurement heights of air temperature and humidity) on the estimate of the potential and actual evaporation and to the numerical stability of each method, in relation to the measurement uncertainty. In particular, numerical problems related to the intrinsic instability of the Bowen ratio equation were deeply investigated and an original method, that identifies the data leading to unphysical results on the basis of the error propagation, was proposed. The obtained results were discussed in order to evaluate the suitability of each approach, taking into account several aspects. Penman’s and Hamon’s methods resulted to be quite stable with respect to the measurement uncertainty and the potential evaporation estimates were not affected by the configuration of the field instruments. Conversely, the Bowen ratio estimates were affected by large uncertainties due to both the intrinsic numerical instability of the method and to the sensitivity to the heights of measurement; nevertheless, the application of a suitable criterion for the data rejection allowed for a significant reduction of uncertainties on the estimates of the actual evaporation.