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

The estimate of the evaporation from the soil is a key-point for the assessment ofthe local recharge to a phreatic aquifer. On this study, we compared different methodsto evaluate the evaporation from a bare soil (Bowen Ratio, Penman and Hamonequations). Experimental data were provided by a field campaign carried out for a periodof 16 months in the area of Parco Lambro, in Milano (Italy); meteorological data(atmospheric pressure, rainfall, humidity, wind velocity and air temperature, net andincident solar radiation) were collected, together with soil data (volumetric water content,temperature and capillary pressure head). In applying the Bowen Ration method,we developed an analysis of error propagation to estimate the expected error on theevaporation; in order to avoid possible unphysical results: the samples for which thecoefficient of variation on evaporation was greater than an assigned threshold wereeliminated from the series. A similar analysis was performed on the Penman results.The evaporation estimates computed by Penman and Bowen Ratio methods were comparedwith each other in order to verify the consistency with the data measured froman evaporation pan and with the actual soil water content measured by TDR probes.Finally, the comparison between Penman and Hamon method allowed us to develop astatistical model for assigning the real day length, which is one of the parameters necessaryto apply the Hamon method. This modified method permitted to obtain goodestimates of the potential evaporation using only standard meteorological data.