How significant is the effect of the surface characteristics on the Reference Evapotranspiration estimates?

Abstract

Estimated Reference Evapotranspiration (ETo), requires the meteorological attributes to be taken above standardized well-watered and vegetation-covered surfaces. However, worldwide such vegetation-reference sites are not common. This results in mistaken estimates of ETo by using the well-known and extensively applied FAO-56 Penman–Monteith formula. Consequently, the use of inappropriate data for ETo estimation from non-ideal surfaces, leads to significant and systematic cumulative errors introducing uncertainties when determining the crop water requirements in a region. Additionally, the existing climatic stations are not spatially distributed but rather concentrated mostly in non-rural urban areas or in local airports, operating above non–standardized surfaces.The purpose of this work is to assess the effect of using daily meteorological data recorded above a well-watered short crop compared to data obtained above dry bare soil, at the same local environment, using two widely used ETo models: the ASCE Penman-Monteith and the Hargreaves-Samani. The results indicate that the meteorological conditions above the different surfaces are quite different, presenting higher temperature and lower relative humidity values above the bare soil surface compared to the well-watered short crop and resulting, finally, to errors in the estimation of reference evapotranspiration. These errors appear to enhance as air temperature, vapor pressure deficit, radiation and atmospheric clearness increases, or relative humidity decreases but are diminishing under adequate soil moisture conditions resulting after rainfall events. The ETo differences vary according to the model each time adopted but the soil substrate influence can be detected by more sophisticated methods, such as FAO56-PM, which consider the energy balance of the surface.