Estimating reference evapotranspiration (ETo) using numerical weather forecast data in central Chile

Abstract

Summary Water demand at a basin level is influenced by many factors such meteorological variables, soil moisture, vegetation type and irrigation system. Among them, climate is the major driver, because weather conditions determine energy balances and vapor pressure deficits that affect the magnitudes of vapor flux from surface to atmosphere. Monitoring evaporation is a great challenge since specific and costly equipments are required. As an alternative, agronomists and engineers use semi-empirical equations such as the Penman–Monteith formula to estimate potential evapotranspiration based on surface weather observations. Unfortunately weather stations are scarce and do not always have the instrumentation to measure relevant variables for its calculation. In this work, we evaluate the use of numerical weather forecasts, obtained from MM5 model, as proxy for surface meteorological data with the specific objective of using them to estimate reference evapotranspiration (ETo) in the Maipo river basin. We compared three procedures to obtain ETo: (a) Raw MM5 estimates of latent heat flux; (b) calculation of ETo from Penman–Monteith equation, using raw MM5 outputs of weather variables; and (c) calculation of ETo from Penman–Monteith using MOS-corrected MM5 weather data. We used class A pan evaporation data and estimates of ETo using observed daily surface data to evaluate the precision of each method. We found that the estimation of ETo based on MOS-corrected weather variables is usually the most effective method to estimate reference evapotranspiration. Since MM5 outputs in this region are available at 25 km grids, the number of monitoring sites can be increased substantially, improving the ability to capture spatial variability of water demands in the basin.