Data error effects on net radiation and evapotranspiration estimation

The objective of this paper is to evaluate the potential error in estimating the net radiation and reference evapotranspiration resulting from errors in the measurement or estimation of weather parameters. A methodology for estimating the net radiation using hourly weather variables measured at a typical agrometeorological station (e.g., solar radiation, temperature and relative humidity) is presented. Then the error propagation analysis is made for net radiation and for reference evapotranspiration. Data from the Raimat weather station, which is located in the Catalonia region of Spain, are used to illustrate the error relationships. The results show that temperature, relative humidity and cloud cover errors have little effect on the net radiation or reference evapotranspiration. A 5°C error in estimating surface temperature leads to errors as big as 30 W m⁻² at high temperature. A 4% solar radiation (R_s) error can cause a net radiation error as big as 26 W m⁻² when R_s ≈ 1000 W m⁻². However, the error is less when cloud cover is calculated as a function of the solar radiation. The absolute error in reference evapotranspiration (ET_o) equals the product of the net radiation error and the radiation term weighting factor [W = Δ(Δ1+γ)] in the ET_o equation. Therefore, the ET_o error varies between 65 and 85% of the R_n error as air temperature increases from about 20° to 40°C. (author)