estimate_ET0_with_TmaxAndTmin {simET}

Estimating ET0 with Tmax and Tmin

Description

When solar radiation data, relative humidity data and/or wind speed data are missing, they should be estimated using the procedures presented in this section. As an alternative, ETo can be estimated using the Hargreaves ETo equation.

Usage

estimate_ET0_with_TmaxAndTmin(Tmean, Tmax, Tmin, Ra)

Arguments

Tmean

mean temperature.

Tmax

max temperature.

Tmin

min temperature.

Ra

extraterrestrial radiation [mm day-1].

Value

A vector for reference evapotranspiration (mm day-1).

Note

Units for both ETo and Ra in Equation 52 are mm day-1. Equation 52 should be verified in each new region by comparing with estimates by the FAO Penman-Monteith equation (Equation 6) at weather stations where solar radiation, air temperature, humidity, and wind speed are measured. If necessary, Equation 52 can be calibrated on a monthly or annual basis by determining empirical coefficients where ETo = a + b ETo Eq.52, where the Eq. 52 subscript refers to ETo predicted using Equation 52. The coefficients a and b can be determined by regression analyses or by visual fitting. In general, estimating solar radiation, vapor pressure and wind speed as described in Equations 48 to 51 and Table 4 and then utilizing these estimates in Equation 6 (the FAO Penman-Monteith equation) will provide somewhat more accurate estimates as compared to estimating ETo directly using Equation 52. This is due to the ability of the estimation equations to incorporate general climatic characteristics such as high or low wind speed or high or low relative humidity into the ETo estimate made using Equation 6. Equation 52 has a tendency to underpredict under high wind conditions (u2 > 3 m/s) and to overpredict under conditions of high relative humidity.