Measured and estimated evapotranspiration from well-watered crops

Abstract

Daily evapotranspiration (ET) measurements from the Bowen ratio-energy balance method are presented for an autumn-sown oat crop and a number of summer crops, all of which were well-watered. The accuracy of both the Penman, and Priestley & Taylor ET estimates were found similar, with errors of 15–20% for daily ET estimates. The Priestley & Taylor method is simpler as it only requires net radiation and temperature data, so it appears preferable. The empirical constant in this method, relating the“equilibrium ET” to ET was found to be 1.21 from data obtained over winter, spring, and summer for a range of crops in the Manawatu. Daylight, rather than 24 h net radiation, data were used in evaluating this constant giving a better-defined constant. Further, daylight net radiation is more accurately predicted than is the 24 h value. Neglect of the nocturnal latent heat flux was of little consequence. Longer term ET estimates, using the Priestley & Taylor method, with net radiation calculated from incoming solar radiation, were in reasonable agreement with drainage lysimeter ET measurements for the oat crop.

Keywords:

• Penman equation
• Priestley & Taylor formula
• radiation
• water balance
• evapotranspiration
• water loss
• water consumption
• evaporation