Comparison of Reference Evapotranspiration Rates Measured by a Weighing Lysimeter and Meteorological Predictions

Weighing lysimeters make direct measurements of evapotranspiration rate, providing data that may be used to assess predictions of evapotranspiration rate calculated from meteorological data. A partially-buried lysimeter located at the University of California West Side Research and Extension Center is near the center of a tall fescue grass field providing a 70 m fetch. A California Irrigation Management Information System (CIMIS) weather station is installed 7 m from the lysimeter. Tall fescue grass grown in the lysimeter is cut weekly to a height of 0.1 m as is the surrounding grass. Daytime subsurface drip irrigation of the lysimeter is regulated to meet the evapotranspirative demand. CIMIS hourly predictions of reference evapotranspiration (ETo) are based on the Pruitt-Doorenbos (PD) and Penman-Monteith (PM) models. Our objective was a comparison of the predictions with hourly evapotranspiration rates calculated from lysimeter data for 2004-05. After correcting for lysimeter drainage, a 7-point Savitsky-Golay filter computed the derivative providing the hourly evapotranspiration rate. CIMIS hourly ETo predictions plotted against lysimeter ETo almost matched a 1:1 line indicating that the CIMIS predictions were accurate for the large data set but the scatter was substantial. The data were selected by hour and a best-fit line was calculated assuming a zero intercept. Slopes of best-fit lines to CIMIS PD ETo vs. lysimeter ETo for data representing each hour from 9 AM through 3 PM were greater than one, reaching a maximum of 1.13, but were consistently less than one at other times during the day. In contrast, the slopes of CIMIS PM ETo vs. lysimeter ETo were close to one between 10 AM and 3 PM but increased in the late afternoon reaching a maximum of 1.11. The PM model includes soil heat flux but the PD model does not. The difference in the slopes during the middle part of the day could be due to lack of downward heat flux in the PD model compensated by an overestimate in the latent heat flux. The CIMIS PM method provides improved hourly ETo predictions compared to the PD method during the middle of the day at this site.