Correlation of Vineyard Atmometer Reference Evapotranspiration Measurements with Weather Station Evapotranspiration Measurements: Adaptation of Simple Tools to Improve Vineyard Water Management Decisions

Atmometers are relatively simple and inexpensive devices for measuring evapotranspiration. They have the potential to improve vineyard irrigation efficiency by providing a locally-accurate estimate of reference evapotranspiration (ETo), which is then used in conjunction with known crop coefficients to determine crop water use. To more accurately estimate ETo, the readings from atmometers installed in a vineyard need to be adjusted with an equation. This research has sought to develop the necessary adjustment equation(s) for conditions along the Central Coast. Daily values of ET from atmometers located at two existing weather station sites and at two nearby vineyards were compared with weather station ETo values during the summer of 2002. Results from two sites on the Central Coast indicate that the atmometers delivered consistent and uniform measurements that correlated fairly well with the nearby weather station data. However, comparison of the atmometer and weather station data from each of the two sites resulted in two significantly different adjustment equations for the two locations. The cause of this difference is likely due to variability in the relative microclimates at the respective vineyard and weather station sites, as well as variability in the ETo calculations themselves. The first cause will be easier to investigate; research in 2003 will evaluate atmometer readings from multiple vineyards surrounding a single weather station.