Evapotranspiration Rates and Crop Coefficients for Lowbush Blueberry (Vaccinium angustifolium)

Lowbush blueberry (Vaccinium angustifolium) yield is strongly influenced by water availability; however, growers need more specific irrigation recommendations in order to optimize water use efficiency. Weighing lysimeters were used to determine actual evapotranspiration (ET) rates of lowbush blueberry at one nonirrigated and two irrigated sites within 7 km of the Maine coast. For the three-year study period, overall mean weekly ET rates (with standard errors) during June, July, and August were calculated to be 2.13 (+/- 0.05), 2.39 (+/- 0.07), and 2.19 (+/- 0.07) cm/wk, respectively. Mean monthly ET rates did not differ significantly between irrigated and nonirrigated sites. Crop coefficient (K-c) values were determined from the ratio of daily Penman-Montieth grass reference ET to measured daily ET. The combined mean monthly K-c values for the two irrigated sites varied between 0.61 and 0.74 from May through September, with an overall mean Kc value for all sites of 0.69 (+/- 0.02). The Kc values showed no significant rotation-year component. Consequently, the Kc value determined in this study can be used in conjunction with meteorological data to provide Maine lowbush blueberry growers with the ability to better predict water demand by their crop. Fog at the irrigated sites was found to contribute approximately 13.5 cm of net water equivalent per growing season. However, the effects of dew formation and fog deposition are already reflected in the Kc value for each site, enabling growers to determine water demand from these values and grass reference ET data alone. These findings may substantially contribute to improved water use efficiency for irrigated lowbush blueberry in Maine.