Evapotranspiration components from energy balance, sapflow and microlysimetry techniques for an irrigated vineyard in inland Australia

Abstract Quantification of the components of evapotranspiration (ET) is key to designing management strategies for improving water-use efficiency of irrigated viticulture. The ET from a drip-irrigated vineyard was determined between January and March in 1995 and in 1996, in order to estimate mean seasonal water-requirements. In 1995 ET was obtained from separate measurements of transpiration ( E c ) with heat-pulse sapflow sensors and soil evaporation ( E s ) at various positions with microlysimeters, while in 1996 it was from the Bowen ratio energy balance (BREB) technique. The ratio of ET to potential evaporation (ET o ) or crop factor ( K c ) was similar in both years with an average of 0.32, while ratio of E c to ET o ( K t ) averaged 0.17. In 1995, the majority of E s occurred along the canopy edge especially on the sunny northern side of the vine rows. Total E s over the 3-month period was similar from positions beneath the vine canopy and at midway between the rows, both of which were about 25% less than that from along the canopy edge. In 1996, a detailed energy balance analysis was compared for warm dry days in mid-February (Period 1) and cool humid days in late March (Period 2). In Period 1, the majority (92%) of available net radiation ( R n ) during daylight hours was split almost equally between latent heat flux from the combined vine canopy and soil surfaces ( λE ) and sensible heat ( H ), with the balance accounted for by the soil heat flux ( G ). In Period 2, the percentage of R n consumed by λE increased to 51%, and that by G to 11%, while that by H declined to 38%. The increase in the partitioning of R n through λE was associated with a reduced vapour pressure deficit of the air ( D ) that enhanced dissipation of energy absorbed by the canopy ( R nc ) as λE through the canopy ( λE c ), thus making the vine canopy cooler in Period 2 compared to Period 1. High D in Period 1 induced stomatal restraint on transpiration, such that λE c R nc , except for brief periods early in the morning or late in the afternoon. Daily rates of ET averaged 2.2 mm, transpiration 1.2 mm and soil evaporation 0.9 mm during January–March period in both years. This study also indicated the need for irrigation practices that make effective use of rain that falls during the season.