Effect of topography on vine evapotranspiration and water status in hillside vineyards

Context and purpose of the study – Many winegrape regions have hillside vineyards, where vine water use is affected by vine age, density and health, canopy size, row orientation, irrigation practices, and by block slope and aspect. Topography affects the amount of solar radiation the vines receive, which is a major “driving force” of evapotranspiration (ET). Nearly all crop ET studies have been conducted on level ground, where the contributions of weather and crop factors to ET are well known. Information on winegrape ET on hillside terrains is scarce but much needed, as growers seek more resource‐efficient production practices and vine water stress monitoring techniques to manage grapes quality, and as future water supplies become increasingly variable, limited and costly. Our UC team measured the seasonal dynamics of actual ET (ETa) and vine water status in two similar vineyard blocks with north and south aspects during three consecutive seasons, with the aim to inform irrigation management decisions.

Material and methods ‐ The vineyard blocks are located in El Dorado County, California, and both are Cabernet sauvignon on 3309 rootstock, planted in 2000 with VSP trellis on approximately 24% (north‐ facing) and 25% (south‐facing) slopes, where the grower managed the irrigation. We determined ETa in the 2016 to 2018 seasons using the residual of energy balance method with a combination of eddy covariance and surface renewal equipment to measure sensible heat flux (H). Reference ET (ETo) data was taken from the nearest weather station to calculate actual crop coefficients (Ka). We also periodically measured midday stem water potential (ΨSTEM). 

Results ‐ The north and south blocks had similar seasonal ETa, but the water use dynamic varied with the slope aspect. Until early May, ETa was slightly higher in the south (Ka between 0.5 and 0.9) than the north block (Ka between 0.4 and 0.7). From mid‐May to June and mid‐July to August, the north block had higher ETa (Ka ~ 0.65 versus 0.55 in the south slope). A progressive decrease in water use was observed from late June onwards in both blocks, with Ka of ~ 0.4 and 0.3 in August and September, respectively. Early and late in the season, we measured lower net radiation in the north block, likely due to the greater incidence angle of the incoming solar radiation. Late in the season, the north block had lower ΨSTEM (more stress) in 2016 and 2017, and the south block had lower ΨSTEM in 2018. Our results show that monitoring ETa and vine water status can inform irrigation and water stress management in hillside vineyards. 

Authors: Daniele ZACCARIA (1), Lynn WUNDERLICH (2), Giulia MARINO (1), Kristen SHAPIRO (1), Sloane RICE (1), Kenneth SHACKEL (3), Richard SNYDER (1)

(1) Department of Land, Air and Water Resources, UC-Davis, One Shields Avenue, Davis, CA. 95616 USA. 
(2) UCCE, 311 Fair Lane, Placerville, CA. 95667 USA.
(3) Department of Plant Sciences, UC-Davis, One Shields Avenue, Davis, CA. 95616, USA. 

Email: dzaccaria@ucdavis.edu 

Keywords: Energy balance, actual water use, slope, crop coefficient, stem water potential

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