Surface renewal (SR) is an interesting and powerful method, particularly for large acreage showing a relative uniformity in leaf area development and thus in water use. It helps to further highlight situations where farmers overuse water. Historically, SR is derived from Eddy covariance method. In 2010, we had invited Prof. Baldocchi (the “father” of Eddy covariance) and Dr. Snyder (one of the “pioneer” of SR) during our Vintage Report conference in Napa to get some early insights. SR is cheaper than eddy covariance because it uses less complex measurements and assumes the values of some parameters instead of measuring them for calibration purposes. However, Eddy covariance method still remains the reference method for ecosystem water use measurements worldwide.
Because SR is a technique initially developed over uniformly covered surface (when soil is completely covered by vegetation like a forest). Difficulties arise when the land surface under monitoring is a row crop (when soil is not uniformly covered by vegetation like in a vineyard). Over row crop, measurement interpretation becomes more complex.
After testing SR against other techniques in the vineyard, we found upsides as well as downsides for practical applications.
The main upside: SR gets the footprint of a large vine population as opposed to a few plants when using plant-based measurement from a selected area.The downsides are related to the complexity of plant response. It is never easy to perform measurements on live ecosystems. Thus, SR is sensitive to:
- “noise” coming from other transpiring and evaporating sources nearby, like a high transpiring vineyard next to yours, a high evaporating soil or a high cover crop water use.
- “topography” like slope orientation, slope aspects, slope angle. Slope affects light interception and the flow of water vapor above the canopy.
- “vineyard layout”: this makes it difficult to compare performances across multiple blocks.
- row spacing effect is difficult to appreciate. Larger row spacing imposes lower water use, even when there is no soil moisture deficit.
- row orientation effect is challenging, particularly as the sun gets lower over the horizon. When more light gets trapped into the canopy, the vineyard needs more water to maintain a same satisfaction level in its water use.
- plant material: some varietals can maintain a high transpiration rate under low soil moisture while other varietals do not. This makes it difficult to adjust a water deficit index for contrasted varietals.
- “vintage effect”: for a same vineyard, total leaf area development can vary from one year to the other. In a wetter year, more leaves can be produced and the vineyard needs to use more water. This makes it difficult to compare vineyard water deficit levels across vintages.
- “practices” like irrigation: plant transpiration increases immediately after one irrigation and declines afterwards. During the declining phase it is difficult to determine when the value gets low enough to trigger a new irrigation particularly if you want to account for row spacing effect.