A critical view on Surface renewal

vineyard surface
The purpose of this discussion is to sharpen critical views of new methods as they arise. It aims at understanding the limitations and benefits of new techniques to improve our ability to optimize water use.


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:

  1. “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.
  2. “topography” like slope orientation, slope aspects, slope angle. Slope affects light interception and the flow of water vapor above the canopy.
  3. “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. 
  4. “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.
  5. “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.
In conclusion, SR method is complementary to other methods scientifically proven like sap flow, water potential, stomata conductance, etc… However, despite its advantage, it is hard to use for practical applications as a stand alone tool. As such, it should be one more useful layer of information to help optimizing water use in viticulture. This should not come as a surprise. You never expect your medical doctor to make a diagnosis based on one single vital measurement. Every method carries limitations with its benefits. As usual, we get only better by combining different information sources.

 

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3 Comments

  1. I think many of the "downside" comments miss the point. Surface renewal measures the crop ET directly so you don't need crop coefficients or a reference ET. All of the points about row spacing, direction, canopy size and slope aspect are irrelevant. Work at the Kearney Ag Station shows good correlation between surface renewal crop ET & the lysimeter crop ET (the gold standard). Surface renewal is like having your own portable lysimeter. What's not to like about that?

  2. Considering the impact of row effects and canopy size on water deficit calculation is justified. It is particularly relevant if you want to improve your irrigation practices by refining your irrigation alerts or by comparing water use variations across contrasted block layouts or contrasted vintages. Below 3 examples to illustrate that point.

    1) When row spacing is the same, but row orientation and slope are different, eddy covariance technique has proven than south west facing vineyards can use at least 20% more water than north east facing vineyard. If you use a lysimeter to trigger irrigation, then you will apply 20% more water in the north east facing!

    2) Impact of row spacing on vine water use variations has been previously discussed (see blog http://blog-en.fruitionsciences.com/2013/08/a-perspective-on-vine-water-deficit.html) and is largely reported in the literature. If you use a lysimeter to trigger irrigation over 2 different row spacings, you have to come up with 2 different thresholds.

    3) The impact of canopy size on irrigation threshold has also been reported in the literature. If you use a lysimeter to trigger irrigation then what you consider being "a low vine water use level" in a vintage where canopy size is large can be perceived by the plant as a "high vine water use level" in a vintage where canopy size is smaller.

    To account for those differences you need a crop coefficient, different for each situation!

  3. I wasn't talking about using a lysimeter literally. I agree with you, it wouldn't be accurate for all the reasons you suggest. I suggested that using SR allows you measure the crop ET directly AS IF you had a lysimeter in every location you put the SR device. You wouldn't use EC equipment because it too expensive. SR equipment is cheap and you get close to or the same resolution.

    So, in your example #1: you would need two SR sensors, one in each row orientation but then you would see the 20% difference in plant water use (or whatever it truly was!) between the two row orientations and hopefully you would use that information to make an informed irrigation decision.

    Your other two examples involve, when to trigger an irrigation. That is a more tricky question. I think empirical soil moisture levels or leaf water potentials are still the best triggers for irrigation events. SR is less of tool for when to water & more of a tool for how much to water.

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