Efficient water management has never been more critical for agriculture and specifically for viticulture. High-end viticulture needs to manage water to not only cut costs, but to keep quality high in a market of oversupply and buyers who hold the upper hand over the grower. High production viticulture may not need the water management finesse for quality that high-end viticulture does, but regulatory demands for groundwater protection as well as limitations on water deliveries push growers into making the most out of every gallon.
Here at AV, we’ve made use of impactful technologies to help growers irrigate efficiently and control vine stress to improve wine quality from their vineyards. Our primary tools have been the soil moisture probe and, more recently, the Florapulse microtensiometer. Both tools have been indispensable and even more so now that we have our own data portal to view and analyze these data streams. More recently, we’ve been working with a new tool, an eddy covariance device, for direct measurement of ET, which gives us another approach for irrigation management that we didn’t have before. Before I introduce this new tool, let me explain what direct measurement of ET is and how it differs from other ET measurements.
Eddy Covariance – who is this guy?
Eddy is not just your buddy down the road with whom you share a good laugh occasionally. Eddy or more meaningful, eddies, are swirls of turbulent air which we experience daily but don’t give a lot of thought. Unless you’re a micro-meteorologist like I am. Yes, I am one of those. I did my Ph.D. in micrometeorology of the vineyard environment so I know a thing or two about it. Let me give you the nickel tour of micrometeorology (let’s call it micromet for short), eddies, and how eddy covariance is used to measure ET.
Wind does not blow in a straight line. As air flows across the earth’s surface, or on a micromet-scale, the surface of a field, forest, desert, or whatever, it experiences friction from that surface. Air flow at the very surface of the earth is zero, because wind cannot penetrate the earth. So, as the wind blows across the surface, the portion closer to the ground experiences drag from the friction, which is enhanced by objects, such as buildings, trees, and of course grapevines. This friction causes a shear force in the air flow field, which creates the turbulent eddies. And these eddies swirl and interact with other masses of air, creating more eddies. These wind shear eddies are smaller than the primary eddies, and these smaller eddies interact with other packets of air, creating their own eddies. And so on and so on. So, indeed air does not flow in a straight line, with some exceptions like katabatic wind during the very still night when cold air sinks. Those flows are not turbulent, but during the day, you can count on turbulent eddies all over the place. You can see this if you look at smoke rising and then swirling: initially it rises quickly because of convection from the heat, but as it cools, it gets taken by the air and you can then see what the eddies look like.
So, now that you know what an eddy is, let’s discuss why It’s important to us, not only for agriculture, but as humans. Without air movement, everything that is generated at the earth’s surface (e.g. dust, CO2, farts, and yes water vapor) would have to rely on diffusion from high concentrations at the surface to lower concentrations in the atmosphere above. Thankfully, that is not the case. The wind, with its turbulent eddies, facilitates the movement of gases and suspended particles (called aerosols) from the surface to the sky.
In the case of water vapor, the water vapor concentration (when it is not raining) is higher within and just above a plant canopy than it is in the air above the canopy. Turbulent eddies mix the moister air below with the drier air above (Fig. 1). In the portion that is swirling upward, vapor is brought upward. In the portion that is swirling downward, drier air is brought downward.
