The current severe drought in California, on the heels of a record breaking drought in 2011-2017, serves as a reminder that in addition to driving temperatures higher, climate change is disrupting historic precipitation patterns. There are two sides to the water equation—supply and demand—and, unfortunately, both are becoming more challenging for winegrowers. On the supply side, some parts of the world are becoming drier while average precipitation remains the same in others and is even increasing in others. Everywhere, however, is experiencing greater year to year variability in precipitation, with fewer small storms and more frequent large ones. Seemingly paradoxically, this means both more droughts and more flooding.
At the same time, demand for water is rising as higher temperatures result in a more rapid loss of moisture from soils and vines via evapotranspiration. And, to make things worse, a vicious circle comes into play as drier soils result in higher local air temperatures, which in turn result in drier soils. As a result, winegrowers who irrigate are finding it takes more water to maintain historical soil moisture profiles while many who dry farm are having to consider irrigating, even as water supplies become less reliable.
One way to reduce the demand for irrigation is to lower the temperature experienced by the vines so some of the measures discussed in the previous article on adapting to higher temperatures, such as changing the row direction and shading the vines, have the additional benefit of also accomplishing that goal. However, there are also many other things that winegrowers can do to reduce how much water their vines use.
The first is simply not to over-irrigate. A paper just published by UC Davis researchers found that irrigating at 50% of evapotranspiration in a dry year is sufficient to keep the vines healthy and even improves grape composition compared to using more water. And, monitoring evapotranspiration is only one of a number of methods available today for determining when the vines actually need water. Others include measuring the rate of sap flow in the vines or measuring the moisture at different depths in the soil profile. And, in addition to better control of irrigation, burying drip irrigation lines can help to ensure that the water gets to the roots rather than sitting on the surface and evaporating.
Anther approach is to encourage the vines to develop a deep root system, when the underlying geology permits that, enabling them to make use of moisture deeper in the soil profile. This can be accomplished by irrigating less frequently with larger amounts of water so that the total amount of water applied is the same but the upper layers of soil have a chance to dry out before the next application. In addition, some researchers believe the response of the vines to warmer temperatures later in the year can be conditioned by the total amount of water applied early in the growing season. Using less water signals the need to conserve water to the vines, leading them to adopt a more cautious strategy and constrain leaf transpiration as temperatures rise compared to what it would have been had more water been used early on.
Some winegrowers have adopted the practice of planting cover crop between the rows early in the year and then mowing or flattening it later in the year to create mulch that reduces moisture losses via evaporation from the soil. More fundamentally, regenerative farming techniques that increase biodiversity in the vineyard and promote a more robust microbial ecosystem in the soil are well known to increase the amount of water retained by the soil. They can also provide the added benefit of increasing the amount of carbon sequestered by the soil.
When developing a new vineyard in a warm climate, choosing a site where the soil tends to retain water and the subsoil is deep enough to allow the development of a deep root system may be advantageous. And, if the soil is also a “cool” soil this will lower its temperature, reducing evapotranspiration and consequently the need for water by the vines. In addition, some winegrowers are choosing to reduce the density of their vines so that the available water goes further.
Switching to drought resistant rootstock or cultivars can also reduce water use, although these may be more vulnerable to extreme heat events because of their lower maximum stomatal conductance, which may also place constraints on photosynthesis in warmer future conditions. In addition, changing to a different cultivar may not be economically attractive if it’s not one for which substantial market demand exists.
In those regions where average annual rainfall is not expected to change while year to year variability is expected to increase, having additional water storage can also play an important role. Irrigation ponds that capture rainfall and subsurface water are a common feature in California vineyards. However, many of them have been depleted during the current drought, underscoring the growing variability in annual rainfall and the accompanying need for even greater storage capacity. In addition, higher temperatures are increasing evaporative losses from ponds, adding to the challenge. In California, as in many other places, managing both surface water and groundwater availability is becoming an increasingly urgent issue at the regional and state levels, suggesting the possibility that some form of shared storage solution may ultimately emerge.
The various measures described above aren’t mutually exclusive, of course. The challenge for winegrowers will be finding the combination of those available to them that’s most compatible with their approach to farming and with their business model.
At Argos Analytics, we’re focused on quantifying climate impacts on winegrowing and evaluating the effectiveness of site selection, vineyard design and cultural practices to mitigate them. If you’d like to learn more, we invite you to contact us.
By Robert Dickinson for The Porto Protocol Foundation
President of Argos Analytics, LLC