by Rivulis

It has never been easy to be a farmer and facing up to the obstacles and the challenges has always been part and parcel of the vocation. The competition, the fresh water resources, land/soil degradation, new diseases, new parasites… the list is exhaustive.

The challenge intensifies

The climate change makes things more difficult with the weather conditions becoming more and more unpredictable, including the frost, which is one of the extreme conditions caused by climate change. The damage caused by the frost has been one of the main threats to agricultural production over these last few years. In 2021, it is estimated that this damage will be among the worst on record!

“At least a third of French wine production, representing a turnover of almost 2 billion euros, will be lost in 2021, following exceptional sub-zero temperatures, which devastated many vineyards and fruit crops throughout the whole of France”. Freshfruitportal.com

Spring frost

The spring frost is the most frequently recorded in agriculture. It is common in temperate countries and causes damage to deciduous crops such as grape vines, apple trees, cherry trees, kiwis, blueberries, etc.

These crops are naturally adapted to cold winters and can withstand sub-zero temperatures for long periods. The cold period is necessary for them to shed their leaves in winter and go dormant. Once flowering and leaf or vegetative growth has ended, they become progressively vulnerable to frost. The further we are from the dormancy and the more we move into spring, the more sensitive they are to frost and the damage will be considerable. These last few years, because of climate change, frosts can occur even in the month of May (northern hemisphere), when crops are already well on their way to becoming engulfed in summer, which leaves them totally unprepared and vulnerable.

Winter frost

The tropical and subtropical crops also require frost protection. Crops such as avocados, litchis, mangos, berries, citrus fruits, etc. have been grown for decades outside their natural habitat. These crops are all of the evergreen type and are not adapted to withstand low temperatures.

Furthermore, more recently, countries with a relatively hot climate have experienced extreme frosts in winter. One single night or even less of exposure to freezing temperatures for these crops can lead to their total destruction. Unlike the deciduous crops (trees), where the frost has a detrimental effect on the annual production, with evergreen trees, the damage caused by the frost can have an effect that lasts for several years or even lead to the complete loss of the tree.

Frost protection by the release of latent heat

The principle: When freezing, the water releases heat into the environment.  For any substance or material, a solid state is always at a lower energy level. When water moves from its liquid state to a gaseous state it will then have a higher energy level, which requires the use of energy. The same applies the other way round. The transition of water from liquid to solid means a reduction in the energy level. The excess energy thus produced is, in fact, released into the environment in the form of heat. Using the latent heat is a method by which water can protect your plants or your vineyard from the frost.

And this not only applies to frost. The double combination! Many regions in the world are experiencing severe winters and extreme summers. If a frost protection system can be effective for winter or spring frost, it can also act as a very good and efficient climate control system. This is possibly the best solution during a heat wave. In this case, where temperatures are high and the relative humidity (RH) is low, having a frost protection system will help save the crop. By activating the system, the level of humidity in the air will rise considerably and the temperatures drop, thus mitigating the level of stress to a certain extent. The cooling systems can use lower flow rates but still will need to be designed properly and an analysis of the water conducted.

Active frost protection with sprinklers

The use of sprinklers for frost protection is one of the most well-known, efficient and reliable methods of frost protection. There are different methods, but the principals are universal. A minimum application rate of 3.0 mm/h is required. This is enough to provide protection at temperatures as low as -3˚C.  Another 0.5 mm/h is required for every additional degree that the temperature drops. For example: if the temperature is -4.0˚C, the minimum application rate will be 3.5 mm/h (35 m3/ha/hour).

Sprinkler frost protection can be categorised into 3 methods: Full cover, individual targeted protection and under-canopy protection

1- Full cover: By using a large sprinkler, for the large spaces or the for smaller spaces, full cover is a good solution for providing frost protection. It can offer very good frost protection for the crops with minimum investment (providing that it is well-designed; the product is good and it is well implemented). The (overhead) full cover method uses high quality resistant sprinklers placed strategically above the plant canopy.

The whole plot receives a high application rate, with no blind spots, and nothing is left to chance. The full cover method requires a relatively low number of sprinklers per hectare and is, therefore, easy to maintain. The disadvantage of this system is that its operation requires a high volume of water.

2- Individual targeted protection: This method uses overhead micro sprinklers. In this case, a single sprinkler is positioned above each tree and its flow rate and the area to be irrigated specifically targets the tree. Targeted protection has the distinct advantage of using lower flow rates compared with the full cover method. Consequently, the water requirements per hectare are also much lower. Apart from the fact that lower water requirements provide operational savings, it also means that the farmer can actually provide protection for a larger surface area with the same amount of water. The targeted protection method provides localized frost protection by only targeting the trees, thus avoiding the irrelevant spaces between the rows. Providing that the system is properly designed, this more targeted protection method is much more water-efficient and profitable. Tropical and subtropical crops that might be exposed to winter frosts lend themselves very well to this approach and can benefit fully from a targeted frost protection.

3- Under-canopy protection: Sprinklers positioned under the trees in a plantation can also provide frost protection. In some cases, this might act as an incentive for selecting one kind of irrigation over another. In some cases, plantations that are already being irrigated by sprinklers installed at the foot of the tree can benefit from the system in place for providing frost protection. Sprinklers are well suited for both irrigation and frost protection purposes. Some people favour this kind of frost protection method because it does not require an additional system and it already exists from the beginning. However, the under-canopy method has its limitations. This method can only provide frost protection when the temperatures are no lower than -2.0˚C. Under certain circumstances, while providing protection for the lower parts of the tree, some damage may occur in the higher parts. In short, under-canopy protections provide an adequate option but only under specific conditions.

When should you start your sprinkler irrigation system for the purpose of frost protection

It is of prime importance to know when to activate your frost protection system. There are multiple theories, models and calculations available to determine the precise threshold for activating the system. We will try and make things as simple as possible.

Be careful. When water evaporates (changing from liquid to gas) it absorbs energy and cools its immediate environment. However, when the sprinklers are first activated, they cause the temperature to drop. It is only when the evaporation ceases that the frost protection really begins. In other words, activating the system too late might cause more harm than good. The drier the air, the longer the evaporation process. The higher the relative humidity (RH), the shorter the evaporation period. Knowing the “dew point” allows you to determine the system’s activation threshold. The “dew points” are sometimes available on-line. However, with a knowledge of the RH and the temperatures, the table below can rapidly enable you to find the dew point. 

Generally speaking, the crop’s sensitivity to freezing temperature increases from the first flowering to the fruit or bud development stages. This is when a crop is most likely to be damaged. Sensitivity is also higher when warm weather has preceded a frosty night. Recommendations given here for temperatures that trigger the starting and stopping of the irrigation can be used for sprinklers positioned above or below the plant. All sprinklers in a protection zone should be activated when the air temperature drops to the temperature selected from the above table. This ensures that the temperature of the wetted bulb will be greater than the critical damage temperature. The sprinklers can be switched off when the air temperature exceeds the value given in the table.