Dealing with Heat & Drought


by Dr. Ed Brotak

With the arrival of spring, farmers and gardeners look forward to the start of the growing season. As temperatures warm, spring planting can begin. Fruit trees will break winter dormancy. Pastures will start to green up. Livestock become more active. But as spring turns into summer, the weather can also provide challenges — the greatest of which are heat waves and droughts. Temperatures may soar past levels where plants and animals begin to be affected and can reach a point where production is negatively impacted. At worst, damage or even death can occur. Drought is an even greater threat to crops. A lack of water causes even more immediate production losses and a total loss is certainly possible. For many locations, heat and drought go hand in hand during the summer, and just about every year somewhere in the country heat waves and drought occur.

What constitutes hot temperatures depends on where you live. For Fairbanks, Alaska, 90°F is rare but has occurred. In Columbia, South Carolina, where it can top 90°F many times in the course of a summer, even 100 degrees is not that unusual. This is important since to a large degree agricultural operations are geared for normal conditions; the type of temperatures normally experienced and expected. With the relatively cool waters of the Pacific just offshore, the West Coast has only brief hot spells when an offshore flow develops in summer. From the Rockies eastward, abnormally hot conditions become more of a periodic threat.

Livestock and poultry can be directly affected by heat. For cattle, temperatures of 80°F to 85°F will start to have an impact. Temperatures above 90 degrees can pose serious health risks. The same is true for sheep and goats. Swine are even more susceptible to the heat. With poultry, egg production will start to fall off with temperatures above 80°F. When temperatures get above 85°F, significant physical effects are noted. Temperatures above 90°F can lead to heat stress, illness and even death.

For plants, heat can also cause problems, but it’s a lack of water that is most critical. As soon as the water needs of a plant aren’t being met, you start having problems. You can have reduced yield for edible plants or crops even without visible damage. Temporary wilting can occur and even if the plant recovers, growth can be stunted. Plants may shed their leaves to conserve water. Permanent wilting means death for annual plants and an end to the growing season for perennials. Water is more critical in certain life stages such as germination and initial development when roots are small. The reproductive phase also requires more water. Water usage varies with plant type with some plants being more or less susceptible to the effects of drought. Whereas other types of drought take weeks or months to develop, plants can begin to feel the effects of reduced water within days of even a good soaking rain.

Like heat, drought is a problem even when it occurs only periodically. In the Southwest, you know it’s going to be dry and you can allow for that. Along the West Coast, you can expect dry summers, increasingly long and dry as you head further south. But for much of the country, rainfall during the growing season is common. It’s the unusual lack of rain that causes problems.

In terms of weather patterns, drought and heat in summer have the same source. An upper-level ridge of high pressure is the culprit. A ridge is a large mound of warm air that extends miles up into the atmosphere. Under the ridge and to its east, the air is sinking. Air warms as it sinks, so any clouds would dissipate in the sinking air. Besides being warm to start with, the air is heated even further by the strong summer sun shining through cloudless skies. What moisture there is in the ground is quickly evaporated. The dry ground heats up even more, warming the air above it and further strengthening the upper ridge. With upper-level weather systems covering hundreds if not 1,000 miles or more, it’s certain that some place in the country will suffer through a heat wave and drought during any given summer.

Predicting Drought

Can meteorologists predict heat waves and droughts in advance? To a certain extent, yes. The complex computer models that are the basis of weather forecasting today are actually pretty good out to two weeks, especially in terms of upper-level features. Just go to the National Weather Service website,, and click on Climatic Outlooks which are produced by the Climate Prediction Center. You can see if your region is headed for hot, dry conditions. The Climate Prediction Center also issues the U.S. Seasonal Drought Outlook. This is a prediction based on long-range mathematical and statistical models. The Outlook is issued on the third Thursday of each month (it’s tied to the running of those models) and updated on the first Thursday. Starting with areas already designated as drought stricken, the Outlook predicts whether things will persist or get worse, improve somewhat, or will improve dramatically. It also highlights areas where drought is expected to develop. The Outlook covers the next three months. Always keep in mind that such long-range forecasts can be off considerably. The science of weather forecasting has not yet developed to the point of making highly accurate long-range forecasts.

So, what can we do to combat these summertime weather extremes?

Mulching can reduce direct evaporation of moisture from the soil. This can be especially helpful for seed germination.

For plants, water is critical. Plants evaporate water through the process of evapotranspiration. They transpire water through their leaves and as it evaporates, it helps cool the leaf. But water is also critical since it is the food/nutrient transport system in plants (similar to blood in an animal). And the water vital for plant existence is taken from the soil by the root system.

One caveat of this is the importance of soil type. Sandy soils drain quickly, not retaining much water for plants. Pure clay soils are often too wet for good root development. Loam soils (a mixture of sand and clay) are best. Mixing in organic matter also helps retain moisture.

Mainly, we have to provide water — watering gardens and irrigating crops. For farmers, the decision to put in an irrigation system is often dictated by economics. One must consider the cost of the system versus the possible crop losses due to drought. The statistical probability of a drought in your area would be a major factor.

How much water do we need to provide? The actual amount of water you should supply depends on the remaining moisture content of the soil. This is often difficult to measure precisely. Certainly you can get an idea of how dry a soil is by just feeling it. Some of the state ag stations actually keep track of soil moisture, but keep in mind this can vary a great deal regionally. The soil moisture supply is a function of rainfall and evaporation. Rainfall can be measured by simple rain gauges which are inexpensive and available at many stores featuring outdoor goods. Evaporation from the soil and evapotranspiration from plants is almost impossible to measure in the real world. Various agricultural weather sites measure “pan evaporation,” evaporation from an open water surface. This gives at least an idea of how much water is being lost. Amounts can be significant. On a hot, dry summer day, one-quarter to one-third of an inch of water can evaporate in one day.

Livestock Considerations

For animals, tolerance to heat is directly related to water supply. Cattle and horses cool themselves by sweating like people do. Chickens and pigs pant like dogs do. In both cases, internal water is evaporated causing a cooling effect. With an adequate water supply, animals can deal with a certain amount of excessive heat. Dehydration is much more of a concern.

In terms of livestock and poultry, we must consider the humidity as well as the temperature in judging heat effects. The Temperature-Humidity Index, now more commonly called the Heat Index, was developed to ascertain the effects of heat on humans but also works for animals. The rate of evaporation and thus the ability of a body to cool itself is a function of the relative humidity of the air. Dry air allows more evaporative cooling. So at the same air temperature, moist air feels warmer to people and animals and puts more heat stress on them.

How can we combat heat stress in livestock and poultry? Basically, we can use the same methods we use for humans (although air conditioning would be a bit extreme). Provide sufficient clean and cool water to alleviate the threat of dehydration. Provide shade. Temperatures in the sun can be 10 to 15 degrees warmer than in the shade. In enclosures, ventilation helps. It will keep the heat from building up and aid evaporative cooling. This can be as simple as having open sides on a shelter or installing ventilation fans. Foggers or misters can also be used.

This article appears in the April 2015 issue of Acres U.S.A.

Online Drought Information

Two excellent sources of drought information are the U.S. Drought Portal; and the National Drought Mitigation Center; Of particular interest is the U.S. Drought Monitor. This shows, at a glance, all regions of the country affected by drought and to what extent. Click on a specific area and you’ll get a close-up view. Drought information as well as other data is available on the government’s Weekly Weather and Crop Bulletin; Weekly/Wwcb. Another great source of agricultural weather information (as well as all kinds of useful tips) would be your local/state agricultural experiment/horticultural research station. Search for your closest site to get specific data for your area.

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