FLASH DROUGHTS

1. Context

• Flash drought is sudden, and extreme dry spells are becoming a big concern for farmers and water utilities.
• Flash droughts start and intensify quickly over periods of weeks to months, compared to years or decades for conventional droughts.
• Still, they can cause substantial economic damage, since communities have less time to prepare for the impacts of a rapidly evolving drought.
• Flash droughts also can increase wildfire risks, cause public water supply shortages and reduce stream flow, which harms fish and other aquatic life.

3. Formation of Flash Droughts

• Flash droughts typically result from a combination of lower-then-normal precipitation and higher temperatures. Together, these factors reduce overall land surface moisture.
• Water constantly cycles between land and the atmosphere. Under normal conditions, moisture from rainfall or snowfall accumulates in the soil during wet seasons.
• Plants draw water up through their roots and release water vapour into the air through their leaves, a process called transpiration. Some moisture also evaporates directly from the soil into the air.
• Scientists refer to the amount of water that could be transferred from the land to the atmosphere as evaporative demand a measure of how "thirsty" the atmosphere is.
• Higher temperatures increase evaporative demand, which makes water evaporate faster.
• When soil contains enough moisture, it can meet this demand. But if soil moisture is depleted for example, if precipitation drops below normal levels for months then evaporation from the land surface can't provide all the moisture that a thirsty atmosphere demands.
• Reduced moisture at the surface increases surface air temperatures, drying out the soil further. These processes amplify each other, making the area increasingly hot and dry.

4. Rapid-onset droughts in the world

• The world has probably always experienced rapid-onset droughts, but only in the past decade or two have they become a significant focus of scientific research.
• New data sources and advances in computer modelling have allowed scientists to home in on the complex physical processes behind them.
• The concept also gained attention in 2012 after a severe drought charged across the United States, ravaging farm fields and pastures and causing over $  30 billion in losses, most of them in agriculture.
• In general, this kind of rapid drying occurs when it is warm and rain would normally be falling but very little.
• In such circumstances, the ground might already be wet from earlier rain or snow.
• When the precipitation suddenly shuts off, hot, sunny and windy conditions can cause large amounts of water to evaporate quickly.
• This is why the humid tropics tend to experience more flash droughts than slow ones.
• The wet seasons there are usually rainy enough to keep land and vegetation damp.
• But when the rains fail unexpectedly, the equatorial heat can desiccate the ground to devastating effect.
• The researchers looked at data from computer models on soil moisture worldwide between 1951 and 2014.
• They focused on drought episodes that were 20 days or longer, to exclude dry spells that were too short to cause much harm.
• The trends varied from place to place, but, looked at globally, they show a shift toward more frequent and more rapid flash droughts.

5. Measure to tackle flash droughts

• Conventional droughts, like the Dust Bowl of the 1930s or the current 22-year drought across the southwestern U.S., develop over periods of years.
• Scientists rely on monitoring and prediction tools, such as measurements of temperature and rainfall as well as models to forecast their evolution.
• Predicting flash drought events that occur on monthly to weekly time scales is much harder with current data and tools, largely due to the chaotic nature of weather and limitations in weather models.
• That's why weather forecasters don't typically make projections beyond 10 days there is a lot of variation in what can happen over longer time spans.
• And climate patterns can shift from year to year, adding to the challenge. For example, Boston had a very wet summer in 2021 before its very dry summer in 2022.
• Scientists expect climate change to make precipitation even more variable, especially in wetter regions like the U.S. Northeast.
• This will make it more difficult to forecast and prepare for flash droughts well in advance.
• But new monitoring tools that measure evaporative demand can provide early warnings for regions experiencing abnormal conditions.
• Information from these systems can give farmers and utilities sufficient lead time to adjust their operations and minimize their risks.