Linda’s post last week about “drought-resistant” plants made
me think about drought and how different types of drought affect gardeners in
different ways. In her article,
she defined drought as “an unusual lack of rainfall”. This is one of four
different kinds of drought that climatologists talk about, and I thought it
might be interesting for you to hear about how the four (or maybe five) types
of drought differ and how they affect gardeners in diverse ways. A great source
of drought information across the U.S. is https://www.drought.gov/.
Meteorological drought
The first type of drought, the one Linda described last
week, is what climatologists consider a meteorological drought. A
meteorological drought is related to how much rain you get compared to usual
conditions at your location. I like to think of it as “too many days of nice
weather in a row”, since in these dry conditions, the sun is shining and it is
a great time to garden, play golf, or do construction. Of course, if you don’t
get rain for a long time, you start to see impacts on plants, water bodies, and
wells, but meteorological drought is usually not identified in terms of
impacts, just on the amount of precipitation measured over weeks, months or
years. Meteorological droughts look different depending on where you are. It is
possible to have drought even in a desert if rain does not fall over an
unusually long time. Droughts in the Pacific Northwest might look quite
different since the frequency and amount of rain looks a lot different there. In
the Southeast, drought can be hard to identify by looks since even when rain
does not fall for a long time, things tend to stay relatively green because in
our worst droughts we still get 35 inches of rain a year. Most gardeners can
cope with meteorological drought by watering their plants at appropriate
intervals and reducing impacts of the dry conditions by mulching to help keep
moisture in the soil.
Agricultural drought
I spend a lot of time talking about agricultural drought
to the farmers and extension agents I work with, because agricultural drought
is always on their mind. Agricultural drought is defined by a negative water
balance that can be related to both lack of rainfall and/or high temperatures
that increase evaporative water stress on growing plants. It occurs mainly in
the growing season because that is when the crops are actively growing and
impacts are most noticeable. A 3-week dry spell may not be a problem for most
gardeners that water their plots, but if you are a dryland farmer without
irrigation, you can lose an entire crop of corn for the year if the dry spell
occurs when the corn is pollinating and the silk dries out before the pollen
can stick to it. Often agricultural drought can occur even when there are no
other impacts to us because it is subtle; most people don’t see the impacts
until months later during harvest. If you have limited access to water for
irrigation or very sandy soil in your garden, then you are more likely to be
affected by agricultural drought since it will be harder to maintain plant
health when the soil is dry.
Agricultural droughts are often related to flash droughts.
Flash droughts are characterized by very rapid development or intensification
over a short time period, and crops are often the first things affected because
of their need for frequent watering. Flash droughts are often characterized by
a lengthy dry spell coupled with very high temperatures, something that is common
when you have a persistent area of high pressure right over your location.
Hydrological drought
Where agricultural drought is related to a shortage of water
over time periods as short as a week to a month, hydrological drought is
related to a shortage of water over months or years. Climatologists measure
hydrological drought as precipitation deficits over periods that range from
three months to multiple years. You can see hydrological drought in dropping
stream, lake, and reservoir levels and in dropping groundwater levels if the
deficit lasts long enough. A hydrological drought can occur even if no
agricultural drought is observed when you get rain at frequent intervals but it
is less than normal over a long time period, as long as the rainfall is enough
to sustain the crops (or if it is winter, when there are not many crops
growing).
Hydrological drought tends to affect gardeners’ access to water for irrigation because the long-term water deficits lead communities to enact water conservation measures to protect drinking water supplies. Most local and state governments have tiered conservation measures that get more strict as the water supplies get lower and lower. They may start by merely providing educational materials on water conservation and then progress to even-odd watering by dates or watering during overnight hours only (since there is less loss of water due to evaporation in cooler night-time temperatures). In the worst droughts, they may cut off the use of water for establishing new lawns and gardens (often with an exception for gardens that are used for food production). If a drought lasts for many years or even decades, then it is considered a megadrought, such as the one that is occurring now in the Southwest U.S. Megadroughts are related to long-period shifts in global atmospheric patterns and can lead to the abandonment of cities because of the loss of water to keep their citizens alive over time.
Socio-economic drought
Socio-economic drought is a little different than the
other kinds of drought mentioned above. It is drought caused by a lack of water
due to overuse, hoarding, or war. An example of a socio-economic drought might
be one caused by one country damming a major river in their country to create a
reservoir, keeping the river water from flowing downstream to other countries
that depend on the water for agriculture or water supply. In the United States,
disagreements between who is allowed to use available water often end up in
court as cases like the Georgia-Florida “water war” that was recently adjudicated
in the U.S. Supreme Court. Locally, disagreements about who is allowed to use
the water sometimes result in tiered water pricing, where the more water you
use, the higher the price. This affects gardeners who have plots that use a lot
of irrigation because of the use of water features, plants with significant
water needs, or lack of mulching or other methods of protecting soil moisture.
Recently, a fifth type of drought called ecological drought has been identified, since a lack of rainfall can affect natural ecosystems in ways that are distinct from gardens, farms, or watersheds. I won’t address it further here, but if you are interested in how natural ecosystems are affected by dry conditions, you will no doubt read about ecological drought in publications in the future.
Drought is a naturally occurring part of the climate across the world, and gardeners must understand the nature of drought in their area to recognize how it affects the weather and climate where they live. Linda’s article last week gives some good guidelines for how to make your garden work in your climate.