What is a drought? When it rains, does that mean the drought is over?
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The official definition of a drought is “a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance in the affected area.”
So firstly it must be abnormal – as in it doesn’t rain as much as it normally does somewhere. If it rarely rains (e.g. a desert) and it still doesn’t rain it doesn’t count as a drought
And the second part is that it must last long enough and be dry enough for there to be a “hydrologic imbalance” which means it must be dry enough to actually impact the environment
This can happen in a few different ways –
Probably the most commonly thought about one is agricultural – when crops start to struggle to grow or die off
Then there is socio-economical which is where the water supply impacts the human population – either with increased cost to industry or a shortage of drinking water supply which can result in water restrictions (or in severe cases places running out of water completely)
There are also environmental droughts which is where the drought starts to impact the natural environment
Most droughts will be a combination of all three depending on where the occur and how severe they are
When it rains it certainly helps to end the drought (commonly referred to as breaking the drought) but it does not necessarily mean it’s over – a drought is not considered over until the “hydrologic imbalance” has been corrected. Which one period of rain might not be enough to achieve.
Or in severe, and cases (like we may see with climate change) until the dry weather is persistent enough to be considered the new normal (note that to be considered persistent enough it would have to be a drought for a very long period of time, probably a generation or two)
Drought is the name given to a weather effect that is significant in that it isn’t typical for the climate
Climate means long-lasting dependable weather patterns that are usually stable trends for a specific area
Because the natural plants (or the planted crops) are usually suited and adapted to the climate that they exist in, a sudden weather event can kill them by changing the living conditions temporarily – but even if the change is only temporary, the dead (or very sick) plants can’t recover, and that can result in problems to the whole ecosystem that sits on the pyramid that exists on top of that producer
Rain doesn’t always end a drought, the rain needs to be back at a level that is either back to typical climate pattern, or at a level to sustain plant life to a recovery, it depends on the definitions you want to use, farmers and scientists will care about different things
When there is a drought many plants die, native, farmer planted. When it does rain the drought is not over. You need a long term period of consistant rain rather than a one off rain event. Things like introduced weeds have adapted to drought conditions so can be the first plants to appear. Not good for anyone.
One rainfall won’t end a drought. Under normal conditions in my home area in Central Europe, during summer it will surely be dry, but if you are digging, there would be a certain amount of humidity being left in the soil deeper down.
When there is a drought, even the humidity deeper down is lost. And even if it is raining then, only the topmost layers of soil are getting wet before the humidity evaporates again. But to have a healthy plant life and enough crops there needs to be enough humidity in the deeper layers. And this is missing more often in the past years.
a drought means that less water falls to sustain life over the year.
It effects the natural vegetation, animal life and even fish in the ocean.
A single rain is welcome but it doesn’t catch the ecosystem up with where it should be. AND with a serious dry spell, much of that water is likely to just run off and not get absorbed into the ground.
(if you let a container plant get toooo dry, the soil will pull away from the pot so when you do water it, most of it runs down the gap on the sides and out the bottom, you have to put the pot in a tub and let it soak up from the bottom till it loosens and swells again)
To add a basic yardstick to determine whether it’s a drought or not (for most areas) would be a combination of assessing currently dam/reservoir levels and rain/snow forecast in the catchment area.
Basically when you can see – given normal water consumption levels combined with evaporation loss and not enough water coming in – that the dam is going to run empty then you’re headed into drought territory.
Each area is different though and some areas can make significant changes – most notably in agricultural sections – to reduce water usage to avoid drought.
Drought is a temporary phenomenon, once you go year on year on year with only low levels of rain it’s no longer a drought and you’ve got to update your climate categorization and plan according – especially in the farming sector.
A drought isn’t necessarily over when it starts raining – it’s important that your forecast and calculations show that your dams are going to start filling up and won’t run down to low levels again between now and when the next rainy season starts.
This can be tricky – especially in many areas that depend on snow melt to supply water for much of the year. So it’s easy to have a situation where you’ve gotten rain but little to no snow in the catchment area – so temporarily you’re fine for water but you’ll be dry again in a couple of months.
Let’s say that, for the past 120 years, the area where you live gets an average of 60cm of rain every year.
Now let’s say that for the past eight years your area has only gotten 30cm of rain every year (average.) Crops are failing, reservoirs are getting low, wetlands aren’t wet enough to support birds and other animals. This is called a drought. You are still getting rain, sometimes in big storms, but overall you aren’t getting enough.
This is a simplistic explanation, (like I’m five!) but others here have spoken to the complexity of the situation. I just wanted you to see that rain does not end a drought. It must rain the normal amount for a few years in order to restore the surrounding environment.
In actual practice, we usually just call it a drought when surface and groundwater levels, and soil moisture contents, decrease significantly over a large region. How do we define significant is not obvious, and there is not a line in the sand where one side is drought and the other is fine and dandy. There are different levels of wetness/dryness which exist, as a spectrum of conditions, so a true drought has to see dry conditions which are extraordinarily unusual for the region (like 5% of less of the time will see such dryness). Kind of the opposite idea of the ten-year and hundred-year flood events (water levels that would be expected about once every ten years or every hundred years, statistically).
To declare a true drought, you have to have a very long history of conditions so you can say, with good statistical support, that this level of dryness is really unusual for the region.
There is a second, more practical, definition of drought which says that water replenishment rates (from natural precipitation) are inadequate to maintain the extraction needs for the region, and this lasts for many months. The weather is not providing for the water needs of the region, and this inadequate provision lasts for many months or more. Natural vegetation suffers as a consequence.
Clearly, you can see that some rain will not be good enough to stop a drought. You need precipitation rates to get back toward normal over time, so the water in the system can return back to normal levels and stay there.
Not that even a bit of rainfall won’t help at all. It will help, but it won’t end the problem on its own. It takes time to dry out the lakes and rivers and ground, and it takes time to provide enough water that the lakes and river and ground become as wet as it usually is.
A rainshower during a drought is like having a tiny sip of water when you are parched from the heat. It helps, a bit, but you will still be thirsty and you will still need more water, a lot more water, if you are to survive.
It is long term, typically measured based on a year or a ‘rainy season’ worth of rain. It can rain many times during the drought but still be significantly below average for the rainy season. So if there is supposed to be 30 inches of rain over the rainy season, there is a drought because last year there was 15 inches of rain in total. And then if there is 16 inches of rain the next rainy season, that has some days of heavy rain, but it still isn’t enough to end the drought, because there is still a big shortfall in the expected rain.
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