Thinking that a drought is over once it starts raining again is understandable. Most people associate the word directly with dry weather, but drought is really an *impact* of the weather over several months or years. Drought is something that takes time to build up and then to fade away again, but exactly how you decide when it starts and stops is an interesting question with several answers. Here’s a brief introduction to some of the details…
Meteorological drought is the simplest type of drought to understand. This simply compares the last, say, 3 month’s precipitation (rain, snow and hail) to historical averages. This doesn’t really describe the impact of the weather but rather the weather itself. Other types of drought relate, for example, the amount of water available for drinking or watering crops. These are more complex because they consider what happens to the precipitation once it has fallen.
The water we use in the areas of the UK currently in drought often comes from underground sources. Normally, rain falls and gradually percolates through the soil (and certain types of rock) to replenish the water we take. When it rains less than normal the balance shifts and reserves go down. Droughts are when these reserves get lower than we would like and might have an impact on us.
When it does start raining again, it takes time for the water to percolate through and rebuild the reserves. When there is heavy rain, the top layers of the soil can’t cope and become water-logged, and any extra water runs off and is lost to the nearest river and eventually the sea. This is why a single wet week doesn’t break a drought.
In other words- drought isn’t about the amount of rain. It’s about how quickly rain replenishes the water sources we use.