It’s a common misconception that all rain is equally useful when it comes to keeping our water supplies going … people have been talking about “the wettest drought on record”. Whilst it’s true that April was a very wet month according to the Met Office there’s more to rainfall than just the amount.
Summer rain vs winter rain
Soil can only hold a certain amount of water, and it needs to be wet all the way down for rainwater to make its way down through the soil to reach aquifers (underground rocks where water is held in tiny holes in the rock).
During the growing season soils tend to dry out due to warmer air temperatures and because of plants taking up moisture as they grow. This drying is referred to as ‘soil moisture deficit‘ and any rain which does fall will go towards topping up the moisture that’s ‘missing’ in the soil. Only once the soil is full will rainfall start to percolate further into the underlying rocks.
Type of rain – downpour or shower
Because moisture is absorbed slowly into the ground, most of the water from very heavy rain will also run off into rivers. It’s better to have an inch of rain spread over several days, rather than a few hours.
One wet month can’t make up for several dry months. Again, according to the Met Office:
“13 of the last 24 months have seen less than 75% of average rainfall, and 6 months have seen less than 50%. Only 2 months – June 2011 and August 2010, have been significantly wetter than average.”
Whilst researching this blog I found a very useful explanation by the BBC – How much rain is needed to ease the drought?, which includes calculations for the South East, as well as a handy diagram about how groundwater supplies are replenished.
In short, despite the rain it’s still important to save water.