July 2022 was the driest July in England since 1935. Combined with record temperatures, we are hearing of a drought comparable to the great drought of 1976, with fears of disruptions to public water supplies and low crop yields, especially for fruit and vegetables . But not all droughts are the same and not all farmers are affected by the same type of drought.
To a meteorologist, drought is generally defined as a period of significantly below average rainfall. However, low rainfall, even for an entire season, does not necessarily mean that water supplies will be low, or that industry or agriculture will suffer, as there may be a lot of water already stored in reservoirs and groundwater.
Of course, these reserves are of little help to pastures, cereals, and other crops that are entirely rain-fed and are greatly affected when we have a dry spring and summer. The last 12 months have been particularly dry across much of the UK and since May 2021 only October and February have seen above average rainfall.
Things are even worse if combined with the high temperatures and abundant sun we’ve seen this year, which increases evaporation and depletes soils of the water needed for plant growth – the so-called “agricultural drought”.
We assess the combined impact of low rainfall and warm, sunny weather using the potential soil moisture deficit (PSMD), which is a cumulative measure (in millimeters) of the balance between rainfall input to the soil and potential losses from evaporation and plant transpiration.
When evaporation exceeds precipitation, soils become drier and PSMD increases. When it rains, it decreases. Typically, PSMD starts to increase from late March or early April, peaking in August or September, when soils are drier. A high PSMD means that rainfed crops such as cereals and grass, as well as our gardens, will suffer.
Using data from weather stations in Cambridge, we estimate that the PSMD in 2022 behaved (so far) very similarly to 1976. The deficit started to widen in early March and continued to grow until late July.
This contrasts with the last drought in 2018, when spring was wetter and soil drying was delayed. PSMD is currently at around 350mm, which is about 50% higher than the average peak between 1981 and 2010. So for farmers who rely solely on rainfall, 2022 looks like it could be such a severe agricultural drought. as 1976.
Irrigated agriculture may be restricted
Most grassland and ‘broad’ crops such as cereals and oilseeds are grown in the UK without irrigation. It’s not that they don’t need water, but it is financially unattractive to invest in irrigation equipment.
However, to ensure yields and particularly crop quality, much of the UK’s potato, vegetable and fruit crops receive extra water from irrigation during periods of drought. Dry soil also means that the demand for water for irrigated crops will be greater, competing with the reduction of water resources available to other sectors.
For the water resources manager, “hydrological drought” is when the water available in rivers, reservoirs and groundwater is insufficient to meet demand – including the demand to maintain a healthy aquatic ecosystem.
Potatoes account for more than half of the UK’s irrigated area and volume of irrigation water used. In a dry year, we estimate that one hectare of potato (just over half a football field) needs more than 2 million liters of irrigation water to maintain productivity and quality. That’s more than 40 liters for every kg of potatoes.
As the UK’s irrigated agriculture and horticulture needs a lot of water but is considered a non-essential user, irrigated farmers are at risk of mandatory restrictions during a drought, with potentially serious financial implications.
Here we see a difference between 1976 – which followed a very dry 1975 – and 2022. The Met Office described the rains in 2021 as “unremarkable”. This, along with better water metering and investment in infrastructure to move water from areas of availability to need, means that water resources are in better shape now than they were in 1976.
The maps below show the status of river flows across the country in February 1976 and February 2022. Pink to red indicate river flows that were below normal (pink) to exceptionally low (crimson) for the time of year.
So while this year’s dry and hot weather was similar to that of 1976, with similar effects on our gardens and agriculture, last winter ended with water resources that were mostly normal for the time of year. This means that we do not expect widespread mandatory restrictions on irrigated farms, although some restrictions may be imposed to protect supplies in certain watersheds.
However, despite the water resources situation not being as severe in 2022 as it was in 1976, demand across all uses needs to be managed to avoid a severe hydrological drought this year. It is also prudent to manage our water resources carefully in the summer of 2022, not only to avoid restrictions this year, but also to reduce the risk of more severe restrictions next year if the UK follows this dry summer with a dry winter.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Tim Hess has received funding from the Natural Environment Research Council, the Biotechnology and Biological Sciences Research Council, and the Economic and Social Research Council. He is affiliated with Water and Sanitation for the Urban Poor.
Ian Holman has already received funding from the Natural Environment Research Council