Droughts have always been part of life

Weather statistics contain data on droughts in years gone by. There was a significant period of drought in Finland in 1939–1942, which hampered farming and hydropower production in a country fighting a war. Old records indicate that several dry and hot summers were experienced in Southwest Finland around the mid-17th century; apparently, it did not rain once in Turku during the growing season of 1652. More recently the country has been hit by droughts especially in 2002–2003 and 2018–2019.

A world tormented by drought

Many farming regions in the world are afflicted by droughts. More than one tenth of arable fields, and an even larger share of pastures, dependent on rainwater suffer from recurring periods of drought. While irrigating crops accounts for approx. 70% of the global water consumption, water shortages affect more than one half of irrigated arable lands. Over three billion people live in farming regions that suffer from scarcity of water, and almost one half of them face severe shortages of water.

Soil moisture index indicates drought

Soil moisture index describes the level of moisture in the soil. The value is usually determined for a layer of surface soil 60 centimetres in thickness. The reference value is a situation with a high level of soil moisture, such as in spring right after snowmelt. Precipitation, relative air humidity, wind speed and solar radiation are factored in to calculate the index value, and information on net radiation and geothermal heat flow is additionally needed to calculate evaporation from the ground.

Plants need water in early summer

Crops are particularly sensitive to drought at the beginning of the growing season as their developing root systems can reach no further than the surface soil. Nearer to harvest time, heavy rains are harmful as they may flatten cereal crops and cause the grains to germinate. According to an old saying, all rain before midsummer increases the crop yield, and all rain after midsummer reduces it.

Landrace varieties can tolerate drought

The old landrace varieties of cereal crops tolerated drought in early summer better than the today’s improved varieties with a higher yield. May and June are often excessively dry in Finland from the farmer’s point of view. In the times of subsistence farming, a seed that could not survive this dry spell in spring did not produce a crop, and its grains were consequently not used for sowing the field the following year. This was a natural way of improving the cereal varieties to tolerate a dry spring.

Soil is still moist but plants are wilting

There usually is plenty of water in the ground between soil particles, which plants absorb through their root hairs. When drought strikes, the water volume in the ground is reduced, and finally water is only present as a thin film around soil particles. The forces of physics hold this film tightly in place against the particle surfaces, and plant roots are unable to access the water. This may make plants wilt even before all water in the soil has evaporated.

Soil organisms need water

An enormous quantity of small organisms that maintain the soil ecosystem live underground. They need water for their vital functions, and some of them actually swim around in water. When the soil starts drying out, life becomes difficult for everyone. Some organisms die, some resort to dormancy, while others burrow deeper down into layers that still retain some moisture. For this reason there usually is less life in the soil in the middle of the summer than in other seasons, but these organisms recover quickly when moisture returns.

Drought causes stress in aquatic habitats

A long dry period reduces the flow rates in rivers and lakes and lowers water levels, changing the living conditions in them dramatically. In shallow rivers and lakes the water temperature may rise to higher values than normal. Water escapes from the shoreline, leaving sections of the bottom high and dry, and small rivers and drains may dry out completely. These changes put aquatic organisms under stress and may impair or change ecosystem functions. The composition of species may change, and reed beds may take over shores.

Drought triggers dormancy

When drought becomes a problem, the grey worm enters a dormant state. It empties its gut, builds a chamber inside a clod of soil and lines it with mucous that hardens to a glass-like consistency. The worm then curls up into a tight knot inside the chamber and waits for better times. Minute roundworms and tardigrades fall into an even deeper dormancy in which they are completely passive and which allows them to survive not only drought but also severe cold and other extreme conditions.