Water balance: when rain exceeds evaporation

Rain against evaporation demand

Rain is easy to measure. One millimetre of rain is one litre on every square metre, or ten cubic metres on a hectare. The other side of the account is harder, because evaporation depends on the surface. Bare soil, a lawn, an olive grove and a pine wood lose different amounts of water under the same sky.

To separate the weather from the plant, the FAO uses a reference surface: a hypothetical grass, 12 centimetres high, actively growing and well watered. The water it would lose is called reference evapotranspiration, ET0, and is given in millimetres per day or per month. Because the surface is fixed, ET0 depends on the weather alone, on four quantities: radiation, air temperature, humidity and wind speed. Sun supplies the energy, warm and dry air takes up the vapour, and wind carries it away.

The Penman-Monteith idea

The standard method for calculating ET0 combines two lines of thought. The English physicist Howard Penman showed in 1948 how evaporation can be calculated from the energy available and from the drying power of the air. John Monteith added in 1965 how the surface itself resists the escape of water vapour. The FAO Penman-Monteith method fixes this resistance for the reference grass and so needs only weather data: daily maximum and minimum temperature, humidity, wind speed at two metres, radiation or hours of sunshine, plus altitude and latitude. An FAO expert consultation recommended it in May 1990 as the single standard method, because it gave consistent results in arid and humid climates alike.

No one needs to solve the equation by hand to use the water balance. What matters is the principle: a sunny, warm, dry and windy day draws much more water out of plants and soil than a cloudy, cool, humid and still one, even if not a drop of rain falls on either.

From the reference to the plant

ET0 describes the demand of the weather, not the consumption of a particular plant. The FAO bridges the gap with the crop coefficient, the ratio between the evapotranspiration of a crop and ET0. A young planting on mostly bare ground uses less than the reference grass, a dense, fully grown canopy can use as much or more. When the soil runs dry, plants close their stomata and transpire less than their coefficient would suggest. They save water at the cost of growth.

For the water balance of a garden or a slope, ET0 is therefore the yardstick, not the exact consumption. It shows when the weather starts to take more than the rain brings.

The soil as a store

Between rain and evaporation stands the soil. The FAO calls the water a soil can hold for plants, between field capacity and wilting point, the total available water. It depends on texture and rooting depth. In the FAO's worked examples a loamy sand holds about 90 millimetres per metre of soil, a silt about 170. Plants cannot use all of it without stress; for many crops FAO takes half as the share they can draw on before growth suffers.

This store explains why the water balance is kept month by month and not day by day. A surplus in autumn and winter first refills the soil. When the account turns negative, plants live for weeks on that reserve. Only when it is used up does drought begin for the roots. Everything that helps water soak in and builds humus enlarges this store.

The year in a Mediterranean climate

In a Mediterranean climate the account swings strongly, and how strongly depends on the place. The station of Genna Cresia lies at 272 metres in Ogliastra, on the east side of Sardinia. According to the climate normals of ISPRA, Italy's national environmental agency, it received an average of 818 millimetres of rain a year from 1991 to 2020. Most of it falls from October to April: 80 millimetres in October, 152 in November, 122 in December, 95 in January, 74 in February and 76 each in March and April. Then the rain almost stops: 36 millimetres in May, 16 in June, 12 in July and 11 in August, before September opens the new rainy season with 67.

Monthly ET0 values for Sardinia were published by the University of Sassari in 1986, calculated with the Penman method for the years 1970 to 1984. For the plain near Oristano on the west coast they add up to 1,145 millimetres a year: 34 in January, 46 in February, 75 in March, 100 in April, 132 in May, 158 in June, 176 in July, 160 in August, 113 in September, 75 in October, 45 in November and 31 in December. These values belong to another place, but they describe the same climate and give the order of magnitude.

Set side by side, the two series divide the year into three parts. From November to February rain clearly exceeds the evaporation demand, by around 290 millimetres in total. In October and March the two roughly balance. From April to September evaporation dominates, by around 620 millimetres in total, most of all in July, when almost fifteen times the rainfall could evaporate. Over the whole year, a good 300 millimetres are missing. The winter surplus is therefore smaller than the summer deficit. The soil can store only part of it, and the rest has to be made up by deep roots, mulch, shade or stored water.

How much the place matters shows in a comparison on the same coast. The station of Capo Bellavista near Arbatax, about 20 kilometres away and close to the sea, recorded only 420 millimetres a year from 1971 to 2000 according to the climate atlas of the Italian Air Force weather service, a little over half. There rain exceeds evaporation only from November to January.

Reading the water balance yourself

Measure the rain. A simple rain gauge, placed in the open and away from roofs and trees, shows within a season how far the local rain departs from the nearest station. On a slope a few kilometres from the coast it can be quite different. The microclimate of a site often matters more than a regional average.

Find the demand. Many agrometeorological services calculate ET0 for their stations by the FAO method. Where no such values exist, the FAO guidelines describe a simpler estimate from air temperature alone. An open pan of water also shows evaporation, but the FAO warns that pan readings depend strongly on how the pan is placed and kept, and are unreliable.

Look into the soil. The decisive figure is not in the sky but in the ground. A spade test shows how deep moisture reaches. The feel method of the US Natural Resources Conservation Service judges a handful of soil by how it looks and how it holds together when squeezed into a ball. Soil that leaves a moist mark and keeps its shape still holds plenty of water; soil that crumbles and runs through the fingers has given most of it up.

What follows for planting, pruning, mulch and storage

Planting. New plants go into the ground at the beginning of the wet half of the year, when the soil has been soaked again. They then have the whole season of surplus to grow roots before the first summer. The Mediterranean garden year follows this rhythm.

Pruning. The pruning rule of this wiki is the water balance turned into practice: pruning happens in the rainy season, from the first heavy autumn rains into early spring. Chop and drop works in this window: cut plants regrow on moist soil, and the cuttings decay instead of drying to fuel. In summer, when the deficit is greatest, the shade stays, because it keeps the burning sun off leaves, bark and soil.

Mulch. Evaporation from bare soil is part of the debit side. A layer of mulch shades the surface and slows that loss, so the stored water stays for the roots. Mulch laid at the end of the wet season carries the soil moisture further into summer.

Storage. A winter surplus that runs off is lost to the site. Swales and other earthworks hold it in the slope, and a cistern keeps roof water for the dry months. Water stored in winter can be spent when the account is empty.

How Terra Humana understands the water balance

For Terra Humana the water balance is the reason behind the calendar. We plant with the rain and prune in the rainy season, because then the soil carries the stress of a cut or a new root. In summer we leave the shade standing, because the deficit is greatest then and the sun burns hardest. The balance is read on the site: in the rain gauge, in published ET0 values and above all in the soil. Where we have no measured values, we say so.