Factbox 4.
Precipitation And Evaporation

By Professor Nils Nykvist

Only part of the precipitation reaches the ground in a plant community, because it is caught in the vegetation and then evaporates. In a low-productivity forest in South Africa about 6% was stuck in the canopy, 4% on grasses and herbs, and 8% on the litter layer [26]. In more dense forests, more than 30 % of the precipitation can get caught in the tree canopy

The evaporation of water from the plant surfaces is a loss of water for the plant community but may be important in warmer climates because of its cooling effects. The rainwater that does not evaporate from the vegetation comes down to the ground as rain, drips from the canopy, and runs down tree trunks. The loss of water from the soil surface can be reduced by covering the soil surface with crop residues (called mulching).

From the sun comes not only the energy that is essential for photosynthesis, but also the energy in form of heat which is necessary for all living organisms. Since plants cannot live when the water freezes inside the cells, the number of frost-free days during the year will be one of the most important conditions for plant growth in the world. But high temperatures can also limit plant growth mainly by less water availability due to greater evaporation from the soil. The importance of temperature and water availability in the soil for plant production is therefore best summarized in a description of the different climate zones in the world.

One of the most commonly used system for classification of climate is the Köppen system which in principle is based on different vegetation types in the world. The rainy (humid) tropical climate which is located at the equator is divided into rainforest climate and savanna climate. The dry (arid) climate where annual evaporation is greater than the annual rainfall is divided into desert climate and steppe climate.

Between the dry climatic areas which exist on both sides of the equator, and the Polar Regions are the temperate humid climate zones situated. The average temperature -3oC during the coldest month is a limit to the warm temperature climate where deciduous forests dominate and the cold temperature climate which is mainly coniferous forests (taiga).In areas with polar climate, the average temperature for the warmest month is below + 10oC which largely coincides with the Arctic tree line.

The Köppen system also includes mountain climate zones where the height over the sea level and the influence of the mountains on wind and precipitation strongly affects the local climate.

The amount of water in the soil is determined primarily by the supply through precipitation, loss through evaporation from wet surfaces on plants and soil, and transpiration of water by living plants that mainly occurs through their stomata which can be regulated by the availability of water in the soil. A summary of the loss of water from wet surfaces and the transpiration of water by plants is evapotranspiration.

One measure of the climate’s impact on the water content of the soil is, in addition to rainfall, the potential evaporation, which is a measure of the evaporation from a free water surface. It can be determined by measuring the water loss from a water-filled open pan, or calculated from temperature and other meteorological data, according to Penman [25].

In areas with high rainfall, the potential evaporation is often less than the current water consumption in the ecosystem, but in areas with low rainfall, it is much higher. A comparison of total precipitation and potential evaporation rates over time provides a good overview of the impact of climate on the amount of water in the soil. How much of this water the plants can use depends on the run-off from sloping grounds, losses to the ground water, and the amount of water bound in the soil in plant-available form.

See the references for this article.

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