2.3 Interception, evaporation and transpiration
Most precipitation reaches the ground, but not all of it, as some is stopped by vegetation, a process known as interception. This is part of a subcycle of the water cycle, involving precipitation, interception and evaporation back to the atmosphere, bypassing that part of the main cycle where water reaches the ground. Evaporation is the process by which water is transferred as vapour from the land or ocean to the atmosphere.
The proportion of the precipitation that does not reach the ground, the interception loss, depends on the type of vegetation, its age, density of planting and the season of the year. The interception loss is 15-35% for coniferous forests and 9-25% for broad-leaved forests (values averaged over a year; the lower values are for tropical rainforest and the higher values for temperate forests). For grassland interception loss is usually lower than for forest, 14-19% for natural grasses. Crops have highly variable values — about 7% for oats, 16% for corn and about 40% for clover, for example. In arid and semi-arid areas, where there is little vegetation, the interception loss is negligible.
The rate of evaporation increases with temperature. The process also depends on the humidity (a measure of how close the air is to saturation with water vapour) and the wind speed. The greater the humidity, the less the evaporation. Wind carries moist air away from the ground surface, so wind decreases the local humidity and allows more water to evaporate. The rate of evaporation is highest from open water. Over the ground surface the rate of evaporation depends on the type of soil and the extent to which the ground is saturated with water. Evaporation from a saturated sandy soil can take place nearly as quickly as it can from open water, whereas evaporation from a saturated clay soil is slower, between 75% and 90% of the rate from open water.
Figure 2.11 shows how evaporation and precipitation vary with latitude. Over two-thirds of total global evaporation occurs within 30° of the Equator, because of the higher temperatures in equatorial and tropical areas. Evaporation reaches its greatest values not at the Equator itself, but between latitudes of 10° and 20° in both hemispheres. The strong trade winds at these latitudes carry water vapour towards the Equator, giving very high precipitation in the equatorial zone where the trade-wind systems converge.
Evaporation also varies with season, because of its dependence on temperature and humidity. In the UK, for example, evaporation is low in winter and high in summer (Figure 2.12). So although precipitation averaged over England is not very seasonal (it is more so in Scotland and Wales; Figure 2.9), the availability of water is due in large part to the seasonality of evaporation.
Figure 2.11 shows that evaporation from the Earth's surface is greater in the Southern Hemisphere than in the Northern Hemisphere. Suggest an explanation for this.
The Southern Hemisphere has more ocean than the Northern Hemisphere, and as evaporation is greatest from open water, evaporation is greater in the Southern Hemisphere.
Vegetation increases the amount of water returned to the air — not only by interception and then evaporation, but also by transpiration. This is the process by which plants draw up water from the soil and transfer it to their leaves, from which it evaporates through pores in the leaf system. Transpiration is controlled essentially by the factors that affect evaporation, and by the type of plant. A considerable amount of water can be transferred to the atmosphere by transpiration: for example, a cabbage transpires about 25 litres of water in total during its growth to full size, and a large oak tree transpires about 400 litres of water each day when in leaf.
Evaporation and transpiration are parts of the hydrological cycle that are difficult to quantify, as it is hard to measure the transfer to water vapour directly. Over land areas it is also difficult to separate the effects of evaporation and transpiration, so the two are usually combined into one parameter, called evapotranspiration. Although actual evapotranspiration is difficult to measure, it is relatively easy to calculate a maximum value of evaporation for a saturated surface, such as open water, using local meteorological parameters such as humidity, temperature and wind speed. This is called the potential evapotranspiration for a particular area. It is the maximum possible evapotranspiration that could take place given an unlimited supply of moisture. Because most land surfaces are neither open water nor saturated, and are partly or wholly covered in vegetation, actual values of evapotranspiration are always less than potential evapotranspiration.
In Britain and other places with a temperate climate, the potential (and actual) evapotranspiration is usually less than the precipitation. The exception is in parts of the east of England where the values of potential evapotranspiration and precipitation are similar (Figures 2.10 and 2.13). However, this is true only on an annual basis, as evapotranspiration is much higher in summer than in winter.
Evapotranspiration is usually greater than precipitation in the summer months, and less than precipitation in winter (Figure 2.14). There are also many areas of the world where the potential evapotranspiration is much greater than the precipitation — usually hot areas where precipitation is low (Figure 2.4). In parts of North Africa and the Middle East the precipitation may be less than 50 mm a year, but the potential evapotranspiration is about 3000 mm a year (Figure 2.15). However, the actual evapotranspiration in these areas is much less than potential evapotranspiration because there is usually little water to be evaporated.
The precipitation that is not intercepted, evaporated or transpired back to the atmosphere either soaks into the ground or becomes surface flow. A rough indication of the quantity of water available from underground or from rivers in any area is given by the excess of precipitation over actual evapotranspiration. This is called the hydrologically effective precipitation.