Water is the most precious substance on earth and this fact has been undisputed throughout history. Water has unique properties that are required for life. In this article we focus on the importance of water in stabilising the earth’s climate.

In a subsequent article we will take a closer look at what you can do to help ensure that there is enough water available in the right places to make the earth habitable and drought proof your farm.

The wonder of water

There are around 1 400 million cubic kilometres of water on earth. It is found in different environments and in three distinct phases.

Next time you drink a glass of ice water consider that you are experiencing water in all three its phases. The solid ice cools the liquid in your glass, and the air you are breathing is full of water vapour. This is possible because it can exist in all three its phases, naturally in the temperature and pressure ranges that are common on earth.

Ice in the polar regions freezes and thaws with the seasons and plays a crucial role in regulating the earth’s climate. Water also has a remarkable ability to absorb and retain heat. The sun delivers a tremendous amount of energy to the earth every day and the tropical oceans absorb much of that energy. In addition, a lot of it goes to creating water vapour, which is the source of all rain and snow. The 13 trillion tons of water in the atmosphere is responsible for absorbing approximately 70% of all atmospheric radiation from the sun.

Water in the different environments

Water’s phase change depends on temperature and pressure. During each phase change a specific amount of latent heat is either absorbed or released without the sensible temperature changing. These different processes contribute to cooling and warming the air in the troposphere (Figure 1).

Figure 1: The phase changes of water.

The oceans contain approximately 97,25% of the earth’s total water volume and cover 70,8% of the planet’s surface. The water in the oceans plays a key role in regulating global temperatures.

Water on land consists of the following:

  • Water is found in solid form in snow and glaciers. This form of water comprises 2,05% of the volume of all water on earth. Moreover, it contains nearly 70% of the world’s fresh water.
  • Only 0,0001% of visible surface water is found in rivers.
  • Lakes and inland seas make up 0,01% of the volume of all water on earth.
  • Groundwater comprises 0,68%.
  • Soil moisture contains 0,005% of the volume of all water on earth.

The volume of water in the atmosphere is almost ten times greater than the volume of water in all rivers. The water in the atmosphere plays a crucial role in regulating local temperature.

Water in living organisms includes water in humans, animals and plants, and makes up approximately 0,00004% of the volume of all water on earth.

The regulating effect of water

Water plays a significant role in balancing the temperature extremes between the seasons, day and night, and different regions. Water vapour is the most abundant greenhouse gas in the atmosphere. Its concentration in the atmosphere varies between 1 and 4%, while the average concentration of CO₂ is 0,0383%.

When there is more water vapour present in the atmosphere, temperatures are moderated to a greater extent. Consequently, there are fewer extreme variations in the weather. Practical experience shows us that where water is lacking in the soil and atmosphere, extreme thermal conditions usually predominate, as experienced in arid and desert regions.

Clouds play a crucial role in regulating the energy balance of the earth’s solar radiation. Clouds usually shadow half the earth’s surface at any given time. Low altitude clouds limit the entry of solar radiation into the atmosphere, which limits evaporation and thus the further formation of clouds.

Thin, high-altitude clouds retain some of the longwave thermal radiation that is emitted from the surface of the earth into space. Thus, it prevents excessive cooling of the planet’s surface.

The earth’s water cycle

The sun provides the energy to facilitate the phase changes of water. The oceans, land and atmosphere store all the water on earth in an equilibrium of different phases and environments. This equilibrium is called the water cycle (Figure 2).

Figure 2: The water cycle by John Evans and Howard Perlman. (Source: United States Geological Survey [USGS])

Water’s ability to absorb and release a large amount of heat allows the earth’s climate to be even-tempered, with an annual turnover of vast amounts of water in the water cycle. There are four main stages in the water cycle, namely evaporation, condensation, precipitation and runoff.

When the sun shines, water evaporates. It turns into water vapour, goes into the atmosphere and turns into clouds. When clouds become dense, the water drops to the earth in some form of precipitation, such as rain, snow, hail or sleet.

Some of it infiltrates into the soil for use by plants or evaporates from the soil. Water that infiltrates deeper could emerge as springs at a lower elevation, while water that does not infiltrate into the soil concentrates on its surface. From here it evaporates or flows to the ocean, from where it evaporates again.

Throughout this continuous movement of water, it is purified, and it drives, cleans and regulates ecosystems. There are two primary components of the water cycle, namely the large water cycle and the small water cycle (Figure 3).

The large water cycle

The large water cycle is the exchange of water between oceans and land. Globally nearly 550 000km³ of water evaporates annually. Around 86% evaporates from the oceans and the remaining 14% from land. Annually, 74% of the precipitation that results from this evaporation fall over the oceans and the remaining 26% on land.

Consequently, roughly 12% of the precipitation that occurs on land is derived from water that evaporated from the oceans. Water vapour is carried high in the atmosphere inland from the ocean over great distances (Figure 3).

Figure 3: A schematic representation of the large- and small water cycles.

The small water cycle

The small or local water cycle is a closed circulation of water on land or over the oceans and is characteristic of a hydrologically healthy area. Water that has evaporated from an area falls as precipitation in more or less the same terrestrial area.

Over the continents water is circulated in many small water cycles simultaneously. Water from the large water cycle is added to these small water cycles. The extent to which moisture saturates soil in a region influences the precipitation in that specific area and its surroundings.

Disturbances in the water cycle equilibrium

Even relatively slight disturbances in the balance of this water system, especially over the long term and spread over many river catchments, could cause problems on land.

This can occur when the volume of rainwater that infiltrates into the soil is reduced. Moreover, these disturbances cause a gradual decrease in the volume of water in the small water cycle over land, resulting in a subsequent reduction in precipitation and an increase in temperature.

This global phenomenon has occurred throughout history and its cause can be attributed to several human activities. Over time the soil moisture content decreases, the subsurface water table declines, vegetation withers and less evaporation takes place from the land.

To ensure that the ecosystem in a river downstream of a reservoir survives, a certain streamflow is allocated and managed according to the ecosystem needs of the remaining length of the river. In the same way it is necessary to consider the water that needs to circulate in the water cycle in order to maintain the entire ecosystem. Consequently, we should not just consider the water that we see, but also the water in the remainder of the water cycle, namely the atmosphere, soil and water in vegetation.

Factors that affect the local balance

There is a perception that over long periods and for large areas this difference in water entering a system and water leaving the system is close to zero. However, evidence shows that sea levels are rising, and experience teaches us that the water reserves on land are declining.

Several external factors beyond the influence of humans affect the global water balance. However, human activities result in a further disturbance of the worldwide water balance, contributing to the desertification of continents. Nevertheless, it could be possible to rectify the problem through deliberate conservation of water on continents. – Fanie Vorster (PrEng), Institute of Agricultural Engineering

For more information and references, phone the author on 012 842 4052 or send an email to vorsters@arc.agric.za.