The water cycle contains the largest chemical flux on earth. Water distributes heat around the globe and thus creates climate, and water is the single most important factor regulating land-plant productivity worldwide. Without water life would not exist (perhaps on ANY planet), and despite the fact that 70% of the earth's surface is covered with water, that water is salty and can't be used for drinking, agriculture, or industry. Only about 0.014% of the water at earth's surface is useable by plants, humans, and other animals.
In attempting to understand element cycles as part of the major functioning of ecosystems, it is useful to follow a specific "approach". This general approach was followed in the previous lecture on the carbon cycle, and it can be used to help understand any element cycle. It consists of 3 parts and is formally outlined below:
Using this approach of gaining knowledge about each of these three components enables you to answer the question of "How will things change?". Gaining this kind of a predictive understanding of ecosystems, or of communities or populations, is the most important goal in basic scientific research. So, let's start with the Accounting in our examination of the global water cycle.
Figure 1. The Laurentian
Figure 2. The distribution of water at the earth's surface.
As you can see from the
Table and from Figure 2, most of the water on earth is tied-up in rocks
and unavailable. Of the water that is at the surface of the earth and available
for cycling, only a very small percentage is fresh water. Of that fresh water,
about 20% is contained solely in the Laurentian Great
Lakes in North America (Figure 1), and another 20% is contained in a
single lake in Siberia,
There are 4 major pathways of cycling in the global water cycle (Figure 3): precipitation, evaporation, vapor transfer from ocean to land, and return flow in rivers and groundwaters from land to oceans. The following gives the flux of these different pathways:
Figure 3. The Global Water Cycle - Pathways and Fluxes. (Values in 103 km3/yr).
There are several major controls on the water cycle, including human consumption, temperature increases, and land use changes.
consumption The consumption of water by humans has
increased dramatically since the industrial revolution, and today water is a
critically lacking resource in certain areas such as deserts and semi-deserts.
In addition to this local vulnerability, it is quite likely that water
shortages due to human consumption will occur at the regional scale in the near
future. For example, the southwestern
Figure 4. Rates of water withdrawn from surface and groundwater sources, and consumption per individual for representative countries.
(B) Temperature The second major control on the
cycling of water on earth is temperature. Increasing temperature increases the rates
of evaporation and ice melting, and causes sea level to rise. Severe droughts,
like in the Sahel in
2. Sea level
rise. Sea level has been rising in the world in recent years. Figures
6 and 7 below show first how large these changes have been in various parts
of the world, and second how much of this increase is due simply to the thermal
expansion of water as temperature Increases. Figure 8 shows the effect of a
rise of 4.7 m in sea level on
Figure 6 & 7 (above). Sea level rise and impacts of temperature (above).
Figure 8 & 9 (below). Impact of sea level rise on
low-lying areas of
As more and more of the ice caps on land melt, there will be an increased river
flow of freshwater from land to the ocean, and especially to the
4. Interactions in the hydrological cycle. One of the important aspects of the hydrological cycle is how temperature will interact with other factors. For example, in 2000 the lake levels in the Laurentian Great Lakes were extremely low, and these low levels had a great impact on shipping and recreation (see pictures below). However, that year the precipitation and temperature were about average, and initially it was unclear just why the lake levels were so low. Based on your knowledge of the main factors involved in the hydrological cycle, can you suggest what might have occurred to cause the lake levels to be so low? (This will be discussed in lecture).
(C) Land use changes Currently most of the land use changes on
earth, such as deforestation, are at a local scale. However, it may soon
become important at regional scales and for the entire globe in the future. For example, in a study done at Hubbard Brook in