Unit 10a.1. Creation of a Global Hydrologic Cycle



The hydrologic cycle is responsible for the largest movement of a chemical substance on our planet. Water movement determines climatic patterns, plant growth, heat energy transfer, erosion rates, and rates of rock weathering. Most of our planet's water (97%) is held in the oceans, while the fresh water we depend on makes up a relatively small percentage of the total global water budget.


Figure 10.a.1



Virtually all of the water on our planet is recycled; a given water molecule can be used over and over throughout the centuries. An understanding of the water cycle is crucial to understanding the movement of nutrients and pollutants across systems. Falling rain intercepts atmospheric pollutants such as nitrous oxide, sulfur compounds, carbon dioxide and bacteria. These are then carried to Earth, where then they can contaminate lakes, rivers, and oceans.


Your assignment is to create a STELLA model using the picture below and the additional information provided. The following is adapted from an illustration that originally appeared in Scientific American (September 1989, p. 82).


Image of Global Water Cycle



Figure 10.a.2

Water cycle



The global water cycle has three major pathways: precipitation, evaporation/transpiration and vapor transport. Water precipitates from the sky as rain or snow, most of which (385,000 cubic kilometers per year) falls into the oceans. It returns to the atmosphere by evaporation. Some flows from the land to the sea as runoff or groundwater; in the other direction, water vapor is carried by atmospheric currents from the sea to the land. Net flow is measured in thousands of cubic kilometers per year. 


All units in Figure 10.a.2 above are x 1012 m3.


You may use data from textbooks, government/educational institution websites, or journal articles to complete your model and answer the lab questions; please include proper citations. Here are two useful sources of information on the hydrological cycle:





Helpful Hints




Questions 10.a.1.1

Two of the components of this model will change with deforestation. Change these two factors in the manner that deforestation would, and state how this affects the amount of water held in the oceans (i.e., increases or decreases) and why.


Questions 10.a.1.2

According to this model, what is the mean residence time of water in the oceans? 


Questions 10.a.1.3

What is the mean residence time of water on land in this model?


Questions 10.a.1.4

Consider the following scenario: Global warming increases ocean evaporation by 50% in the next 50 years.


Questions 10.a.1.5

How will this affect the clouds? (You can answer this question by modeling it).


Questions 10.a.1.6

How do you think that this will affect ocean precipitation? (This is a thought question--as we have modeled ocean precipitation as a constant, your model will not answer this question).


Questions 10.a.1.7

How do you think you could change this model so that the increase in ocean evaporation would affect ocean precipitation?


Questions 10.a.1.8

How would global warming and a melting of all the ice caps and glaciers affect the global hydrologic cycle?





Scientific American (September 1989, p. 82).


Back to index