Hydrologic Cycle Java Applet

Directions and Activities:

This runnable model is a mathematical representation of the graphic below, adapted from an illustration which originally appeared in Scientific American (September 1989, p. 82). The purpose of this model is to provide a modeling environment to investigate several simple "what if" conditions in the hydrologic cycle.

This model is set up to run for a period of 100 years, starting in Year 0. You can change any of the parameters either by sliding the "slider", or by typing in the value you desire. The applet responds automatically to changes in parameters. Currently all four of the main variables -- amount of water in the oceans, the surface/groundwater, the clouds (atmosphere) over the land, and the clouds over the water -- are being plotted.

Java Applet - The Hydrologic Cycle

The illustration and the STELLA model representation are shown below. The boxes represent reservoirs that hold some initial amount of material, in this case cubic meters of water. The thick arrows with the bubble attached are flows, representing fluxes, or movements, of water from one reservoir to another. The circles, or converters, hold constants or algebraic formulas. In this model, we have two converters: one, Temperature, holds the value of 50 degrees Celsius, the "normal" temperature of the Earth's surface; and a converter "Change in Evaporation" that changes the temperature into a percentage change in the amount of evaporation that occurs. For example, a surface temperature of 75 degrees results in a 5o% increase in evaporation.

The values shown at the end of each variable are the amounts that the model uses. For example, the value "8200" after "Surface and Groundwater" says that, at the beginning of the simulation, there are 8.2 x 1015 (or 8200 x 1012) cubic meters (m3) of water total on the surface of the Earth and in the groundwater of the Earth. Likewise, for the flows/fluxes, the values represent the amount of water (in 1012 m3 per year) that moves from one place to another. For example, 71 x 1012 m3 of water per year is transported from the land to the land clouds via evapotranspiration.

Illustration - The Hydrologic Cycle

Image of Global Water Cycle

Figure 1: The global water cycle has three major pathways: precipitation, evaporation/transporation 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 the picture above are x 1012 m3. 

The following information is also included in the model: 


LAND (groundwater and surface water)  8,200 x 1012 m3
OCEANS 1,350,000 x 1012 m3
ATMOSPHERE (ocean clouds) 10 x 1012 m3
ATMOSPHERE (land clouds) 3 x 1012 m3

STELLA Representation - The Hydrologic Cycle

STELLA representation of water cycle

Sample Investigations

  1. Investigate the behavior of each reservoir individually at a surface temperature of 50 degrees. To do this, click on the variable you wish to investigate, and change the Y-axis maximum value according to the range chart below:
    Plottable Variable Y maximum value
    Atmosphere over land 20
    Atmosphere over oceans 20
    Surface/groundwater 10000
    Oceans 1.4e6 (1.4 x 106

  2. Investigate the effect on each of the reservoirs if there is a 25 degree increase in surface temperature during the modeling period (i.e. the temperature becomes 75 degrees). By how much to you have to change the Y-maximum value axis to see the effect of the change over the 100-year modeling period?

  3. How might deforestation (removal of forests) change the amount of water in the oceans? Specifically, what two variables (hint: both are fluxes) might change if deforestation occurs?