Simple Hydrologic Model Assignment
Introduction
In class, we have developed a stock and flow diagram for the natural portion of the hydrologic cycle
(Figure 1). Water moves continually between the various compartments, but the amounts in each
compartment remain relatively unchanged (i.e., at steady state). Is this guaranteed to always be the
case? What might happen if the system is altered by human intervention or climate change? In this
assignment your team will use a system dynamics model to investigate possible perturbations to the
hydrologic system and evaluate the potential outcomes. The goal of this assignment is twofold: 1) to
become familiar with system dynamics models, and 2) to deepen your understanding of the hydrologic
system.
Figure 1. Conceptual model of the hydrologic system.
Tasks
1.
Familiarize yourself with the hydrologic model. The stock and flow diagram that we developed
in class for the hydrologic system (Figure 1) was transformed into a system dynamics model
using software called Stella. The Stella program is available on computers in the third-floor
computer labs, and the “Simple Hydrologic Model” can be downloaded from the Blackboard site
(Assignments folder). A diagram of the model structure is given in Figure 2 below. Once you
access the model file, please save it to a thumb drive so that you can make changes. Familiarize
yourself with the program and the model by viewing the model structure, investigating model
linkages, running the model, changing options in the interface tab, and viewing and adjusting
graphs. A quick tutorial of the model will be given in class to help with this task.
Figure 2: Structure of the Stella Global Hydrologic Model
2. Evaluate climate change scenarios. Turn on the climate change lever, and view the results under
the climate change scenario. Then answer these questions.
a. Compared to the original scenario, what impact does climate change have on the
amount of water in each stock over time? Can you explain why this happens by
referring to the original water cycle in Figure 1 and the model structure in Figure 2?
b. Note that the model simulates 300 years (from 2011 through 2311). Is there evidence
during this time frame that the system is moving toward a steady state? How can you
tell? How do you think this new steady state will compare with the original baseline
case?
c. Simulate a longer time period of, say, 1000 years to test your answer to part 2b (To do
this click on the Adjust Sim Yrs button and adjust the “To” year to something much later
than 2311, say 3011 or even later. What happens? Was your conjecture in 2b right?
How can you explain these results based on your knowledge of the hydrologic cycle?
d. Next, alter the climate change scenario by using the graphical function on the interface
page. Describe your new climate change scenario and describe how this compares with
what you’ve already seen in the baseline case and default global warming scenario. Can
you explain why this happens?
e. Lastly, based on the results of all three scenarios, answer the following questions: How
sensitive are various portions of the hydrologic cycle to climate change scenarios (i.e.,
which stocks are most sensitive)? Given that we don’t know exactly what the
magnitude of future climate change might be, how could a model such as this be used
to help develop policies for mitigating the impacts of climate change?
3. Evaluate groundwater extraction scenarios. Turn on the groundwater extraction lever. Before
running the model, make sure you reset things to their default values by turning off the global
warming scenario, clicking the Clear Graphs button and clicking on the Adjust Sim Yrs button to
reset the model to simulate through year 2311. Run the model and view the results under this
new scenario. Answer the following questions.
a. Compared to the baseline case, what impact does water withdrawal have on the
amount of water in each stock over time?
b. Does it reach a new steady state? How is this new steady state different?
c. How can you explain these results based on your knowledge of the hydrologic cycle?
d. In the model, water extraction only comes from groundwater. How would the results
above change if extraction was primarily from surface water instead of groundwater?
4. Evaluate the completeness of the model. This is a relatively simple hydrologic model. What
important variables are not accounted for in this model (name at least three)? For these
additional variables, how would they be connected to the existing model structure? How might
the results above change if these variables were included?
Deliverables
Prepare a written report that answers all of the questions posed in numbers 2-4 above. Provide
thorough and thoughtful responses. You will be evaluated on the depth and quality of your answers,
not merely the accuracy of your answers. Where appropriate, include graphs to support your responses
(Hint: You can save any graph from the model by right-clicking on the graph and selecting Save as
Image. Then save the graph as a “jpg” or “gif” image that you can import into your write-up).