System Dynamics and hydrology

[Antarpreet anj024 mail.usask.ca]

Posted by Antarpreet <anj024@mail.usask.ca>
I am a graduate student in University of Saskatchewan and am applying SD
principles to model hydrology of reconstructed watershed. I am just curious
to know how does SD principles differ from other models. E.g., there is feed
back mechanisms in conventional watershed models. Water is not allowed to go
into soil when it is saturated. Tank model by Suguwara is based on these
principles. Even HSPF make use of such principles. Except for transparency
in methodology, I completely failed to understand to understand how the
models differ? What I feel is that SD methodology makes models less complex
in terms of understanding the processes and modeling. SD refines art of
modeling than the conventional models.

Can you please suggest me some references that compares differences between
conventional and SD models? Sorry for any inconveniences caused, but I will
appreciate if you can be in position to send some relevant material or
suggest some relevant literature.

Thanks and regards

antar
Posted by Antarpreet <anj024@mail.usask.ca>
posting date Sunday, April 17, 2005 3:33 AM

[Dudley Richard rgd6 cornell.edu]

Posted by ""Dudley, Richard "" <rgd6@cornell.edu>
Antar:

That is a great question, and there are others better qualified to answer
it. Nevertheless I will give you my opinion. In fields that have a long
history of modeling it may be harder to justify an approach different from what is already used.

SD models are essentially sets of differential equations (now) set in a user
friendly graphical interface and solved by the software using a specified
time step. Thus very complex models that can not be solved analytically
can be constructed (e.g. with many state variables = levels). There are
mathematical software packages that can also do this, but the SD software is
more user friendly and can be used to assist in modifying the model in
cooperation with ""domain experts"". That is, in creating _new_ models of
specific complex situations in cooperation with people from other fields.

If the models constructed are merely going to be used by technical personnel
then the usefulness may be less, especially if these people are already
familiar with another approach that is accepted as useful. This is
especially true if you are merely recreating a standard model used in your
field.

On the other hand using the SD approach it is fairly easy to build rather
complex models with lots of feedbacks.

The SD philosophy emphasizes feedbacks and the software helps to create and
examine these. I am not sure if pure hydrologic models have a lot of
feedbacks, but coupled hydrologic reservoir / power generation / flood
control models would -- I assume.

I am not sure what models you are using and how they are constructed. It is
possible to use SD software to construct models that are not true to SD
""quasi-standards"". For example some people use SD software but have models
constructed with many ""if then else"" statements..... more like a computer
code model. In such cases it may be better to stick with other approaches.

Richard
Posted by ""Dudley, Richard "" <rgd6@cornell.edu>
posting date Mon, 18 Apr 2005 15:30:43 +0700

[Paul Newton plnwtn gmail.com]

Posted by Paul Newton <plnwtn@gmail.com>
Donella Meadows' and Jennifer Robinson's 1985 book, ""The Electronic Oracle:
Computer Models and Social Decisions,"" compares the views of modelers from
four different schools of modeling: econometrics, input-output analysis,
optimization and system dynamics. The book's description of the
characterisitics that distinguish system dynamics will probably be useful in
helping you answer your question.

For me, the ultimate distinguishing characterisitic of system dynamics is
its focus on the role of feedback loop structure in creating dynamics. It is
not that other models don't contain feedback, but rather the emphasis that
system dynamicists place on the importance of feedback dynamics. However, in
their Electronic Oracle book, Meadows and Robinson also discuss additional
salient distinguishing characteristics system dynamics practice.

Hope this helps...

Paul Newton
607-273-0569
paulnewton@StewardshipModeling.com
Posted by Paul Newton <plnwtn@gmail.com>
posting date Mon, 18 Apr 2005 09:59:57 -0400

[Malczynski Leonard A lamalcz san]

Posted by ""Malczynski, Leonard A"" <lamalcz@sandia.gov>
Antar,

We have a small group here at the laboratory that partners with several
universities to build 'water' models.
Both of the primary modelers are out of town at the moment so let me try
to summarize what we have been doing in relation to hydrology.

Most of the models have a strong hydrology component. In most cases this
is in order to build confidence in the model outcomes. River reaches,
reservoirs, agricultural and municipal diversions, ground and surface
water interaction, precipitation, and population have all been modeled.
Nutrient loads, dissolved chemicals, riparian benefits, etc. are
currently being worked on. In at least two cases there has been
significant citizen and interest group participation in the model
construction.

As Richard said, in many cases SD tools are being used to model non-SD
problems in this area. We have started to model some of the social
implications of water use such as: water rights banking (buying, selling
and storing water or water rights), water compact agreements, and
non-use benefits.

We have found that the interfaces to the model that we have built have
been very useful with the public and policy makers in first of all,
understanding the complexity of the water use situation in New Mexico.
It one case the conservation of water in Albuquerque has been
detrimental to the in-stream flow in the Rio Grande. Many people did not
realize that the city primarily uses ground water and the treated water
is sent into the Rio Grande. Conserving water in the city has reduced
return flows to the river. This becomes a large issue in times of
drought (now) and when the city may violate the Endangered Species Act
here in the USA. This was an 'Aha!' moment for many.

You might also want to check the SD bibliography. There are several
papers by Simonovic on broader water issues.

Len
Posted by ""Malczynski, Leonard A"" <lamalcz@sandia.gov>
posting date Mon, 18 Apr 2005 14:46:52 -0600

[Jay W. Forrester jforestr MIT.ED]

Posted by ""Jay W. Forrester"" <jforestr@MIT.EDU>
Perhaps you will find useful material below.

Information on System Dynamics
Jay W. Forrester
Information revised
February 4, 2005

System Dynamics Bibliography: http://www.systemdynamics.org/biblio/sdbib.html

To order the system dynamics bibliography of over 4100 entries,
specify IBM type PC, or Macintosh

System Dynamics Society
Roberta Spencer, Executive Director
Milne 300--Rockefeller College
State University at Albany
Albany, NY 12222 USA

tel: 1-518-442-3865Ý
fax: 518-442-3398
email: System.Dynamics@albany.edu

Three formats are available:

1. For Endnote, a very effective bibliography
software available for either Macintosh or PC
from:

ISI ResearchSoft
www.endnote.com
Tel: 760-438-5526

I use Endnote and recommend it and use it to search for the references.

2. An exported version with field delimiters that
presumably can be loaded into some other kind of
database.

3. A listing that one can look at in a word
processor and do some simple finding operations.

-----------------------------------------------

Membership in the System Dynamics Society and
subscription to the System Dynamics Review are
US$90 per year for regular members
and US$45 for students.
Send application to the System Dynamics Society (above)

To contact the office of the System Dynamics
Society and to order copies of the ""Beer Game""
group simulation exercise: tel: 1-518-442-3865
fax: 518-442-3398
email: System.Dynamics@albany.edu
----------------------------------------------

MIT history of system dynamics development:
From the System Dynamics Society can be obtained
a DVD disk containing several thousand memos
written during the first several decades of
developing the field of system dynamics. The
disk also contains more than 200 theses that were
written in that same time period.


-------------------------------------
There is a system dynamics discussion group on
the Internet. To join, send email to:
listserv@listserv.albany.edu In the body of the
message, enter the following line:

Subscribe sdmail your name


------------------------------------
The next annual international conference of the
System Dynamics Society will be in Boston,
Massachusetts, USA, July 17-21, 2005. Write to
the System Dynamics Society,

System Dynamics Society
Roberta Spencer, Executive Director
Milne 300--Rockefeller College
State University at Albany
Albany, NY 12222 USA
tel: 1-518-442-3865Ý
fax: 518-442-3398
email: System.Dynamics@albany.edu
----------------------------------------

The publisher for books in the following block
has changed from Productivity Press to Pegasus
Communications.

Pegasus Communications, Inc.
One Moody Street
Waltham, MA 02453-5339

Within the U.S
tel:1-800-272-0945
fax: 1-800-701-7083
Outside the U.S.
tel: 781-398-9700
fax: 781-894-7175
Web page: www.pegasuscom.com

Alfeld, Louis Edward, and Alan K. Graham. 1976.
Introduction to Urban Dynamics. Waltham, MA:
Pegasus Communications. 333 pp.

Forrester, Jay W. 1961. Industrial Dynamics.
Waltham, MA: Pegasus Communications. 464 pp.

Forrester, Jay W. 1968. Principles of Systems.
(2nd ed.). Waltham, MA: Pegasus Communications.
391 pp.

Forrester, Jay W. 1969. Urban Dynamics. Waltham,
MA: Pegasus Communications. 285 pp.

Forrester, Jay W. 1971. World Dynamics. (1973
second ed.). Waltham, MA: Pegasus Communications.
144 pp. Second edition has an added chapter on
physical vs. social limits.

Forrester, Jay W. 1975. Collected Papers of Jay
W. Forrester. Waltham, MA: Pegasus
Communications. 284 pp .

Forrester, Nathan B. 1973. The Life Cycle of
Economic Development. Waltham, MA: Pegasus
Communications. 194 pp.

Goodman, Michael R. 1974. Study Notes in System
Dynamics. Waltham, MA: Pegasus Communications.
388 pp.

Lyneis, James M. 1980. Corporate Planning and
Policy Design: A System Dynamics Approach.
Waltham, MA: Pegasus Communications. 520 pp.

Mass, Nathaniel J., ed., 1974. Readings in Urban
Dynamics: Volume I, Waltham, MA: Pegasus
Communications, 303 pp.

Mass, Nathaniel J. 1975. Economic Cycles: An
Analysis of Underlying Causes. Waltham, MA:
Pegasus Communications. 185 pp.

Meadows, Dennis L. 1970. Dynamics of Commodity
Production Cycles. Waltham, MA: Pegasus
Communications. 104 pp.

Meadows, Dennis L., et al. 1974. Dynamics of
Growth in a Finite World. Waltham, MA: Pegasus
Communications. 637 pp.

Meadows, Dennis L., and Donella H. Meadows, ed.,
1973. Toward Global Equilibrium: Collected
Papers, Waltham, MA: Pegasus Communications, 358
pp.

Morecroft, John D. W., and John D. Sterman, ed.,
(1994). Modeling for Learning Organizations,
Waltham, MA: Pegasus Communications, 400 pp.

Randers, Jorgen, ed., 1980. Elements of the
System Dynamics Method, Waltham, MA: Pegasus
Communications, 488 pp.

Richardson, George P., and Alexander L. Pugh III.
1981. Introduction to System Dynamics Modeling
with DYNAMO. Waltham, MA: Pegasus Communications.
413 pp.

Roberts, Edward B. 1978. Managerial Applications
of System Dynamics. Waltham, MA: Pegasus
Communications. 562 pp.

Roberts, Nancy, David Andersen, Ralph Deal,
Michael Garet, William Shaffer. 1983.
Introduction to Computer Simulation: A System
Dynamics Modeling Approach. Waltham, MA: Pegasus
Communications, 562 pages .

Schroeder, Walter W., III, Robert E. Sweeney, and
Louis Edward Alfeld, ed., 1975. Readings in Urban
Dynamics: Volume 2, Waltham, MA: Pegasus
Communications, 305 pp.

---------------------------------------------------
Books from other publishers include:

Coyle, R. G., 1996. System Dynamics Modelling--A
Practical Approach, London: Chapman & Hall. 413
pp.

Fisher, Diana M. (2001). Lessons in High School
Mathematics: A Dynamic Approach. Hanover, NH,
isee Systems, Inc.

Fisher, Diana M. (2005). Modeling Dynamics
Systems: Lessons for a First Course. Hanover, NH,
isee Systems, Inc.

Ford, Andrew, 1999. Modeling the Environment: An
Introduction to System Dynamics Modeling of
Environmental Systems, Washington, D.C.: Island
Press. 415 pp.

Mandinach, Ellen B., and Hugh F. Cline, 1994.
Classroom Dynamics: Implementing a
Technology-Based Learning Environment, Hillsdale,
NJ: Lawrence Erlbaum Associates. 211 pp.

Richardson, George P., 1991. Feedback Thought in
Social Science and Systems Theory, Waltham, MA,
Pegasus Communications. 374 pp.

Richardson, George P., 1996. Modelling for
Management: Simulation in Support of Systems
Thinking, Brookfield, Vt.: Dartmouth Publishing.
493 & 447 pp.

Sterman, John D. (2000). Business Dynamics:
Systems Thinking and Modeling for a Complex
World. New York: Irwin: McGraw-Hill. 982 pp.

----------------------------------
A self-study guide to system dynamics, called
""Road Maps,"" is available for downloading from:

http://sysdyn.clexchange.org

or in paper copy from:

Creative Learning Exchange
Ms. Lees Stuntz, Director
1 Keefe Road
Acton, MA 01720, USA
tel: 1-508-287-0070
fax: 1-508-287-0080
email: stuntzln@tiac.net
------------------------------------------------

For those wanting information on introducing
system dynamics in kindergarten through 12th
grade education:

1. The Creative Learning Exchange is a nonprofit
foundation that acts as a clearinghouse to
provide information on system dynamics in
precollege education and to help teachers share
their experiences. They can be reached at:

Creative Learning Exchange
Ms. Lees Stuntz, Director
1 Keefe Road
Acton, MA 01720, USA
tel: 1-978-287-0070
fax: 1-978-287-0080
email: stuntzln@tiac.net
web: http://www.clexchange.org

2. The System Dynamics in Education Project at
MIT has transferred its web page to the Creative
Learning Exchange at http://sysdyn.clexchange.org

3. An Internet discussion group on K-12 issues
related to system dynamics can be joined:
Contact: StuntzLN@CLEXCHANGE.ORG
To subscribe,
Please provide the following information:
First Name:
Last Name:
E-mail:
Title:
Organization:
Address:
City:
State or Province:
ZIP or Postal Code:
Country:
Day Phone Number:
Evening Phone Number:
Fax Number:

5. The summer 93 issue of the System Dynamics
Review, vol 9 no. 2, was a special issue on
""Systems thinking in education"" It contains many
interesting pieces including reports from the
field by teachers.

----------------------------------------------

There are now three good software packages for
system dynamics. You can request information:
--------------------------------------------
STELLA for Macintosh or PC:

High Performance Systems
45 Lyme Road, Suite #300
Hanover, NH 03755, USA

Phone: 1-603-643-9636 customer support
tel: 1-800-332-1202 product inquiries
fax: 1-603-643-9502
email: support@hps-inc.com
http://www.hps-inc.com/

--------------------------------------
Powersim for PC:

Powersim Corporation
1175 Herndon Parkway Suite 600
Herndon, VA 20170
Phone: (703) 481-1270
Fax: (703) 481-1271
Email: powersim@powersim.com
http://www.powersim.com

Norway Address:
Powersim AS
PO Box 206
N-5100 Isdalstø
Phone: +47 56 34 24 00
Fax: +47 56 34 24 01
Email: powersim@powersim.no
http://www.powersim.no
-------------------------------------------

Vensim for PC or Macintosh:

Ventana Systems, Inc.
60 Jacob Gates Road
Harvard MA 01451

tel: 1-508-651-0432
fax: 1-508-650-5422
email: vensim@world.std.com
http://www.vensim.com/

A ""Personal Learning Edition"" of Vensim and its
manual can be downloaded free from:
http://www.vensim.com/ This downloadable Vensim
PLE is the one used for teaching system dynamics
at MIT.
-----------------------------------------------------------
Jay W. Forrester Professor of Management Sloan School
Massachusetts Institute of Technology
Room E60-156
Cambridge, MA 02139
Posted by ""Jay W. Forrester"" <jforestr@MIT.EDU>
posting date Mon, 18 Apr 2005 10:32:05 -0400

[fadl aqww fadlmaster1 yahoo.com]

Posted by fadl aqww <fadlmaster1@yahoo.com>

I have exactly the same question , what are the valuable of using sd
methodology for simulation. For example in studying the glucose
regulatory system there are two models have done by dynamo language
:foster 1970 & guyton 1978. the causal loop diagram did not present
at that time which I think make both works more complex.

there are a lot of models in literature using another approaches.
Please if any one interested in this field and have comparison
between all approaches used in modeling glucose regulatory system
or have relevant literature to send to me .


Thanks and regards

Fadl M. Ahmed
Graduate student, Systems and Biomedical Engineering Department
Faculty of Engineering, Cairo University, Egypt
Fadlmaster1@yahoo.com
Posted by fadl aqww <fadlmaster1@yahoo.com>
posting date Mon, 18 Apr 2005 08:32:03 -0700 (PDT)

[Michael Evans Michael.Evans une.]

Posted by ""Michael Evans"" <Michael.Evans@une.edu.au>
Hi Antarpreet,

I also was surprised to discover the near-equivalence of SD models and
hydrologic models. The intuitive appeal of the ""bathtub dynamics"" idea is
doubly resonant when applied to the physical movement of water. Through my
postgraduate studies and now in professional practice I have found few
situations where SD modelling is not directly applicable to hydrology, with
a valuable pay-off in the increased understanding of models by
non-specialists.

However, one thing that SD modelling can't do, and which is worth mentioning
again to the SD community as of general relevance, is the issue of iterative
solutions to equations that are mathematically as opposed to dynamically
non-linear. This is a bit of a mouthful. What it means is that some
equations are structured in such a way that the only way to find a solution
is through iteration based on an initial guess. For example solutions to
Manning's equation for steady streamflow. To solve Manning's equation in the
midst of a dynamic model you would have to pause time and run these
iterations within a time step, which the SD software I have seen simply
cannot do. In a previous discussion on this list I believe we concluded that
such equations represent an equilibrium rather than a dynamic solution - the
iteration is a mathematical trick that is unrelated to the physical dynamics
- hence in these cases SD is not an appropriate tool.

Still, I wistfully imagine someday the capacity to pause time and
surreptitiously get a few other things done, even if only in the imaginary
world of modelling!

Regards,
Dr Michael Evans
Centre for Ecological Economics and Water Policy Research
The University of New England, NSW Australia
Posted by ""Michael Evans"" <Michael.Evans@une.edu.au>
posting date Tue, 19 Apr 2005 09:39:09 +1000

[lanhai aed47039 yahoo.com]

Posted by lanhai <aed47039@yahoo.com>
>> Still, I wistfully imagine someday the capacity to pause time and
>> surreptitiously get a few other things done, even if only in the imaginary
>> world of modelling!


Michael raised a good issue. I discussed with my colleague Dr. McInnis. He offered some
valuable comments as follows:

I think that the problem being hinted at here may be described more generally as follows.

You have a system in which there are coupled processes whose transient response times are
much different. Suppose there are 2 coupled processes, one with much faster response time
than the other. The slow response time process is the one which is dynamically non-linear
and the fast one is the one that is mathematically non-linear. If the fast one is very
fast we may take it as always in equilibrium (eg manning's equation as a constraint in a
larger hydrological system).

In the whatIf? software environment (see below) problems of the type above pose no
difficulty since whatIf?, not being committed to any mathematical paradigm, can easily
accommodate components with differing time steps even including ones with no time step
(that is just mathematical constraints). The solution for completely coupled components
would involve a double iteration something like;

1) assume a solution to the fast process
2) solve the slow process
3) solve the fast process by iteration
4) if converged stop otherwise go to 2

I would note that this iterative structure could be done for each time step incrementally
or it could also be done for all times in parallel. There are many problems for which the
second method is faster. This second method also cannot be done within current SD
software.

Of course there is a price to be paid for ""not being committed to any mathematical
paradigm"". It is of course that there can be no general solution method as there is in
current SD software.

whatIf? (www.robbert.ca) is a software environment for modelling and simulation that
supports multidimensional data objects and scenario analysis

Dr. Bert McInnis can be reached by:

Regards
Bert McInnis
ROBBERT Associates
whatIf? Decision Support Tools
338 Somerset Street West, Third level
Ottawa, Canada
K2P 0J9
Bus. 1- 613 - 232-5613
Res. 1- 613 - 730-4982
Bert McInnis <bert.mcinnis@robbert.ca>
www.robbert.ca
Posted by lanhai <aed47039@yahoo.com>
posting date Tue, 19 Apr 2005 08:49:14 -0700 (PDT)

[Bob Eberlein bob vensim.com]

Posted by Bob Eberlein <bob@vensim.com>
Hi Everyone,

This has been a very interesting discussion and clearly most of it
applies to lots of things besides hyprology.

One thing that needs to be emphasized is that system dynamics is not
defined by the software one uses to create models, or even necessarily
by the style of those models. System Dynamics as an area of study got
started when Jay Forrester decided to apply the control theory
principles that he had learned in working with servo-mechanisms to
social systems. This suggests to me, more than anything else, that the
goal should be the pursuit of interesting tools to apply to interesting
problems. That the tools developed for social systems can be so helpful
in the area of hydrology is a testament to Jay's original vision.

I don't particularly want to take a stance on what is and what is not
system dynamcis. I don't think there is actually a common view on this -
and that does not bother me. I would like to point out however, that the
issue of iterations at a point in time is hardly new to work in the
field. Erik Mosekilde long ago did a model of a rats kidney that has
this precise property. Business models also often step outside the pure
differential equations view of the world to solve for market
equilibriums, run a linear program or perform some other manipulation.

Certainly there are a variety of ways of doing this type of thing with
Vensim. There is a simultaneous function to converge solutions, a
function to find the zeros of a set of equations, a variety of
allocation and market clearing functions and the ability to add in
programming language extensions to the set of model functions. I can't
speak authoratatively on other software available, but the tools
developed for system dynamics do not strongly limit the types of
problems that can be addressed. More than anything it is usually a
question of efficiency and the ability to not make too many mistakes.
You can, after all, write almost any imaginable model using a
spreadsheet if you sufficiently persistent (and don't know better).

Bob Eberlein
Posted by Bob Eberlein <bob@vensim.com>
posting date Tue, 19 Apr 2005 16:41:26 -0400

[Malczynski Leonard A lamalcz san]

Posted by ""Malczynski, Leonard A"" <lamalcz@sandia.gov>
Greetings,

As Bob Eberlein emphasized, SD is not defined by the software, but by
the methodological approach. Fortunately or unfortunately we
sometimes/always add extra-methodological features to many of our
models.

Software-wise, Powersim Studio has a VBFUNCTION capability that permits
the inclusion of VB Scripts in a model. We used this feature for a great
circle distance calculator that iterates to an acceptable error. The
model was concerned with the physical distance between various pairs and
how that distance had an effect on the non-linear (to distance) cost of
doing business. I'm sure we could have done this work with stocks and
flows however the calculation was what I term extra-methodological and
not central to the system dynamics nature of the problem. It was a great
confidence builder when the customer saw that it was indeed possible.
Bob mentioned the questions of efficiency and ability, I would also add
creativity.

Len Malczynski
Posted by ""Malczynski, Leonard A"" <lamalcz@sandia.gov>
posting date Wed, 20 Apr 2005 09:45:37 -0600

[Paula Antão pantao lnec.pt]

Posted by =?ISO-8859-1?Q?Paula_Ant=E3o?= <pantao@lnec.pt>
Dear Antar,

I am very glad to hear from someone interested in ""modeling water""
taking account of some aspects related.

In fact, some time ago, I told this very important group of discussion,
that I am preparing my PhD work on water resources management within the
watershed geographical unit. And I am applying System Dynamics Theory
for doing so.

To give some more details on the work in course, it is supported by a
small monetary fund from The National Portuguese Foundation for Science
and Technology and we have still one more year to finish the Project -
""*DEVELOPMENT OF A TOOL **TO SUPPORT SUSTAINABLE WATERSHED MANAGEMENT"".
It has been an enjoyable work with the participation of main
stakeholders belonging to a small watershed in the Southern of Portugal
(in the Algarve region).

Hope to hear from you soon and all the people working on the subject.

Best regards,

Paula Silva
*

-- Paula Antão da Silva Eng. do Ambiente (Environmental Eng.)
Estudante de Doutoramento, FCT/UNL
(PhD Student, New University of Lisbon)
Assistente de Investigação (Research Assistant)
LNEC (National Laboratory of Civil Engineering)
Av. do Brasil, 101 PT 1700-066 Lisboa PORTUGAL
Posted by =?ISO-8859-1?Q?Paula_Ant=E3o?= <pantao@lnec.pt>
posting date Thu, 21 Apr 2005 11:55:47 +0100

[ali kerem saysel ali.saysel boun]

Posted by ali kerem saysel <ali.saysel@boun.edu.tr>
Hello Everyone,

Antar points to the feedback mechanisms present in models of hydrology.
I think, many good dynamic models of environmental phenomena, models of
hydrologic and atmospheric processes, models representing geochemical
cycles, pollutant transport etc., all comprise the essential feedback
mechanisms. In the models of physical systems the feedback loops are
closed by material cycles and energy flows. Sometimes, an explicit
analysis of the feedback structure in these systems helps describing the
fundamental processes and have implications on learning (example: the
causal loop diagrams representing the driving forces of global climate
change in Andy Ford's book, Modeling the Environment; I once studied the
feedback processes in irrigated soil salinization in an article in
Ecological Modeling, V139- 2,3).

However, the ""system dynamics"" emphasis on feedback causality is on
information feedback, where the loops are closed by information and
human actions. System dynamics is more about the study of social systems
and social dynamics. This information feedback is certainly what the
models of physical systems would lack.

Best wishes,
Ali Saysel
Posted by ali kerem saysel <ali.saysel@boun.edu.tr>
posting date Fri, 22 Apr 2005 14:22:25 +0300