Editing Stock and flow (section)

==More general uses==
{{Main|System dynamics}}
Stocks and flows also have natural meanings in many contexts outside of economics, business and related fields. The concepts apply to many [[conserved quantity|conserved quantities]] such as [[energy]], and to [[material]]s such as in [[stoichiometry]], [[water reservoir]] management, and [[greenhouse gas]]es and other durable [[pollutants]] that accumulate in the environment or [[bioaccumulation|in organisms]]. [[Climate change mitigation]], for example, is a fairly straightforward stock and flow problem with the primary goal of reducing the stock (the concentration of durable greenhouse gases in the atmosphere) by manipulating the flows (reducing inflows such as [[greenhouse gas emissions]] into the atmosphere, and increasing outflows such as [[carbon dioxide removal]]). In living systems, such as the human body, [[energy homeostasis]] describes the linear relationship between flows (the food we eat and the energy we expend along with the wastes we excrete) and the stock (manifesting as our gain or loss of body weight over time). In [[Earth system science]], many stock and flow problems arise, such as in the [[carbon cycle]], the [[nitrogen cycle]], the [[water cycle]], and [[Earth's energy budget]]. Thus stocks and flows are the basic building blocks of [[system dynamics]] [[model (abstract)|models]]. [[Jay Forrester]] originally referred to them as "levels" rather than stocks, together with "rates" or "rates of flow".<ref name="worlddynamics">{{cite book |last=Forrester |first=Jay |author-link=Jay Wright Forrester |date=1973 |title=World Dynamics|edition=2 |location=Cambridge, Massachusetts |publisher=Wright-Allen Press, Inc. |pages=18–19}}</ref>

A '''stock''' (or "level variable") in this broader sense is some entity that is accumulated over time by inflows and/or depleted by outflows. Stocks can only be changed via flows. Mathematically a stock can be seen as an accumulation or integration of flows over time – with outflows subtracting from the stock. Stocks typically have a certain value at each moment of time – e.g. the number of population at a certain moment, or the quantity of water in a reservoir.

A '''flow''' (or "rate") changes a stock over time. Usually we can clearly distinguish inflows (adding to the stock) and outflows (subtracting from the stock). Flows typically are measured over a certain interval of time – e.g., the number of births over a day or month.

'''Synonyms'''

{| class="wikitable"
|-
| Stock  || Flow
|-
| Level || Rate
|-
| Integral || Derivative
|-
| [[State Variable]] || 
|}