Stocks and flows
A measure of stocks refers to the resources that are available at a particular moment, whereas the flow measure relates to an amount during a specified interval, or period of time. Flows refer to processes or values occurring during a period of time. Knowledge of the time element is essential if the, volume of a flow is to make sense. For example, a reservoir is a stock of water while a river leading out of it is a flow of water. Capital is a stock whereas investment, monetary aggregates such as broad money and gross domestic product (GDP) are flows.