Agricultural practice should be considered as a business, which is economically justifiable only when it shows a profit. Water is one of the major inputs of agriculture and in many cases may prove to be the limiting factor in an agricultural system. It is therefore important that any irrigation project be fully analyzed from the economic viewpoint and shown to be profitable, so that its output should have a clearly higher value than the sum total of all inputs. Moreover, any separable subsystem of such a project (i.e., any part of the project which may be included or excluded from the project without changing the feasibility of the rest of the system) should be able to pass this test and be shown to be economically justifiable, or else should be rejected.