Precipitation (P) - The total water received through hailstorm, snowfall, rainfall, etc. over the area.
Imported water (Wi) - Surface water flowing in the streams, and ground water inflow in the area.
Stream outflow (R) - The quantity of water outflowing through streams from the area.
Evapotranspiration (Et) - The quantity of water lost through evaporation from the ground surface and transpiration through the leaves of the plants.
Groundwater outflow (Wo) - Ground water flowing out of the area.
Change in groundwater storage (<Sg)-- The change in the volume of saturated thickness of the aquifer.
Change in soil moisture storage (<Ss)- The change in moisture content in top-soil and sub-soil. This is generally taken as negligible for larger time periods.

The Presently Suggested System: The above water balance equation does not take into account the water quality factor, and the requirement of water for various human purposes. For the water balance study, which takes into account all the requirements and environmental considerations the following equation can be used.

Requirement of water (quality and quantity-wise) for all the purposes in the area

<------->

Availability of water (quality and quantity wise) from all the sources in the area

If the requirement is more than the availability, the area is deficient in water and on the other hand if the availability of water is more than the requirement the area is surplus in water. Further, if in a region it is seen that a specific quality water is falling short while some other quality water is available more than its requirement, it may be enquired whether this extra water may be treated to upgrade its quality to meet the shortage of better quality water. This treatment may be techno-economically more feasible than meeting the shortage by water transportation. Hence the availability should be assessed and arranged in terms of both quality and quantity. Using this concept the exercise for balancing can be done in the manner described later.

As is evident, it is necessary to identify all the water sources and then to assess the availability of water from them both in terms of quality as well as quantity. Similarly, it is also necessary to assess the requirement of water for purposes both quantitatively as well as qualitatively.

A very important point, though understood, worthy mentioning here is, any water balance study should be made for a complete watershed or a mini- or micro-watershed and definitely not for a country, state or district.

Identification of Water sources: The water sources in any area can be classified into the following two categories.
1. Surface water sources
2. Sub-surface and underground water sources

The `Surface Water Sources' include the followings.
* Rainfall.
* Rivers and their tributaries.
* Water storages in the form of lakes, ponds, tanks, etc.

The `Sub-surface and Underground Water sources' include the followings.
* Aquifers, both confined and unconfined.
*Water being pumped out of the workings below land surface of the mines in mining areas (if it is used through recycling).

Availability of Water: In most area, specially in those having monsoon type climate or seasonal or occasional rainfall, the total quantity of water available from the various sources varies. It is expected to be more in rainy seasons/months and less in dry seasons/months. Therefore, to assess the availability of water from the sources it is necessary to establish an `availability pattern' for each source separately.

The availability pattern can be established on the basis of month-wise water availability data. The data for about 10-years should be taken into consideration to establish this pattern, which then can be used to anticipate the availability of water for future.

In the Indian context it will not be possible to get the water availability data except rainfall for 10-year period and the exercise of water availability assessment may have to depend on the data for one year only.

Month-wise average availability of water can be computed by the available standard methodologies, some of which are briefly given next.