The word ‘water balance’ was first of all used in 1944 by the famous meteorological and climate scholar C. Warren Thornthwaite (1899-1963), who meant that it is a balance between precipitation, water obtained by melting of snow and evaporation, groundwater recharging and surface flow of water.
For its assessment, statistics of soil moisture, transpiration, groundwater recharging and figures of flow of rivers are collected from meteorological department and of moisture and vegetation. Water balance is useful for assessment of demand for irrigation water and geographical as well as climatic image of soil moisture.
Water balance study gives information about increase or decrease of water of lakes and water ponds as well as their salinity. For utilization of land and water resources and to assess possibilities of economic and ecological study, statistics regarding soil moisture, groundwater and river water flow can be obtained only on the basis of water balance.
Water balance also explains the effect of human activities on hydrological cycle. Changes in climate, vegetative cover etc. can be studied on the basis of water balance , hence water balance is an important tool for analyzing water problems in a geographical region.
Measurement of Water Balance:
The following formula can assess measurement of water balance of a micro drainage area:
P = I +AET+OF + ▲SM+▲GWS+GWR
It is an area of non-permeable rocks up to a certain depth.
Here, these symbols explain changes in precipitation, interception, transpiration, sub-soil surface flow, soil moisture, quantity and flow of groundwater. First of all, precipitation and interception are measured.
According to the above formula, actual transpiration can also be assessed:
P = Precipitation, I = Interception, AET = Actual Evapotranspiration, OF= Over Land Flow▲ ASM= Change in Soil Moisture, ▲GWS = Change in Ground Water Storage, GWR= Ground Water Runoff
Computation of Water Balance:
The method first suggested by C. Warren Thornthwaite had suggested the method of assessing water balance which has been transformed many times for different objectives. Thornthwaite and Mather (1957) have given suggestions from time to time regarding changes in the formula related to water balance.
For assessing water balance, analytical study of precipitation, interception, transpiration, evaporation, overland flow, changes in soil moisture, changes in groundwater storage and groundwater flow is made, the detailed description of which is as under:
It is the quantity of water which reaches the surface of earth after movement in different forms in the atmosphere.
Evaporation is that seasonal process in which water from independent ground level evaporates and reaches in the atmosphere in the form of vapour. This process takes place when temperature reaches boiling point of thermal energy.
Factors like quantity of solar radiation on earth, temperature of air, quantity of water vapour existing in the atmosphere, nature of ground level, current air etc. determine the rate of evaporation. Quantity of evaporation is maximum in case of high temperature, fast flow of air and presence of vast water surface.
It is the total moisture which mixes with atmosphere as a result of evaporation and action of transpiration from plants. Rate of evapo-transpiration is mainly based on temperature. It is an important factor in climatic studies because it decides all the atmospheric conditions.
This process depends on the availability of water. If sufficient moisture is available to meet the needs of vegetation, the resulting evaporation is called potential evapo-transpiration.
Actual evapo-transpiration is found in special conditions. Its quantity is found out by adding precipitation and changing soil moisture.
AET = P + ▲ASM
where AET = Actual Evapo-transpiration
P = Precipitation
▲SM = Changing soil moisture
Infiltration is the flow of water into the ground through the soil surface. After rain or irrigation, as soon as the water comes on the surface of the earth, it inclines to enter soil from the nearest point. Thus, entry of water into the soil is called infiltration.
That part of water from rain or of melted snow which flows as drainage on the surface, is called ‘run-off.
Percolation is downward movement of water in soil profile. Percolation is not the same as infiltration. It is the first stage of infiltration. Thus, till water does not percolate, its infiltration cannot take place.
Method of Assessment of water Balance:
1. First of all, assessment of potential evapo-transpiration is done.
2. P = PET is counted where P = Precipitation.
3. Loss of storage water is counted., which is added by reducing analysis method (P = PET). Only for dry season, moist climate is counted in the end.
4. Soil moisture (Soil Moisture – SM) is counted from figures of potential loss of storage water and soil and land utilization.
5. Changes in soil moisture (▲SM) is counted from soil moisture rules, i.e., difference between current month and next month.
6. Actual evapo-transpiration (AET) is counted Precipitation –i- changes in soil moisture. If rainfall is more than potential evapo-transpiration, then actual transpiration is equal to actual transpiration.
7. Loss of soil moisture (D) is counted by deducting actual evapo- transpiration from potential evapo-transpiration (PET-AET).
8. Excess soil moisture (S) is analyzed. Once the shortage of soil moisture is calculated, then in humid climate additional rainfall after collection of water soil moisture becomes excessive. Thus, more than required soil moisture is counted.
9. Additional moisture mixes with underground water sources. It cannot flow fully in the same month. Its remaining part flows in the next month. Its quantity differs according to texture and depth of soils of that basin. Thus, during any month, actually 50 per cent of water becomes available for flow.