In its simplest sense, effective rainfall means useful or utilizable rainfall. Rainfall is not necessarily useful or desirable at the time, rate, or amount in which it is received. Some of it may be unavoidably wasted while some may even be destructive. Just as total rainfall varies, so does the amount of effective rainfall.
The term effective rainfall has been interpreted differently by specialists in different fields. To a canal irrigation engineer, the rain which reaches the storage reservoir directly and by surface runoff from the surrounding area is the effective portion. Geo-hydrologists would define as effective rainfall that portion of rainfall which contributes to groundwater storage. Agriculturists consider as effective that portion of the total rainfall which directly satisfies crop -water needs. In the field of dry land agriculture, when the land is left fallow, effective rainfall is that which can be conserved for the following crop.
Thus, it may be seen that even though the concept is the same in all the above cases, the values of effective rainfall are different for different agencies for the same total rainfall. Since there are such variedinterpretations of what may be regarded as effective rainfall, it is difficult to develop a definition to suit all the interested disciplines. Rainfall which is ineffective according to one may be effective according to another. From the point of view of the water requirement of crops, the Food and Agricultural Organisation of the United Nations (Dastane, 1974) has defined the annual or seasonal effective rainfall as that part of the total annual or seasonal rainfall which is useful directly and/ or indirectly for crop production at the site where it falls, but without pumping. It therefore includes water intercepted by living or dry vegetation, that is lost by evaporation from the soil surface, the precipitation lost by evapotranspiration during growth, that fraction which contributes to leaching, percolation or facilitates other cultural operations either before or after sowing without any harm to yield and quality of the principal crops. Consequently, ineffective rainfall is that portion which is lost by surface runoff, unnecessary deep percolation losses, the moisture remaining in the soil after the harvest of the crop and which is not useful for next season's crop. The growing season is counted from the start of the first tillage operation until the harvest.
The above concept of effective rainfall is suggested for use is planning and operation of irrigation projects. The irrigation water supply in a given year should be planned to complement rainfall. Since annual rainfall varies from year to year, an irrigation project is to be planned based on data, over a long period to calculate the effective rainfall on the basis of probability of occurrence.