Effective Precipitation And Its Effect On Agriculture

All the intense rain received in a single occasion may not be useful for crop.part of it may be lost as surface runoff. deep percolation beyond root zone or by evaporation of rain intercepted by foliage. The extent of its utility in any given region determines effectiveness of rainfall of rainfall efficiency. Fraction of the total rainfall available for the consumptive use of crop is called effective precipitation (EP) or effective rainfall (ER).

(1) Rainfall characteristics - amount, intensity and distribution of rainfall.

(2) Land and soil characteristics - land topography, soil texture, structure, bulk density, permeability and infiltration.

(3) Groundwater contribution.

(4) Management factors- tillage, bunding, terracing, ridging, mulching etc aimed at minimizing surface runoff.

(5) Crop characters - root zone depth, degree of ground cover, growth stage etc.

Frequent light rains intercepted by foliage with full ground cover are close to 100 percent effective. With large surface soil, rainfall up to 8 mm per day may be all lost by evaporation. Rainfall around 30 mm per day , with low ground cover, may be only 60 percent effective.

To be reasonably safe from evaporation, the rain must penetrate to a depth of at least to 10 to 12 cm. Rainfall needed for this purpose has been estimated at 15 to 20 mm for a single occasion. The annual effective rainfall commonly amounts to about one-third or 60 percent of total rainfall.

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Moving average of 10 years annual rainfall is calculated to know rainfall trend of a particular region. Around 50 years rainfall is necessary for this purpose. If rainfall data is available from 1965 to 1974, 1966 to 1975, 1967 to 1976 etc can be calculated and plotted in  a graph from which the increasing and decreasing trend of rainfall can be determined.

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Precipitation, especially rain, has a dramatic effect on agriculture. All plants need at least some water to survive, therefore rain (being the most effective means of watering) is important to agriculture. While a regular rain pattern is usually vital to healthy crops, too much or too little rainfall can be harmful, even devastating to crops. Drought can kill crops and increase erosion, while overly wet weather can cause harmful fungus growth. Plants need varying amounts of rainfall to survive.

In areas with wet and dry seasons, soil nutrients diminish and erosion increases during wet season. Animals have adaptation and survival strategies for wetter regime. Previous dry season leads to food shortages into the wet season, as the crops have yet to mature. Rainfall and its influence on crop production can be summarized as indicated below:

(1) Rainfall patterns determines cropping systems.

(2) Decisions on preparatory tillage, time of sowing, scheduling of irrigation, plant protection, time of harvesting etc depends on rainfall.

(3) Rain analysis is necessary for designing farm ponds, tanks and irrigation projects.

(4) Amount , distribution and intensity of rainfall dictate crop productivity.

(5) High intensity rains leads to floods and soil erosion.

(6) Deficit and untimely rain leads to drought.

(7) Food grain production and fodder needs of a country (adequacy, surplus or deficit) depend on untimely rainfall during crop growing season.