Factors Influencing Effective Rainfall
- (i) Rainfall characteristics: A soil has a definite and limited infiltration and moisture holding capacity. Hence, greater amount and intensities of rainfall normally reduce the effective fraction of rainfall and increase runoff. A well distributed rainfall in frequent light showers increases the effective rainfall and is conducive to crop growth than heavy downpours. In India the intensity, frequency and amount of rainfall are high during July and August and hence the effective fraction is low. From November to April most of the rainfall is effective due to its low intensity, frequency and amount.
- (ii) Land slope: The slope of the land has a profound influence on the time available for the rain water to infiltrate into the soil (infiltration opportunity time). Water stays longer on flat and levelled land and thus has a longer opportunity time than on slopping land where there is a rapid runoff.
- (iii) Characteristics of the soil: The soil properties influencing infiltration and moisture retention influence the effective fraction of rainfall. High values of infiltration rates- and hydraulic conductivity of the soil enhance the infiltration and reduce runoff. The moisture content of the soil at the time of occurrence of the rain (initial moisture content), affects the effective rainfall considerably. The higher the moisture content, the lower the infiltration rate and higher the surface runoff which reduces the effective rainfall.
- (iv) Management practices: Any management practice which influences runoff, infiltration, hydraulic conductivity or evapotranspir- ation also influences the degree of effective rainfall. Bunding, terracing, contour tillage, ridging and mulching reduce runoff and increase effective rainfall.
- (v) Crop characteristics: Crops with high water consumption create greater deficits of moisture in the soil. The effective rainfall is directly proportional to the rate of water uptake by the plant. Crop characteristics influencing the rate of water uptake are the degree of ground cover, rooting depth and stage of growth. Soil moisture stored in deeper layers can be tapped only when roots penetrate to these depths. Deep-rooted crops, therefore, increase the proportion of effective rainfall in a given area.
- (vi) Carry-over soil moisture: It is the moisture stored in the crop root zone between cropping seasons or before the crop is planted. This moisture is available to meet the consumptive water needs of the crop. The contribution of a fairly good rain occurring just prior to sowing may be equivalent to one full pre-sowing irrigation. An additional benefit of excess pre-sowing season rain is the leaching of salts accumulated in the root zone in the summer season. Carry-over of soil water is of great importance in rainfed farming in water scarcity areas.
- (vii) Groundwater contribution: Soil moisture contribution from the groundwater table is determined by the depth of groundwater below the root zone and the capillary and conductive properties of the soil.
Measurement of effective rainfall: Evaluation of effective rainfall involves the measurement of rainfall and/or irrigation losses by surface runoff, percolation beyond root zone and soil moisture use by crops. Precise measurements are often made by weighing type lysimeters.
Groundwater contribution: The contribution to the root zone soil moisture from the groundwater table is determined by the depth of groundwater below the root zone, the capillary and conductive properties of the soil and the soil moisture content in the root zone. Both the rate and the distance of water movement are important criteria. For heavy soil the distance of movement is high and the rate low, while for light textured soil the distance of movement is low and the rate high.
Surface and sub-surface in- and out-flows: Computation of surface inflow normally does not apply, except for areas subject to occasional flooding. Under efficient irrigation practices surface outflow is small. Management losses and waste of water due to technical faults are normally accounted for in irrigation efficiency. Sub-surface inflow is only of local significance in areas where there is upward movement of water from deeper subsoil caused by seepage from reservoirs and canals. Subsurface inflow may also occur locally on or near the toe of sloping lands.
Deep percolation: Deep percolation is that amount of water which passes below the root zone of the crop and can continue for a long time after field capacity has been reached in the crop root zone, following irrigation or heavy rain. Total water loss by deep percolation in irrigated conditions can account for 20% or more of the total amount of water applied. However, soil water movement in arid below the root zone, after an initial downward outflow, can later be reversed to an upward inflow from the wet sub-soil to the drying root zone above.