Rainfall and irrigation water are the main source of water. rainfall is the basic source of water. However, ground water can also be made available to crops. After precipitation or application of irrigation water, it is lost from soil in four ways.
Percolation (downward movement of excess water).
Evaporation from the soil surface
1) Surface run off: The loss of water through run-offs is the largest and is almost damaging because it causes soil erosion. The rate of loss of water through run-off depends upon the soil type, intensity of precipitation or quantity of irrigation water. When the intensity of rainfall is more for longer period, the loss of water from the soil surface through run-off is greater. The infiltration capacity of sandy soil is more than heavy soils and hence in sandy soil, loss of water through run-off is low.
2) Percolation: When rainfall is high and water-holding capacity of soil is less, the losses due percolation are very great. Such losses are very rapid particularly when the soils are sandy and porous. In heavy soils, percolation is low because of more water holding capacity. Besides rapid percolation of water, there is also heavy loss of plant nutrients viz. Ca, Mg, S, K etc. resulting in soil becoming acidic. Percolation losses are maximum in humid climate. When high rainfall is received, the loss of water through percolation is necessary otherwise, poor drainage conditions and water logging may develop in heavy soils. When water is in excess of water holding capacity of soils, it percolates through the soil due to gravity.
3) Evaporation losses: Evaporation is the process during which a liquid changes into a gas. The process of evaporation of water in nature is one of the fundamental components of the hydrological cycle by which is one of the vapour through absorption of heat energy. This is the only form of moisture transfer from land and oceans into the atmosphere.
Considerable quantity of water is lost by evaporation from the soil surface. Sunlight, temperature, wind velocity and humidity are the main climate factors influencing the rate and extent of evaporation. More the fine aggregates of black soil, more the heat absorbed resulting in more loss of water.
Man can do maximum control over such losses by adopting suitable soil management practices. The basic principle is to cover the soil with vegetation, mulching, keeping soil surface loose by tillage operation, use of wind brake etc. that can help to reduce evaporation losses.
4) Transpiration losses: Transpiration is the process by which water vapour leaver the living plant body and enters the atmosphere. It involves continuous movement of water from the soil into roots, through the stem and cut through the leaves to the atmosphere. The process include cuticular transpiration or direct evaporation in to the atmosphere from moist membranes through the cuticle, and stomatal transpiration or outward diffusion into the atmosphere through the stomata and lenticels vapour previously evaporated from imbibed membranes, into intracellular space within the plant.
Transpiration is an evaporation process. However, unlike evaporation from a water surface, plant structure and stomata behavior in conjunction with the physical principles governing evaporation modify transpiration.
The loss of water through transpiration is governed by temperature, humidity, wind velocity, moisture content in the soil and inherent characteristic of the plant. Since transpiration is a physiological process, which must continue, if the plant has to grow, the only way to save this loss is by growing such crops and their varieties whose transpiration co-efficient is low. Transpiration can be checked to some chemicals. Transpiration produces energy gradient, which causes movement of water into through plants.
Effective rainfall is a part of rainfall available for the consumptive use of the crop.
Part of the rain may be lost as a surface run-off, deep percolation below the root zone of the crop or by evaporation of rain intercepted by foliage. When rainfall is of high intensity, only a portion of rainfall can enter the soil and stored in the root zone. In case of light rains of low intensity depending on the amount of moisture already present in the root zone of the crop, even the amount and intensity of rainfall, rate of consumption use, moisture storage capacity of soil, initial moisture content and infiltration rate of the soil. It is difficult to predict effective rainfall because of variation of soils, crops, topography and climate. However, in India it is assumed that 70% of the average seasonal rainfall to be effective in arid and semi-arid regions while 50% considered effective humid regions.
The upper surface of the zone of saturation is called the water table. At the water table, the water in the pores of the aquifer is at atmospheric pressure. The hydraulic pressure at any level within a water table aquifer is equal to the depth from the water table point and is referred to as the hydraulic head. When a well is dug in a water table aquifer, the static water level in the well stands at the same elevation as the water table. The water table is not a stationary surface moves up and down rising when more water is added to the saturated zone by vertical percolation, and dropping during drought periods when the previously stored water flows out towards springs, streams, well and other points of ground water discharge.