Loss of Water from Soil: 2 Processes

This article throws light upon the two processes by which water is lost from soil. The processes are: 1. Percolation Losses 2. Evapotranspiration.

Process # 1. Percolation:

The losses of soil water through percolation are maximum under humid climate where precipitation is high. Different essential plant nutrients are lost from soils through such percolating water. Percolation losses of water can be reduced with the application of organic matter in the soil especially in soils containing moderate to high amount of sand.

Process # 2. Evapotranspiration (ET):

Water vapour losses from soils occur in two ways:

(a) By the evaporation of water at the soil surface, and

(b) By transpiration from the leaf surfaces of water absorbed by the plants and trans-located to the leaves. The combined loss resulting from these two processes is called evapotranspiration (ET) and it is responsible for most of the water removal from soils under normal field conditions.

The rate of evapotranspiration increases when the air is dry (low relative humidity), warm, or moving (winds), and if the soil water is near the field capacity to the absorbed by roots or evaporated from the soil surface. Evapotranspiration losses can be reduced by selecting more growth efficient plants, reducing total growth by shading (mulching) or cooling the area or by using a moisture barrier.

Consumptive use and Water use Efficiency:

Consumptive use is the quantity of water lost by evapotranspiration (ET) plus that contained in plant tissues. Consumptive use of water is almost equal to the evapotranspiration.

Water use Efficiency (WUE):

The amount of water (transpiration, plant growth, evaporation from the soil, drainage loss) required to produce a unit of dry weight material (a) kilogram corn, for example) is a measure of water use efficiency. Water use efficiency (WUE) increases as the amount used per unit of production decreases and is expressed as the transpiration ratio.

The transpiration ratio is the weight of water transpired divided by the weight of dry plant material produced (only above ground plant portions are used except for root crops—e.g. sugar beet and potato etc.). Transpiration ratios range from 200 to 1000 (200: 1 to 1000: 1) and it may be used to compute water needed for irrigation of a particular crop.

The efficiency in the use of water means the maximum crop production with a given amount of water available either through irrigation or rainfall.

The water use efficiency is thus defined mathematically as follows:

WUE = DW/ET

where, DW = Dry weight of crop per acre

ET = Water used in evapotranspiration in acre inches per acre.

From the above formula it is clear that if we can increase the ratio of the production of dry weight to evapotranspiration, then the water use efficiency (WUE) will be increased. Any practice that increases the production of dry matter will lead to increased efficiency in water use. Whether the ET increases or not depends upon the plant figure, i.e. the size, cover, colour etc. and their interaction with the available energy.