In India the fertility of the soil is often reduced or destroyed by the formation on the surface of the ground of an efflorescence called “reh.” It consists of various salts or compounds of sodium and occurs chiefly where there is an impervious layer of subsoil. The salts exist as an ingredient of the upper soil. This becomes saturated with rain or canal water and as the water evaporates the salts are left on the surface. Remedies are drainage, or flooding the soil and running the water off, or deep tilling, or chemical treatment with lime or gypsum. (Indian Engineering, 8th Jan., 1910).
The inundation canals of the Punjab have been described in Punjab Rivers and Works. All descriptions and remarks in the present book regarding Indian canals must be assumed to refer to perennial canals unless the contrary is stated or implied.
4. Losses of Water.
—When water flows or stands in an earthen channel or tank, or is spread over a field, losses occur from evaporation, percolation and absorption. Of these, absorption is by far the most important and, unless the contrary is stated or implied, it will be taken to include the others. The losses by evaporation are very small. The loss by evaporation from the surface of the water, even in the hot season in India when a hot wind often blows, does not exceed half an inch in 24 hours and on the average in India is only about a tenth of an inch in 24 hours.
Fig. 3.
Percolation and absorption are described as follows by Beresford in Punjab Irrigation Paper, No. 10, “The Irrigation Duty of Water.” Percolation consists in flow through the interstices of boulders, shingle, gravel or coarse sand. The flow is similar to that in pipes. The water percolating into the soil from a channel, extends downwards and spreads outwards as it descends. None of it goes upwards. In fine sand and ordinary soil the interstices act like capillary tubes. The water is absorbed as by a sponge and it remains in the soil by virtue of capillarity. Owing to the combined action of capillarity and gravity the water spreads in the manner shown by the dotted lines in [Fig. 3]. The amount of absorption from a channel will be greater the greater the area of the wetted surface. In a high embankment with narrow banks, the absorption ceases when the water reaches the outer slopes, except in so far as it is evaporated from the slopes. Moreover high embankments are generally in clayey soil. If banks of sand are constructed on a layer of clay ([Fig. 4.]) and well rammed, the absorption ceases as soon as the banks are saturated and the channel then holds water as well as any other except for evaporation from the outer slopes, but if the bed and subsoil are also of sand the absorption of the water will be far greater. Absorption ceases when the water extends nearly down to the level of the subsoil water, i.e., to a point where the effect of absorption from above plus gravitation is equal to the effect of absorption from below minus gravitation. If a bottle is filled with water and a small sponge jammed into the neck and the bottle turned upside down, the sponge becomes saturated but no water will be given out. But if a dry sponge is placed in contact with the wet one it will absorb moisture until saturated.
Fig. 4.
It is known that the loss of water is greatly influenced by the nature of the soil. When water is turned into a dry channel or onto a field, the loss is at first great. It decreases hourly and daily and eventually becomes nearly constant, tending to reach a fixed amount when the water extends down to nearly the level of the subsoil water. Observations made by Kennedy on loamy fields near the Bari Doab Canal in India showed that on a field previously dry the rate of absorption is given by the equation