M. H. Panhwar
The water logging and salinity problem is highly complex, when it comes to actual execution in the field. In general the main factors involved in water logging and salinity control need highly to chronological efforts for the success of programme.
Causes of Water Logging.
Water logging is caused by rising water table and this in turn is mainly caused by irrigation. Irrigation waters invariably contain some amount of salts, which seep unto ground and create serious problems in the future. If the quantum of irrigation water applied is so small, i.e., only meets evaporation as well as evapotranspiration requirements of crops, salts in irrigation water will remain in the soil because evaporation and evapotraspiration processes essentially are vaporization of water and vapours do not carry any salts form water into the atmospheres.
The river Indus water contain 150 to 250 parts per million (ppm) parts of salts at different times of the year. If annual irrigation dose of water with 150 ppm amounts to a column of 60 inches or 5 feet or about 1.5 meters, salts built up will be 3,260 cubic feet per acre in the top 3-4 feet of soil. These salts will make the land totally unproductive within 3-4 years, unless they are removed.
How farmer ever comes salinity.
n In actual practice the farmer knows the problem and puts approximately 37% more water to leach down the salts out of the root zone. Addition of these salts to water table increases salinity of ground water. If ground water is used again and again and allowing 37% to seep back in to the ground, within a period of time ground water in turn will become unsuitable for further irrigation. n To overcome this problems, whenever ground water is used it is advisable to mix it with surface water to keep the salinity down. n But yet a stage is bound to come when ground water will turn unusable. Luckily for us ground water column is many hundreds Copyright © www.panhwar.com 1
feet deep and it may take as much as 50-100 years to make it unusable. As could be seen from above, adding of salts to ground water by irrigation water raises salt content of ground water and also raises the water table. Applying 5 feet of water column a year as irrigation, will add 37% or 1.85 feet of water column to the soil. The porosity of sand lying below the top alluvium is about 33.3%. This means that water column or water table will rise by (1.85% 100 + 33.3) or 5.5 feet in each year. This is how our sills get water logged. Once the ground water tale is within 8 feet, water rises by capillary action and evaporates leaving salts contained in it at the surface. These are to be leached down by irrigation during the next year. The process is thus continued year after year. Capillary rise of water however is also beneficial in another way. In some areas large quantities of water evaporate and water table goes down. It is to every body knowledge that in rice areas water table reaches the surface latest on 1st October when irrigation is cut off. By the end of May, in the following year, it goes down to 8 feet. This makes next rice crop a possibility. There are other areas where ground water is saline and when water table due to irrigation rises and comes close to the surface, large quantity of salts are left at the surface by evaporation. The salts accumulation makes cropping of the area un-economical due to low yields on account of high salinity and lack of aeration of the root zone.
Rise of water table a necessary evil.
The above discussions have leaded us to the following conclusion: n Rise of water table is to necessary evil, if we want to irrigate land for raising crops. n If rain fall was enough to meet irrigation requirement and not in excess, of no problem will occur as rain water is formed from vapours and contains no salts. Thus there is no build up of salts in the soil. n However where...