By Mr. A. RAJAMOHAMED AMBALAM GEOPHYSICIST COASTAL ENERGY Pvt Ltd, CHENNAI +919443619352
The output from the bore well supply depends on the water level that stand in the early morning and the underlying water bearing and water giving properties of the earth in the vicinity of the bore well. Technically it is the storage coefficient and the transmissivity and these are the two important factors that determine the output supply of the bore well.
Second aspect is the pump. While pumping out the water from the bore well the water level inside the bore well start falling continuously. During first 10 to 20 minutes the fall will be very deep and later on gradually it will fall and it will reach a near static condition after one hour or so, depending on the terrain condition or so called basin characteristics. Hence your selection of the pump, or your output supply in addition to the natural factor also depends on at what depth the pumping water level falls, while pumping.
Hence if your bore well is capable of supplying more water, ( Your neighbour’s well) that is if you had obtained the so called yield of 3 inches say at the depth of 150 feet ( as in the case of your neighbour’s bore ). Then if you put as you said a 2 h.p pump, say at the depth of 200 feet, then the fall in water level while pumping will be slow, Because, the bore well is capable supplying more water to compensate the pumping rate in litres per minute. After certain time the fall in water level would be a bare minimum.
But in your case, you have obtained only one inch of water and hence the fall in water level will be sharper and it will go deep and deep at every minute of pumping. When the water level falls, that implies the motor has to push up more weight to the ground level and the discharge will be lesser and lesser with respect to time.
Secondly when the water level falls , the hydrostatic pressure in your area will try to bush the water through the fracture where you got water while you drilled the bore. In your neighbour’s case more water per minute will be pushed as he had 3 inches yield and in your case less water will be pushed to compensate the water that was pumped out. Nature will try to obtain a equilibrium state. What I meant by the instantaneous yield is this.
Assume that no such bush or compensation.
In your case a one meter depth of water in the bore will is about 17.68 litres of water. Your bore well depth is about 462 feet,. Hence assuming a summer water level of 90 feet, your bore well will hold about 370 feet of water that is about 112 meter of water column and hence you have a storage reservoir of about 1994 litres of water. Suppose if you use a 1.5 h.p pump, this can discharge about 30 litres per minute from a depth of about 200 feet.
Hence if your need is only 500 litres, then you will be only pumping out 500 litres of water from the bore whatever pump you install. This 500 litres of water implies a water column of about 28.5 meters only. Hence assuming a summer water level of 90 feet, at summer months, your pumping water column will reach only about 183 feet. This is when we assume only from the storage we are pumping. Your one inches water at 150 feet will bush about 10 litres of waters per minute because of hydrostatic pressure.
Hence for 500 litres sintex tank I have recommended installation depth at 180 feet. Whatever pump, 1.5 or 2 h.p. you install if only 500 litre tank if you want to fill then this is enough and twice a day if you pump it will suffice the need of 7 member families domestic need. This is the optimum solution. In case if you need more water you need to install at deeper depth. I do n0ot think in summer the wat5er level goes below 100 feet in your area.
Under the worst condition you may install the pump at 250 feet. Assuming a 500 litres tank which need only maximum of 100 feet water column of stored water. If the water level falls below 150 feet, where you struck the water yielding zone then the bore will become dry. Or just very very meagre quantity of only you will get. That is why in your case the optimum depth of installation will be about say 200 feet.
Kindly consultant a hydro geologist in your locality and decide upon. We can only provide suggestions. It is always better to consult a professional and seek his guidance which we do not know instead of experimenting.
