Subrat The answers to your ...

Subrat

The answers to your question will be very much determined by the geology. If you are in an area where mostly farmers are using dug wells, then I am guessing you are on metamorphic basement rocks, with water held in the weathered zone.

1. The amount of water that infiltrates in the monsoon is very much affected by soil type and topography, but for a very very approximate initial guess, you might assume that 10% of the rainfall recharges groundwater - so in your case 140 mm (it could be higher or lower) . Now this water that recharges is not all available for shallow wells - some may flow out of the zone, or infiltrate deeper, some may support dry season water demands of trees and vegetation, or provide baseflow in streams. In India a figure of 85% of recharge being available for abstraction is sometimes used, but many authors would consider this too high. For an initial guess, assume 50%  so an available water of about 70mm per year.

2. How long water is available post monsoon is again very much about geology and topography, but this time will also depend on how wells are dug and how much water farmers use.  As water naturally evaporates and farmers pump water the level in the aquifer will fall. If wells are shallow and pumping rates high, this may happen quickly - or with deep wells and little pumping, it may be that water lasts all year round. 

3. Again - you can't ignore the geology and soil texture.  If we assume that the weathered soils have 10% porosity, 70 mm of  infiltrated rainfall will raise the water table by 700 mm. So if the water table had fallen by 2 metres over a wide region, it would take nearly 3 years of average rainfall with no pumping to recover.

4.  In most cases if the geology is weathered metamorphic rocks pumping borewells will certainly affect shallow wells. The borewells will gather water from fractures in the rock, but those fractures will get their water from the same rainfall as the shallow wells, so the overall budget of water is just the same.  It is possible (usually in sedimentary or volcanic rock provinces) that the deep borewells draw water from rocks where water travels horizontally (e.g. from a hilly area to the farmland), but you can only tell this with careful geological investigation.

Now all of this is speculative.  The calculation of recharge depends on the soil and geology, and I haven't accounted for irrigation water losses flowing back to the aquifer. The connectivity between shallow and deep aquifers is equally speculative.

In your position I would probably seek some local geological advice (from the CGWB or state authority) - but also try and get a good inventory of how many wells there are, and how much water is pumped. Monthly monitoring of water levels in wells can help you understand how the system is working, by observing how levels rise in the monsoon and fall in the dry season.

Andy