Question

why does the rainfall decrease from the east to the west in northern India

Answer 1

rainfall decreases from East to West in northern India because most of the rain gets precipitated in eastern Indian only and when the clouds goes to western India there is nothing to pricipitat

Answer 2

the rate of precipitation decreases form eastern India to western India because the moisture laden wind bring heavy showers to the eastern coast and get deflected to the northern India which results in less rain or no rain in western India.
hope it helped!