Another different type of thinking!! underneath water level is decreasing due to increasing number of plant!! as we know that plant collect water from the soil and prepare food by means of photosynthesis!! is is also true that the source of oxygen which plant give us is water!! plant also take oxygen from the atmosphere to keep its huge number of cell living!! is the amount of released oxygen by phothosynthesis process is more than the oxygen plant takes!!
I am not a botanist to answer ur second part of the question.I beg to disagree with ur observation that water level r decreasing because of increasing number of plants.The water level is decreasing becaus of irresponsible pumping of the water by most of the farmers who enjoy free electricity.To the best of my general knowledge,the water which is used by the plants; returns to us in the form of rain.