Because at different times of the year, the underground layers are differently saturated with water. Precipitation (melt snow, rain) is absorbed into the ground and replenishes groundwater. That is why in spring there is usually a lot of water in the well. At the same time, in the summer, especially in arid regions, the well becomes shallow. In addition, an important role is assigned to the composition of the soil. After all, as you know, clay and shales are resistant to water penetration. While sand and gravel allow sediments to seep out and fill ground water.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.