What does hydrology and water quality have to do with each other? EVERYTHING! Hydrology is study of how water moves through the environment and water quality (what compounds are dissolved in the water) has everything to do with where that water has been and what it came in contact with along the way.
The water we have on Earth today is the same water that was here when the dinosaurs roamed the Earth. New water is not created; it only changes forms (vapor, liquid, ice) as it moves through the environment in what we call the hydrologic cycle, or more commonly, the water cycle. The hydrologic cycle doesn't have a beginning or an end, but typically we think of water coming first from the sky as rain which happens when water vapor in the air cools and changes to liquid form, a process called condensation. After the rain reaches the ground it generally has two pathways it can go: infiltration down through the soil and into the aquifer, or runoff over land. What path the water takes has a lot to do with how much water there is and the geology or make up of the soil and rock underneath it. Some areas in Northern Florida are confined, which means that there is a layer of clay preventing water from infiltrating down. When that happens, water will runoff downhill until it reaches a lake or stream, or until it reaches an unconfined area of sandy soil where it can infiltrate down into the aquifer. Once in the aquifer, water continues to flow through pathways in the rock until it is either pumped out of a well for drinking water or irrigation, taken up by plant roots for growth and photosynthesis, or until it reaches an area with the right geology to form a spring or seep. When the water is drawn back up to the land surface it is then heated by the sun and evaporates or released by plants during the process of transpiration through their leaves. The term evapotranspiration (ET) in general is defined as the sum of water loss into the air from both evaporation and transpiration. However, many scientists use the term evaporation for surface water bodies like lakes, rivers, and oceans, which make up 90% of the moisture release to the atmosphere, and reserve the term evapotranspiration to specify water loss associated with plants and the ground surface, which makes up most of the remaining 10% of atmospheric moisture.