Groundwater is our Most Abundant Fresh Water Resource
One reason why groundwater is important is that a large percentage of the Earth's fresh water is in the underground aquifers. To put groundwater and surface water quantities into perspective, let's consider the entire inventory of water at and near the Earth's surface. The information in this section came from a 1970 article in the Scientific American. Full reference information is given at the end of the article.
First we'll consider relative quantities of salt water and fresh water. If you've looked at a globe lately, you'll probably predict that there is more salt water than fresh water. Indeed, there is a lot more salt water. Approximately 96% of the Earth's water inventory is salt water in the oceans and seas. This leaves 4% of the Earth's water as fresh water.
The fresh water is present as ice, liquid water, and water vapor. The polar ice caps contain a lot of ice. About 75% of the fresh water is ice (3% of the total water inventory) and 25% (1% of the total) is liquid water. Only about 0.001% of the Earth's total water inventory is in the form of water vapor.
Some of the liquid fresh water (most of it in fact) is hidden underground as groundwater. About 98% of the Earth's fresh water is groundwater. The other 2% of the Earth's fresh water rmakes up all of the rivers, lake, reservoirs and streams, known as surface water. The diagram above shows the distribution of the major part of the Earth's water inventory.
A couple of important forms of water are present as very small percentages of the total. About 0.0003% is soil moisture, which is available to plant roots and necessary for all plant life to grow. About 0.0000004% is in plants and animals. (That well known 75% of your body that is made up of water!)