Driving across the western half of Kansas you see them, long metallic caterpillars riding atop fat knobby tires, anchored to the ground at a central point. Water spigots dangle from under large diameter pipes, raining a steady mist of water as they move at a glacial pace around the field. These giant sprinklers, some up to a quarter mile long, endlessly circle acre upon acre of land, irrigating the wheat fields and other crops that help feed the country. Viewed from the air, the land is a patchwork of vibrant green circles, half-circles, and quarter circles on a background of dry brown as the sprinklers bring life to a land once referred to a "The Great American Desert". The land now produces much of the world's grain, and signs on the Interstate proudly proclaim, "1 Kansas farmer feeds 110 families plus you!" Where did the water come from that changed "The Great American Desert" into "America's Breadbasket"? Western Kansas, and most other western states, have few large rivers and get little rainfall during the year. With little water available on the surface, farmers, ranchers, and cities, have to look underground for the water they use.
The water found underground, simply referred to as
groundwater, is nearly 60 times as plentiful as the fresh water found in surface lakes and rivers (excluding water found in glaciers). Groundwater is an important and valuable commodity, providing most of the water used by communities living in arid regions of the world. Groundwater is often less polluted than water taken from rivers, making in an important source of fresh water.
In our study of the Hydrologic Cycle, we learned that a portion of the water that falls to the earth as precipitation percolates into the ground where it is slowly cycled back to the oceans and returns to the hydrologic cycle. How easily and quickly the water moves through the ground is dependent on two important factors, porosity and permeability. Porosity is the percentage of open (or pore) space in a volume of rock. It is often a measurement of a rocks ability to hold water. Permeability is the capacity of a rock to transmit a fluid, like water or petroleum. It determines how easily fluids flow through the rock. As you can see from the table, different rocks and sediments have different values of porosity and permeability. Knowing the type of rock, its porosity and permeability is important when looking for places where groundwater might be found. It's also important to understand the difference between these two terms. A rock can be porous, holding lots of water, but not permeable (often referred to as impermeable), so the water cannot travel through the rock. Rocks, such as shale, and sediment, like clay, are examples of highly porous but impermeable rock and make for poor places to look for groundwater.
