The Ogallala Aquifer, also known as the High Plains Aquifer, is a colossal underground water source beneath the central United States. Functioning as a massive natural reservoir, its sheer scale makes it one of the world’s largest aquifers. This immense body of freshwater sustains vast agricultural production and provides drinking water for millions across the semi-arid American Great Plains.
Defining the Geographic Footprint
The physical size of the Ogallala Aquifer is one of its defining characteristics, covering approximately 174,000 square miles across eight states in the Great Plains region. This enormous footprint stretches from South Dakota to Texas, underlying Nebraska, Wyoming, Colorado, Kansas, Oklahoma, and New Mexico. The aquifer underlies nearly 112 million acres of land, profoundly shaping economic activity across the High Plains. Its vast horizontal extent was created by sediments eroded from the Rocky Mountains filling ancient valleys over millions of years.
Measuring the Water Volume and Structure
The true measure of the Ogallala’s size lies in its capacity and the volume of water it stores. The aquifer is not an underground lake but a shallow water table contained within unconsolidated layers of sand, silt, clay, and gravel. The water fills the pore spaces between these sediment grains, resembling a massive, saturated sponge.
Estimates of the total water storage vary, but the U.S. Geological Survey (USGS) calculated the volume to be around 2.9 to 3.3 billion acre-feet in the early 2000s. An acre-foot is the amount of water needed to cover one acre of land to a depth of one foot, equivalent to over 325,000 gallons.
The thickness and depth of the aquifer are highly variable, affecting storage and access. The saturated thickness—the depth of sediment holding water—can range from a few feet to over 1,000 feet. The deepest parts are found in the Northern Plains, particularly under Nebraska, which holds about two-thirds of the aquifer’s total water storage. Southern portions, such as in Texas and New Mexico, tend to be thinner.
The Rate of Depletion
Despite its immense size, the Ogallala Aquifer is largely considered a “fossil water” source, meaning its current volume is finite. The water accumulated over thousands of years, primarily from glacial meltwater and precipitation. The natural recharge rate is exceedingly slow, often averaging less than an inch per year across the region.
The issue is the dramatic imbalance between the slow natural recharge and the rapid rate of extraction for human use. Since large-scale pumping began after World War II, driven by advancements like center pivot irrigation, water has been withdrawn at rates far exceeding replenishment. This has led to a net overdraft of groundwater, causing water levels to decline significantly.
Depletion is not uniform; it is most severe in the southern and central parts, notably Texas and Kansas. In some areas, water levels have dropped by 100 to over 250 feet since the 1950s, and parts of the aquifer have already been dewatered. Since 1950, agricultural irrigation has reduced the aquifer’s saturated volume by an estimated 9%.
The Ogallala’s Role in US Agriculture
The scale and volume of the Ogallala Aquifer translate directly into its importance to the nation’s food supply and economy. The resource underpins a thriving agricultural economy in the semi-arid High Plains region. Approximately 27% of all irrigated land in the United States lies over the aquifer, providing about 30% of the country’s groundwater used for irrigation.
This water supports the production of high-value crops, including a significant portion of the nation’s corn, wheat, and cotton. The reliability of this groundwater source has transformed the High Plains from a drought-prone region into one of the world’s most productive agricultural areas. The economic output generated by agriculture dependent on the Ogallala is estimated to be in the tens of billions of dollars annually.
The reliance on the aquifer is heavily skewed toward irrigation, which accounts for up to 95% of the water pumped out. This concentration of water use means the fate of the Ogallala is linked to the continued productivity and economic structure of the Great Plains.