<P> An aquifer is a groundwater storage reservoir in the water cycle . While groundwater is a renewable source, reserves replenish relatively slowly . The USGS has performed several studies of the aquifer, to determine what is coming in (groundwater recharge from the surface), what is leaving (water pumped out and baseflow to streams), and what the net changes in storage are (rise, fall or no change). </P> <P> Withdrawals from the Ogallala Aquifer for irrigation amounted to 26 km (21,000,000 acre ft) in 2000 . As of 2005, the total depletion since before development amounted to 253,000,000 acre feet (312 km). Some estimates indicate the remaining volume could be depleted as soon as 2028 . Many farmers in the Texas High Plains, which rely particularly on the underground source, are now turning away from irrigated agriculture as they become aware of the hazards of overpumping . </P> <P> The rate at which recharge water enters the aquifer is limited by several factors . Much of the plains region is semiarid, with steady winds that hasten evaporation of surface water and precipitation . In many locations, the aquifer is overlain, in the vadose zone, with a shallow layer of caliche that is practically impermeable; this limits the amount of water able to recharge the aquifer from the land surface . However, the soil of the playa lakes is different and not lined with caliche, making these some of the few areas where the aquifer can recharge . The destruction of playas by farmers and development decreases the available recharge area . The prevalence of the caliche is partly due to the ready evaporation of soil moisture and the semiarid climate; the aridity increases the amount of evaporation, which in turn increases the amount of caliche in the soil . Both mechanisms reduce the amount of recharge water that reaches the water table . </P> <P> Recharge in the aquifer ranges from 0.024 inches (0.61 mm) per year in parts of Texas and New Mexico to 6 inches (150 mm) per year in south - central Kansas . </P>

What is the ogllala aquifer what is happening to it