Long Range Planning For Drought Management - The Groundwater Component

The National Water Management Center (NWMC) located in Little Rock, Arkansas is involved in agriculture-related ground water use studies and ground water quantity/quality management planning across the nation. In the United States, agriculture relies on ground water supplies to produce an abundance of inexpensive food and fiber. Of the total ground water use in the United States, irrigated crop agriculture accounts for 65%. In the United States, 81% of total consumptive use is for irrigation. Ground-water use for irrigation of row crops has grown from 34,000 million gallons per day (mgd) in 1950 to 76,400 mgd in 1995. The highest ground water use year was 1980, which exceeded 83,000 mgd. The ratio of total water use for irrigation in 1950 was-72% from surface water and 28% from ground water. In 1995, ground water withdrawals have increased to account for 37%, (1). Agricultural production has increased on decreasing field acreage. This abundance, efficiency and economic prosperity has been possible in many areas due to the availability of shallow ground-water reserves. Increasing dependence on ground-water resources has exceeded the safe yield of many aquifers and caused water quality problems elsewhere. Several areas are losing irrigated cropland acreage due to dwindling ground water supplies and/or quality degradation. Some aquifers have been permanently damaged because full recharge of depleted aquifer storage will not be possible where compaction and subsidence have occurred. Mining of ground water reserves to sustain agricultural production is temporary. Declining water levels will increase pumping costs and aquifer depletion will decrease well yields to the point where aquifers will be abandoned for irrigation usage. Long-term solutions must be investigated and evaluated today to maintain United States (US) food and fiber production for tomorrow.

Areas impacted by overdraft of ground-water resources include-the High Plains of Texas, the Ogallala aquifer is near depletion. The state of Texas has lost 1.435 million acres of irrigated cultivated cropland over the period, 1982-1997 (2). Most of the loss is due to dwindling ground-water supplies from the Ogallala aquifer. Aquifer level declines have ranged from 50-100 feet since 1980 with saturated thickness reductions of 50%. Depth to water is highly variable but commonly exceeds 100 feet. Well yields are down from 1000 gallons per minute (gpm) to 250 gpm and less in some areas. At current rates of pumpage, current irrigated acreage is predicted to drop 50% by 2030, (3). In south central Arizona, ground water depletion is a primary concern. Since development, water table declines of 200 feet have been observed. Land subsidence caused by aquifer depletion has reached 18 feet in some areas near Phoenix, indicating considerable and potentially irreversible impacts on aquifer storage and conductivity, (5). In the southern section of the Central Valley of California- (Kern, Kings and Tulare Counties) overdraft of 800,000 acre feet (acft) per year has resulted in declines of over 200 feet in some areas. Subsidence has been measured at 29 feet in Mendota, California indicating aquifer damage, (4). Irrigated, cultivated crop acreage for the state of California has declined 700,000 acres from 1982 to 1997, (2). Many states that have traditionally been perceived as having abundant water resources are beginning to experience ground water shortages. In Arkansas, the Mississippi River Alluvial Aquifer has declined 100 feet in 90 years in the Grand Prairie region and well yields have declined accordingly. Declines in saturated thickness there have limited water availability for agriculture. In Rhode Island, agricultural irrigation combined with municipal, commercial and other uses have impacted stream flows.

With agriculture’s critical dependence on the ground water resource base, long-range planning must be conducted that includes ground water. Often, information is readily available to make predictions on aquifer health and longevity. Other times, data may have to be collected and analyzed.

Water supply predictions that include ground water may be used to guide conservation activities. The safe yield of ground water aquifers is a quantity of water that may be harvested on a sustainable basis and is equivalent to annual replenishment/recharge. Continuing overdraft exceeding the safe yield will require conservation practice implementation to reduce demand, protect water quality and increase water use efficiency. In some areas, conservation will not reduce ground-water use to the safe yield of the aquifer and land-use changes will be inevitable. Surface water diversion may be necessary where feasible to reach sustainability. Whether the planning task you have is associated with PL 83-566, River Basin Studies, Drought Management. plan or a generic plan; incorporate the ground water resource into the planning process. Ground water could be used to augment existing supplies in some areas during drought and lessen the impacts to society. However, with ground water problems widespread, good aquifer assessments and drought action plans are required prior to the onset of drought.

Ground water expertise is available at the National Water Management Center to provide assistance to the states (Natural Resources Conservation Service (NRCS)), districts and state governments in ground water resources planning. Ground water professionals are available to answer your questions concerning: Aquifer assessments, basin delineation, water level declines, saturated thickness, long range availability projections, quality problem identification, contaminant characterization, critical aquifer criteria, perennial yield-recharge, karst studies, artificial and enhanced recharge, and more.

Sustained agricultural production relying on ground water resources will require greater NRCS involvement in ground water studies and incorporation of ground water data into future long-range planning.

References:

(1) Solley, W.B., Pierce, R.R., and Perlman, H.A., 1995, Estimated Use of Water in the United States in 1995, US Geological Survey Circular 1200, 71p.

(2) National Resources Inventory, 1997, US Department of Agriculture, Natural Resources Conservation Service.

(3) Ryder,P.D., 1996, Ground Water Atlas of the United States, Segment 4, Oklahoma and Texas, US Geological Survey Hydrologic Investigations Atlas 730-E, 30p.

(4) Planert,M., and Williams, J.S., 1995, Ground Water Atlas of the United States, Segment 1, California and Nevada, US Geological Survey Hydrologic Investigations Atlas 730-B, 28p.

(5) Robson, S.G., Banta, E.R.,1995, Ground Water Atlas of the United States, Segment 2, Arizona, Colorado, New Mexico and Utah, US Geological Survey Hydrologic Investigations Atlas 730-C, 32p.

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