Agricultural irrigation in northern Indiana has increased rapidly since 1975 and might double by the year 2000. A 16.5 square-mile area in north-central Indiana was studied to determine possible effects of increased irrigation on local water supply. In 1982, an average of 2 inches of water was used to irrigate 975 acres of sandy soil overlying highly transmissive outwash deposits. Irrigational pumpage was 75 percent of the summer water use but was less than potential irrigational pumpage because (1) only one-third of the suitable land was irrigated, and (2) precipitation was near normal for the year.

A three-dimensional digital flow model, calibrated with data collected in 1982, was used to simulate four hypothetical pumping plans representing various irrigational schemes and possible rainfall conditions: (1) 1982 acreage irrigated and 1982 (above normal) precipitation; (2) 1982 acreage irrigated and below-normal precipitation; (3) maximum acreage irrigated and normal precipitation; and (4) maximum acreage irrigated and below-normal precipitation. A fifth pumping plan was used to simulate maximum year-round water use. Plan 5 was not designed to simulate irrigational development but rather a maximum rate of withdrawal sustainable year-round until steady-state is reached.

Of the four pumping plans that simulated irrigational pumpage, plan 4 had the greatest effect on ground- and surface-water supply. Compared with 1982 pumpage, this plan represented a thirteenfold increase in the volume of water pumped for irrigation from wells and from Turkey Creek, a stream bordering the area of study. The model predicted a potentiometric decline of as much as 20.7 feet over an 8-acre area of the aquifer. This decline was one-fourth of the available drawdown and would not dewater the source aquifer. Streamflow in Turkey Creek would be reduced 39 percent by simulated ground-water and surface-water pumpage but remaining flow would still be twice the 7-day, 10-year low flow. However, the model predicted that flow in two smaller streams would be reduced to zero.

The rate of pumping used in plan 5 was nearly 4 times the pumping rate in 1982. Potentiometric decline for plan 5 was as much as 40 percent of available drawdown, and predicted streamflow reduction would cause flow in Turkey Creek to decrease below the 7-day, 10-year low flow.

Results of plans 1, 2, 3, and 4 indicate that the outwash system provides adequate water for current (1982) needs and substantial growth for irrigation. However, maximum irrigational development might cause temporary, local competition for water in several parts of the area. Plan 5 indicates .that water use could increase substantially before effects of pumping would prevail year-round.