ANTECEDENT CONDITIONS AND PRECIPITATION TIMING - TWO IMPORTANT CONTROLS ON GROUNDWATER DISCHARGE AT BIG SPRING, NORTHEAST IOWA
H. Liu, R. D. Rowden, and R. D. Libra
The Geological Society of America
33rd Annual North-Central Section Meeting
Clarion Hotel and Convention Center, Champaign, IL, April 22-23, 1999
1999 Abstracts with Programs, v. 31, no. 5, p. A-31
The Big Spring basin is a 103 mi2 (267 km2) groundwater basin located in a dynamic karst region in Clayton County, northeast Iowa. Landuse in this basin is essentially agricultural. The main aquifer used by residents within the basin is the Ordovician Galena aquifer. Sinkhole areas occupy about 11% of the basin. Over 85% of the groundwater that exits the basin is discharged through Big Spring, the largest spring in Iowa.
Precipitation, groundwater and surface water discharge, and water quality within and around the basin have been monitored since 1981. The long-term monitoring results from Big Spring indicate that groundwater discharge is not highly correlated to precipitation (r2 = 0.44). Significant discordance occurred in Water Years (WYs) 1988, 1990, and 1997. Precipitation in the basin was 37.87 inches for WY 1990 and 38.29 inches for WY 1997. These totals were 4.90 and 5.32 inches greater than the long-term average of 32.97 inches. The annual groundwater discharge at Big Spring was 17,476 acre-feet (ac-ft) for WY 1990 and 22,943 ac-ft for WY 1997, which was 56% and 74% of the average annual discharge, respectively. The discharge as a percentage of precipitation was only 8.0% for WY 1990 and 10.9% for WY 1997, which were significantly lower than the long-term average of 16.3%. These discrepancies occurred because both WYs 1990 and 1997 followed dry years, and much of the rainfall during these water years went toward replenishing previously depleted groundwater. A reverse example occurred in WY 1988, the driest year in the states history. Precipitation for this water year was 22.94 inches, or 70% of the long-term average. The annual groundwater discharge for WY 1988 was 26,000 ac-ft, or 83% of the long-term average, with a discharge/precipitation ratio of 20.5%. These suggest significant discharge of groundwater from storage occurred during WY 1988.
In addition to local antecedent conditions, the monitoring results from Big Spring show that groundwater discharge can also be affected by the timing and intensity of precipitation.