A HYDROLOGIC BUDGET OF A PERENNIAL STREAM SYSTEM IN IOWA

A water budget analysis for the Cedar River Watershed, located in eastern Iowa, was conducted to determine the influx and out flux of water during the summer months of the year 2000. The Cedar River Watershed extends from southern Minnesota to southeastern Iowa, where it joins with the Iowa River and flows into the Mississippi River. The watershed has seven major tributaries that comprise a drainage area of 20,242 km², of which 81% is primarily agricultural land.

Water budgets are essential when examining the movement of agricultural chemicals, as well as nutrients within the system. The water budget was determined using the Hydrologic Mass-Balance Equation, which states that [inflow=outflow ± storage]. The precipitation data was obtained for each month from the Iowa Climate Review. The effective uniform depth of precipitation was determined by the Thiessen Polygon method. The discharge, of each of the seven tributaries, was determined from the direct measurement conducted weekly from July to September 2000. Evaporation pans were used to determine loss of water by evaporation.

The results of the study indicate a larger portion of water entering the watershed (5.32x10⁹m³) than leaving (1.59x10⁹m³), within the three months studied. The surplus of incoming water is stored in the geologic materials, which is significant in terms of sustaining biogeochemical processes in the soil. The predominant mechanism of loss is the infiltration into the streambed. It is observed that Cedar River exchanges a significant amount of water with the bedrock aquifer during its seasonal flow. These initial results may be key in determining the nutrient balance that occurs within the watershed itself, and the movement of those nutrients from land to stream water and finally into the groundwater.