The development of a hydrologic budget to determine the nitrogen and phosphorus loads of the Cedar River watershed in Iowa

Abstract

A water budget analysis for the Cedar River watershed in northeastern Iowa was conducted to determine the water balance during the summer months of 2000. The watershed has eight major tributaries that comprise a drainage area of 20,242 km2, of which 81% is 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 inflow components were measured individually and included precipitation, tributary and Cedar River baseflow. The outflow components included evaporation, transpiration, tributary and Cedar River discharge. The results of this study indicate a slightly larger volume of water leaving the watershed (6.24×109 m3) than entering (6.21×109 m3). The surplus of the outgoing water (0.5%) is most likely due to an overestimation of transpiration, or the contribution of water from the intermittent streams not measured during the study. Calculations of nutrient flux showed that approximately 2.99×106 kg of nitrogen and 2.39×105 kg of phosphorus were lost from the watershed during the study.