Nutrients in Off-Stream Reservoirs

Map and Data for This Study

• Objective:
  Characterize nutrient concentrations and loads in agricultural reservoirs during one complete reservoir operation season (filling and releasing) and determine if the reservoirs are a source or sink of nutrients to irrigation-supply water.
• Approach:
  Five agricultural reservoirs were selected for study.
  
  Initial nutrient conditions in each reservoir were determined during the first initial round of sampling.
  
  Inflows and releases were sampled for nutrients on a monthly basis and continuous flow records were obtained from reservoir operators.
  
  Nutrient concentrations within the reservoirs were characterized at 4 times during the study. A minimum of five points in each reservoir at multiple depths were sampled during each sampling event.
  
  Bottom sediment cores were collected twice (winter and summer) at each reservoir to examine chemical and biological processes between the water-sediment interface.
  
• Timing:
  February through September 1995
• Targeted physical and chemical data:
  Light, temperatue, pH, specific conductance (salinity), and nutrients
• Selected Results:
  
  During the irrigation season, nitrogen concentrations in agricultural reservoirs were highest in March and decreased through September as a result of dilution from river inflows and biological uptake of nitrate.

Phosphorus concentrations in the reservoirs did not consistently decrease from March through September. Phosphorus was probably recycled through processes such as excretion from fish, decay of aquatic plants and animals, and release of orthophosphate from bottom sediments during periods of low oxygen.

The practice of storing South Platte River water in off-stream reservoirs leads to decreased nitrogen concentrations during the summer. However, storage may also contribute to the growth of nuisance algae that could affect the recreational use of these reservoirs.