Hydrologic effects of proposed changes in management practices, Winnebago Pool, Wisconsin

Various changes in the management practices for the Winnebago pool have been proposed. The objectives of the proposed changes are protecting wetlands adjacent to the lakes in the pool and assuring adequate flow in the Fox River downstream from Lake Winnebago. Three proposed operation plans for the Winnebago pool were studied to determine the effects on lake stage and discharge. A digital reservoir model simulated daily lake stage and discharge from the Winnebago pool.

Inflow for this simulation was computed from daily discharge, recorded at Rapide Croche Dam, and daily stage, recorded at Oshkosh. This computed inflow is net inflow, combining the effects of streamflow, precipitation, evaporation, and net ground-water inflow.

The model was adapted to simulate operation of the Winnebago pool according to each of the proposed plans. Because the important features of each operation plan were expressed in terms of stages to be attained at certain dates, simulated discharge was adjusted daily to keep simulated lake stage close to that specified in the plan.

The simulated lake stage for each of the proposed plans exceeded the allowable maximum in some years. However, the simulated stage exceeded the allowable maximum less frequently and for shorter periods than did the observed stage.

For the 61 years simulated (water years 1918-78), the simulated 7-day, 10- year low flow ((57,10) at Rapide Croche Dam was 1,000 ft^/s compared with 900 ft^/s for the observed record. The 7-day, 2-year low flow (Qy 2) was 1,500 ft^/s (simulated) versus 1,600 ft3/s (observed).

The model predicted that executing the proposed operation plan would result in monthly Q7 JQ for August and September of 1,100 ft-Vs. This is no change for August and a 12-percent increase for September.

In addition, the study illustrated that the discharge from the Winnebago pool could be managed to guarantee that the discharge in the Fox River downstream would not fall below a specified discharge up to 1,500 ft^/s. In most years this would have no effect on the stage of the pool. In a few years there would be a conflict between releasing water from the pool to maintain the desired discharge and maintaining water levels above the required minimum stage. The frequency of this possible conflict varies with the level of discharge desired in the Fox River from 1 year in 60 for a discharge of 1,100 ft^/s to about 1 year in 15 for a discharge of 1,500 ft^/s.