As part of the development of the northeast side of Iowa City, engineers and planners are considering whether a small lake (20 acres) could be built in the North Branch Ralston Creek watershed to provide flood control, water quality, and recreational benefits.  The city has hired you to "determine the lake storage required to maintain the state-mandated minimum instream flow (0.25 cfs) during a critical dry summer period (June through August)".  After searching through monthly streamflow measurements for Ralston Creek, you discover that the driest summer in the past thirty years was in 1985 (i.e., June-August 1985).  For this 3-month design period, carry out a monthly water budget to determine the minimum lake storage required to maintain the instream flows during this historical "critical dry summer period".

1. Estimate the monthly lake evaporation (in acre-feet) for the 3-month design period based on the monthly pan evaporation for Iowa City.  Assume a pan coefficient of 0.75.
2. Estimate the monthly lake inflow volume (in acre-ft) for the 3-month design period based on the monthly streamgage measurements for the North Branch Ralston Creek.
3. Estimate the monthly lake outflow volume (in acre-ft) for the 3-month design period.  Assume that the release from the lake is equal to the minimum instream flow.
4. Estimate the lake preciptation (in acre-feet) for the 3-month design period based on the monthly precipitation measurements for Iowa City (in inches).
5. Compute the change in storage (DS) for each month (in acre-ft).
6. What is the minimum lake storage (S) (in acre-ft) required to maintain the minimum instream flows during this 3-month period?  (That is, what must the storage be at the beginning of the three month period so that the lake does not empty before the end of the period?)