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Mr. Mike Grochala January 21, 2016 Page 2 The purpose of the water balance is to determine if the lake holds enough water for irrigation purposes throughout the irrigation season. Therefore, a conservative approach was taken when estimating the drainage area; only the immediate drainage within the development was considered for the water balance. Calculating Precipitation Rainfall data was obtained from the Minnesota Climatology Working Group. Data was available beginning on May 1, 1959, and extending to June 30, 2015. Days in which rainfall data was missing were assumed to have received no precipitation. Calculating Runoff The water balance uses the United States Department of Agriculture (USDA) Natural Resources Conservation Service method for determining runoff. This method, commonly known as the SCS Runoff Curve Number method, has widespread use in hydrologic modeling. The curve number for the drainage area was calculated as part of the Lino Lakes Northeast Study. The curve numbers for the drainage area within the development was estimated using soil type and single - and multi -family residential published curve numbers. Using the calculated curve numbers, an S -value (the amount of maximum soil moisture retention after runoff begins) was determined using the following equation: 1000 _ S 10 Curve Number From the S -value, the initial abstraction (Ia) was calculated as 0.2*S, per the USDA. To predict the amount of runoff (Q) from a particular rain event, the USDA gives the equation: Q = {0 for P < IQ (P-102 for P > 1 P—Ia+S The equation dictates that there is no runoff from a storm where the amount that falls (in inches) is less than the initial abstraction. In the single-family residential area, a storm of 0.47 inches is required, and in the multi -family residential (townhome) area, a storm of 0.26 inches is required. In the model, runoff from the three areas was considered independently to ensure the most accurate estimate of total runoff volume possible. Using the equation above, the runoff entering the lake was evaluated for each rain event from the historical rainfall data. Calculating Pond Volume Pond volume was determined using contour data from the grading plan for the lake. First, a stage to surface area relationship was developed using surface area from the grading plan. Once the surface area was evaluated at several elevations, the trapezoidal method was used to estimate overall pond volume as well as the volume at several elevations. The storage capacity of the pond is approximately 122 million gallons, with a volume of 86 million gallons at the normal water elevation. The total possible volume was assumed to be the volume at the overflow