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Mr. Mike Grochala January 21, 2016 Page 3 elevation. The grading plans assume a normal water level of 900 feet, which was set as the initial lake volume for modeling purposes. Calculating Evaporation Based on the estimated volume contained in the lake, the surface area was calculated in Excel using the Forecast function. The Forecast function gives a predicted value of a variable using a linear regression analysis. In this case, the function uses the elevation-surface area data calculated in the pond volume analysis to predict what the surface area would be for any volume of water. This is a necessary component for the model because the volume analysis as outlined in the previous section gives volume at one foot increments only. Once the surface area is estimated, the amount of evaporation is predicted using the pan evaporation method with a pan coefficient, as outlined by the University of Minnesota. The guiding document recommends a pan coefficient of 0.75 for Minnesota. The equation used is below: ܧݒܽ݌݋ݎܽݐ݅݋݊ ൌ ܹܽݐ݁ݎ ܵݑݎ݂ܽܿ݁ ܣݎ݁ܽ ሺ݂ݐ ଶ ሻݔ 0.75 ݔ 36.98 ݄݅݊ܿ݁ݏ 12 ݄݅݊ܿ݁ݏ/݂ݐ The value of 36.98 inches is the average pan evaporation for the State of Minnesota between 1974 and 2004. The equation above provides annual evaporation. Therefore, in the water balance, the value was divided by 180, or the approximate number of days in which evaporation occurs annually. Calculating Irrigation Requirements Using aerials and plans provided by the developer, the irrigated area was estimated for the berms, park, townhomes, and single-family homes. It is estimated that to maintain a lawn, one inch of water is required per week (including precipitation). 1-inch of irrigation weekly results in an average of 1/7, or 0.14, inches per day. Therefore, if the rainfall is greater than 0.14 inches in a given day, it was assumed in the water balance that irrigation did not occur that day. Rice Creek Watershed District rules dictate that stormwater irrigation occurs, at maximum, from April 15 to October 15 for the generation of volume reduction credits. The water balance calculates irrigation between these dates. Phase Irrigation Volume per Day (gal) 1. Park and berms only 42,000 2. Park, berms, and townhome area 88,000 3. Park, berms, townhome area, single family area 546,150 Water Balance: Final Calculation and Results Once all of the above parameters were calculated, the balance was conducted to determine lake levels throughout the irrigation season. The overall water balance equation is as follows: ܲ݋݊݀ ܸ݋݈ݑ݉݁ ൌ ܫ݊݅ݐ݈݅ܽ ܲ݋݊݀ ܸ݋݈ݑ݉݁ ൅ ܵݐ݋ݎ݉ݓܽݐ݁ݎ ܴݑ݊݋݂݂ ൅ ܦ݅ݐ݄ܿ ܫ݂݈݊݋ݓ െ ܦ݅ݏ݄ܿܽݎ݃݁ െ ܧݒܽ݌݋ݎܽݐ݅݋݊ െ ܫݎݎ݅݃ܽݐ݅݋݊ The average annual precipitation, including snowmelt, was 33.5 inches. The model predicted that on average, irrigation will occur 156 days each year, out of 184 possible irrigation days. This