COLBY BI 493 - WATER BUDGET
School name Colby College
Course Bi 493-
Pages 22

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Water BudgetIntroductionMethodsResults and DiscussionPhosphorus BudgetIntroductionMethodsResults and DiscussionPopulation TrendsHistoric Population TrendsFuture Population ProjectionsGeneral Development TrendsPhosphorus Model PredictionsIntroductionMethodsResults and DiscussionLand-Use and DevelopmentWatershed ManagementBuffer strips/ErosionRoadsSeptic SystemsLand-UseIn-Lake ManagementRecreationWater QualityCommunity Awareness and EducationWATER BUDGETIntroductionA water budget is a series of calculations that account for all the inputs and outputs of waterin a lake. The budget generates a flushing rate, which represents the number of times the volume of water in a lake is replaced throughout a year (Chapman 1992). This knowledge can be helpful in determining lake vulnerability to pollution and nutrient loading. Lakes with a low flushing ratetend to retain the same water through the course of the year, allowing pollutants and nutrients to accumulate. Buildup of this material could lead to algal blooms, unswimmable waters, and other problems. On the other hand, the water within a lake with a high flushing rate will have a higher turnover rate, clearing detrimental pollutants, particles and excess nutrients that may build up in a lake with a low flushing rate. This high turnover rate prevents the build up of pollutants and excess nutrients and allows an accelerated recovery from depositional events (George et al. 2007).Another important insight gained from a lake water budget is the percent input of water thatother water bodies contribute to the lake. With this information, the relative impact of major water bodies contributing to the lake can be identified and assessed. For example, Long Pond South receives water directly from Long Pond North, Ingham Pond and two smaller ponds within the Long Pond South watershed (Doloff Pond and Unnamed Pond) meaning any change in water quality of these four water bodies could influence Long Pond South.MethodsThe water budget calculation for Long Pond South includes all inputs of water entering the lake and deducts lake water loss due to evaporation to create a net input (Inet) of water into the lake, measured in cubic meters per year and a flushing rate measured in flushes per year. The following formulae were used to calculate Inet and flushing rate for Long Pond South (see Appendix C):Inet= (runoff x watershed land area) + (precipitation x lake and ponds area) – (evaporation x lake and ponds area)Colby College - Long Pond South Basin Report 147Flushing Rate= [(Inet Long Pond South) + (Inet Long Pond North) + (Inet Ingham Pond)]/ (mean depth x Long Pond South surface area)Although lake water levels constantly change due to droughts and storm events, this study assumed that the amount of water entering the lake was equal to the amount leaving the lake at any given time over the course of the year because lake size was not increasing. Inet values of Long Pond North and Ingham Pond were included in the flushing rate calculation because they both flow directly into Long Pond South, making them an indirect part of the Long Pond South watershed. It is important to note that Moose Pond, Doloff Pond, and Unnamed Pond are also located within the Long Pond South but were not directly included in the flushing rate calculation for Long Pond South (see Appendix C). Moose Pond was not directly included because it flows directly into Ingham Pond and not direcly into Long Pond South. Consequently, its contribution to the Inet of Long Pond South was included in the Inet of Ingham Pond. Because Doloff Pond and Unnamed Pond are part of Long Pond South watershed their areas were added to the precipitation and evaporation portions of the Inet equation. By doing this, runoff and storm events adding water and evaporation subtracting water to these ponds were included in the study.Parameters for the inputs of the Long Pond South water budget were derived from many sources. The runoff coefficient (0.508 m/yr) and mean lake depth (8.4 m) for the Inet calculation of Long Pond South were obtained from the Maine DEP vulnerability compilation of Long Pond South (MDEP 2007b). A study of the Lower Kennebec Basin produced an evaporation constant of 0.56 m per yr that was used in this study (Prescott 1969). Mean precipitation was measured over a 10-year period by the National Oceanic and Atmospheric Administration (NOAA 2006) from a recording station located at the Waterville Treatment Plant. Watershed land area was calculated using ArcGIS®9.2 with layers received from Steve Harmon from the Maine DEP (Harmon, pers. comm.). The flushing rates of Ingham Pond and Long Pond North were obtained from the Maine DEP (PEARL 2007) and CEAT (2007) respectively.Results and DiscussionLong Pond North contributes the most water to Long Pond South (79%) with land runoff (12%), Ingham Pond (6%), and storm events (3%) also contributing. In contrast, water exits Colby College - Long Pond South Basin Report 148Long Pond South via Belgrade Stream and evaporation. Because most of the water entering Long Pond South begins in Long Pond North, the quality of water within Long Pond South is heavily influenced by this input. A study of Long Pond North by CEAT (2007) categorized GreatPond as one of the most important inputs to Long Pond North. The position of Long Pond South in the Belgrade Lakes chain suggests that the water quality of Long Pond South is indirectly affected by the water quality of most other Belgrade Lakes. Although most other Belgrade Lakes have relatively low flushing rates, the flushing rate of Long Pond South was found to be approximately 3.5 flushes per year (Table 8). In an independent study the Maine DEP also found the flushing rate of Long Pond South to be 3.5 confirming the accuracy of the flushing rate (MDEP 2007). The flushing rate of Long Pond South is much higher than the mean rate of the Belgrade Lakes and of Maine lakes in general (Table 8). Because the water in Long Pond South is replaced at this high rate, high concentrations of nutrient and pollution loading are less likely to occur in the water column and ahigh cleansing potential is possible. A high flushing rate may be one important reason Long PondSouth is generally considered a healthy lake. Despite its recent trend in declining dissolved oxygen, Long Pond South has yet to experience algal blooms. PHOSPHORUS BUDGETIntroductionThis study used a phosphorus loading model to estimate the amount of phosphorus


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COLBY BI 493 - WATER BUDGET

Course: Bi 493-
Pages: 22
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