COLBY BI 493 - WATER BUDGET
School name Colby College
Course Bi 493-
Pages 51

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Colby College: China Lake Report 158 WATER BUDGET INTRODUCTION A water budget is used to calculate the flushing rate of a lake, which is a measure of how often the total volume of water in the lake is replaced, and is inversely proportional to residence time. By measuring the inputs and outputs of the lake, it is possible to track the movement of nutrients into and out of the lake. A lake with a flushing rate equal to one will fully replace its total volume in one year. The flushing rate can provide some indication of the recovery or self-purification rate of lakes (Chapman 1992). A water budget is important in assessing the physical and chemical features of a lake. Lakes that have large watersheds or many inputs from other ponds, rivers or streams will have more water volume flowing in, and more out-flow volume. Flushing rate is directly tied to nutrient loading capacity. Lakes have low flushing rates compared to rivers and streams, which are constantly replenishing their water volume. A lake is more vulnerable to the accumulation of pollutants and nutrients both in its water column and in its organisms than a river or stream (Chapman 1992). Low flushing rates exacerbate nutrient loading problems and accelerate eutrophication because the water is not replenished often enough to prevent accumulation of nutrient-rich runoff from the watershed, leading to increased amounts of nutrients in the sediments. METHODS In calculating a water budget the following formulas were used: I net = (runoff* land area) + (precip.* lake area) – (evaporation* lake area) Flushing rate = Inet / (mean depth* lake area) The water level in China Lake is not static throughout the year. In fact, it is adjusted seasonally and controlled at the dam in Vassalboro (see Historical Trends). Rainfall and runoff are not consistent throughout the year, but over the course of many years, a mean approximates what is typical during a given year. Inet is the net increase in water in the lake each year contributed from direct precipitation into the lake as well as the watershed runoff. It is based onColby College: China Lake Report 159 rainfall average that was taken as a 10-year mean calculated using NOAA rainfall data collected at the Augusta airport from June 1995 through May 2005 (NOAA 2005). The other factors used in calculating Inet, runoff and evaporation rates, were obtained from the North Kennebec Regional Planning Commission (NKRPC unpublished data) and a U.S.G.S. study of the Lower Kennebec River Basin, respectively (Prescott 1969). Runoff is the mean rate of water flow off land, and the evaporation is a mean of water evaporating from the surface of the water. Using ArcGIS®9.0 GIS maps obtained from MEGIS, CEAT calculated the boundary of the watershed and its land area, as well as lake area. A mean depth was also calculated using our ArcGIS®9.0-created bathymetry map (CEAT 2005). RESULTS AND DISCUSSION The first step in calculating a water budget is to determine Inet, the rate at which water flows into the lake, taking into account rainfall and runoff from the land in the watershed, and precipitation and evaporation from the lake surface. The figures used to calculate Inet are listed in Appendix D. Using these data, the Inet was calculated to be 59,356,148 m3 per year. This represents the volume of water contributed by the runoff over the watershed area plus the precipitation over the lake area minus the evaporation that comes off the lake area over the course of one year. Using this Inet, it was possible to then calculate the flushing rate for China Lake, which is 0.35 flushes per year. This means that in 12 months, China Lake only replaces 35% of its water volume. In relation to other lakes in the Kennebec River Basin, this flushing rate is very low (Figure 54). Lakes with low flushing rates are less able to wash away nutrients flowing in from the watershed, and are particularly vulnerable to even slight amounts of external nutrient loading. Furthermore, nutrients in the water column, along with decaying organic matter, and any pollutants sink to the bottom, rather than be swept away, to become part of the sediment. This fact increases contributions of phosphorus from the sediments in China Lake. The problems that China Lake faces are not solely caused by its low flushing rate. There is no concrete relationship between water quality and flushing rate, because there are so many other factors involved. However, a low flushing rate can exacerbate some of these problem factors, especially by accelerating the accumulation rate of organic sediment and because the lake will not be able to flush out nutrients released from the sediment into the lake water.Colby College: China Lake Report 160 Figure 54. Flushing rate of China Lake and seven other lakes in the Kennebec Valley. All but China Lake were taken from previous Colby Environmental Assessment Team studies (CEAT 1997, 2000, 2001, 2003, 2004).Colby College: China Lake Report 161Colby College: China Lake Report 162 PHOSPHORUS BUDGET INTRODUCTION A phosphorus loading model was used to estimate the total amount of phosphorus entering China Lake from specific sources in the China Lake watershed. This model helped to identify problem sources of phosphorus loading in the watershed, and was a critical tool in assessing overall water quality as well as developing strategies to address water quality problems. The model was also used to project changes in phosphorus input to the lake as a result of potential future land use change and population growth. METHODS The model used for China Lake was adapted from Reckhow and Chapra (1983), as well as from past studies on similar regional lakes (CEAT 2000, 2003, 2004, and 2005). The amount of phosphorus entering the lake from various sources within the watershed was determined using the following equation: W = (Eca x As) + (Ecmf x Areamf) + (Eccp x Areacp) + (Ecp x Areap) + (Ecg x Areag) + (Ecw x Areaw) + (Ecrl x Arearl) + (Eccm x Areacm) + (Eccr x Areacr) + (Ecsr x Areasr) + (Ecs x Areas) + (Ecn x Arean) + [Ecss x #capita years x (1-SR1)] + [Ecns x #capita years x (1-SR2)] + [IA x (1-SR3A)] + [IB x


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COLBY BI 493 - WATER BUDGET

Course: Bi 493-
Pages: 51
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