WATER AND NUTRIENTS MASS BALANCE OF CATCHMENT - LAKE SYSTEM - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 119

6.1. INTRODUCTION
A hydrologic or water balance is of considerable importance in water quality analyses and
management. A water budget for a lake is an expression of the conservation of water mass for the lake,
and can be simply stated as the inflow equals the outflow plus the change in storage of the lake for a
given period of time (Lee and others, 1991, p. 14). Construction of a hydrologic budget involves a
description and quantification of inputs and outputs. The basic objective of conducting the mass
balance is to account for the water gains or losses for the lake, or for some sub-area of interest - and to
give a description of the mechanisms by which water is redistributed within the lake. A thorough study
of water balance evaluates all the processes which contribute to the inputs or outputs of water to or
from the lake system. Calculation of water balance of the lake system is a requirement for the
estimation of the lake residence time. The average length of time water remains within the boundaries
of an aquatic system is a key parameter controlling the system's biological and chemical behavior.
This time scale provides a first order description of the multiple and complex processes that drive the
transport of the water and dissolved or suspended substances. Another important item which depends
on the water balance of the lake is the nutrients balance. This depends on integrating the estimated
inflow and outflow from the water budget with the water quality data.
This chapter gives an overview of the estimated water budget of the lake, the calculation of the
residence time and the estimation of the nutrients mass balance, specifically for nitrogen and
phosphorous compounds. The calculations are limited to the available historical data and discharge
field measurements.
6.2. LAKE WATER BALANCE
The source of a lake's water supply is very important in determining its water quality and in choosing
management practices to protect that quality. If precipitation is the major water source, the lake will be
acidic, low in nutrients, and susceptible to acid rain. (This includes many seepage lakes.) If
groundwater is the major water source, the lake is usually well buffered against acid rain and contains
low to moderate amounts of nutrients. (This includes all groundwater drainage lakes and some seepage
lakes.) Local septic systems or other groundwater contamination could cause problems. Water
exchange is fairly slow.
If streams are the major source of lake water, nutrient levels are often high and water exchange takes
place more rapidly. These lakes have the most variable water quality depending on the amount of
runoff and human activity in the watershed. Managing the watershed to control nutrients and soil that
enter the lake is essential for protecting water quality. Controlling water that runs from the land's
surface or streams into the lake is important for drainage lakes and impoundments, and some seepage
and groundwater lakes. Protecting groundwater quality is particularly important for seepage and
groundwater drainage lakes. Watershed management becomes especially critical in impoundment
lakes. Lake managers measure inflow and outflow to determine a lake's water budget. A general and
simple hydrologic budget equation for a given water body such as a lake is given by:
dV/dt = Q in - Qout + PA s - E v A s
where V = lake volume [L 3 ],
As = lake surface area [L 2 ],
Qin and Q [L 3 /T] represent net flows into and out of the lake due to tributary inflows and releases,
P [L/T] is the precipitation directly on the lake,