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 CHAPTER 1
WATERSHED - RESERVOIR PROCESSES: RELATION TO WATER QUALITY
1.1 OVERVIEW OF WATERSHED PROCESSES
Monitoring and modeling are tools to improve understanding of watershed processes.
Monitoring provides empirical data for understanding important components. Modeling extends the
knowledge gained from the empirical and provides a method of estimation or prediction of components.
When combined, monitoring and modeling provide a picture of watershed processes that could not be
possible with either tool in isolation.
1.1.1 WATERSHED HYDROLOGY
A useful daily watershed hydrology model is proposed by Arnold et al. (1998). The equation
for the hydrology model is:
n
∑ (R
SWt = SW +
- Qi - ETi - Pi - QRi )
i
i=1
(summation I = 1 through n )
where SW is the soil water content minus the wilting point water content, t is the time in days, and R,
Q, ET, P and QR are the precipitation, runoff, evapotranspiration, percolation and return flow, with
units in mm.
1.1.1.1 Precipitation
Rainfall or snowmelt serve as precipitation for the daily hydrology model. Precipitation gauge
data from the watershed or weather records from a nearby site are potential data sources. Methods for
precipitation measurement and analysis of spatial variability in rainfall may be found in by Dunne and
Leopold (1978) and Schwab et al. (1993). A stochastic weather generation model can also be used to
simulate weather inputs. Precipitation statistics and monthly probabilities of receiving rainfall if the
previous day was wet or dry are used in the weather generator developed by Nicks (1974) and
Williams et al. (1984).
1.1.1.2 Evapotranspiration
Objectives and data requirements help in the selection of a method to estimate potential
evapotranspiration (PET). The Priestly-Talyor (1972) method requires daily minimum and maximum air
1.1-1
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