Cooperators:
US Environmental Protection Agency - Region 6
Dallas, Texas
Blackland Research Center
Texas Agricultural Experiment Station
Temple, Texas
Period:
Budget:
Objectives . Methodology . Results
Comments:
Please send your comments/questions by email to Dr. Srinivasan or
Dr. Ramanarayanan
The water quality parameters simulated by the instream water quality component of SWAT were algae as Chlorophyll-A (Chl-A), dissolved oxygen (DO), carbonaceous biochemical oxygen demand (CBOD), organic nitrogen (OrgN), ammonium nitrogen (NH4N), nitrite nitrogen (NO2N), nitrate nitrogen (NO3N), organic phosphorus as P (OrgP), and soluble phosphorus as P (SolP).
Each reach was divided into several computational elements and for each time step the resultant first-order rate equation was solved numerically by implicit backward difference technique.
In the instream water quality component of SWAT, the diffusion was ignored assuming complete mixing within a reach. The advection component was considered in the SWAT's existing routine. Thus, only the first-order rate decay was considered in the instream water quality component, which depends on the travel time of the constituent within a reach by considering the reach as a single computational element.
First-order decay relationships for Chl-A, nutrients, CBOD, and DO used in QUAL2E were adopted in SWAT with necessary modifications.
Water temperature was not physically based, but was estimated from the air temperature based on a relationship developed in the literature.
Prior to the addition of the instream kinetics, SWAT did not predict Chl-A, CBOD, and DO loads from subbasins to the streams. Appropriate loading estimations for these variables were established as a function of flow, nitrogen, phosphorus, organic matter, and temperature whose dynamics are already defined in SWAT.
The input data necessary for the simulation were accumulated by the
SWAT/GRASS interface and the SWAT model was used to simulate the hydrology
and water quality in the Wister lake from 1991 to 1994. All the model runs
were made starting from 1989 using synthetic weather data for 1989 and 1990.
A minimal calibration for stream flow was conducted.
Figure 1 shows the time series plot of cumulative stream flow from 1991 to 1994 for the station at Fourche Maline river at Red Oak (Station 1). In general, SWAT overestimated the stream flow but the simulated trend matches well with the observed. Prediction of cumulative flow at Poteau river at Cauthron (Station 2) is not as good as that for station 1.
Simulated water temperatures are reasonably close to the observed data and the trends of simulations match closely with the observed (Figure 2). SWAT predicted unreasonably high total nitrogen concentrations compared to the observed values and consistently lower total P concentrations. Also, SWAT predicted lower DO concentrations during high flow periods and higher concentrations during low flow periods in station 1 and the DO predictions at station 2 during high flows are reasonably close to the observed values, but they were overpredicted during low flows (Figure 3).
There are only a very few models that are linked with a GIS and capable of
simulating both hydrology and water quality on a river basin scale. This
study shows that the GIS-linked SWAT with the instream water quality
component csn be used as a effective tool to predict the effects
of landuse activities on surface water bodies. It can also be used
to analyze the effects of alternative management strategies and aid
regulatory agencies in decision making.
