1. Water Quality Modeling
Evaluation of Nitrogen and Phosphorus Land Surface Loads for Nueces Bay
Zsuzsanna Iwanicka
University of Texas at Austin

2. Project Objectives
Learn about GIS application in water quality modeling
Learn about data processing needs for GIS- aided water quality models

3. A few reasons to develop water quality models
Gain a better understanding of ecosystems
Determine the impact of human activities on water quality
Determine what is needed to improve water quality
Provide an analysis tool supporting policy decisions

4. Data layers for a mass balance model
Point Source Loads
Atmospheric Loads
Land Surface Loads
Waterbody
Inflow Loads
Outflow Loads
Sediment Loads
A complete GIS based water quality model was developed by Ann M. Quenzer
It is available at:

5. Term Project Focus
Develop a model for mean annual non-point source pollution loads in ArcGIS …
…and make it user friendly

6. Modeling Steps I Determine the annual surface runoff in
Following Ann Quenzer’s methodology
Determine the annual surface runoff in
[mm precipitation/year] through establishing a statistical relationship between:
land use type
annual precipitation
streamflow
Equation: lnQ=lna+b*P+c*Ln(%A)

7. Q=ax*exp(bx*P) [mm/year]
Modeling Steps II
Get the equation for different land use types and compute the runoff
Q=ax*exp(bx*P) [mm/year]
Determine the P and N concentration* depending on land use type and compute the annual loads
for the defined watershed
*Study conducted for watershed provides the constituent’s concentration for different types of land use

8. Modeling Steps III Getting into GIS… Pull up the following layers:
Delineated watershed, river network…
Land use polygons
Annual Precipitation

9. Modeling Steps IV Intersect land use and precipitation layer
Create the button to run the model…

10. Modeling Steps V
Nueces Bay - Study Area

11. Acknowledgements Imane Mirni for sharing her data
Ann Quenzer for her GIS-based water quality model
Dr Maidment for the wonderful GIS classes and for his quick answers to my questions
My fellow students in the GIS class for their advice and encouragement
My husband, Jan Iwanicki, for helping me to develop the “button” and for taking care of our children during my studies

12. Questions?