This document outlines a research project to improve the simulation of nitrogen and phosphorus fate and transport in irrigated agricultural watersheds in semi-arid regions. The project aims to: 1) Enhance the Soil and Water Assessment Tool (SWAT) by integrating it with MODFLOW and RT3D to better simulate surface-subsurface flow and reactive transport processes. 2) Improve understanding of the key processes governing nitrogen and phosphorus movement, transformation, and storage in agricultural watersheds. 3) Assess remediation strategies for nitrogen and phosphorus contamination under varying climate scenarios. The enhanced model will be tested and results disseminated to help watershed managers.

1. Mazdak Arabi, Ryan Bailey, Timothy Gates, Xiaolu Wei (PhD student)
Dept. of Civil & Environmental Engineering, Colorado State University
Improved Assessment of Nitrogen and Phosphorus Fate and
Transport for Irrigated Agricultural Watersheds in Semi-Arid Regions
1. Integrate SWAT with MODFLOW and UZF-RT3D to enhance simulation
of surface-subsurface flow and reactive transport processes
2. Improve understanding of the processes that govern N and P
movement, transformation, and storage in agricultural watersheds
3. Assess remediation strategies for N and P contamination in semi-arid
agricultural watersheds under varying climate scenarios
4. Work closely with the SWAT development team to enable broad
dissemination and use of the enhanced model
Project Objectives
Project Timeline
Modeling Framework
Principal Study Region
Additional Study Regions
Conceptual Model
Model Coupling HRU-Grid Linkage2013-2014 Modeling Framework / Field Work / Model Testing
2014-2015 Finish Model Testing / Assess Fundamental Processes
2015-2016 Remediation scenarios / Climate Scenarios
2016-2017 Remediation scenarios / Distribute Results (SWAT Team)
Lower Arkansas River Basin, Colorado
Upper Klamath Basin, Oregon
SWAT + MODFLOW + RT3D
Previous Results (MODFLOW + UZF-RT3D)
Model and code are open source and are now available.
They can be obtained by contacting the project participants.
Little River Watershed, Georgia
Nitrate Groundwater
Loading
SWAT + MODFLOW + RT3D
Dissemination of Model
Conference Presentations
Working with SWAT Modeling Team
June 2015 October 2015
1. Working on test watersheds (Little River, GA; Middle Bosque, TX)
2. SWAT-MODFLOW Workshop at SWAT Conference (Purdue University; October 13, 2015)
3. Developing User Interface (prepare MODFLOW model, SWAT-MODFLOW linkage)
Nitrate Concentration in Groundwater (UZF-RT3D)
Bailey, R.T., Gates, T.K., and M. Ahmadi (2014), “Simulating reactive transport of selenium
coupled with nitrogen in a regional-scale irrigated groundwater system.” Journal of Hydrology
515, 29-46.
Nitrate Groundwater Concentration
SWAT Model
MODFLOW Model
DEM  Grid
HRU  Grid
River 
Subbasins
Model Linkage
Water table
elevation
Groundwater
discharge