The Automated Geospatial Watershed Assessment tool

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Abstract

A toolkit for distributed hydrologic modeling at multiple scales using two independent models within a geographic information system is presented. This open-source, freely available software was developed through a collaborative endeavor involving two Universities and two government agencies. Called the Automated Geospatial Watershed Assessment tool (AGWA), this software is written for the ArcView GIS platform and is distributed as an extension via the Internet. AGWA uses commonly available GIS data layers to fully parameterize, execute, and visualize results from both the Soil and Water Assessment Tool (SWAT) and Kinematic Runoff and Erosion model (KINEROS2). These two distributed hydrologic models operate at different time scales and are suitable for application across a range of spatial scales. Descriptions of the GIS framework, hydrologic models, spatial analyses and algorithms that control the modeling process are given. Model requirements, limitations on the model applications and calibration techniques are described with examples of the use of AGWA for watershed modeling and assessment at a range of scales.

Introduction

Over the past decade numerous significant advances have been made in the linkage of geographic information systems (GIS) and various research and application models (e.g. Shen et al., 2005, US Army Corps of Engineers (ACE), 2003, He et al., 2001, DHI, 2000, Pullar and Springer, 2000, Arnold et al., 1998). These GIS-based systems have greatly enhanced the capacity for research scientists to develop and apply models due to the improved data management and rapid parameter estimation tools that can be built into a GIS driver. This project started after a review of models appropriate for multi-scale hydrologic modeling in support of landscape analysis (Hernandez et al., 2000) determined that none of the existing GIS tools provided a suitable interface for both research and application development. In this manuscript we present a GIS-based tool for the rapid application of two widely applied hydrologic models, and visualization of their results. The Automated Geospatial Watershed Assessment (AGWA) tool is used to provide input to the Soil and Water Assessment Tool (SWAT2000; Arnold et al., 1998, Srinivasan et al., 1998; http://www.brc.tamus.edu/swat/) and the Kinematic Runoff and Erosion (KINEROS2; Smith et al., 1995, Goodrich et al., 2002; http://www.tucson.ars.ag.gov/kineros) hydrologic model. These models operate at different temporal and spatial scales and may be applied in a range of environmental conditions (Miller et al., 2002) to evaluate the impacts of land-cover change on hydrologic and erosion response.

A primary goal of research hydrology is to develop methodologies for accurately depicting the processes driving runoff and erosion at a range of scales. Both process-based models (such as KINEROS2) and more empirical models (such as SWAT) provide insight into the response of watersheds to land-cover and managerial change, provided they are used properly and their input files have appropriate parameters. However, these models are highly dependent on spatially distributed data, and the subdivision of watersheds into response units and the assignation of appropriate parameters are both time-consuming and computationally complex. To apply these models on an operational basis, there is a critical need for automated procedures that are repeatable, accurate, and relatively straightforward. Towards satisfying those needs, AGWA was developed under the following guidelines: (1) that its parameterization routines be simple, direct, transparent, and repeatable; (2) that it be compatible with commonly available GIS data layers, and (3) that it be useful for assessment and scenario development (alternative futures) at multiple scales.

AGWA is an extension for the Environmental Systems Research Institute's ArcView versions 3.x (ESRI, 2001), a widely used and relatively inexpensive PC-based GIS software package (trade names are mentioned solely for the purpose of providing specific information and do not imply recommendation or endorsement by the USDA or U.S. EPA). The GIS framework of AGWA is ideally suited for watershed-based analysis in which landscape information is used for both deriving model input and for visualization of the environment and modeling results. AGWA shares the same ArcView GIS framework as the U.S. EPA Analytical Tool Interface for Landscape Assessment (ATtILA; Ebert and Wade, 2000; http://www.epa.gov/nerlesd1/land-sci/attila/), and can be used in concert with this and similar environmental assessment tools to improve scientific understanding of hydrologic processes and controlling influences of soil and landscape parameters (Miller et al., 2002). AGWA is available either for download directly as an individual program suite and is also included as a standard component of the Better Assessment Science Integrating Point and Nonpoint Sources (BASINS software v3.1; Lahlou et al., 1998; http://www.epa.gov/waterscience/basins/), which provides access to data and several additional environmental models. Interdisciplinary studies may benefit from the integration of multiple model outputs as this approach facilitates comparative analyses and is particularly valuable for interdisciplinary studies, scenario development, and alternative futures simulation work. The primary distribution method for AGWA is via the Internet as a free, modular, open-source suite of programs (www.tucson.ars.ag.gov/agwa or www.epa.gov/nerlesd1/land-sci/agwa/).

Ongoing research efforts are tied to enhancing AGWA features and moving the code from ArcView 3.x (Avenue) to ArcGIS 9.x (Visual Basic, VB.NET, Python). Upcoming beta releases are planned for AGWA 2.0 (ArcGIS 9.x) and an Internet-based version of the toolkit referred to as DotAGWA. The essential elements of the AGWA coding and methods for performing watershed assessment, change analysis, and hydrologic modeling, will remain the same, although the software engine will be updated to reflect more modern GIS releases and to enhance Internet accessibility. Detailed design documents for AGWA 2.0 and DotAGWA (Cate et al., 2005) are available on the AGWA web sites.

AGWA provides the functionality to conduct all phases of a watershed assessment for SWAT and KINEROS2. SWAT2000 is the current version of SWAT and is a continuous-simulation model for use in large (river-basin scale) watersheds. KINEROS2 is an event-driven model designed for watersheds characterized by predominantly overland flow. The AGWA tool combines these models in an intuitive interface for performing multi-scale change assessment, and provides the user with consistent, reproducible results. Data requirements include elevation, land-cover, soils, and precipitation data, all of which are available at no cost over the Internet. Model input parameters are derived directly from these data using optimized look-up tables that are provided with the tool.

Section snippets

Component models

The key components of AGWA are the hydrological models used to evaluate the effects of land-cover and land use on watershed response. In this section, a description of the basic structure of each model is provided as well as their simplifying assumptions, strengths, and weaknesses. The KINEROS2 and SWAT models are able to simulate complex watershed representations that explicitly account for spatial variability of soils, rainfall distribution patterns, and vegetation.

Overview of the AGWA tool

AGWA is loaded into an ArcView project in the typical manner for all extensions. Once loaded, however, AGWA mandates that the user specify a new project name, and a complete directory structure is created based on user input. The creation of a specific directory structure allows AGWA to locate model inputs, results, and newly created GIS data layers to ensure project integrity. A fundamental requirement of AGWA is that the user has previously compiled the necessary GIS data layers, all of which

Watershed delineation

Numerous approaches have been developed for automated extraction of watershed structure from grid digital elevation models, with the earliest techniques still being used today (e.g. Gyllenhammar and Gumbricht, 2005, Martz and Garbrecht, 1993, Moore et al., 1988, Band, 1986, Mark et al., 1984). The most widely used method, and that which is used in AGWA, for the extraction of stream networks is to compute the contributing area upslope of each pixel through a network of cell-to-cell drainage

Watershed modeling with AGWA

There are several primary intended uses of AGWA. For one, AGWA can be used in a research environment as a hydrologic modeling tool. In this setting, the user would be expected to alter the look-up tables or estimated parameters manually to allow for more rigorous quantitative assessment. In the absence of a rigorous training set for calibration and validation, AGWA is well suited for watershed assessment using spatially distributed hydrologic response as a metric of change. If multiple

Conclusions

A GIS-based hydrologic modeling toolkit called the Automated Geospatial Watershed Assessment (AGWA) tool has been developed for use in watershed analysis. This tool has been released as open-source and is fully modular and customizable. AGWA automates the process of converting commonly available GIS data to input parameter files for the SWAT and KINEROS2 hydrologic models. Rainfall files for both models can be prepared within AGWA depending on the availability of rainfall data. Results from

Acknowledgements

This research was funded under an interagency agreement (DW1293940901) from the U.S. Environmental Protection Agency. Infrastructure and support was also provided by the University of Arizona Advanced Resources Technology (ART) Lab. Ian (Shea) Burns, Soren Scott, Lainie Levick, and Averill Cate contributed to the development of AGWA including the modeling code, user manual, and web site. Carl Unkrich provided technical assistance with linkages to KINEROS2 including manipulation of its code

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