Integration of SSURGO maps and soil parameters within a geographic information system and nonpoint source pollution model system

Computer based frameworks, including geographical information systems (GIS), associated digital data sets and integrated hydrological models, have become essential tools for the practical deployment of watershed assessment projects and ultimately for providing support for water quality protection and conservation. Within these frameworks digital soil information and data sets play a key role in defining the spatial distribution of important hydraulic variables and consequently of fundamental hydrologic functions connected with NPSs (Nonpoint Sources) and their modelling. This paper illustrates background and results from the development of a GIS based tool required for the use of Soil Survey Geographic (SSURGO) data sets (an undergoing development and delivery effort of digital soil maps by the Natural Resources Conservation Service) and primarily designed in specific support of the Soil and Water Assessment Tool (SWAT) model and its applications in the United States. The tool is developed as an addition or extension to the model companion, the ArcView GIS interface in its latest version (AVSWAT-X) which is designed to define watershed hydrologic features; store, organize, and manipulate the related spatial and tabular data; and analyze management scenarios. Within this framework the tool expedites the otherwise complex inclusion operations of the SSURGO data, such as: (1) downloading, via the Internet, up-to-date SSURGO data sets (version I or II); (2) processing and managing variously formatted data sets in order to create the needed digital soil maps; (3) generating and/or storing the required soil physical and hydraulic model input parameters derived from pedo-transfer functions; and (4) seamlessly including them in any watershed modelling framework. Besides the operational advantages, the tool addresses the trend of employing soil maps with increasing detail in order to improve the landscape representation and reduce the modelling drawbacks involved with the aggregation of the spatial input. Soil map features with unprecedented detail along with the elaborated attributes have a potential for enhancing modelling assessments in support of water quality programs (i.e. Total Maximum Daily Load) at the watershed as well as stream reach scale, as indicated in basic yet important highlights in the paper.