Changes in model results as a function of changes in soil survey data a GIS comparison of soil rating for plant growth (SRPG) values derived from the 1966 and 1999 versions of the Reno County, Kansas soil surveys

Abstract

Soils are one of the most essential natural resources required to maintain the health of the environment and the continued prosperity of mankind. The mapping of soil resources can generate a better understanding of soil composition and its functions while aiding the development of improved soil management practices. Traditionally, hardcopy formats of soil survey reports were the end products of field mapping investigations and their interpretations were largely agricultural in nature. Rapidly changing technologies have initiated the need to satisfy increasing numbers of technical soil survey information users. Among these new users of soil survey information are the geographic information systems (GIS) technicians. SSURGO is the digital format of soil survey information currently available in the United States for use with GIS software. GIS technologies provide an effective means to interpret, analyze, and display digital soil survey information but few GIS practitioners stop to consider the implications of temporal changes in soil survey information on their interpretations from digital soil surveys. The opportunity to compare two versions of a digital soil survey area to evaluate these temporal changes in mapping within the same study area is rare. The Soil Surveys of Reno County, Kansas published in 1966 and 1999 are both available in digital format and therefore provide a unique opportunity to evaluate temporal differences in soil surveys within a single area. This type of spatial comparison is best conducted using GIS to evaluate the possible differences in soil properties and soil map unit delineations because a GIS approach allows spatial variations of map unit delineations and associated soil attribute information to be investigated simultaneously. The incorporation of a model application, Soils Rating for Plant Growth (SRPG) (NSSC Staff, 1999), which utilized soil survey information to estimate soil productivity, was used to enhance GIS comparison capabilities in the context of a practical application for soil survey data. SRPG indices, calculated for individual map units, reflect a soil's ability to produce commodity crops. Recently, the Division of Property Valuation for the Kansas Department of Revenue has started using SRPG indices to help them assess the tax value of agricultural land. As a function of re-mapping the land valuations based on soil survey information will potentially change. The cause and extent of these types of changes is currently unknown. SRPG indices, soil map unit delineations, and soil properties are investigated in this study, at a variety of spatial levels, in order to assess the spatial differences between SRPG indices calculated using the 1966 and 1999 versions of the Reno County Soil Survey. These evaluations conclude that it is difficult to compare the changes between two versions of a digital soil survey area at the county extent. A more detailed investigation is recommended, preferably at the map unit level. Fluctuations in SRPG indices were contributed more to changes in map unit delineations rather then changes in soil property values. The 1999 version of the Reno County Soil Survey area displayed a slightly larger mean SRPG index value when summarized by township polygons. This may be contributed to more knowledge about soils themselves, better mapping techniques and equipment, and/or the result of mapping to a greater detail.