Land use distributions and changes in the Willamette Valley in relation to soil characteristics Public Deposited


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  • A study was undertaken in the Willamette Valley, Oregon, to document 1971 land use patterns as well as land use change in a smaller study area in the 16 years prior, and to relate these to soil characteristics. Quantification of these inter-relationships was aided by use of computer tabulation and graphic methods. Use of the data base generated and methods developed was expanded somewhat and applied to calculation and mapping of composite indices of suitabilities and limitations of soils for selected uses. Mapping of land use for 1971 and 1955 was done on color and black and white photos respectively, having a scale of about 1:62,500. To avoid creating artificial, apparent land use changes by differences in delineation and classification, mapping was done first on the higher quality 1971 color photos and then by comparison on 1955 black and white photos. Importance of various photo-interpretive factors to legend development and consistent delineation and classification became apparent and was documented. The legend utilized was, with some modification, the 1972 U.S.G.S. proposed national system for use with remote sensor data. To facilitate comparison of land use and land use change with soil survey map information, photo mapped land use information and available soil association map information was transferred to transparent mylar film over planimetric U.S.G.S. 15' topographic quadrangle sheets. This also allowed use of the square, 1000 meter Universal Transverse Mercator (UTM) grid as reference for point sampling and geo-referenced coding of land use and soil information for computer storage. Tabulation of land use classes for the entire study area for 1971 showed the largest proportion of land to be in agricultural (53.5 percent) and forest land (38 percent) use. A further 6.5 percent was in urban use. Within the selected townships of the land use change study area the proportions were different; 65.7 percent, 14.3 percent and 17.2 percent for agricultural, forest, and urban uses respectively. Analysis of changes in land use between 1955 and 1971 in the land use change study area was done by means of transition matrices; diagonal elements represent areas of land not subject to change; off diagonal elements represent components of change. In total, between 55.9 and 72.4 percent of the land in the area studied experienced no use change, the range being due to uncertainty of classification of some land on the 1955 black and white photos. Urbanization was shown to have occurred largely at the expense of agricultural land and particularly non-irrigated cropland, pasture and orchard land uses. Some conversion of forest to urban land use (0.9 percent of the area) was documented, however. The most significant agricultural use change was a net increase in irrigated land of between 5 and 8.4 percent. A net decrease in orchard land was also recorded. Land use for 1971, not unexpectedly, showed significant concentration in certain physiographic areas and in areas with particular soil characteristics. Thus for example, urban uses were found more frequently than expected on valley-floor and stream-cut terraces, and forest land uses more frequently on foothills and mountainous uplands. Irrigated cropland was somewhat concentrated on soils in sandy, coarse loamy, coarse silty and fine silty particle size classes. Urban and cropland agriculture use classes have obviously avoided steeply sloping areas. Irrigated cropland and orchard land uses have avoided soil areas with high shrink-swell potential whereas grass seed producing croplands have not. Numerous other examples could be given. In summary, Chi-square analysis on the contigency tables of land use class and physical land characteristic classes gave highly significant values (eg. for 1971 land use/physiographic areas x ² calc = 9108.25 and x ² tab = 137.68), and thus the null hypotheses of no association between land use classification and these physical land characteristics were rejected. Three-way tabulation of soil characteristics and physiographic area classification against 1971 and 1955 land use classification allowed analysis of land use change in relation to these physical properties. The idea of composite model soils was used to provide a framework for summary of the kinds of soils subject to smallest and largest changes in particular kinds of urban, agricultural, and forest land uses. As examples, it appeared that greatest relative increases of residential and industrial uses occurred on poorly drained, fine textured, slowly permeable soils; greatest relative losses of agricultural land occurred on soils having the highest capability for agriculture, and; greatest relative increases in irrigation cropland uses occurred on soils rated good to excellent for irrigation. Other examples and trends were observed. A method for computer calculation of composite soil suitability or limitation indices for various uses was developed. Ranked indices were compared to interpretive groupings by best subjective judgement. Class frequency correlations were found to be generally good. Indices were further mapped using a computer graphics system, GRIDS. The index calculation method interfaced with a mapping capability was shown to have potential for rapid, flexible display of various soil, land use, as well as soil and land use patterns for land use planning purposes.
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