MAP SCALE EFFECT ON THE STREAM ORDER ANALYSIS

Abstract

It can readily be imagined that the result of morphometry of a given region will be different according to the scale of topographical map used in morphometry. Stream order analysis is attempted on the drainage basin of the Ôguri River, a tributary of the Tama River. Four kinds of topographical maps of different scale (1:3, 000, 1:10, 000, 1:25, 000, 1:50, 000) are used. Drainage system maps are drawn from these maps respectively. The Strahler system of stream ordering is adopted and the whole basin is subdivided into 26 subbasins. The drainage networks of these subbasins are shown in Figures 1, 2, 3 and 4. The result of the stream order analysis obtained from each map system is tabulated on Table 1.
Comparison of these figures enables us to recognize the effect of map scale especially at the head of stream network where the number of first order streams decreases with the de crease in map scale. As can be seen from Fig. 7, plots of the logarithms of the number of first order streams against denominator of map scale show a linear relationship between them. The regression equation for this line is expressed approximately in the form of
LogN=LogK-LogS
where K denotes the vertical intercept of the line.
Although the Strahler's stream number assigned to a particular stream segment depend on the scale of topographical map used in ordering the stream network, the Horton-Strahler straight line for the stream number is found to be independent of map scale as shown in Fig. 8. The result obtained supports the Yangs' conclusion that the Horton-Strahler relationship obtained from topographical maps of different scales for the same stream system have the same bifurcation ratio of stream number. Concerning stream length, most of the stream net works of subbasins do not conform to the law of stream length. Sometimes in some basins, the higher order stream segments are shorter than they should be. It may be that, in this case, lack of agreement with the Horton's law is a result of the Strahler method of ordering. Comparison of actual stream system and that of the largest scale map suggest that stream system obtained from larger scale maps includes intermittent streams, which is considered to be less effective as an erosive agents. Therefore, maps of larger scales are not necessarily indispensable for stream order analysis.