No abstract

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The unit‐source watershed is an intermediate step between plots, in which certain runoff generative processes can be isolated, and large watersheds, the yields of which are controlled by the hydraulics of their complex channel systems. Several unit‐source watersheds have been instrumented within the Walnut Gulch experimental watershed in southeastern Arizona. Their comparative data indicate some basic hydrologic relationships between net runoff and size of drainage area, the significance of storm patterns, the relation of runoff and sediment yield to vegetational cover, and hydrograph characteristics.

Soil carbon data were collected from published sources for 50 measurement sites spanning the globe's major climate and vegetation types. For each site, climate, vegetation, and land-use variables were determined and entered into a multiple curvilinear regression program to predict soil carbon. The best model incorporates an estimate of site disturbance, annual actual evapotranspiration, and annual soil moisture deficit, and yields an R = 0.803. The curvilinear regression equation was coupled with a large climatic database and computer cartography programs to produce first-generation maps of estimated soil carbon. These maps correctly portray soil carbon as high in boreal and cool temperate zones arid low in deserts and tropical zones.Computer planimetry of maps of soil carbon for an 'undisturbed' world and for a 'disturbed' world resulted in estimates of 1457 x lo9 mtC and 504 x lo9 mtC respectively. These estimates compare favourably with recent estimates using other approaches. Clearly, the disturbance factor is critical to future refinements in estimates, suggesting the need for detailed studies of the relationship between land-use history and the creation and destruction of this important carbon pool.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. . Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Chuska Wash, 55.3 per cent sediment. 2. From peak of flow in Catron Wash, 21.8 per cent sediment. 3. From peak of flow in Figueredo Wash, 20.0 per cent sediment. 4. From peak of flow in Mexican Springs Wash, 2:30 P.M., 26.0 per cent sediment. 5. From same flow, 2:45 P.M., 14.3 per cent sediment. 6. From same flow, 3:10 P.M., 13.2 per cent sediment. 7. From same flow, 4:35 P.M., 7.6 per cent sediment. All percentages of sediment are by weight. portion-less than 1 per cent-to the total runoff. The water carries, also, considerable amounts of debris, especially at the beginning of a flow, as is illustrated in figure 2, which shows one of the ephemeral streams on the Station area. Weather data collected since 1934 indicate that the average annual precipitation ranges from approximately 9.5 inches at an elevation of 6200 feet to 17.5 inches at 8000 feet. A Class A weather station, established in April, 1934, is maintained at an elevation of FIG. 2. Beginning of a flow in Mexican Springs Wash. Note debris.