摘要:

ABSTRACTThe Usambara Mts., like other Tanzanian mountain areas, have a high potential for crop production, thanks to a favourable climate and fertile soils. This has resulted in a high population density and, in consequence, heavy pressure on the land resources. Deforestation and soil erosion are increasing threats to the long-term potential of the mountains to sustain a growing population.The aim of the present report has been to compare surface runoff and soil loss, and soil characteristics influencing these, in natural montane forest and under small-scale agricultural farming in the Usambara Mts., Tanzania.Two sites of undisturbed natural forest, one in Intermediate Evergreen Forest and one in Highland Evergreen Forest, were selected, together with one very well-kept agricultural site. On each site, four runoff plots of 12 sq.m each were installed—two on“gentle”slopes (10–15°) and two on“steep”slopes (20–25°). On these twelve plots (3 sites×4) surface runoff and soil loss were recorded together with rainfall/throughfall in 1974 and 1975 (24 months). Soil samples down to a depth of 50 cm were collected and analysed for pH, organic carbon content, bulk density, texture, water-stable aggregates, porosity, and water-retention properties. In addition, infiltration capacity was measured, with a double-ring infiltrometer, at each site plus at a badly eroded area nearby.Non-stable soil parameters of importance to erosion are normally slightly less favourable in the agricultural topsoil (0–10 cm) than in the forest topsoils. Thus, organic matter content was appr. 25 % in the Highland forest topsoil and appr. 10 % in the Intermediate forest, compared with appr. 8 % in the agricultural topsoil. Bulk density was appr. 0.4 g/cm3in the Highland forest topsoil, appr. 0.8 g/cm3in the Intermediate forest and appr. 0.9 g/cm3in the agricultural topsoil. The forest topsoils showed high porosity values, the Highland forest had appr. 80 % pores of the soil volume, the Intermediate forest appr. 70 %, and the agricultural land appr. 60 %. In addition, the percentage of water-stable aggregates was lower in the agricultural soil than in the two forest soils. Infiltration capacities of the two forest sites were 850 mm/h and 830 mm/h respectively, of the agricultural land 635 mm/h and of the badly eroded agricultural land only 270 mm/h, all after 90-minutes measurements. Water-retention properties differed greatly between the three sites. At field capacity, for example, the plant-available moisture in the top 50 cm of the soil was 129 mm in the Highland forest soil, 101 mm in the Intermediate forest soil, but only 52 mm in the agricultural soil. Certain soil properties, such as pH and clay content, indicate that there might be inherent soil differences between the agricultural site and the forest sites.Average annual rainfall at the Intermediate forest site was 1115 mm, at the Highland forest site 820 mm, and in the agricultural area 635 mm for the two years of study. This is lower than normal. Average surface runoff as a percentage of rainfall was 0.4 % on the gentle-slope plots and 1.0 % on the steep-slope plots in the Intermediate forest. The corresponding figures for the Highland forest were 0.8 % and 1.3 %, and for the agricultural area 0.1 % and 0.1 %. Average soil loss in g/m2/year was 4.2 from the gentle-slope plots and 10.1 from the steep-slope plots in the Intermediate forest. The corresponding figures for the Highland forest were 3.7 and 7.5, and for the agricultural area 1.0 and 0.9. The very low values of surface runoff and soil loss from the small-scale agricultural land are discussed in the report. Good land management, including, for example, no burning and leaving weed trash as mulch, is the most probable explanation.Large seasonal variations of runoff as a percentage of rainfall/throughfall, with high percentages (up to 14 %) at the onset of the rainy seasons, were observed. This was especially pronounced on the Highland forest plots. Retarded rainfall acceptance of the strongly humic topsoils when they are dry is a probable explanation.The study has shown that with good land management it is possible to minimize surface runoff and soil loss from agricultural land on moderate slopes, at least during years of below average rainfall. Since no extreme rainfall events occurred during the two years of study it is impossible to say how the agricultural soil would have reacted to this. It is well-known, however, that rare and extreme rainfall intensities cause most of the erosion that occurs on badly managed land, a fact that has been observed also in the Usambara mountains.

ISSN:0435-3676    

DOI:10.1080/04353676.1980.11880005

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

ABSTRACTA series of features is ranged down a hillslope at Peak Scar, in North Yorkshire, England, which illustrates stages in the development and degradation of a consecutive-block-failure type of landsliding which is widespread in the surrounding hills. A cliff 30 m high is riven by widened joints and downslope there are massive tilted blocks of rock. The forms suggest consecutive operation of three sets of mechanisms which give rise to:a)splitting of the rockb)sliding towards the valley over essentially horizontal slipplanes, of rock masses separated from the parent hill by the splitting mechanismc)tilting of the slid rock masses towards the valley.Degradation then proceeds by weathering, surface creep, surface wash and occasional debris flow.

ISSN:0435-3676    

DOI:10.1080/04353676.1980.11880006

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

ABSTRACTThis paper is a preliminary presentation of some of the results from a study of soil erosion and sedimentation in Lesotho, started in 1973. Sedimentation of six small (0.4-2.8 104m2) reservoirs within Roma Valley and Maliele catchments was studied between 1973 and 1977. The reservoirs have catchment areas varying in size from 0.4 km2to 26 km2. Total (dry matter) accumulation was found to vary from 0 to 4000 metric tonnes with mean annual rates of up to 1100 metric tonnes.

ISSN:0435-3676    

DOI:10.1080/04353676.1980.11880007

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

ABSTRACTIn this study three sets of scanning electron micrographs are presented: (a) after grinding of quartz crystals in a rotating rock mill, (b) after (a) plus aqueous transportation of two durations (600 and 1000 hours), and (c) after (a) plus eolian transportation of three durations (5, 50 and 100 hours). Sand composition were mixed according to the size distributions of certain Polish river and dune sands. Each of the two compositions was run in its respective laboratory apparatus.The original ground quartz grains demonstrate the effect of the shearing motion of the millplates. The surfaces of these grains show conchoidal fractures known as“glacial”features, indicating that natural grains with these features are formed in an ice with a shearing motion.In the experiment on aqueous transportation formerly reported mechanical V-shaped patterns were formed. The abrasion of the grains was slight.In the eolian experiment the rounding of the corners of the grains was greater than in the aqueous experiment. There was a deposition of fragments on the larger grains in the eolian experiment. Upturned plates are noted as well as what may be small molten and recrystallized quartz fragments.

ISSN:0435-3676    

DOI:10.1080/04353676.1980.11880008

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

ABSTRACTEight14C dates have been obtained from a 5 cm thick Ah, horizon of a podzol soil buriedin situbeneath the outermost end moraine of the Haugabreen gletschervorfeld, southern Norway. The dates ranged from 880±35 to 3140±55“C years B.P.; a strong increase in age with depth was indicated; various fractions of soil organic matter were found to differ significantly in age. Ten problems in the interpretation of these results were considered. Date of burial of the palaeopodzol was discussed using multiple working hypotheses, assuming different forms of age/depth relationship and the presence or absence of contamination. Burial during the“Little Ice Age”is most likely, although a conservative maximum estimate of time elapsed since burial is given as 140014C years. The recognition of numerous and precisely dated pre-“Little Ice Age”Neoglacial glacier maxima on the basis of14C dates from well-developed palaeosols is criticized. Apparent mean residence times within such soils are likely to increase steeply with depth and may be of the order of 1000 years within the top 2.5 cm. Some other pedological implications are outlined; it is inferred that at least 3000 years were required for formation of this soil horizon.

ISSN:0435-3676    

DOI:10.1080/04353676.1980.11880009

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

ABSTRACTThe Holocene shorelines of the Mälar valley were according to G. De Geer formed by strong gales during occasional high water levels. They were then uplifted and preserved due to land uplift. Another hypothesis is that the shorelines were formed during eustatic transgressions when the rise of sealevel, at least to some extent, compensated for the isostatic land uplift. In such case several different storm surges would be responsible for the development of a particular shoreline. In order to find out which of the ideas is the more correct one, a study of shorelines in two areas with different land uplift could be of great value. For this study, two regions about 700 km apart, were selected. Some levels of shore-marks are possible to trace at the two areas of investigation. This gives some support to the idea that the shorelines, or rather levels of shoremarks, were formed during times of eustatic transgressions.

ISSN:0435-3676    

DOI:10.1080/04353676.1980.11880010

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor

摘要:

ABSTRACTA mountain area in northern Sweden (lat.68°N, long. 19°E) was surveyed on stereoscopic Landsat imagery concerning avalanche landforms and general geomorphology. The result is presented on two maps to the scales of 1/100,000 and 1/400,000. An evaluation of the mapping was made with the use of existing topographical and geomorphological maps as ground truth. Mainly large-scale geomorphology could be mapped from the imagery. Avalanche boulder tongues and some other medium-scale landforms could be surveyed but not very accurately. The restricted resolution and the problem of obtaining optimum imagery for large areas are present limitations of the application of Landsat MSS imagery to the survey and mapping of landforms. Stereo viewing of the imagery increases its potential in applications related to geomorphology and geology.

ISSN:0435-3676    

DOI:10.1080/04353676.1980.11880011

出版商:Taylor&Francis    

年代:1980

数据来源: Taylor