摘要:

ABSTRACTWithin the scope of the DUSER project (1968–74) research on land degradation and related problems was carried out in four catchments with reservoirs in Dodoma district, Tanzania. In this paper are presented findings from one of the catchments. Imagi. The results include quantitative indications of the seriousness of soil erosion and its implications on range conditions, surface water storage and water supply in central Tanzanian drylands.The catchment area covers 2.2 km2. It is situated in semi- arid environment with inselbergs and pediment slopes as predominant landforms. Mean annual rainfall is 528 mm and annual potential evaporation is well above 2000 mm. Rainfall is strongly seasonal and confined to the months December-April. The vegetation is dense thicket and miombo woodland on the inselberg slopes and shrub and sparse grass on the pediment. The soils are of low fertility. Present dominating land use is grazing by livestock.For a period of more than 20 years, beginning in 1929, when the reservoir was built, the area was reserved as catchment for the Dodoma town water supply. In that period hydro-meteorological research was carried out in the catchment. Much of the rainfall was found to be lost by evaporation from interception storage and transpiration from the catchment vegetation. Annual runoff from the catchment varied between 3 and 17 % of the rainfall. Losses through leakage and evaporation from the reservoir amounted in average to 50 % of the max. volume stored in the respective years. The small size of the catchment, the low rainfall and the great water losses all contribute to the reservoir being filled in one year out of five only. During the period 1928–55 the responsible authorities in Dodoma had to enforce rationing of water in altogether 10 seasons.The reservoir is gradually losing capacity through sedimentation due to soil erosion in the catchment. As part of the DUSER programme the volume of sediment was assessed through levelling and sounding of profiles across the reservoir. Inventories of vegetation and erosion features in the catchment were made through air photo interpretation and field studies.Erosion was found to be slight on the densely vegetated inselberg slopes while on the pediment surfaces runoff and soil loss are more prominent. Splash, sheet wash and rilling are the dominating types of erosion. Gullying occurs along cattle tracks.The observed variations in sediment yield are shown to be directly related to changes in land use. When the earlier protected catchment was opened for cattle grazing the sediment yield rose by 50 %. The average annual sediment yield for the period 1934–71 was 602 m3or 900 ton/km2which is comparatively low considering the steep slopes and the small catchment size. The rate of soil denudation distributed over the whole catchment varied during the same period between 0.50 and 0.83 mm p.a. which means a denuding effect measured on the pediment only between 1 and 3 mm p.a.The reservoir has on an average lost 0.78 % p.a. of its original volume and the maximum depth has decreased from 10.8 to 5.7 m. Provided the average rate of sedimentation remains the same in the future the total life of the reservoir will be 120–130 years.As the ground water resources of the Dodoma area are unsufficiently known and permanent water for domestic purposes and watering of stock is scarce it is recommended not to discontinue the construction of surface water reservoirs but each project should include a prognosis of expected rate of sedimentation and a plan for soil and water conservation measures in the catchment. And unless control can be obtained over cultivation practices and the number of animals grazing in the area little can be done to prevent land degradation.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879986

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor

摘要:

ABSTRACTRainfall erosivity parameters such as rainfall intensity and kinetic energy and (heir combination and the temporal distribution of erosive rains, are examined for 35 stations in Kenya, Tanzania and Uganda. Intensity and kinetic energy parameters show the highest erosivity hazards to be in Uganda, the Lake Victoria area and parts of the Kenyan Highlands and the coast (R values of over 400). The lowest erosivity hazards occurred in the drier areas of Kenya and Tanzania (R values less than 150). The hazards in these latter areas are magnified because about 70 per cent of the erosive rains often occur within 30 days of the onset of the rains. In this period, crop and grass cover is low with much bare soil exposed and few crop management practices are available to increase the cover.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879987

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor

摘要:

ABSTRACTOwing to favourable soil and climatic conditions, the naturally forested highlands of Tanzania have a comparatively high potential for agriculture. Major limiting factors are drought damage and erosion. The severity of these depend on the interaction between land use practices, soil/ topography and climate. As a result of rapid population increase land use practices are becoming more intensive, resulting in deteriorating soil structure. This in turn will lead to changed relations between climatic features on the one hand and crop and soil damage incidence on the other. The report is based on analyses of rainfall, interception and evaporation data in a forest and a small clearing in the Mazumbai Forest Reserve in the Usambara Mts. in NE Tanzania, obtained from measurements carried out 1971–1976, and of monthly rainfall data from the period 1945–1976 from the nearby Mazumbai Estate. The mean annual rainfall in Mazumbai is 1227 mm but the variability is great. Standard deviation from the mean (based on a record of 32 years) is 324 mm and the rainfall likely to be exceeded in 4 years out of 5 is 950 mm. The highest annual rainfall recorded in the period is 1864 mm and the lowest 721 mm. April and May have high (180 and 250 mm) mean monthly rainfall, all other months have an average between 50 and 100 mm, but the variation from year to year is very high and any month, except April and May, may be completely dry. Prolonged droughts, i.e. two or more consecutive months with less than 50 mm of rain are frequent. In the 32-year period two very long such droughts have occurred (5 and 6 months), when only 30–35 % of normal rainfall was received.The longest dry spell (i.e. four or more consecutive days with less than 0.25 mm rain) recorded in the clearing lasted for 35 days. Spells of shorter duration are frequent and may occur any time, also during the long rains (April—May). Mean number of rain-days (>0.25 mm rain) per year in the forest clearing was 125, the maximum monthly value occurring in May with 20 rain-days and the minimum in January with 7 rain-days. The mean rainfall per rain-day is 10.3 mm (max. in May with 15.4 mm and min. in August with 4.9 mm). The highest daily rainfall recorded was 102.4 mm. Rain-days with less than 5 mm rain account for 50 % of the total no. of rain-days but only 11 % of the total rainfall. Total throughfall in the forest, i.e. rainfall at 0.5 m under closed canopy, during 2 1/2 years measurements, amounted to 78 % of rain received in the open. Neglecting stemflow, this gives a rain interception of the forest canopy of 22 %. Evaporation measured at 1.5 m height above ground with Andersson evaporimeters was five times higher in the clearing than in the forest–677 mm and 127 mm respectively per year. The seasonal variation in evaporation was much larger in the clearing than in the forest. In the final discussion it is pointed out that the incidence of land damage, in the form of erosion or reduction of crop growth, caused by climatic events such as intensive rains and prolonged droughts, will increase rapidly in the Usambara Mts., as a result of land use practices causing deterioration of soil structure.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879988

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor

摘要:

ABSTRACTTalus slopes, especially their particle orientation, were studied in order to illustrate the variations in talus forming processes in relation to climate.An arid and a periglacial area were chosen for this study. As case studies certain desert talus slopes were investigated in parts of the Negev and Sinai deserts close to the Gulf of Aqaba and a periglacial talus cone was studied in Kärkevagge, northern Scandinavia.The studies were concentrated to measurements of such indicators of past and present talus forming geomorphic processes, as particle form, size, orientation and imbrication. While the periglacial field area includes only one type of bedrock, the other includes a larger variety of bedrock types and cone morphology.The interpretation of particle orientation and imbrication was made from plottings on Schmidt-nets showing the true dip and an orientation of the talus material.The study of the different talus cones has clearly illustrated the difference between the two contrasting environments as regards slope profile, particle shape—transport relations, particle size and orientation, all indicating a difference, between the two areas in type of process and frequency.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879989

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor

摘要:

ABSTRACTAn objective classification of daily weather maps for the Canadian High Arctic was developed with a view to identifying those synoptic situations which greatly affect ablation season temperatures and annual precipitation totals. This classification was used to catalog synoptic types for the period January, 1946 to August, 1974. 22 basic types were recognized, accounting for∼96 % of days in the period. Most types have distinct seasonal maxima. Using data from Alert, Isachsen and Thule, monthly mean temperature characteristics of the types were obtained, enabling them to be ranked, warmest to coldest. Although“warm”and“cold”types were different for each station, the circulation characteristics of cold types, and of warm types, were similar. Stepwise multiple regression analysis was used to indicate the synoptic types which are closely related to inter-annual variations of mean monthly maximum temperatures. Generally, the maximum reduction of variance in the temperature record was achieved with a minimum number of synoptic types in the months April-August, suggesting greater“control”on temperature by a few types in spring and summer months. Ablation season climatic data were used to identify synoptic types which were“cool and wet”,“warm and dry”,“cool and dry”or“warm and wet”. Cool, wet types have increased in frequency over the last 10–15 years whereas warm, dry types were slightly less frequent. Such changes are not sufficient to account for the deterioration in summer climate of the area in the 1960s; within-type changes are probably significant factors in this deterioration.Stratification of precipitation data by synoptic type indicates a small number of types account for most of the annual precipitation at each station, though this is really a function of type frequency. Other, less frequent, types are more efficient precipitation-bearing situations, in terms of precipitation per day of type occurrence. In many of these situations, low pressure close to the station dominates the circulation; these may be North Atlantic depressions regenerated along the Siberian coastline. Even small changes in the frequency of these systems would have important consequences for High Arctic precipitation.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879990

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor

摘要:

ABSTRACTA minimum date of 8480±15514C yr B.P. on the deglaciation is reported. The date was obtained on a sample of wood fragments and is therefore not affected by old carbonate. The date is regarded as a reliable minimum date of the deglaciation. Another slightly older minimum date (8900±14014C yr B.P.) obtained on peat from the same locality is probably also reliable. Dates on early postglacial organic sediments from ten other localities are discussed. It is concluded that a large section of northern Lappland was deglaciated before 9000–850014C yr B.P. The number of high-quality14C dates is at present insufficient to determine the pattern of deglaciation in Lappland.Problems involved in the14C dating technique are discussed. It cannot be excluded that short-term fluctuations in the atmospheric content of14C occurred around 9000 B P., and this may have influenced dates from this period.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879991

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor

摘要:

ABSTRACTThe retreat of a cliff coastline in sediments of glacial origin is described. Rates of erosion are discussed, related in volumetric and percentage terms to removal from the complete surface area of the cliffline, and the processes involved are ranked with respect to the magnitude-frequency of events. The influence of temporal and spatial scales are stressed and a model of coastal slope development for this cliffline is forwarded.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879992

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor

摘要:

ABSTRACTConcern over loss of shore property and local deterioration of the physical condition of beaches on the west coast of Barbados has led to extended analysis of the coastal subsystems. The main features of the west coast are inherited from events associated with complex sea-level fluctuations and extension of coral terraces in the Quaternary. The contemporary coast was established at the close of the Flandrian transgression along an early Wisconsin rocky shoreline. Introduction of sand into the shore system from cliff erosion (2 to 10 m retreat during the period), reef destruction and terrestrial sources has formed a sand beach that has moved seawards with time leaving a narrow sand terrace between the shore and the earlier coastline. Nineteen beach cells were identified along the west coast and intensive studies of wave and current processes leading to changes in beach planform and profile were made at two (Gibbs and Sandy Lane bays). Whilst the volume-shape mechanism operating through an annual cycle leaves a beach cell in quasi-equilibrium, overall loss of beach sand in the past 13 years has been accompanied by a 45 % linear increase of exposed beach rock.

ISSN:0435-3676    

DOI:10.1080/04353676.1979.11879993

出版商:Taylor&Francis    

年代:1979

数据来源: Taylor