No description available.

This thesis aims to study evaporation, the depletion of soil moisture during the rainy season and to test the applicability of various evaporation estimation methods in the hot semi-arid conditions of the Gedaref Region of the Sudan (annual rainfall between 300mm and 700mm). Evaporation variables and some climatic factors were measured in the field in a rainfed area, 40KM east of Gedaref Town, from July 30th to December 31st 1983. The methods employed in the measurement of evaporation include a Class A pan, a Piche atmometer, and 20 weighable lysimeters for the assessment of actual and potential rates of bare soil evaporation and evapotranspiration. The four evaporation variables (Pan, Piche, PE and potential bare soil evaporation) increased in magnitude from August towards December. Monthly mean Piche evaporation, for example, increased from 9.7 mm/day in August to 13.5 mm/day in December. This increase of evaporation towards December, despite the decrease in solar radiation, is attributed to the effects of advected heat in the dry months (October to December). Established evaporation estimation methods tested for applicability (which include the Penman Formula, the Thornthwaite method and the Blaney-Criddle method) proved to be inadequate in the Gedaref Region. The original Penman (1948) equation, for example, greatly under-estimated measured PE with mean monthly deviations as high as 5.41 mm/day. New regression models were therefore developed and the Penman Formula was revised in a search for better estimation methods. The four regression models developed were found to approximate measured PE values very closely such that the highest mean monthly deviation was only 0.38 mm/day. The newly revised Penman Formula gave good results too, with the highest mean monthly deviation being 0.89 mm/day. In addition to these five estimation equations developed, the 10-day averages of the wet bulb depression in degrees Celsius (without the application of any correction factor) also closely approximated the measured PE with the highest mean monthly deviation being only 0.89 mm/day.

Slopewash, a combination of rainsplash and overland flow erosion, is one of the principal soil erosion processes in rain-forest areas. Conventional theory suggests that slopewash in rain-forest environments should increase systematically with increasing slope angle. if soil and ground cover variables themselves change with slope angle, however, relationships between erosion and slope angle may be more complex. When rain forest is logged, whether and for how long rills and gullies initiated on heavily distributed and compacted terrain components continue to enlarge is critical. This thesis examines these issues on slopes of 0-40° in primary and selectivity logged (in 1988-89) rain forest at Danum Valley, Sabah (Malaysian Borneo). Measurements were made of: (a) actual erosions, deposition and changes in surface roughness at over 100 pre-existing and new sites over periods of 1-9 years using the erosion bridge (microprofiler) technique; (b) infiltration capacity, overland flow, splash detachment and overland flow erosion at small (30 x 30 cm) plots at key sites using a rainfall simulation programme; (c) ground cover and soil properties; and (d) overland flow occurrence at free-standing slope sites under natural rainfall conditions. Overland flow on primary forest slopes was found to be more widespread and frequent than previously thought. Tentative relationships between overland flow, erosion and slope angle, to higher angles than previous studies and theory have proposed, are presented. The dominance of extreme rainstorms in the temporal pattern of both primary forest and post-logging erosion is demonstrated. Eight to twelve years after logging, landslides along logging roads in higher slope areas and roads (particularly when unsurfaced) are the main sediment sources to the stream network. Erosion rates at skid trails and logged slopes, however, are now close to those in primary forest. Results highlight the importance of organic carbon within the soil to maintain structure and aggregate stability, and, therefore, to increase soil resistance to erosion.

No description available.

Pedological research in the Sudan is scanty and almost non-existent in the area west of the White Nile. This dissertation attempts to throw some light on the soils developing in this semi-arid region in terms of genetic and behavioural characteristics using air photographs, satellite imagery, field work, laboratory and computer assisted statistical analyses. The study considers, for the first time in the Sudan, soil relationships using multivariate analysis. 'Phyaiographic napping' at a reconnaissance level of survey is employed to delineate geographical facets that are pedologically uniform. Four map units were identified and broadly classified on the basis of the FAO Soil Map of the World Legend. Thirty profiles, randomly selected and distributed proportional to the unit sizes, were sampled at three depth intervals. Several field, physical, chemical and mineralogical properties were analyzed. Multivariate analysis (Principal Component Analysis) was applied separately for the laboratory and field data sets using a layer-by-layer approach to find intercorrelations between the variables. These data were utilized further to discover the magnitude of variability and correspondence between the units delineated and their respective soil profiles. The tests revealed clearly that it is feasible to map the area physiographically as indicated by the congruency between analyzed and remotely sensed data. Classifying soils on the basis of surface horizon properties matched closely with the prevailing morphodynamic phenomena. Field data results revealed more in this respect than the complex laboratory data. Differences between surface soils and subsoils have proved more realistic however, in giving the true magnitude of sand cover in the area, suggesting that desertification studies need a thorough consideration of the depth factor. The dissertation has further emphasized the problem of land use in this part of the Sahcl Zone by comparing the existing soils with standards used by the FAD and Soil Survey Administration, Sudan. Suitability classes with possible arable crops are suggested for both irrigated and rain-fed agriculture. The classes are derived from entirely pedological information and are hence subject to minor alterations when socio-economic information becomes available.

A number of geomorphic studies were conducted on Jebel Arashkil, an inselberg and pediment complex in central Sudan. The various explanations of inselbergs are summarised and some testable hypotheses formulated. The environment of central Sudan is described and its variability emphasised. The morphology of the inselberg is described and the slopes analysed using Rock Mass Strength criteria. The inselberg slopes are not simply controlled by bedrock strength characteristics and clearly have a developmental history. Data are also presented concerning drainage density, infiltration capacities and the nature of surface material for the inselberg and pediment. Field and laboratory techniques are used to assess spatial patterns in the nature and intensity of weathering. The techniques employed include descriptive analysis, x-ray diffraction of clay minerals, abrasion pH, quartz:felspar ratios, SEM analysis of quartz grains and gypsum weathering tablets. Weathering indices are produced which show that the most intense weathering is taking place on the inselberg slopes. This is due to both concentration of water and rate of throughput. The concentration of runoff and solution in regolith pockets on the slopes enhances slope microrelief. Schmidt hammer 'R' values were similar for all exposed bare rock surfaces regardless of location. Erosion measurements using simple repeat survey techniques show that significant erosion occured near the break of slope, mainly through headward growth of and lateral erosion by ephemeral gullies. Vertical incision and planar wash are insignificant in comparison. The form of the inselerg is due to the local balances between erosion and weathering and does not require novel explanations. Inselbergs have long evolutionary histories, and may be subject to equifinality.

No description available.

No description available.

No description available.

On the North Gower saltmarshes soil samples were collected (at a series of depths) from 40 pits, these being arranged in linear transacts at right angles to the shoreline. From the samples, data for the following were obtained: soil pH, calcium carbonate content, total and hygroscopic soil moisture contents, loss on ignition, particle size analysis, clay mineral fractions, water soluble sodium and chloride, and degree of soil ripeness. Percent cover data were also collected for the plant species present along the transacts. Field examination and the analytical results show that distinct vertical and horizontal (i.e.-seaward) gradients exist in the saltmarsh soils. It was also established that 8 of the factors studied were strongly related to geographical location on the saltmarsh. Statistical analysis revealed a high degree of interrelationship between these factors. Using the plant data, Ward's agglomerative clustering technique produced species clusters. These were found to represent plant communities occurring on the saltmarsh. It was possible to describe these communities in terms of the soil factors measured. Multiple discriminant analysis showed that the soil factor most important in terms of vegetation distribution in the North Gower saltmarshes, was pH. The soils and vegetation of adjacent reclaimed and open saltmarsh sites, were also studied. Here, seawall construction had imposed a new set of environmental conditions upon an area of enclosed saltmarsh. As a result, soil development at these adjacent sites had diverged, and the soils studied on either side of the seawall were found to differ greatly. This difference was reflected in the vegetation present. It was concluded that the saltmarsh soils of North Gower represent the interplay of a complex set of factors along a land to sea continuum. This graded interplay is expressed visually in the vegetation zonation observed on the saltmarsh. Soil salinity may determine which plants grow on the saltmarsh, but pH was the factor which accounted for the actual distribution of the vegetation present. Enclosure was shown to constitute a truncation of the soil continuum described.