摘要:

AbstractAgricultural production from steep terrain is frequently limited to pasture systems, where forage harvesting is done by grazing animals. Intercepted solar radiation determines the energy available for forage production. A model was developed for estimating potential direct beam radiation in areas with a wide range of aspects, landscape positions, and various shadow sources. The model was applied to a small, steep pasture watershed in central Appalachia to determine the spatial distribution of potential direct beam radiation. Effective horizon was an important parameter affecting the energy budget of the study watershed. A simple means for estimating global radiation from the modelled potential direct beam radiation, given atmospheric transmissivity and cloudiness, was presented. When the proportion of global radiation that was diffuse radiation was increased the effective horizon was less important and the spatial variability of modelled radiation decreased.

ISSN:0197-9337    

DOI:10.1002/esp.3290160102

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractDifferences in lithologic erodibility and sediment storage within a drainage basin affect the relationship between alluvial fan area and drainage basin area along the western White Mountains. Large fans are produced by basins underlain by resistant rocks, which have steep and narrow trunk stream canyons with little sediment in storage. Small fans are produced by basins composed of erodible lithologic units, which have wider valley floors, lower valley‐side slopes, and considerably more sediment stored along trunk stream canyons than is the case in basins underlain by resistant rock

ISSN:0197-9337    

DOI:10.1002/esp.3290160103

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe ‘velocity‐reversal hypothesis’ is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool‐riffle sequences in gravel‐bedded streams. The literature in support of the hypothesis together with published adverse criticism is reviewed. It is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable. Continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow, high stage, and stable morphology. These observations are substantiated by a detailed study of the hydraulic geometry of stable pool‐riffle sequences in the River Severn, England. Neither the sectionally‐averaged velocity nor the near‐bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow. Instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases. A detailed investigation of the three‐dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles. Unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles. The hydraulic data are used to comment on the stability of the test reaches in the context of the development of the River Severn in the vicin

ISSN:0197-9337    

DOI:10.1002/esp.3290160104

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractApplications of landform recognition in map‐making, geographical information systems, and in improving the storage and representation of earth surface information are described. Reasons are given for the use of digital elevation models for landform recognition: increasing availability of data, easier data integration, and automation of landform recognition.An automated technique for recognizing valley heads from digital elevation models is presented. The technique has been developed for valley head recognition because valley heads are known locations of geomorphological activity and because areal features have been largely ignored in the automated landform recognition literature. The problem of valley head recognition is divided into valley head location and valley head delineation, and the technique is developed using a DEM representing a simple artificial landscape.Two methods are given for valley head location. In the first, valley heads are located at valley ends. In the second, valley heads are located at cells which would have water flowing into them from many directions if water was flowing over the terrain. The latter method locates all the valley heads on the artificial landscape. The cells locating valley heads are iteratively ‘grown’ to adjacent cells that are concave in plan and not flat, to delineate the valley head. At best, 82 per cent valley head cells on the artificial landscape are correctly recognized with a 43 per cent commission error.The method is tested on real data where it is less successful because of the variability of real landscapes and the inadequacy of the methods chosen for valley network delineation. Machine learning of thresholds and a method involving inspection of profiles across a DEM cell are suggested as methods to improve the automated valley head recognition tech

ISSN:0197-9337    

DOI:10.1002/esp.3290160105

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractMany tafoni occur on the faces of marine cliffs on three uplifted shore platforms with different altitudes and with known ages of emergence. The mean value of the ten largest depths of tafoni,D(cm), was obtained and the period of their formation,t(years), was estimated. The relationship between the two variables was found to beD= 20·3 × (1−e−0.

ISSN:0197-9337    

DOI:10.1002/esp.3290160106

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe Schmidt hammer has for some years been used to measure the hardness of different rock surfaces and to date relatively boulders in moraine deposits. In this paper applications on isostatically raised boulder‐beaches and rocky shores are described. The research area is the northern Swedish Bothnian coast where present isostatic uplift is between 0.8‐1.0 cm year−1. Elevations above sea‐level can be converted to a timescale. Rebound values (R) of the Schmidt hammer are correlated with the altitude and the degree of rock weathering can be estimated for each site.Eight sites with boulder‐beaches and nine sites with rocky shores have been analysed. For boulder‐beaches correlation‐coefficients of 0.82‐0.97 for weathering values (R) and the elevations above sea‐level were obtained. For the rocky shores these values were calculated to 0.75‐0.97.This method has applications in both geomorphology and archaeology. For example, a deep layer of weathered dolerite in the Nordingrå region, which previously has been regarded as a post‐Weichselian formation, can be shown to be of pre‐Weichselian origin. Several archaeological applications are als

ISSN:0197-9337    

DOI:10.1002/esp.3290160107

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractChanges in channel character along a small river in the Scottish Highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0.02 to 0.00015 because of local baselevel control. This decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed. The channel pattern alters progressively rather than abruptly, in the sequence (1) near‐braided, (2) meandering with active point‐bar chutes, (3) meandering with active outer‐bank talweg, (4) stable equiwidth sinuous. The changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical concepts than by purely empirical discriminant or trend equations. Measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload. It is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions, are typical of many streams in mountainous areas and can be understood in terms of slope‐induced changes in hydraulic pro

ISSN:0197-9337    

DOI:10.1002/esp.3290160108

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractShort‐term (1‐30 months) weathering rates were obtained for steep cliffs of Cretaceous Ardingly Sandstone in southern England, using a new photographic method. Mean rates for an exposed and a sheltered outcrop were 0.11 and 0.033 mm a−1respectively. Significant (p<0.1) positive correlations were obtained with illumination and algal biomass and a significant negative correlation (p<0.05) with slope angle. There was no correlation with grain size, porosity, or mean surface temperature. Highest weathering rates were observed on south‐ and west‐facing exposed cliffs. The results are discussed with reference to algal growth, climate, and wind

ISSN:0197-9337    

DOI:10.1002/esp.3290160109

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290160110

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

ISSN:0197-9337    

DOI:10.1002/esp.3290160101

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY