Constraints on spatial transference of rainfall-runoff relationships in semiarid basins drained by ephemeral streams

Abstract

Field data on drainage basin response have a characteristic scale which is determined by the size of the basin investigated. As a rule, information obtained at one particular scale can be extrapolated over a limited scale range only. This study identifies the nature of constraints on spatial scale transference in a series of semiarid badland drainage basins ranging in area from < 1 to 202 260 m². Research focussed on the rainfall-runoff relationship during a single rainstorm so that the temporal scale was kept constant. Spatial scale transference between systems of differing scale was restricted by morphological and functional constraints. Morphological constraints are caused by morphological elements present in large scale systems but absent in small scale ones. Functional constraints arise solely from the characteristics of the matter and energy flows in the systems of interest. Limits imposed upon spatial scale transferences by morphological and functional constraints are fuzzy rather than sharp in character.     

Structural Petrology of the Ronda Peridotite, SW Spain: Deformation History

Solid bodies of upper-mantle peridotite, emplaced in the Betic-Rifchains of SW Spain and North Morocco, show a variety of structuresdeveloped under different metamorphic conditions. These structuresand related metamorphism reflect tectonic processes in the WestMediterranean mantle during orogeny in the Betic-Rif realm.The largest of the peridotites, the Ronda massif, has preservedthree structural domains which are spatially associated withmetamorphic domains previously defined by Obata (Journal ofPetrology, 21,533–572, 1980). These structural domainsinclude: (1) porphyroclastic spinel peridotites (spinel tectonites)and mylonitic garnet-spinel peridotites (garnet-spinel mylonites),developed during progressive strain localization at ambientconditions changing from the Arigite subfacies to garnet peridotitefacies; (2) seemingly undeformed granular peridotites, developedduring a phase of extensive recrystallization affecting thespinel tectonites and garnet-spinel mylonites at Seiland subfaciesconditions, and separated from the spinel tectonites by a well-preservedrecrystallization front which forms a marked structural, metamorphicand possibly geochemical boundary probably unique to orogenicperidotites; (3) porphyroclastic plagioclase peridotites (plagioclasetectonites) developed at the expense of the granular peridotitesduring progressive shear localization allied to ductile emplacementof the Ronda massif into the crust. Our structural and microstructural data from the Ronda massifallow us to assess the relative ages of the different metamorphicfacies seen in the West Mediterranean peridotites. In orderof decreasing relative age, these are: Arigite-subfacies, garnetperidotite facies, Seiland subfacies and plagioclase peridotitefacies. In addition, the associated microstructures providesome insight into the microphysical conditions controlling thedevelopment of the different structures and, as a result, thestructural and chemical heterogeneity of the West Mediterraneanperidotites. KEY WORDS: structural geology; peridotite; Betic Cordillera; Ronda; recrystallization; strain localization ^*Corresponding author. Present address: Philips Electron Optics BV, Applications Laboratory, Building AAE, PO Box 218, 5600 MD Eindhoven, The Netherlands     

AN EVALUATION OF FRACTAL DIMENSIONS TO QUANTIFY CHANGES IN THE MORPHOLOGY OF FLUVIAL SUSPENDED SEDIMENT PARTICLES DURING BASEFLOW CONDITIONS

The morphology of suspended sediment particles reflects the origin of the suspended load and any modifying processes that may have occurred during transport and storage in the aquatic system. The objective of this study was to evaluate the use of four fractal dimensions to quantify visually observed changes in the morphology of fluvial suspended sediment particles during baseflow conditions. Samples were collected during summer low flow in a small stream on the Canadian Prairies. Particle morphology data were obtained with a transmitted light microscope, a CCD camera and an image analysis system. The morphology of the particle population was characterized using four fractal dimensions (D, D_K, D₁ and D₂). D was derived from the area–perimeter relationship and showed an increase from 1·26±0·02 on 30 June, to 1·34±0·02 on 4 July, to 1·42±0·01 on 7 July. Visually, the increase in D represented the formation of large particles with intricate shapes and increased perimeters. D_K was determined from the area–rank relationship and varied from 1·86±0·01 on 30 June, to 1·90±0·00 on 4 July, to 1·74±0·00 on 7 July. The decrease in D_K between 4 July and 7 July would indicate a greater concentration of the particle area over a small number of large particles. Although the decrease in D_K is consistent with observed changes in the particle size distributions, D_K should be used with the considerable caution because D_K varied more than one standard error between replicates. D₁ and D₂ were determined from the length–perimeter and length–area relationships, respectively. D₁ proved to be of little value for quantifying changes in particle morphology as it showed little change with time, despite considerable visual changes. D₂ however, was useful, despite some variation between replicates. Over the sampling period, D₂ for the composite data sets showed a steady decrease from 1·74±0·02 on 30 June, to 1·68±0·02 on 4 July, to 1·60±0·01 on 7 July. This change in D₂ indicates that, through time, the larger particles became longer and thinner relative to the smaller ones. This study shows that temporal changes in D, D_K and D₂ were consistent with, and allow quantification of, observed changes in particle morphology. D₁ did not reflect observed morphological changes, and is therefore of little value for this type of study. The changes in particle morphology coincided with an increase in primary production in the form of algae. © 1997 John Wiley & Sons, Ltd.