Richard J. Chorley

THROUGHFLOW, OVERLAND FLOW AND EROSION

Abstract The Horton (1945) infiltration model of surface runoff and erosion is shown to be of much more limited geomorphic application than has been recognised hitherto. It is most applicable to clay badlands with low infiltration capacities and little weathered cover, and is one end-member of a wide spectrum of erosion models. The other end-member applies to slopes with high infiltration capacities and thick soil covers where throughflow dominates, and overland flow, with its attendant channel initiation, only occurs in a few restricted areas. Throughflow is capable of producing peaks in some river hydrographs, as is demonstrated both by controlled field experiments (Whipkey, 1965) and, by inference, by the “partial area” concept of runoff (Betson, 1964; Tennessee Valley Authority, 1963, 1964 and 1966). Experimental results of Hewlett (1961; Hewlett and Hibbert, 1963) are shown to be consistent with infiltration theory (Philip, 1957–8), indicating different response of throughflow to rainfall in the unsa...

Trend-Surface Mapping in Geographical Research

ALTHOUGH GEOGRAPHERS are traditionally concerned with the description, analysis and explanation of areal distributions of phenomena, much of the most vigorous development of new techniques in this field has come from outside geography. The harnessing of the vast potential of computer systems to mapping problems has been pressed forward both in subjects like meteorology where mechanical graph-plotters, line printers, and cathode-ray tube displays are being used to map directly the output of digital computers (Sawyer, 1960; Wippermann, 1959), and in botany where field information on the occurrence of vascular plants are being rapidly processed and printed out (for example, the 1,623 maps of the Atlas of the British Flora (Perring and Walters, 1962)). The most striking adaptations of computer technology to traditional mapping problems are being employed in the earth sciences, however: for example, in geophysical prospecting, structural mapping and sedimentary petrology (Krumbein, 1958). Here the most fundamental developments in mapping techniques lie less in the automation of traditional techniques (a field reviewed for geographers by Tobler (1959)) but in the evolution of new ways of extracting more information from map data. This paper reviews developments in one of the most promising of these techniques, trend-surface mapping, and attempts to assess its significance for, and applications in, the wider field of geographical research.

Comparison of Morphometric Features, Unaka Mountains, Tennessee and North Carolina, and Dartmoor, England

The established morphometric relationships and measures permit the theoretical construction of an idealized drainage basin embodying the average linear, areal, and relief characteristics of a homogeneous geomorphic region. Such basins have been constructed from map data for two contrasting regions in which crystalline and metamorphic rocks maintain a high relief: the Unaka Mountains just east of the Great Smokies in Tennessee and North Carolina, and the Dartmoor region of England. The visual representation of these typical basins by means of block diagrams drawn to scale provides a useful tool for regional geomorphic description and comparison and helps to bridge the gulf between the quantitative and qualitative approaches to geomorphology. The striking contrast in drainage densities between the two regions, on which most of the remaining morphometric differences hinge, is explained mainly on the basis of the consistently differing rainfall intensities which probably have been experienced by the Unaka Mountains and Dartmoor at least since the Miocene.

CLIMATE AND MORPHOMETRY

An analysis of the morphometry of three areas of similar gross lithology, structural effect, and stage of dissection shows that there is a significant difference between each of the equivalent landscape unit forms. A climate/vegetation index (7C) was obtained for each region, employing the mean annual rainfall, the mean monthly maximum precipitation in 24 hours, and Thornthwaites precipitation effectiveness index. It is found that the climate/vegetation index bears a remarkably consistent relationship to the mean logarithms of stream lengths, basin areas, and drainage densities.

MR properties of water in saturated soils and resulting loss of MRI signal in water content detection at 2 tesla

This paper reports a systematic MRI study at 2 tesla of 23 soils, each separately saturated with a known amount of water. The percentage of that water which could be detected using various MR methods was determined by comparison with a liquid reference sample. A pulse-acquire sequence gave a bulk detection of between 47 and 94% of the known water content of saturated soil. Also for bulk measurements, the inversion-recovery sequence used for determining T1 values detected a range of 0.7–75% of the existing soil water. The CPMG sequence with an echo time (TE) of 1 ms used for determining the bulk T2 values gave lower values, in the range of 0.4–66% overall. A spin-echo MRI sequence with a TE of 2.9 ms gave an even lower bulk detection, ranging from 0.2 to 57%. These low values for the detectable water content of bulk saturated soil water reflect the loss of water magnetization which occurs even during short echo time MR sequence at 2 tesla field strength. The source of the above findings was investigated by measurements of the longitudinal (T1) and transverse (T2) relaxation times and spectral linewidths of the soil-water protons, and by conventional analysis of soil properties. The MR parameters of critical importance to water quantification are T2 and T2∗, shorter values of which lead to a progressively greater loss of signal intensity for all MR protocols. Those parameters are affected by the following soil chemical and physical features: soil magnetic susceptibility, and the content of free iron oxides, clay, sand, exchangeable cations (K, Na and Ca), and organic matter. The implication of this work is that the only soil water which can be detected quantitatively at 2 tesla using a conventional spin-echo MRI protocol with echo times of 3 ms or longer is that located in the relatively large soil pores. Using the protocols investigated in this work, water in smaller pores will only be detected accurately for soils which have relatively low paramagnetic-metal impurities and/or have low clay content. Future MR studies of soil water should consider the use of other MRI protocols (e.g. for solid state), and measurement at low magnetic fields.

Study of flow and hydrodynamic dispersion in a porous medium using pulsed–field–gradient magnetic resonance

This paper reports on the use of pulsed–field gradient (PFG) magnetic resonance (MR) techniques to obtain displacement and velocity spectra of steady–state, saturated flow through a column packed with glass beads. The displacement spectra obtained by PFG MR correspond to travel–distance probability–density functions (PDF) for initial conditions of a concentration impulse in a column with zero concentration. These spectra show strong dispersion–time dependence, and depart from Gaussian–shaped PDFs for short dispersion times. These data provide estimates of the dispersion–time dependence of transverse and longitudinal dispersion coefficients. The longitudinal dispersion coefficient reaches its long–time behaviour more slowly than the transverse coefficient; long–time values obtained from MR data agree well with those calculated using existing empirical correlations. A model based on three components of apparent velocities and dispersion coefficients is sufficient to describe the time dependence of displacement spectra for water flow through the bead column. The short–distance component arise because of convection–dispersion–diffusion processes within the narrow necks between particles. The long–distance component, on the other hand, represents a macroscopic convection–dispersion process. This study shows that PFG MR flow spectroscopy is a simple but potentially useful method for the investigation of flow and hydrodynamic dispersion in porous media, especially for time–dependent phenomena.

STUDY OF INFILTRATION INTO A HETEROGENEOUS SOIL USING MAGNETIC RESONANCE IMAGING

In this paper, the feasibility of using magnetic resonance imaging (MRI) to study water infiltration into a heterogeneous soil is examined, together with its difficulties and limitations. MRI studies of ponded water infiltration into an undisturbed soil core show that the combination of one- and two-dimensional imaging techniques provides a visual and non-destructive means of monitoring the temporal changes of soil water content and the moisture profile, and the movement of the wetting front. Two-dimensional images show air entrapment in repetitive ponded infiltration experiments. During the early stages of infiltration, one-dimensional images of soil moisture profiles clearly indicate preferential flow phenomena. The observed advance of wetting fronts can be described by a linear relationship between the square root of infiltration time (√t) and the distance of the wetting front from the soil surface. Similarly, the cumulative infiltration is also directly proportional to √t. Furthermore, from the MRI infiltration moisture profiles, it is possible to estimate the parameters that feature in infiltration equations.

Studies of soil-water transport by MRI.

Sequential spin-echo spin-warp MRI pulse sequences have been used to study soil-water transport processes including infiltration, redistribution, and drainage of water in soil columns. Those images provide a means for monitoring and quantifying spatial and temporal changes of soil-water distributions and the movement of wetting fronts. In addition, temporal-geometric changes of unstable wetting fronts during water redistribution were estimated from 2D images and the temporal development of the longest length of finger was described by a fractal relation t approximately L1.38. Bulk dispersion-time-dependent displacement and velocity spectra, as well as 2D maps of flow velocities and dispersion coefficients in soil macropores during saturated steady-state flow, were reconstructed from data obtained using the alternating-pulsed-field-gradient (APFG) pulse sequences.

Magnetic resonance imaging of soil-water phenomena

2D and 3D images of static and dynamic water phenomena in packed clay soil columns were obtained by MRI. A spin-echo spin-warp Fourier imaging sequence, together with prefocused 90 degrees selective pulse for 2D imaging, was selected. The images demonstrate the potential of using MRI for studies of static and dynamic water phenomena in soil with modest iron content and adequate water content.

Cartographic problems in stream channel delineation

The acquisition of data for stream network analysis is largely dependent on the availability of topographic maps and the accuracy with which data is portrayed thereon. When Yoxall (1969) examined the 1:250,000 series of maps of Ghana, he found inconsistencies in the selection of rivers for mapping. Scheidegger (1966, p. 57) reported that the blue stream lines printed on maps are extremely arbitrary and appeared to depend on the channel flow observed to be present by the surveyor or by means of air photographs. When Morisawa (1957) mapped the channel system of a part of the Appalachian Plateaus in spring she found much more running water in channels than was depicted on the U.S. Geological Survey 1:24,000 topographical maps, even when they were produced from air photographs, it being particularly apparent that many flowing fingertip tributaries had been obscured from the air by overhanging trees. The policy of the U.S. Geological Surveys has been to classify all streams as either perennial (i.e. ones that ...