Soil erosion by overland flow, resulting from infiltration‐excess rainfall, generates rill networks on hillslope areas. The way in which these networks emerge and develop suggests that hillslope erosion functions as a self‐organizing dynamic system. Based upon this argument, a model for soil erosion (RillGrow 1) has been developed: this operates at the spatial scale of raindrops and microtopography. In this paper the second generation of the model (RillGrow 2) is described and applied to four different soil surfaces. Results suggest that, even at this early stage in its development, RillGrow 2 is capable of replicating the success of the earlier model and in some cases of extending them. The success of both models suggests that this self‐organizing view of rill generation may capture some fundamental aspects of the operation of real erosional systems. Copyright © 2000 John Wiley & Sons, Ltd.

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Digital photogrammetry provides a tool with which to automatically generate digital elevation models (DEMs). The necessary equipment is now both readily available and affordable: thus there is considerable potential for this technique to be widely adopted in geomorphological studies. But is it possible for geomorphologists without a background in photogrammety to use it successfully? As part of a larger study into rill initiation by overland flow, a non-metric digital camera and ERDAS IMAGINE OrthoMAX software were used to generate small-scale DEMs of soil surface microtopography. This paper reports on the procedure used, highlights potential pitfalls, and comments on the quality of the resultant DEMs. Whilst acquisition of high-quality images using a digital camera is relatively straightforward, problems were subsequently encountered due to the small size of the internal imager and the need for camera calibration. Potential stumbling blocks in the use of the software lay in the setting-up of ground control points and the use of tie-points and check-points, as well as several software glitches not identified in the current manual. Nonetheless, once these problems were overcome the technique proved to be a simple, effective and fast tool for generating high quality microtopographical DEMs. This methodology shows great promise for future geomorphological studies that require these kinds of surface data.

Rainfall simulators are widely used yet there is little evidence in the literature to show that their spatial and temporal variability has been adequately taken into account. For experiments that are concerned only with some aggregate or mean effect of simulated rain then such variations may be unimportant. However, where rainfall simulation is being used to study (and perhaps model) small-scale processes that are themselves spatially variable (such as rill initiation) then knowledge of the simulator's inherent variability is vital. A first aim of this paper is therefore to examine this variability, and to appraise methodologies by which it may be quantified. A second aim is to evaluate the implications for spatially explicit rainfall simulation experiments.Two simulators were used, a portable drip-screen simulator and a laboratory-based full-cone nozzle simulator. Neither produced a spatially uniform distribution of rainfall depth: both produced distributional patterns that were fairly consistent despite varying intensities and run times. Small-scale, apparently random variations were superimposed on these more deterministic patterns. However, despite this marked spatial variability, calculation of uniformity coefficients (1ÀSD/mean) resulted in high values. Thus it appears that the uniformity coefficient gives little real indication of the spatial uniformity of simulated rainfall, despite its established usage in the literature. Additionally, spatial distributions of raindrop size ±and hence kinetic energy ±were calculated for the full-cone nozzle simulator. These show that zones of high rainfall amount do not necessarily relate to zones of high energy reaching the surface.The presence of such variability raises a number of issues for spatially explicit rainfall simulation experiments. While there has been little work on the spatial variability of natural rainfall at field scale and smaller, it appears that the spatial heterogeneity of simulated rainfall depths observed in this study does not differ greatly from that of natural rain. But since a major attraction of rainfall simulation experiments is additional control over rainfall's many variables, the spatial non-uniformity of depth observed in this study is unwelcome. The existence of an apparently deterministic component to this non-uniformity nonetheless suggests that it can, at least in principle, be corrected by calibration. Less easily handled is the discrepancy between spatial distributions of rainfall depth and energy, since this will certainly affect rainfall simulation experiments that are, for example, concerned with erosion processes due to raindrop impact.

The first five papers in this volume were presented at a workshop held at the University of Leicester from 18 to 21 April 1999. The workshop was both the final meeting of the BGRG Rainfall Simulation Working Group and the first scientific meeting of COST Action 623 Soil Erosion and Global Change. The workshop was attended by 72 delegates from 15 countries.Rainfall simulation is widely used in geomorphological research. There are a range of simulator types and designs, and yet there has not, until recently, been any coordination of efforts to collate all the available information regarding the design and purpose of such simulators, or to discuss future developments relating to the use of this technique. The BGRG Rainfall Simulation Working Group was set up in 1995 to address these issues. The Working Group had the aims of producing a) a catalogue of simulators in use and their specifications; b) details of the performance data of these simulators; and c) a catalogue of research undertaken using simulators of different types. The group also wished to design, conduct and report on standard evaluation tests for simulators and produce a standard form for the submission of simulator details for collation. Seven meetings were held to discuss specific issues, report on progress, and plan the way forward. These discussions highlighted areas that had not as yet been fully addressed by the scientific community. Questions concerning the appropriateness of rainfall intensity and dropsize distribution for particular experimental designs, the relationships of data obtained from simulation experiments to needs of erosion modelling, significance of plot size, spatial variability and validity of upscaling of results were raised. Overall, the need for a catalogue of simulators, their specifications and the research they have been used for has partly been met by the creation of a website, http://www.geog.le.ac.uk/bgrg/index.html, which currently includes details of 20 simulators. Some of the issues concerning spatial uniformity in simulation experiments, the relationships between rainfall simulation experiments and erosion modelling, and of scaling and data on raindrop spectra of natural rainfall are still outstanding. However, the papers presented here go some way towards addressing them.