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Soil surface roughness greatly affects surface sealing and runoff generation, yet little information is available about the effect of roughness on the spatial distribution of runoff and on flow concentration. This study tested the hypothesis that runoff distribution and flow concentration differ with roughness and affect the amount of soil loss. Sequences of four rainstorms of constant rainfall amount but decreasing intensity (60, 45, 30, and 15 mm h−1) were applied to the Ap horizon material of a loess soil (Glossic Fragiudalf) packed into a 0.6 by 3.7 m flume. Rough, medium, and smooth soil surfaces were studied at 17, 8, and 2% slope steepness. Surface roughness and flow pathways were visualized with digital elevation maps obtained from laser microrelief measurements. On the smooth surfaces, runoff was uniformly distributed during the first two rainstorms and soil losses were 0.23, 0.07, and 0.12 kg m−2 for the first and 2.26, 0.35, and 0.2 kg m−2 for the second storm at 17, 8, and 2% slope steepness, respectively. On the rough and medium surfaces, flow concentrated in pathways between clods and soil losses were up to eight and three times that on the smooth surfaces during the first and second storms, respectively. During the last two storms, flow concentrated also on the smooth surfaces and soil losses were similar for the three initially different surface configurations. Surface roughness effects on runoff amount were minor, but roughness affected the spatial distribution of runoff, thereby affecting the amount of soil loss.

Abstract. Experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas. In a laboratory channel, packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow. The rainfall produced a well-developed surface seal that minimized surface soil detachment. During overland flow, soil erosion occurred exclusively at the headcut, and after a short period of time, a steady state condition was reached where the headcut migrated at a constant rate, the scour hole morphology remained unchanged, and sediment yield remained constant. A fourfold increase in flow discharge resulted in larger scour holes, yet aspect ratio was conserved. A sediment bed was deposited downstream of the migrating headcut, and its slope depended weakly on flow discharge.

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The Heaslet‐Alksne technique solved the nonlinear diffusion equation by expansion around the wetting front for power law diffusivities. Essentially the same technique has been applied when a well‐defined wetting front exists at a finite distance. In this paper, the method is extended for an arbitrary diffusivity and to the case when there is no well‐defined wetting front at a finite distance. Two illustrations for exponential and power law diffusivities show the excellent accuracy of the method.

An analytical solution to the kinematic wave approximation for unsteady flow routing is presented. The model allows time‐dependent lateral inflow with piecewise spatial uniformity and can be applied to complex kinematic cascades. Kinematic shocks are considered as manifestations of higher‐order effects such as rnonoclinal flood waves, bores, etc. Within the context of kinematic approximation therefore we retain their dynamic effects by routing the discontinuities as they appear. Certain simplifying assumptions are made which permit closed form solutions and an efficient numerical algorithm, based on the method of characteristics, is employed. The resulting model, called an approximate shock‐fitting scheme, preserves the effect of the shocks without the usual computational complications and compares favorably with an implicit finite difference solution. The efficiency and accuracy of the new method are illustrated by computing a variety of unsteady flows, ranging from simple cascades to complex natural watersheds.

rangement and distribution of the cracks. Moreover, White (1972) explained the energy implications that The theory of water movement in high shrink/swell soils has expericause the cracks to reappear in the same position from enced consistent revision since Haines first presented the topic in 1923. Several aspects of the infiltration process in cracking soils have event to event. Johnston and Hill (1944) observed that proven to be difficult to measure; seal/crust formation and properties, cracks develop between crop rows where the water concrack network patterns, preferential flow zones and contributions, tent was higher (due to less uptake by plants) and, thus, and soil moisture determinations within the profile (near crack and the strength of the soil was lower. Since dry clay soils near center of prismatic column) to name a few. Here, we used simutend to moisten more rapidly to a greater depth along lated rainstorms, laser measurements of surface elevation, needlea crack than at a distance from a crack, cracks transmit penetrometer measurements, and mass measurements of infiltrating more water deeper under heavy precipitation events, water over a 206-and 145-d period to examine water movement and making the soil wetter in this area. cracking patterns in a large sample box filled with a swelling clay soil.

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