Hortonian Overland Flow Research Articles

Assessing the impact of the hydraulic properties of a crusted soil on overland flow modelling at the field scale

Soil surface crusts are widely reported to favour Hortonian runoff, but are not explicitly represented in most rainfall-runoff models. The aim of this paper is to assess the impact of soil surface crusts on infiltration and runoff modelling at two spatial scales, i.e. the local scale and the plot scale. At the local scale, two separate single ring infiltration experiments are undertaken. The first is performed on the undisturbed soil, whereas the second is done after removal of the soil surface crust. The HYDRUS 2D two-dimensional vertical infiltration model is then used in an inverse modelling approach, first to estimate the soil hydraulic properties of the crust and the subsoil, and then the effective hydraulic properties of the soil represented as a single uniform layer. The results show that the crust hydraulic conductivity is 10 times lower than that of the subsoil, thus illustrating the limiting role the crust has on infiltration. Moving up to the plot scale, a rainfall-runoff model coupling the Richards equation to a transfer function is used to simulate Hortonian overland flow hydrographs. The previously calculated hydraulic properties are used, and a comparison is undertaken between a single-layer and a double-layer representation of the crusted soil. The results of the rainfall-runoff model show that the soil hydraulic properties calculated at the local scale give acceptable results when used to model runoff at the plot scale directly, without any numerical calibration. Also, at the plot scale, no clear improvement of the results can be seen when using a double-layer representation of the soil in comparison with a single homogeneous layer. This is due to the hydrological characteristics of Hortonian runoff, which is triggered by a rainfall intensity exceeding the saturated hydraulic conductivity of the soil surface. Consequently, the rainfall-runoff model is more sensitive to rainfall than to the subsoil's hydrodynamic properties. Therefore, the use of a double-layer soil model to represent runoff on a crusted soil does not seem necessary, as the increase of precision in the soil discretization is not justified by a better performance of the model. Copyright © 2005 John Wiley & Sons, Ltd.

Influence of storm damage on the runoff generation in two sub-catchments of the Sperbelgraben, Swiss Emmental

The project “Lothar and Mountain Torrents” investigates the effect of storm-originated deforestation on the hydrology on three scales within the Sperbelgraben catchment (Swiss Prealps). This article focuses on runoff measurements during a 3-year period in two differently affected sub-catchments (≈2 ha) and on 2-year surface runoff measurements on smaller plots (50–110 m2). The link between these two scales and the results of irrigation experiments on 1 m2 areas are interpreted using a detailed map of forest site types describing soil and vegetation characteristics. Plot results show that surface runoff is generated in two distinct ways. On the one hand, high amounts of saturation overland flow were observed on wet areas of gleyic soils. On the other hand, hardly any surface runoff was measured on Cambisols, with the exception of a short hydrophobic reaction at the beginning of storms occurring on areas with a thick organic litter layer (temporary Hortonian overland flow). On the long term, the lightly damaged sub-catchment (SC1) yields less runoff than the highly damaged one (SC2). This is confirmed when direct runoff volumes during flood events are considered. However, short and intensive showers surprisingly lead to higher discharge peaks in SC1. This occurrence is explained by different geomorphologic characteristics (mainly the channel density) and the spatial distribution of the moist to wet forest site types. Effects of deforestation and local soil compaction due to forest clearing remain small on both plot and sub-catchment scale.

Soil hydraulic properties in a marly gully catchment (Draix, France)

Runoff is one of the main factors controlling gully development. Concentrated flow erosion occurs where flow discharge and velocity exceed critical values. In Southern French Alps, in black marl gully catchments, runoff from hillslopes occurs in the form of Hortonian overland flow because rainfall intensities exceed the infiltration capacity of the soil. In such a situation, spatial variability of infiltration capacities is critical for describing the runoff production. This study reports detailed field measurements of near-saturation and saturated infiltration properties: capillary sorptivity ( S) and hydraulic conductivity ( K). A total of 140 infiltration tests were conducted using tension disc and ponded ring infiltrometers on 6 different regoliths (weathering profile) encountered in a marly gully catchment. The main objectives were to improve our knowledge of the spatial variability of infiltration in black marl areas and to compare different field and data analysis methods. Hydraulic conductivity values K( h) at supply pressure heads ( h) ranging from h = − 100 up to h = 0 mm were calculated using steady state flow and Wooding's equation [Wooding, R.A., 1968. Steady infiltration from shallow circular pond. Water Resour. Res. 4 (6), 1259–1273]. K( h = 0 mm) was also estimated from the ponded ring infiltrometer data set. The estimation of sorptivity was based on transient flow and the solution of Haverkamp et al. [Haverkamp, R., Roos, P.J., Smettem, K.R.J., Parlange, J.Y., 1994. Three dimensional analysis of infiltration from the disc infiltrometer. 2. Physical based infiltration equation. Water Resour. Res. 30 (11), 2931–2935]. Three methods were used to analyse tension disc infiltrometer data: multiple radii, multiple potential and single test. Only the multiple radii method was found not suitable in our context due to a large spatial variability in the infiltration properties over small distances. The other methods gave similar estimates of the hydraulic conductivity. The hydraulic conductivity K( h) ranged up to two orders of magnitude. This result confirms the role played by structural voids when the regolith is at near saturation. The dispersion of the estimates of field saturated hydraulic conductivity is larger than unsaturated values. The spatial differences found are explained by the topographical position and by the regolith structure observed for the 6 sites. An analysis based on the ponding time show that the regolith type plays an important role in the dynamic of the runoff production.

Hydrological Characteristics at Hilly Catchment in a Semi-Arid Region, Chile

The Alto Loica experimental site was established to achieve some understanding of the hydrological characteristics and the effects of afforestation on soil and water conservation in semi-arid regions in Chile. Hydrological measurements were taken in the 16.25-ha catchment for seven years. Because of the high variability of annual rainfall, there was a drought year in which streamflow disappeared throughout the year. As the catchment contains intermittent streams, mean annual baseflow is only 5.9% of mean annual rainfall, while mean annual evapotranspiration is 62.6% of that. The seasonal fluctuations of soil moisture storage suggest that moisture conditions during the period of December through March may be the driest ones for the growth of vegetation. While Horton overland flow occurs over grassy and shrub covered hillside during high rainfall intensities, it contributes largely to sharp hydrograph peaks. During large rainstorms, the mean runoff coefficient was more than 50%. Hence, measures to reduce overland flow are required for afforestation as well as soil and water conservation.

Comparison of infiltration models to simulate flood events at the field scale

Most runoff simulation and infiltration models have been developed at the global-catchment scale or the local-soil column scale. Few models have been specifically developed at the scale of agricultural fields and there are no guidelines to help modellers choose an adequate model to simulate overland flow and hence analyse the impact of different soil management practices on flood generation. A comparison is undertaken to select and calibrate models that simulate Hortonian overland flow at the field or small plot scale. The proposed methodology couples a runoff production model to a unit hydrograph transfer function. Four different models were tested: Philip, Morel-Seytoux, Horton and SCS. These models differ by their mathematical structure and the parameters to be calibrated while input hydrologic data are the same site data: rainfall/runoff and initial water content. The models are calibrated on 14 events and validated on 14 others. The results of both the calibration and validation phases are compared on the basis of their performance with regards to six objective criteria, three global criteria and three relative criteria representing volume, peakflow, and the root mean square error. The first type of criteria gives more weight to strong events whereas the second considers all events to be of equal weight. The results show that the calibrated parameter values are dependent on the type of objective criteria used. Furthermore, when analysing the performance of the six objective criteria used, it can be seen that the global volume, global RMSE and relative peakflow criteria give the best compromise between bias and precision. Within the selected modelling framework, Morel-Seytoux's model performed better than the other three and the SCS gave the worst results. Horton's model showed to be more consistent in overall performance than Philip's model. Results also highlight problems related to the simulation of low flow events and intermittent rainfall events.

Physically-based hydrological modelling for non-point dissolved phosphorus transport in small and medium-sized river basins / Modélisation hydrologique à bases physiques et du transport de phosphore dissous diffus en bassins versants de petite et moyenne tailles

Current research suggests that strategies to control sediment and phosphorus loss from non-point sources should focus on different runoff components and their spatial and temporal variations within the river basin. This is a prerequisite for determining effective management measures for reducing diffuse source pollution. Therefore, non-point source models, especially in humid climatic regions, should consider variable hydrologically active source areas. These models should be able to consider runoff generation by saturated overland flow, as well as Hortonian overland flow. A combination of the hydrological model WaSiM-ETH and the erosion and P-transport model AGNPS was chosen for this study. The models were run in the WaSiM runoff generation mode (Green & Ampt/TOPMODEL or Richards equation approach) and the SCS curve number mode to assess the effect of these different runoff calculation procedures on the dissolved phosphorus yield. A small and a medium-sized river basin, of the area of 1.44 and 128.9 km2, respectively, in central Germany were selected for the investigation. The results show that the WaSiM–AGNPS coupling produces more accurate results than the SCS curve number method. For the spatial distribution, the more physically-based model approach computed a much more realistic distribution of water and phosphorus yield-producing areas.

Toward understanding the cumulative impacts of roads in upland agricultural watersheds of northern Thailand

This paper describes the interactions of various physical processes that allow unpaved roads to contribute disproportionately to basin-wide runoff and stream sediment in the 93.7 ha Pang Khum Experimental Watershed (PKEW) in northern Thailand. Many road sections in PKEW are constant sources of sediment entering the stream during most rain events because: (1) Horton overland flow is generated on the compacted surfaces after small depths of rainfall; (2) surface preparation processes, including vehicle detachment and maintenance activities, renew the supply of easily transportable surface sediment on inter- and intra-storm time scales; (3) erosion of the road surface is accelerated in locations where slopes are steep, overland flow distances are long, and/or vehicle usage is high; (4) surface runoff typically exits from the road directly into the stream. Owing to these collective processes, sediment delivery rate on PKEW roads is more than an order of magnitude higher than that on adjacent fields (≈120 Mg ha −1 per year versus 9 Mg ha −1 per year). Thus, unpaved roads appear to be on the same order of importance as agricultural lands in contributing sediment to the stream network, despite occupying a fraction of the total surface area in the basin (≈0.5% versus 12%). A more thorough assessment of linkages between all hillslope runoff/sediment sources and the stream network, however, is still needed to fully evaluate the relative impacts of roads versus those of agriculture practices in PKEW. © 2004 Elsevier B.V. All rights reserved.

Scale dependent thresholds in hydrological and erosion response of a semi-arid catchment in southeast Spain

Land degradation and soil erosion are perceived as important problems in the dryland zones of the Mediterranean. Three-year measurements of hydrological and soil erosion data from a series of nested experimental watersheds in a semi-arid area of SE Spain are discussed. The aim was to study the role and effects of thresholds on the spatial connections between different system compartments, such as response units and sub-catchments that act at different levels of scale (plot to watershed scale). It was also the aim to quantify runoff and erosion at these different scales. Several types of thresholds are described and these are related to vegetation type and pattern, soil surface roughness, distance to the main channel, land use and tillage effects (intrinsic properties of the landscape) as well as rainfall intensity, duration and depth (external influence). The expansion of runoff generating areas under Hortonian overland flow is discussed in relation to vegetation structure and rainfall. Results showed that runoff and sediment yield results highly depend on the vegetation structure. The relation between rainfall intensity and rainfall depth and the hydrological response were established at five levels of scale. Three spatio-temporal process domains were analysed: the spot- and plot-processes at the finest scale, the hillslope, micro- and sub-catchment processes at the intermediate scale and catchment scale and main channel network processes at the broadest scale. An event with a 5-year recurrence period is discussed to illustrate the importance of scale related thresholds, explaining the relative importance of high intensity rainfalls. Soil erosion was found to be a magnitude larger on terraced valley bottoms (2500–3000 g m−2) when compared to the semi-natural hillslopes, where erosion figures were less than 10 g m−2. This indicated that the contribution of sediment from terraced cultivated lands is important and are an underestimated part of the sediment budget of semi-arid catchments.

Hydrological consequences of landscape fragmentation in mountainous northern Vietnam: evidence of accelerated overland flow generation

Measurements of saturated hydraulic conductivity ( K s) and indices of Horton overland flow (HOF) generation are used to assess the influence of landscape fragmentation on near-surface hydrologic response in two upland watersheds in northern Vietnam. The fragmented landscape, which results from timber extraction and swidden agriculture, is a mosaic of surfaces having distinct infiltration characteristics. In general, human activity has reduced infiltration and altered near-surface flow paths on all disturbed land covers. Compacted roads, paths, and dwelling sites, for example, have the propensity to generate HOF for small rainfall depths. Although these surfaces occupy a small fraction of a basin land area (estimated at <1%), they contribute disproportionately to overland flow response during typical rainfall events. Recently abandoned fields have the lowest K s of all non-consolidated, post-cultivation surfaces tested. Beginning 1–2 years following abandonment, diminished K s recovers over time with the succession to more advanced types of secondary regrowth. If a grassland emerges on the abandoned site, rather than a bamboo-dominated cover, K s recovers more rapidly. The decrease in K s with depth below disturbed surfaces is more acute than that found at undisturbed sites. This enhanced anisotropy in near-surface K s increases the likelihood of the development of a lateral subsurface flow component during large storms of the monsoon rain season. Subsequently, the likelihood of return flow generation is increased. Because the recovery time of subsurface K s is greater than that for the surface K s, the impact human activity has on hydrologic response in the fragmented basin may linger long after the surface vegetation has evolved to a mature forested association.

Tests of two physically based filters for base flow separation

A physically based filter for separating base flow from streamflow time series is tested using a simple dynamical model of streamflow generation from a hillslope. The model is a modified version of an integral water balance model developed for hillslopes and small basins. It does not include Hortonian overland flow, but it represents the physics of runoff generation from a hillslope better than the physics used in developing the filter. Rainfall time series data are used to run the model. The filter is applied to streamflow simulated from the model and filter estimates of base flow are compared to model‐simulated base flow. The filter performs well on average, but base flow estimates include a strong high‐frequency component that is not found in simulated base flow. The filter assumption that overland flow and recharge are constant fractions of rainfall is found to be a main source of estimation error. A new filter is developed using the modified model in which recharge is a delayed and attenuated response to rainfall because it is a function of integrated soil moisture. Comparing this new filter with the original one shows that base flow estimates only marginally improve, based on one criterion, for streamflow data from a first‐order basin. However, this new filter provides time series estimates of integrated soil moisture and estimates of soil moisture are similar to observed trends in the basin. The formulation of this filter provides a new framework for developing better filters for estimating time series of base flow and integrated soil moisture.

Modelling the effects of hillslope–channel coupling on catchment hydrological response

AbstractA new two‐dimensional hydrological model has been developed that accounts for dynamic interactions between hillslope and channel flows. The model is specifically designed for semi‐arid areas dominated by Hortonian overland flow, and has a dynamically active channel belt. Sensitivity analyses of the model have been carried out to assess the relative importance of topographic linkages, surface characteristics and rainfall characteristics on catchment hydrological response. Attribute sensitivity analyses suggest that hillslopes are more sensitive than floodplains to all parameters except surface roughness. However, decoupling through the presence of floodplains or other barriers will reduce the relative importance of the sensitivity of hillslope parameters. Spatial sensitivity analysis suggests that sensitivity to the spatial variability of infiltration decreases with an increase in the magnitude of the runoff event. On the other hand, variability in output discharge at the catchment outlet increases with an increase in the spatial variability of infiltration. Rainfall intensity is thus an important factor in controlling the overall coupling characteristics. Catchment runoff production is affected by a complex interaction of topographic, surface and rainfall characteristics. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Soil degradation and overland flow as causes of gully erosion on mountain pastures and in forests

In the European Limestone Alps, “Lithic Borofolists”, i.e. soils in a thick, peat-like organic surface layer, occur widespread in the subalpine vegetation belt in northern Italy. The most important properties of the Lithic Borofolists are their field capacity of more than 600% of the dry weight and an extremely high infiltration capacity. These soils prevent any hydraulic erosion and equilibrate the water budget of forests and the discharge of mountain torrents. Forest fires, clear-cutting, or overgrazing by cattle and horses could result in the collapse of the ecosystem within a short period of time. In the study area, overgrazing and trampling by cattle lead to a rapid degradation of Lithic Borofolists to “Histic Rendzinas” on the summer pastures. This causes a serious decrease in both field and infiltration capacity. On such degraded soils, the rainfall of an average thunderstorm may cause Horton overland flow and erosional features on the slopes as well as on the valley bottoms. Overland flow from the valley heads carries enormous quantities of silt, resulting in an extensive siltation of the summer pastures on the valley bottom and thereby lead to further reduction of the infiltration capacity. On the valley floor, the overland flow will be concentrated in shallow depressions, where it causes the erosion of steep-sided gullies. Even a beaten hunter path in a wooded area led to the formation of a deep gully during a thunderstorm.

Threshold conditions for initiation of valley-side gullies in the Middle Veld of Swaziland

Gullied and ungullied catchments in four areas of deeply weathered saprolite in the Middle Veld of Swaziland can be successfully separated ( p<0.01) using discriminant analysis based on area and slope of their basin heads. The discriminant scores for the catchments indicate that those on the Manzini land system differ in their behaviour from those on the Lobamba land system. No differences exist within each land system with respect to land use when the basins are classified as arable, rangeland or mixed. The discriminant functions indicate that gullies on the Lobamba land system have developed by Hortonian overland flow whereas those on the Manzini land system should be attributed to subsurface processes. Since no evidence exists for subsurface activity, it is suggested that gullying on the Manzini land system has been more active and, though initiated by overland flow, has developed to a stage where seepage at the base of the saprolite is now the main process. Erosion control measures should be based on dense ground vegetation to reduce flow velocity on the Lobamba land system and deep-rooted vegetation with high evapotranspiration rates to reduce flow volumes on the Manzini land system.

A process based assessment of the potential to reduce flood runoff by land use change

Various runoff processes with widely varying infiltration and retention capacities, such as Hortonian overland flow, saturation overland flow and fast subsurface flow, form storm runoff in catchments. Areas can be classified according to these different runoff processes on the basis of information on soil characteristics, geology, topography, and land use. The results of such classifications can be verified with infiltration experiments combined with tracer techniques. A reduction of storm runoff by a change of land use or land use management practices is only feasible on sites, where infiltration and matrix wetting can be enhanced. In order to estimate the effects of possible land use changes on storm runoff, the spatial distribution of the dominant runoff processes and the actual land use was assessed in a meso-scale catchment in the state of Rheinland Pfalz, Germany. Based on assumptions on how changes in land use might influence the runoff processes, the potential for flood runoff reduction could be estimated.

Scale effects of Hortonian overland flow and rainfall‐runoff dynamics: laboratory validation of a process‐based model

AbstractHortonian runoff was measured in the laboratory from uniform slopes of lengths of 1·5, 3·0, and 6·0 m for steady, high‐intensity rainstorms with durations of 1·0 to 7·5 min. A clear reduction in runoff per unit slope length was found as slope lengths were increased. This effect becomes more pronounced with decreasing storm duration. The runoff data were used to validate a simple process‐based model that combines the Philip‐two‐term infiltration equation with the kinematic wave overland flow principle. The predicted and experimental results agreed well. Laboratory findings were extrapolated with the aid of the model to slopes and rainfall durations similar to those found under West African conditions. The calculated reduction of runoff per unit length is similar to reported observations. Thus, this process‐based model can largely explain the phenomenon of runoff reduction with increasing slope length. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Stormflow‐hydrograph separation based on isotopes: the thrill is gone — what's next?

Hprovedanirresistibletemptation, and was a vast improvement over graphical separationand solute tracer methods that were prevalent at the time. Eventu-ally, hydrologists realized that this new method entailed a plethoraof assumptions about temporal and spatial homogeneity of isotopiccomposition (many of which were commonly violated). Nevertheless,hydrologists forged ahead with dozens of isotope-based hydrograph-separation studies that were published in the 1970s and 1980s.Hortonian overland flow was presumed dead. By the late 1980s,the new isotope-based hydrograph separation technique had movedinto adolescence, accompanied by typical adolescent problems suchas confusion and a search for identity. As experienced hydrologistscontinued to use the isotope technique to study stormflow hydrol-ogy in forested catchments in humid climates, their younger peersfollowed obligingly—again and again. Was Hortonian overland flowreally dead and forgotten, though? What about catchments in whichpeople live and work? And what about catchments in dry climatesand the tropics? How useful were study results when several of theassumptions about the homogeneityof source waters were commonlyviolated? What if two components could not explain the variation ofisotopic composition measured in the stream during stormflow? Andwhat about uncertainty? As with many new tools, once the initialshine wore off, the limitations of the method became a concern—oneof which was that isotope-based hydrograph separations alone couldnot reveal much about the flow paths by which water arrives at astream channel during storms.During the late 1980s and early 1990s, isotope-based hydro-graph separation techniques matured into young adulthood. Unlikethe early studies that separated hydrographs into two componentsbased only on time, increasingly complex approaches emerged thatallowed stormflow-hydrograph separation into three-componentsthat reflected the geographic source areas of runoff. Hydrologistsalso began to incorporate error into their studies, and, most impor-tantly, began to combine isotope-based hydrograph separations withphysical measurements such as groundwater levels and soil watercontent to learn more about hydrologic flow paths. As the sciencematured further in the 1990s, a point was reached at which isotope-based hydrograph separations alone were insufficient to guarantee

Simulating hydrological response for the R‐5 catchment: comparison of two models and the impact of the roads

AbstractIn this paper the performance of two hydrological‐response models is evaluated and compared based upon simulations for a single rainfall–runoff event. The two models are QPBRRM, a relatively simple model of Horton overland flow, and InHM, a comprehensive physics‐based model of each of the known streamflow generation mechanisms. The rainfall–runoff event focused upon in this study is from the small rangeland catchment in Oklahoma known as R‐5. When calibrated, both QPBRRM and InHM are shown to effectively simulate the R‐5 event. The calibration procedures used in this study for QPBRRM and InHM were quite different. The calibration of QPBRRM was a curve fitting exercise, whereas the calibration of InHM was based upon an internally valid estimate of the continuous head field. In this study QPBRRM did not perform well outside of the calibrated range. The impact of the roads cutting across the R‐5 catchment is simulated with InHM and discussed for the first time in the study reported here. The relative merits of QPBRRM and InHM are each discussed. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Horton overland flow contribution to runoff on unpaved mountain roads: A case study in northern Thailand

AbstractTwo indirect methods are used to detect evidence of intercepted subsurface flow (ISSF) by the road prism in the Pang Khum Experimental Watershed (PKEW) in northern Thailand. During the 12‐month study period we failed to observe a soil moisture change that corresponds with ISSF being generated by the water table rising above the road surface. In support of the soil moisture data, δ18O signatures of rain water, road runoff, and stream water (a proxy for soil water) suggest Horton overland flow (HOF) generated on the road surface, not ISSF, is the dominant source of observed road runoff during typical rainfall events in the study area. This finding, which is contrary to the ISSF‐dominant road runoff regime found typically on unpaved mountain roads in the US Pacific NW, suggests that the use of a HOF‐based model to simulate runoff and sediment transport on unpaved roads in PKEW provides not only lower bound estimates of these processes, but realistic approximations for typical events. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Acceleration of Horton overland flow and erosion by footpaths in an upland agricultural watershed in northern Thailand

Through field rainfall simulation experiments in an upland mountainous watershed of northern Thailand, we have identified two phenomena that increase the potential for Horton overland flow (HOF) generation on agricultural lands. First, there appears to be a transition period of 12–18 months, extending from the time of abandonment until the formation of a dense vegetation layer capable of intercepting rainfall and ponding surface water, during which HOF generation is accelerated. Simulation data indicate these recently abandoned fields may have runoff coefficients (ROCs) as high as 40% during large seasonal storms with wet antecedent soil moisture conditions. In comparison, actively cultivated lands and advanced (>16–18 months) fallow fields, the land surfaces existing before and after the threshold period, have ROCs≤4%. Secondly, compacted path surfaces initiate HOF within agricultural fields, which have saturated hydraulic conductivity ( K s) values that are 100–200 mm h −1 higher. In the study area, path/furrow networks, comprising 8–24% of field surface areas, are designed to provide walking access within fields and channel excess surface flow from the fields. Compared with hoed surfaces, path/furrows reduce the time to HOF generation by about 85% and have ROCs that are six times higher. Access paths have the lowest K s values of all watershed surfaces, but conveyance efficiency of HOF generated on these surfaces is low. Even recently created field paths are capable of reducing runoff generation time by 40–90% and producing sixfold increases in ROCs. Collectively, the data suggest that agricultural erosion rates are accelerated during the 12–18-month threshold period following abandonment and during storms where path-generated HOF interacts with adjacent planting surfaces. Despite having periods of increased HOF generation, the total HOF contribution from agricultural fields to the basin stream hydrograph is similar in magnitude to that of unpaved roads, which occupy 95% less land area.

Runoff and sediment loss responses to rainfall and land use in two agricultural catchments on the Loess Plateau of China

AbstractSoil erosion is a severe problem hindering sustainable agriculture on the Loess Plateau of China. Plot experiments were conducted under the natural rainfall condition during 1995–1997 at Wangdongguo and Aobao catchments in this region to evaluate the effects of various land use, cropping systems, land slopes and rainfall on runoff and sediment losses, as well as the differences in catchment responses. The experiments included various surface conditions ranging from bare soil to vegetated surfaces (maize, wheat residue, Robinia pseudoacacia L., Amorpha fruticosa L., Stipa capillata L., buckwheat and Astragarus adsurgens L.). The measurements were carried out on hill slopes with different gradients (i.e. 0 ° to 36 °). These plots varied from 20 to 60 m in length. Results indicated that runoff and erosion in this region occurred mainly during summer storms. Summer runoff and sediment losses under cropping and other vegetation were significantly less than those from ploughed bare soil (i.e. without crop/plant or crop residue). There were fewer runoff and sediment losses with increasing canopy cover. Land slope had a major effect on runoff and sediment losses and this effect was markedly larger in the tillage plots than that in the natural grass and forest plots, although this effect was very small when the maximum rainfall intensity was larger than 58·8 mm/h or smaller than 2·4 mm/h. Sediment losses per unit area rose with increasing slope length for the same land slope and same land use. The effect of slope length on sediment losses was stronger on a bare soil plot than on a crop/plant plot. The runoff volume and sediment losses were both closely related to rainfall volume and maximum intensity, while runoff coefficient was mainly controlled by maximum rainfall intensity. Hortonian overland flow is the dominant runoff process in the region. The differences in runoff volume, runoff coefficient and sediment losses between the catchments are mainly controlled by the maximum rainfall intensity and infiltration characteristics. The Aobao catchment yielded much larger runoff volume, runoff coefficient and sediment than the Wangdongguo catchment. Copyright © 2001 John Wiley &amp; Sons, Ltd.