Hortonian Overland Flow Research Articles

Holocene dune-sourced alluvial fans in the Nebraska Sand Hills

The large, stabilized dunes of the Nebraska Sand Hills are in a phase of degradation. The deposition of small-scale alluvial fans composed of well-sorted, fine- to medium-grained sand occurs when sand is transported via gullies on the lee side of large barchanoid-ridge dunes during infrequent, intense summer rain storms (>5 cm/h). The hydraulic conductivity of the dune sand itself is too high to allow Hortonian overland flow, but organically bound mats and crusts on the surface of the dunes help to impede infiltration. Secondary eolian bed forms at the crests of the dunes focus shallow groundwater and overland flow that erodes gullies. Internally, alluvial fans are dominated by laminated sands interpreted as sheetflood deposits. Depositional couplets of structureless (bottom) and laminated (top) sands may form from a combination of hyperconcentrated flow and sheetflood. Thickness of deposits varies with storm intensity and sediment supply in the gullies. The alluvial fans are Holocene in age, and gully initiation may have begun at the onset of a wetter climate when vegetation was less dense than at present. Water-laid sandstones associated with large-scale eolian cross-strata have been recognized in Precambrian, Devonian, and Cretaceous sequences. The alluvial fans from the Sand Hills may provide a new modern analogue for interpreting these ancient sandstones.

A comparison of thematic mapping of erosional intensity to GIS-driven process models in an Andean drainage basin

Evaluation of predicted patterns of erosional processes using GIS techniques can be problematic in areas where few field data are available. Yet it is in such areas where the ability to extrapolate using GIS could be most useful in practical applications. We compare patterns of low and high intensity erosion predicted by topographically driven, process-based models to geomorphological mapping in a mountainous 350 km 2 sub-catchment of the Iruya Basin in northern Argentina. Following Dietrich et al. (Channelization thresholds and land surface morphology. Geology 1992, 29, 675–679), areas most susceptible to different erosion processes were identified by plotting thresholds for soil saturation, erosion by overland flow and landsliding on a graph of drainage area versus slope. Within areas potentially subject to shallow landsliding, relative hazard was further stratified using a simple slope stability model. For areas dominated by overland flow, relative erosional intensity was modeled based on Hortonian overland flow. We compared model predictions to the distribution of areas identified from thematic mapping as subject to weak or strong erosion. Each of the process models predicted that large drainage areas and steep slopes produce energetic erosion associated with features that appear in classic geomorphological mapping. However, fine-scale differences in the patterns of predicted and mapped erosional intensity reflect fundamental differences between the generalized polygons generated by thematic mapping and pixel-by-pixel analysis generated by process models. Our analysis illustrates how DEM-driven process models and thematic mapping provide complementary tools for predicting landscape-scale patterns of erosional processes.

Distributed sediment and phosporus transport modeling on a medium sized catchment in central germany

Current research suggests that strategies to control nitrogen as well as sediment and phosphorus loss from nonpoint sources should focus on different runoff components and their spatial and temporal variation within the catchment. Therefore nonpoint source models, especially in humid climatic regions, should consider variable hydrological active source areas. These models should also be able to consider runoff generation by saturated overland flow as well as Horton overland flow. A combination of the hydrologic model WaSiM-ETH and the erosion and P-transport model AGNPS Version 5 was chosen for this study. The models were run in the WaSiM-ETH runoff generation mode (Topmodel and Green a. Ampt approach) and the SCS curve number mode, modified by Lutz (1984), to assess these different runoff calculation procedures. A medium sized catchment in the lower Hessian uplands of Germany was selected for the present investigation. The results show that the combined Topmodel/Green and Ampt runoff approach produce reasonabe results for the simulation of runoff volume for single runoff events at the catchment outlet. The more physically based model approach computed a much more realistic distribution of runoff and P yield producing areas than the SCS curve number method, modified by Lutz (1984). Reasonable results were achieved for sediment yield calculations. Particulate P calculations showed larger deviations between measured and calculated values.

Hydrophysical degradation associated with hiking‐trail use: a case study of Hawai'iloa Ridge Trail, O'ahu, Hawai'i

AbstractRecreational activities can have significant impacts on geomorphic and hydrologic processes in drainage basins, often out of proportion to their areal extent. With increased stress on hiking trails nationwide, there is a need to characterize the impacts of human trampling on soil properties. We examine an 810 m segment of Hawai'iloa Ridge Trail (O'ahu, Hawai'i, USA). Soil compaction and surface erosion on moderate to steeply sloping sites have degraded the trail environment. Bulk density, penetration resistance, and vane shear strength were significantly higher on the trail than in adjacent undisturbed areas, with median differences ranging from 29 to 120 per cent. With compaction and exposure of subsoil on the trail, void ratio, air‐filled porosity, saturated hydraulic conductivity, effective and preferential porosity were significantly lower, with relative change values ranging from 23–93 per cent. No significant changes were noted in meso‐ or micro‐porosity, but macropores with a radius of >110 μm decreased significantly by 58 per cent for on‐trail locations. This comprehensive dataset indicates that hiking stress is deleterious to the soil–hydrologic system. Data point to a trail system that would be dominated by Hortonian overland flow and this was supported by field evidence during a storm event. Increased runoff has incised rills on some trail segments and there is evidence that run‐on to adjacent side slopes has lead to accelerated erosion. Management on most trails in Hawai'i, including the one studied, is limited, but from our data it is apparent that on‐trail sites directly influenced by an overhanging canopy of rapidly growing (aggressive) exotics were least impacted due to increased organic contributions to the surface and root network development. These data will allow land managers to more effectively address the potential geomorphic and hydrologic impacts of recreational land use. Copyright © 2001 John Wiley & Sons, Ltd.

Thresholds for gully initiation and sedimentation in Mediterranean Europe

In Mediterranean areas the dynamics of gully development act as an important indicator of desertification. However, little is known about the influence of climate and land-use changes, and almost no field data exist to assess the sensitivity of a landscape to gully erosion. Two important components of gully erosion studies are the prediction of where gullies begin and where they end. To address some of these issues, topographical thresholds for gully initiation and sedimentation in six different Mediterranean study areas were established. Field measurements of local soil surface slope (S) and drainage-basin area (A) at the point of initiation of ephemeral gullies in intensively cultivated fields (five datasets) and permanent gullies in rangelands (three datasets) were carried out. A negative power relationship of the form S = aA−b was fitted through all datasets, and defined as the mean topographical threshold for gullying in the respective area. Topographically controlled slopes of sedimentation at the gully bottom were also measured. Compared to theoretical relationships for channel initiation by overland flow, relatively low values for b are obtained, suggesting a dominance of overland flow and an influence of subsurface flow. The influence of landsliding at steeper slopes appeared from the flattening of the overall negative trend in the higher slope range (S > 0·30) of the integrated dataset. Comparing the threshold lines of our datasets to the average trend lines through data found in literature revealed that vegetation type and cover could better explain differences in topographical thresholds level than climatic conditions. In cultivated fields, soil structure and moisture conditions, as determined by the rainfall distribution, are critical factors influencing topographical thresholds rather than daily rainfall amounts of the gully-initiating events. In rangelands, vegetation cover at the time of incision appears to be the most important factor differentiating between topographical thresholds, overruling the effect of average annual rainfall amounts. Soil texture and rock fragment cover contributed little to the explanation of the relative threshold levels. Differences in regression slopes (b) between the S–A relationships found in this study have been attributed to the soil characteristics in the different study areas, determining the relative importance of subsurface flow and Hortonian overland flow. Sedimentation slopes where both ephemeral and permanent gullies end were generally high because of the high rock fragment content of the transported sediment. A positive relationship was found between the rock fragment content at the apex of the sedimentation fan and the slope of the soil surface at this location. Copyright © 2000 John Wiley & Sons, Ltd.

Runoff generation and sediment production on unpaved roads, footpaths and agricultural land surfaces in northern Thailand

Rainfall simulation was used to examine runoff generation and sediment transport on roads, paths and three types of agricultural fields in Pang Khum Experimental Watershed (PKEW), in mountainous northern Thailand. Because interception of subsurface flow by the road prism is rare in PKEW, work focused on Horton overland flow (HOF). Under dry antecedent soil moisture conditions, roads generated HOF in c. 1 min and have event runoff coefficients (ROCs) of 80 per cent, during 45 min, c. 105 mm h−1 simulations. Runoff generation on agricultural fields required greater rainfall depths to initiate HOF; these surfaces had total ROCs ranging from 0 to 20 per cent. Footpaths are capable of generating erosion-producing overland flow within agricultural surfaces where HOF generation is otherwise rare. Paths had saturated hydraulic conductivity (Ks) values 80–120 mm h−1 lower than those of adjacent agricultural surfaces. Sediment production on roads exceeded that of footpaths and agricultural lands by more than eight times (1·23 versus < 0·15 g J−1). Typically, high road runoff volumes (owing to low Ks, c. 15 mm h−1) transported relatively high sediment loads. Initial road sediment concentrations exceeded 100 g l−1, but decayed with time as loose surface material was removed. Compared with the loose surface layer, the compacted, underlying road surface was resistant to detachment forces. Sediment concentration values for the road simulations were slightly higher than data obtained from a 165 m road section during a comparable natural event. Initial simulation concentration values were substantially higher, but were nearly equivalent to those of the natural event after 20 min simulation time. Higher sediment concentration in the simulations was related to differences in the availability of loose surface material, which was more abundant during the dry-season simulations than during the rainy season natural event. Sediment production on PKEW roads is sensitive to surface preparation processes affecting the supply of surface sediment, including vehicle detachment, maintenance activities, and mass wasting. The simulation data represent a foundation from which to begin parameterizing a physically based runoff/erosion model to study erosional impacts of roads in the study area. Copyright © 2000 John Wiley & Sons, Ltd.

Hydrological implications of soil water-repellency in Eucalyptus globulus forests, north-central Portugal

Soil water-repellency (hydrophobicity) is a widespread property of Eucalyptus globulus and Pinus pinaster forest soils in central and north littoral Portugal and is particularly severe during the summer dry conditions. This paper attempts to assess the impact of water repellency on overland flow and runoff generation at plot and catchment scales for two types of terrain with differing land management and degree of soil hydrophobicity: (i) highly hydrophobic land with regenerating eucalyptus forest following fire; and (ii) largely hydrophilic land on which deep-ploughing prior to planting eucalyptus seedlings had eliminated hydrophobicity. Overland flow responses were monitored over a 40-month period for two 8m×2m plots and streamflow was recorded continuously at gauging stations for two small catchments of predominantly regrowth eucalyptus and ploughed/planted eucalyptus, respectively. Soil hydrophobicity was assessed using the Water Drop Penetration Time (WDPT) test. Seasonal variations in the factors influencing plot overland flow response were assessed for each land management type using multivariate analysis. For the regrowth eucalyptus plot, overland flow generation was found to be negatively correlated with antecedent soil moisture in summer (suggesting that hydrophobicity-linked Hortonian overland flow is then dominant), but positively related to throughflow in winter (suggesting that saturation overland flow generation in a hydrophilic-phase soil was at that time the dominant mechanism). In the ploughed/planted areas, negative correlations with soil moisture were found neither in summer nor winter. Rainfall amount (and in winter also antecedent precipitation) were found to be the variables most strongly and positively related to overland flow volume. The plot results are compared with streamflow responses for the small catchments.

Runoff generation and sediment production on unpaved roads, footpaths and agricultural land surfaces in northern Thailand

Rainfall simulation was used to examine runoff generation and sediment transport on roads, paths and three types of agricultural fields in Pang Khum Experimental Watershed (PKEW), in mountainous northern Thailand. Because interception of subsurface flow by the road prism is rare in PKEW, work focused on Horton overland flow (HOF). Under dry antecedent soil moisture conditions, roads generated HOF in c. 1 min and have event runoff coefficients (ROCs) of 80 per cent, during 45 min, c. 105 mm h−1 simulations. Runoff generation on agricultural fields required greater rainfall depths to initiate HOF; these surfaces had total ROCs ranging from 0 to 20 per cent. Footpaths are capable of generating erosion-producing overland flow within agricultural surfaces where HOF generation is otherwise rare. Paths had saturated hydraulic conductivity (Ks) values 80–120 mm h−1 lower than those of adjacent agricultural surfaces. Sediment production on roads exceeded that of footpaths and agricultural lands by more than eight times (1·23 versus < 0·15 g J−1). Typically, high road runoff volumes (owing to low Ks, c. 15 mm h−1) transported relatively high sediment loads. Initial road sediment concentrations exceeded 100 g l−1, but decayed with time as loose surface material was removed. Compared with the loose surface layer, the compacted, underlying road surface was resistant to detachment forces. Sediment concentration values for the road simulations were slightly higher than data obtained from a 165 m road section during a comparable natural event. Initial simulation concentration values were substantially higher, but were nearly equivalent to those of the natural event after 20 min simulation time. Higher sediment concentration in the simulations was related to differences in the availability of loose surface material, which was more abundant during the dry-season simulations than during the rainy season natural event. Sediment production on PKEW roads is sensitive to surface preparation processes affecting the supply of surface sediment, including vehicle detachment, maintenance activities, and mass wasting. The simulation data represent a foundation from which to begin parameterizing a physically based runoff/erosion model to study erosional impacts of roads in the study area. Copyright © 2000 John Wiley & Sons, Ltd.

Overland flow and infiltration modelling for small plots during unsteady rain: numerical results versus observed values

This paper reports the development and application of a two-dimensional model based on an explicit finite difference scheme coupling overland flow and infiltration processes for natural hillslopes represented by topographic elevation and soil hydraulics parameters. This model allows modelling of hortonian overland flow and infiltration during complex rainfall events. Original procedures have been developed in order to simulate complex rainfall events on natural slopes. The accuracy of the results is tested by comparison with experimental field data on the basis of calibrated soil and surface friction parameters. Good agreement between the calculated result and the measured data was found. The scheme proposed was found to be appropriate. The results of tests presented illustrate the effect of microtopography on the distribution of the flow depths, the magnitude and direction of flow velocities and infiltration depths.

The importance of single events in arid zone rainfall-runoff modelling

Two high magnitude rainstorm floods are simulated by a distributed rainfall-runoff model in the 1400 km2 arid catchment of Nahal Zin, Israel. Only the processes dominating arid zone flood generation (generation and spatial concentration of Hortonian overland flow on the terrain and transmission losses into the dry channel alluvium) are described. No calibration with measured flow data is performed. During one event (October 1991) almost the entire catchment was covered by high intensity rainfall as detected by rainfall radar. During another (October 1979) only one ground station in the uppermost headwaters recorded heavy precipitation, while the majority of the catchment remained dry. For this event the distributed model serves as ‘runoff-rainfall model’ to reconstruct characteristics of the rainfall. Different event characteristics directly affect parameter sensitivity and model uncertainty. Maximum model uncertainty of the diminished October 1979 peak is governed by transmission loss parameters and exceeds 300 %. During 1991 only 90 % is determined for this value and infiltration characteristics of the terrain are more relevant. Also a paleoenvironmental scenario on the hydrological effects of widespread loess deposition is highly event dependent. It may be concluded that the separate analysis of single events is crucial for the understanding of high magnitude floods in arid catchments.

Scale effects of Hortonian overland flow and rainfall-runoff dynamics in a West African catena landscape

Hortonian runoff was measured from plots with lengths of 1·25 and 12 m, and at watershed level for rainstorms during the 1996 rainy season in cental Côte d'Ivoire, Africa. A clear reduction in runoff coefficients was found with increasing slope lengths, giving order of magnitude differences between runoff measurements at point level (1 m2: 30–50% of total rain) and watershed level (130 ha: 4% of total rain). Runoff reduction from 1·25 and 12 m slopes was reproduced for each major runoff-producing rainstorm at two different sets of plots, but the reduction was erratic for rainfall events which produced little runoff. In addition, runoff reduction varied wildly from one rainstorm to the next. In the analysis, we show that the spatial variability of runoff parameters causes the erratic behaviour during rainstorms with little runoff. During the more important, larger runoff-producing events, which give 78% of total runoff, the temporal dynamics of the rainfall–runoff process determine the reduction of runoff coefficients from longer slopes. A simple infiltration/runoff model was used to simulate the field results, thereby confirming the importance of rainfall dynamics as an explanatory factor for measured reduction of runoff coefficients. Copyright © 2000 John Wiley & Sons, Ltd.

A noncalibrated rainfall‐runoff model for large, arid catchments

A distributed, field‐based rainfall‐runoff model was developed for the 1400‐km2 arid catchment of Nahal Zin, Israel. No calibration with measured flow data was performed. The model used rainfall radar input applied over a catchment that was spatially disaggregated into different terrain types according to hydrologically relevant surface characteristics. Hortonian overland flow generation on each type was parameterized independently using values of initial loss and temporal decay of infiltration determined from existing field experiments. Delimited by topography, this catchment wide pattern of rainfall excess was distributed over 850 tributary catchments (model elements). Runoff delivery from the model elements to the adjoining channel segments was timed by applying a mean response function determined in an environmentally similar experimental catchment. Inside the channel network the MVPMC3 method of the Muskingum‐Cunge technique was used for streamflow routing, accounting for channel dimensions and roughness. For each channel segment a constant infiltration rate was applied to account for transmission losses and discontinued when the wetting front reached the bottom of the available alluvial storage. Within two model tests, one separate for the routing component (October 1979) and one for the complete model (October 1991), observed hydrographs and reconstructed peak discharges were successfully simulated. The spatially distributed model output showed that during the October 1991 test, tributaries produced preceding peaks that wetted the channel alluvium before the main flood had arrived and transmission losses lost their significance downstream. Total maximum model uncertainty was estimated including the uncertainty ranges of each model parameter. In general, this study shows that field‐based data on generation and losses of runoff may be incorporated into a distributed hydrologic model to overcome calibration with the poor data records of arid high‐magnitude events.

Hydrologic Response: Kaho'olawe, Hawaii

AbstractKaho'olawe, the eighth largest island in the Hawaiian chain, has experienced major surface erosion during the last century due to overgrazing and military activity. In this paper we report the results from a series of physically‐based numerical simulations of hydrologic response, to large rainfall events, that were performed for Kaho'olawe. An event‐based rainfall‐runoff and erosion model (i.e., KINEROS) was applied, in a geographic information system (GIS) framework, to make quantitative and distributed estimates of infiltration, Horton overland flow generation, and erosion. A digital elevation model (DEM) was employed to delineate individual catchments across the island, and define areas of uniform slope within each catchment. A Landsat MSS scene of Kaho'olawe was used to classify the island into three distinct landcover categories via the normalized difference vegetation index (NDVI). The landcover categories were then used to further approximate the spatial pattern of near‐surface soil hydraulic properties across the island. Distributed estimates of saturated hydraulic conductivity and sorptivity were obtained by ordinary kriging of 135 field measurements from the island. Sensitivity analysis was performed to characterize what impact the uncertainty in soil‐hydraulic property and rainfall data had on simulated runoff. Horton overland flow and the related transport limited erosion were shown to be highly variable. The introduction of vegetation to denuded areas on Kaho'olawe was found to significantly reduce erosion.

Hydrogeochemical responses to rainfall inputs in a small rainforest basin: Rio de Janeiro, Brazil

This work provides basic information about hydrogeochemical contributions to different pathways of water in a small tropical rainforest catchment (3.5 km2) in Rio de Janeiro, Brazil. Precipitation and discharge measurements were conducted at the basin outlet, and at five other stream sites that were selected according to their hydrological behaviour and lithology. Precipitation and streamflow samples were collected weekly, over seven months, and continuously during one storm event. These streamflow samples were analyzed for Ca2+, Mg2+ Na+, K+ and SiO2. Precipitation, throughfall and topsoil water samples were also collected after various storm events, over six months, within a small sub-catchment and were analysed for Ca2+, Mg2+, K+, Na+ and pH. Results from this research showed that rainfall was acid, with low cation content. Throughfall was neutral and cations enriched. Analysis of topsoil water indicated that litter was an important source of Ca2+ and K+ by means of saturation overland flow. In relation to throughfall, streamflow output was characterized by lower concentrations of K+, Ca2+, and Mg2+. Biotite gneiss weathering was the main source of Mg2+ to streamflow. During stormflow periods SiO2 and Na+ concentrations decreased due to Hortonian overland flow contributions to the stream channel. The increasing concentrations of Ca2+ and K+ were due to litter leaching, especially in the zones producing saturation overland flow. Streamflow hydrogeochemistry was not only a product of weathering processes, but also of biotic processes.

Influence of revegetation efforts on hydrologic response and erosion, Kaho'olawe Island, Hawai'i

Measurements of soil physical and hydrological properties provide the first evaluation of the success of revegetation eAorts in reducing surface runoA and accelerated erosion on the largely barren plateau region of Kaho‘olawe Island, Hawai‘i. Saturated hydraulic conductivity and sorptivity data, collected within four of the largest restoration sites, suggest revegetated areas have significantly higher infiltration capacities compared with those of the bare areas surrounding the project sites. Furthermore, comparison of modeled steady-state infiltration capacity to one-min rainfall intensities demonstrate erosion-producing Horton overland flow is very rare on the vegetated areas compared with the barren landscape. Thus, recently, established vegetation forms zones of high infiltration capable of absorbing both rainwater and surface flow exported from upslope areas. However, the current areal extent and spatial arrangement of vegetation is not suAcient to significantly reduce watershed-scale runoA and erosion. #1998 John Wiley & Sons, Ltd.

Influence of revegetation efforts on hydrologic response and erosion, Kaho'olawe Island, Hawai'i

Measurements of soil physical and hydrological properties provide the first evaluation of the success of revegetation efforts in reducing surface runoff and accelerated erosion on the largely barren plateau region of Kaho'olawe Island, Hawai'i. Saturated hydraulic conductivity and sorptivity data, collected within four of the largest restoration sites, suggest revegetated areas have significantly higher infiltration capacities compared with those of the bare areas surrounding the project sites. Furthermore, comparison of modeled steady-state infiltration capacity to one-min rainfall intensities demonstrate erosion-producing Horton overland flow is very rare on the vegetated areas compared with the barren landscape. Thus, recently, established vegetation forms zones of high infiltration capable of absorbing both rainwater and surface flow exported from upslope areas. However, the current areal extent and spatial arrangement of vegetation is not sufficient to significantly reduce watershed-scale runoff and erosion. © 1998 John Wiley & Sons, Ltd.

On the interaction between infiltration and Hortonian runoff

Abstract A model which couples the kinematic wave approximation for Hortonian overland flow and the conceptual approach developed by Corradini et al. (1997) [J. Hydrol., 192, 104–124] for local infiltration was used to investigate the effects of random spatial variability of saturated hydraulic conductivity, K s , on the outflow hydrograph at hillslope scale. The model incorporates a representation of infiltration of overland flow running over pervious downstream areas (“run-on” process). Single rainfall pulses and complex storms over two soils, representative of a silty loam and a sandy loam soil, were considered. Our results suggest that for realistic values of the coefficient of variation of K s the run-on process cannot be disregarded, because it produces a significant decrease of overland flow during both the rising and the recession limb of the hydrograph. Furthermore, the role of the level of spatial correlation of K s was found to be typically minor and the run-on process concurred to this result. The possibility of simplifying the stochastic problem by a deterministic approach based on the use of a uniform lumped value of K s was also examined and rainfall patterns adequate for this simplification were deduced in terms of scaled storm intensity and storm duration.

An adaptive modeling and monitoring approach to describe the hydrologic behavior of small catchments

An adaptive combination of field mapping, simple hydrologic monitoring, and hydrologic modeling was used to describe the hydrologic behavior of two zero-order catchments: one undeveloped forest, the other suburban. Field mapping was used to delineate hydrologic process zones and provide guidance for the location of monitoring stations. Hydrologic monitoring included continuous streamflow and precipitation, as well as limited piezometer information. The hydrologic model was developed in conjunction with the measurement program to assure that the level of model detail was consistent with the level of field measurements used for model input and testing. At the start of the work it was anticipated that saturation overland flow would drive peak discharge in the forested catchment and Horton overland flow from impervious surfaces would dominate storm runoff in the suburban catchment. Instead, subsurface flow was found to dominate both annual and peak discharge in the forested site, and discharge from lawns and other pervious areas accounted for roughly 60% of annual and storm runoff from the suburban catchment. The adaptive combination of modeling and measurements was essential to elucidate the various components of flow production and flow paths in these catchments.

Importance of rural roads as source areas for runoff in mountainous areas of northern Thailand

Unpaved road surfaces have extremely low infiltration rates compared with other watershed land surfaces and, therefore, are significant source areas for erosion-producing Horton overland flow. The hydrologic role of roads is an important issue in mountainous areas of the tropics where erosion control efforts are predominately focused on deforestation and agricultural practices. We report on an investigation of soil physical properties that control excess rainfall (rainfall intensity in excess of infiltration capacity) on rural roads and surrounding lands in a mountainous watershed in northern Thailand. The results of our disk permeameter measurements indicate that saturated hydraulic conductivity on unpaved roads is about one order of magnitude lower than on any other land-surface type. Median saturated hydraulic conductivities were not exceeded by measured rainfall intensity on any land use except road surfaces and roadside margins. By simulating excess rainfall, we found that in contrast with other areas of the watershed, the road surface tends to generate excess rainfall early in a rain event, and on nearly all of its area. Despite the relatively small areal extent of road-related surfaces (<0.5% of basin area), they contribute a large portion of basin-wide total excess rainfall during frequently occurring, small rainfall events. However, during larger events, agricultural, secondary vegetation, and forested areas assume greater importance because of their larger areal extent. ©1997 Elsevier Science B.V.

MEASURING AND MODELLING OF SOIL WATER DYNAMICS AND RUNOFF GENERATION IN AN AGRICULTURAL LOESSIAL HILLSLOPE

Surface runoff may be generated when the rainfall intensity exceeds the infiltration capacity, or when the soil profile is saturated with water. Indications exist that both types of overland flow may occur in hilly agricultural loess regions. Here, for a loessial hillslope under maize in the southern part of The Netherlands, it was shown, with pressure head and runoff measurements, that Hortonian overland flow occurs during typical summer rain events. Surface runoff was initiated after saturation of the top 5–10 cm of the soil. Deeper in the soil, unsaturated conditions prevailed while runoff took place. Peak runoff discharges at the outlet of the subcatchment occurred a few minutes after peak rainfall intensities were measured. It appeared that SWMS_2D, a two-dimensional water flow model, was capable in simulating observed pressure head changes and runoff. Simulated potential runoff for the transect studied was higher by a magnitude of three than the measured areal average. This indicates effects of surface ponding, and the probable location of this particular transect in a region with high runoff production.