Hortonian Overland Flow Research Articles

Contribution of intercepted subsurface flow to road runoff and sediment transport in a logging‐disturbed tropical catchment

AbstractHydrological and sediment fluxes were monitored for a 1 yr period in a tropical headwater catchment where a 3 yr old logging road caused substantial Hortonian overland flow (HOF) and intercepted subsurface flow (ISSF). On a 51·5 m road section, ISSF became an increasingly important component of total road runoff, up to more than 90% for large storms. The proportion of ISSF contributed by road cuts along more or less planar slopes compared with ISSF from a zero‐order basin (convergent slopes) truncated by the road declined with increasing rainfall. During the monitored storms that generated ISSF along the road, on average, 28% of sediment export and 79% of runoff from the road section were directly attributable to ISSF. Estimates of total sediment export from the road surface (170 t ha−1 yr−1) and suspended sediment export from the logging‐disturbed catchment (4 t ha−1 yr−1) were exceptionally high despite 3 yr of recovery. ISSF caused not only additional road‐generated sediment export, but also exacerbated HOF‐driven erosion by creating a poor foundation for vegetation recovery on the road surface. Copyright © 2007 John Wiley & Sons, Ltd.

Hortonian overland flow from Japanese forest plantations—an aberration, the real thing, or something in between?

AbstractThere is a growing opinion that poorly managed plantation forests in Japan are contributing to increased storm runoff and erosion. Here we present evidence to the contrary from runoff plots at two scales (hillslope and 0·5 × 2 m plots) for several forest conditions in the Mie and Nariki catchments. Runoff coefficients from small plots in untended hinoki forests were variable but typically higher than from better managed or deciduous forests during small storms at Nariki; at Mie, runoff during small events was highly variable from all small plots but runoff coefficients were similar for hinoki plots with and without understory vegetation, while the deciduous plot had lower runoff coefficients. Storm runoff was less at the hillslope scale than the plot scale in Mie; these results were more evident at sites with better ground cover. During the largest storms at both sites, differences in runoff due to forest condition were not evident regardless of scale. Dynamic soil moisture tension measurements at Nariki indicated that during a large storm, flow in the upper organic‐rich and root‐permeated soil horizons was 3·2 times higher than measured overland runoff from a small hinoki plot with poor ground cover and 8·3 times higher than runoff from a deciduous forest plot. On the basis of field observations during storms, at least a portion of the monitored ‘Hortonian overland flow’ was actually occurring in this near‐surface ‘biomat’. Therefore our field measurements in both small and large plots potentially included biomat flow in addition to short‐lived Hortonian runoff. Because overland flow decreased with increasing scale, rill erosion did not occur on hillslopes. Additionally, runoff coefficients were not significantly different among cover conditions during large storms; thus, the ‘degraded’ forest conditions appear not to greatly enhance peak flows or erosion potential at larger scales, especially when biomat flow is significant. Copyright © 2007 John Wiley & Sons, Ltd.

Changes in catchment runoff after harvesting and burning of a Pinus caribaea plantation in Viti Levu, Fiji

A catchment study was conducted between January 1990 and April 1992 to quantify the impact of harvesting a mature Pinus caribaea plantation forest on total runoff, baseflow recession, peak discharge, stormflow volume and sediment concentration. As rainfall differed between pre- and post-harvesting periods, a streamflow prediction model (HBV) and statistical methods were used to evaluate observed changes and to test if these were significant. Most runoff occurred as baseflow (64–74% of total runoff). Baseflow increased significantly after harvesting, from 10% to 16% of rainfall. The minimum daily flow increased by 70% after harvesting, in spite of lower rainfall in that period. Stormflow volumes and peak discharges also increased significantly as a result of Horton overland flow generation on new road and log-landing surfaces and a higher wetness of the catchment due to lower evapotranspiration. Stormflow volume doubled from 2.7% to 5.5% of rainfall after harvesting. The combined effect amounted to estimated increases in flows from 252 mm under forest to 580 mm after harvesting for the 324-days period of 1990 (total rainfall of 1613 mm) and from 90 mm to 194 mm for the drier 309-days period in 1991/1992 (total rainfall of 992 mm). Sediment concentrations increased significantly during stormflow events after harvesting, but not under baseflow conditions. Harvesting of the pine plantation therefore increased stormflow, baseflow and total water yield significantly. Stormflow water quality was affected negatively, though, through increased sediment concentrations. These aspects are important for water resources management considerations related to plantation forestry in areas with a distinct dry season.

Surface runoff as affected by soil water repellency in a Japanese cypress forest

AbstractRecent studies have suggested that soil water repellency can be one of the important factors affecting hydrological processes on headwater catchments. In Japan, water repellency is known to occur under Japanese cypress (Chamaecyparis obtusa) forests, a typical plantation type in Japan, however, previous studies have not evaluated the severity of water repellency and its effects on surface runoff generation. To quantify water repellency and its effects, this study combined the critical surface tension (CST) test with a new spraying experiment in which the infiltration rates of water and ethanol solutions sprayed over 0·09‐m2 plots were compared. Long‐term intensive hydrological observations of surface runoff from 2‐m2 plots, soil water potential, and soil water content were conducted concurrently.The spraying experiment revealed that strong water repellency in surface soils, as quantified by the CST test, caused Hortonian overland flow despite the high conductivity measured under saturated hydrophilic conditions. Generally, the surface runoff coefficient for a storm event was negatively correlated with initial soil moisture conditions. However, during a period of successive storm events separated by short intervals, the coefficient decreased gradually even when the initial moisture conditions were similar, indicating a weakening of water repellency by repeated wetting. On the other hand, a drying period with long inter‐rainfall intervals and increasing air temperature was associated with increases in the surface runoff coefficient. These results suggest that the water repellency and the resultant surface runoff depended on the history of rainfall at the site. Relationships between soil water potential and soil water content indicate that changes in the soil water repellency and consequently surface runoff coefficient could occur during a single storm event. Copyright © 2007 John Wiley & Sons, Ltd.

A numerical model for simulating Hortonian overland flow on tropical hillslopes with vegetation elements

AbstractThis paper describes a two‐dimensional hydrodynamic model that characterizes surface runoff process resulting from a varying rainfall intensity event, on an infiltrating soil surface. The soil surface has spatially varied soil physical, hydraulic and microtopographic characteristics. Infiltration process is modelled with the Philip two‐term equation and the time before ponding approximated with the time compression algorithm. Vegetation is modelled as a dynamic component with the modified Gash model. The equation is solved with a modified second order Leapfrog explicit finite difference scheme with centred time and space derivatives. The model was validated with standard analytical solutions. Evaluation with results from field campaigns in the Volta Basin of West Africa during the 2002 rainfall season indicates good agreement, with r2 values ranging from 0·89 to 0·96. The developed method will be useful in studying the dynamics of surface runoff generation under complex microtopographic conditions, spatially varying soil hydraulic characteristics and temporally dynamic rainfall intensity, as found in many tropical catchments. Copyright © 2007 John Wiley & Sons, Ltd.

Effect of temporal rainfall distribution and soil type on soil moisture and runoff generation in semi-arid Zimbabwe

This paper examines the effect of temporal rainfall distribution on soil moisture and runoff generation in the 5.9 km2 Mutangi catchment in semi-arid Zimbabwe. Rainfall, soil moisture and runoff were measured during the 1999/00 and 2000/01 rainy seasons during which periods 755 mm and 615 mm of rainfall were received, respectively. The percentage of rainfall totals in these periods were 58% and 69%, respectively, in February. The total catchment runoff was 102 mm and 63 mm, of which 52% and 49% were recorded over 6 and 4 d in 2000 and 2001, respectively. Baseflow was negligible. Rainfall intensities were generally low. In the 1999/00 season there were 2 and 8 h with intensities >20 mm h−1 and 10 mm h−1, respectively. Some runoff appears to be generated by Hortonian overland flow (HOF), mainly in the early wet season before ploughing creates a rougher soil surface. The dominant process of runoff in this catchment was saturated overland flow (SOF), which occurs when the soils become saturated from below. The sodic soils along the stream channels appear to generate most of the runoff because of their small capacity to store water before saturation. The ridge soils are coarse sands, with a large capacity to store rainfall. The transitional (slope) soils have an intermediate capacity to store water. If there is a sequence of daily events that completely fills the storage available in both the sodic and transitional soils, and which begins to saturate the ridge soils, there could be very large amounts of runoff (>50% of the daily rainfall). The occurrence of such runoff events depends very heavily on the distribution of rainfall. Dry spells between rain events create storage, thereby reducing the risk of runoff from the next events.

Numerical calculation of secondary discharge peak from a small watershed using a physically based watershed scale infiltration simulation

Numerical infiltration simulations were performed to reproduce secondary discharge peaks in a mountainous forest watershed (watershed area, 1.89 ha; average topsoil depth, 2.61 m; and bedrock geology, Mesozoic–Paleozoic) using a simplified physically based three-dimensional saturated and unsaturated water-flow model based on Richards' equation. We were able to calculate the quick discharge during rain and a secondary discharge peak at the watershed simultaneously, using observed topographical information, the topsoil depth distribution, and soil hydraulic characteristics, and by dividing the watershed by 2.5 m horizontally and ten cells vertically. Although the calculated hydrograph did not agree entirely with the observed hydrograph, we conclude that the characteristics of the observed hydrograph were explained with better accuracy using the smaller soil porosity patterns than using the observed patterns. We verified that the simulation method based on Richards' equation was effective to analyze the rainfall-runoff processes toward the intended watershed. Computational comparisons clarified that lower soil porosity quickens the timing of secondary discharge peaks and increases their volume. Additional examinations, such as the distribution of soil hydraulic characteristics and the actual condition of Hortonian overland flow, are necessary to simulate rainfall-runoff processes precisely at the intended watershed.

Evolution of overland flow after a severe forest fire, Point Reyes, California

Forest fires on granitic soils often increase overland flow and erosion. Runoff generation was monitored on a small hillslope plot on Mt. Vision near Point Reyes Peninsula, California, after it had been burned by a wildfire on October 3, 1995. After the fire, the ground surface was covered with up to 2 cm of ash, which overlaid a 5–20 cm thick hydrophobic (water repellent) soil layer. We used nine recording tensiometers to monitor soil-water potentials during infiltration and runoff. Surface-runoff rates were determined by diverting the flow into a collection tank. The subsurface flow through the upper 6 cm of soil was collected and measured in a second tank. The surface runoff was diverted to a tank in order to record its rate. The initial intense rainfall infiltrated into the base of the ash-bed; here, the hydrophobicity limited deeper penetration and led to both subsurface and shallow saturation overland flow. The preferential flow paths through the ash layer contributed to deeper water penetration. As the ash was eroded and consolidated with successive rainstorms, the preferential flow paths clogged, the infiltration capacity reduced, thus preventing the storage of shallow permeable soil; therefore, the runoff generation changed to Hortonian overland flow. Correspondingly, the runoff ratio increased from approximately 0.2 during the early storms to 0.8 during intense rain bursts. These results suggest that runoff mechanisms evolve simultaneously with the eroding soil surface.

Runoff generation in a degraded Andean ecosystem: Interaction of vegetation cover and land use

Tropical mountain regions are affected by rapid land use/-cover change, which may threaten their (eco-)hydrological functions. Although there is a growing interest in evaluating the effect of land use/-cover change on mountain hydrology, quantitative assessments of the impact of land use/-cover on hydrological processes are hampered by the lack of field measurements characterizing runoff generation processes. In this paper, we present results from field experiments of rainfall runoff mechanisms in the southern Ecuadorian Andes. A rainfall simulator was used to quantify the hydrological response of distinct land use/-cover types to intense rainfall (about 40 mm/h). The rainfall runoff experiments indicate that degraded and abandoned land generate surface runoff within a few minutes after the start of the rainfall event. These lands have a very rapid and sharp hillslope hydrological response, as Hortonian overland flow is the dominant runoff generation mechanism. In contrast, surface runoff on arable and rangelands is rare, as their soils are characterized by a high infiltration capacity (i.e. > 29 mm/h). Our experiments provide evidence that runoff generation in degraded Andean ecosystems is mainly controlled by the surface vegetation cover and land management. When reducing the surface vegetation cover, the soil is increasingly affected by rapid hillslope runoff as the presence of large amounts of smectites in the outcropping soft rocks makes the material very prone to sealing and crusting, thereby enhancing runoff generation.

Persistence of road runoff generation in a logged catchment in Peninsular Malaysia

AbstractMeasurements of saturated hydraulic conductivity (Ks) and diagnostic model simulations show that all types of logging road/trail in the 14·4 ha Bukit Tarek Experimental Catchment 3 (BTEC3) generate substantial Horton overland flow (HOF) during most storms, regardless of design and level of trafficking. Near‐surface Ks(0–0·05 m) on the main logging road, skid trails and newly constructed logging terraces was less than 1, 2 and 34 mm h−1, respectively. Near‐surface Ks on an abandoned skid trail in an adjacent basin was higher (62 mm h−1), owing to the development of a thin organic‐rich layer on the running surface over the past 40 years. Saturated hydraulic conductivity measured at 0·25 m below the surface of all roads was not different (all <6 mm h−1) and corresponded to the Ks of the adjacent hillslope subsoil, as most roads were excavated into the regolith more than 0·5–1 m. After 40 years, only limited recovery in near‐surface Ks occurred on the abandoned skid trail. This road generated HOF after the storage capacity of the upper near‐surface layer was exceeded during events larger than about 20 mm. Thus, excavation into low‐Ks substrate had a greater influence on the persistence of surface runoff production than did surface compaction by machinery during construction and subsequent use during logging operations. Overland flow on BTEC3 roads was also augmented by the interception of shallow subsurface flow traveling along the soil–saprolite/bedrock interface and return flow emerging from the cutbank through shallow biogenic pipes. The most feasible strategy for reducing long‐term road‐related impacts in BTEC3 is limiting the depth of excavation and designing a more efficient road network, including minimizing the length and connectivity of roads and skid trails. Copyright © 2007 John Wiley & Sons, Ltd.

Hydrological consequences of landscape fragmentation in mountainous northern Vietnam: Buffering of Hortonian overland flow

We use a hydrology-based fragmentation index to explore the influence of land-cover distribution on the generation and buffering of Hortonian overland flow (HOF) in two disturbed upland basins in northern Vietnam (Tan Minh). Both the current degree of fragmentation in Tan Minh and the current spatial arrangement of buffers (relative to HOF source areas) provide only limited opportunities for infiltrating surface runoff from upslope source areas, in part because of the high connectivity of swidden fields on long hillslopes. The intentional placement of buffers below HOF sources and the reduction of the down-slope lengths of swidden fields could reduce the occurrence of HOF on individual hillslopes. Reduction of the total watershed total depth of HOF would require maintaining a sufficient area of buffering land covers; and this may necessitate the use of longer fallow periods. These measures are, however, counter to the land-practice trends witnessed in the last several decades (i.e., no buffers, cultivation of long slopes, and increasingly shorter fallow periods). The two most likely scenarios of future land-cover change in Tan Minh—one representing increased fragmentation, the other decreased—both lead to an increase in HOF because of reduced buffering potential. The unlikely scenario of abandonment of agriculture and subsequent regeneration of forest, leads to both less fragmentation and less HOF. The study highlights the hydrological impacts associated with fragmentation at Tan Minh, which is the product of decades of local and regional forcing factors that have dictated the degree and timing of timber removal and swiddening at the site.

Mechanisms affecting stormflow generation and solute behaviour in a Sahelian headwater catchment

The aim of this study was to analyse stormflow processes and the behaviour of solutes therein (Ca 2+ , Mg 2+ , Na + , K + , alkalinity, NO 3 - , SO 4 2 - , Cl − , Si), during flood events in tropical semi-arid conditions. The study site was a small Sahelian catchment (1.4 ha) located in northern Burkina Faso. Runoff and rain water was sampled over a 2-year period (1999 and 2000). In addition to dissolved load, suspended load was measured in the stream water collected at the outlet of the catchment. Isotopic tracing using δ 18 O was also conducted during two very different flood events. The results indicated that: (i) event water was by far the major contributor to the stream stormflow, with Hortonian overland flow being the main stormflow process at work; (ii) a small fraction of pre-event soil water may have contributed during the recession of heavy flood with wet antecedent conditions; (iii) solute concentrations were higher in runoff compared to rainwater. With the exception of NO 3 - and Cl − , the highest concentrations were measured at the onset of floods, and almost always decreased during the rising stage of the hydrographs; (iv) a good correlation was found between suspended load and the concentrations of Ca 2+ , Mg 2+ , alkalinity and Si. It was concluded that fast physico-chemical interactions between event water and reactive suspended phases may explain most of the chemical changes between rainwater and floodwater.

Application of physical modelling of debris flow triggering to field conditions: Limitations posed by boundary conditions

Debris flows are often triggered by Hortonian overland flow during high-intensity rainstorms. Data derived from debris flow trigger zones in the southern French Alps were fed into a physical model of debris flow triggering based on Takahashi. Using a Monte Carlo approach with 1000 runs, the results show a wide distribution of safety factor values, indicating that physical modelling based on actual field measurements may not always be practical. As all safety factor values obtained are well below 1 even though debris flows only occur during very high-intensity rainstorms, the model used must be inappropriate. Apparently, the composition of the overland flow plays an important role: during high-intensity rainstorms it usually has a very high sediment content and contains stones. This prevents it from flowing through the pores of coarse debris accumulations in the central gully of a trigger zone; it will rather run over the debris. This situation is more stable than with the fluid flowing through the pores. The behaviour switch of the fluid above a certain sediment and stone content thus drastically changes the triggering conditions for debris flows and it is concluded that debris flow triggering in the area requires the occurrence of both overland flow and landsliding.

Spatial variability of soil hydrophobicity after wildfires in Montana and Colorado

Increases in runoff and erosion after wildfires are often attributed to the development of hydrophobic soils. The potential for increased overland flow depends on the spatial contiguity of the hydrophobicity as well as its overall strength, but there is limited information on the spatial variability of soil hydrophobicity. We conducted spatially intensive hydrophobicity measurements in 225 m 2 and 1 m 2 plots in forested areas of Montana and Colorado burned at moderate to high severity, and in unburned control plots. Both the burned and unburned 225 m 2 plots contained 10–23 hydrophobic soil patches in which hydrophobicity was strongest at the surface and declined rapidly with depth. The hydrophobic patches were closer together and up to 3 times larger in the burned plots. Consequently, 19% to 76% of the burned plots were hydrophobic compared to just 11% of the unburned plots. In five of the six burned plots, the patches were not laterally connected, suggesting that in most cases Hortonian overland flow generated from hydrophobic patches will infiltrate near its point of origin. The 1 m 2 plots were smaller than most of the hydrophobic patches, so they did not capture the spatial characteristics of soil hydrophobicity. Characterization of the spatial variability of soil hydrophobicity should be based on measurements conducted at ∼ 1 m intervals across areas of > 100 m 2. Due to the patchiness of soil hydrophobicity at the 10 0 to 10 1 meter scale, overland flow measurements in small (∼ 1 m 2) plots may overestimate the magnitude and variability of runoff from burned catchments.

Transport of water, solutes and nutrients from a pasture hillslope, southwestern Brazilian Amazon

AbstractA conceptual model of water and solute transport pathways was developed and applied to a pasture hillslope in the southwestern Brazilian Amazon basin using select field measurements. Infiltration‐excess or Horton overland flow (HOF), saturation overland flow (SOF), and groundwater in both the near‐stream zone and upslope were sampled on a hillslope draining a 3·9 hectare pasture for a total of ten storms during the first half of the rainy season (October–November) in 2002. A Soil Conservation Service SCS curve number model of HOF and an annual water balance of both upslope and near‐stream zones were used to calculate the contribution of each flowpath to solute export. HOF occurred in rainstorms greater than 5 mm and accounted for ∼8% of annual rainfall. Flow generated in the near‐stream zone was ∼8% of annual rainfall. Sub‐surface flow from upslope groundwater dominated annual runoff (∼19–30% of annual rainfall). Solutes fell into three categories according to flowpath. HOF from upslope positions dominated the export of total phosphorus (TP) and total dissolved phosphorus (TDP, 51–72% of total annual export). The near‐stream zones controlled the export of K (58–65%), total dissolved nitrogen (TDN, 76–80%), and total nitrogen (TN, ∼75%) owing to relatively high solute concentrations and the large volume of water that flowed through the near‐stream zone. Na and Si export was via groundwater from upslope (50–67% of annual export). The flux calculations were based on a small number of storms and are preliminary estimates designed to identify broad patterns in solute export via different hydrologic pathways. Additional processes, especially N removal at the groundwater‐stream interface and in the stream channel, may affect actual export rates at the watershed scale. Whereas HOF production is negligible in Amazon forests, it represents a significant pathway for additional loss of elements, especially phosphorus, from mature pasture systems. The evidence presented here shows that biogeochemical perturbations and enhanced solute fluxes continue for decades following deforestation for pasture. Copyright © 2006 John Wiley & Sons, Ltd.

Water storage and runoff processes in plinthic soils under forest and pasture in eastern Amazonia

AbstractExtensive areas of the Amazon River basin are underlain by soils with shallow impeding horizons. To evaluate how the distinctive hydraulic properties of soil with a plinthic horizon under forest and pasture affect water storage and runoff process, two first‐order catchments drained by ephemeral streams were instrumental in eastern Amazonia. Field measurements showed the presence of a strong vertical gradient of saturated hydraulic conductivity, which declines to extremely low values (median <1 mm h−1) at the plinthite layer, limiting both vertical and lateral flow, and keeping the soil water content close to saturation throughout most of the wet season. This scenario led to the frequent occurrence of saturation overland flow (SOF) under both land covers and very small amounts of shallow sub‐surface flow (SSF). The annual flow in the exit channels was 3·2% of throughfall (2·7% of annual rainfall) under forest and 17% of annual rainfall for pasture, while the frequency of days with overland flow (OVF) was about 60% of the days for both catchments during the wet season. In the forest, all OVF originated from saturated areas, while in the pasture, infiltration‐excess OVF accounted for 40% of the runoff and SOF accounted for 55% of runoff. The higher flow generation in the pasture could be explained by the higher water storage compared to the forest, promoting more frequent SOF, and additionally by the lower hydraulic conductivity near the surface favouring the occurrence of Horton overland flow (HOF). Copyright © 2006 John Wiley & Sons, Ltd.

Multiple modes of storm runoff generation in a North Carolina coastal plain watershed

AbstractThe results of field measurements conducted in a small (19·37 ha) agricultural watershed on the North Carolina coastal plain during the summer of 1996 are presented. The objective of the study was to develop a more complete understanding of basin response in the region with respect to stormflow generation and, in particular, to identify the processes that determine storm runoff and the conditions under which such processes occur. Twenty‐four storm events were monitored, including two tropical storm systems and two hurricanes. The data demonstrate considerable spatial and temporal heterogeneity in runoff generation within the watershed. Surface flowpaths, in the form of Hortonian overland flow and saturation overland flow, were found to be the dominant runoff processes during the storm events measured. The hillslope flowpaths had the same response time as the basin streamflow, but significantly shorter time of rise and lag times. The importance of Hortonian flow in a basin with sandy, permeable soils, as well as the rapid stormflow response in a low‐relief area with a humid climate, was contrary to expectations. This, coupled with the contingency of runoff response, suggests that it may be difficult to generalize about runoff generation mechanisms in broad terms, and that a synoptic approach may be more appropriate. Copyright © 2006 John Wiley & Sons, Ltd.

A Modified Hortonian Overland Flow Model Based on Laboratory Experiments

Overland flow, which is examined here, is an important hydrological response to storms of a catchment. And it is also a main component of hillslope hydrology. Through controlling of rainfall intensities and changing of artificial slope angles, 49 overland flow hydrographs are obtained from rainfall-runoff simulation experiments conducted on a manmade hillside. Based on the measured overland flow processes, the relationship between the average water depth on the hillslope and that at the outlet, which is nearly a quadratic curve, is obtained. The relationship is different from the assumption, in which the average water depth on the hillslope is equal to that at the outlet, proposed by Horton. Finally, a regression equation is also presented, which provides both the theoretical and experimental basis to modify the time delay of peak flow for a distributed rainfall-runoff modeling.

Gullying and erosion control at archaeological sites in Grand Canyon, Arizona

AbstractGully erosion of cultural sites in Grand Canyon National Park is an urgent management problem that has intensified in recent decades, potentially related to the effects of Glen Canyon Dam. We studied 25 gullies at nine sites in Grand Canyon over the 2002 monsoon–erosion season to better understand the geomorphology of the gully erosion and the effectiveness of erosion‐control structures (ECS) installed by the park under the direction of the Zuni Conservation Program. Field results indicate that Hortonian overland flow leads to concentrated flow in gullies and erosion focused at knickpoints along channels as well as at gully heads. Though groundcover type, soil shear strength and permeability vary systemat‐ically across catchments, gradient and, to a lesser degree, contributing drainage area seem to be the first‐order controls on gully extent, location of new knickpoints, and ECS damage. The installed ECS do reduce erosion relative to reaches without them and initial data suggest woody checkdams are preferable to rock linings, but maintenance is essential because damaged structures can exacerbate erosion.Topographic data from intensive field surveys and detailed photogrammetry provide slope–contributing area data for gully heads that have a trend consistent with previous empirical and theoretical formulations from a variety of landscapes. The same scaling holds below gully heads for knickpoint and ECS topographic data, with threshold coefficients the lowest for gully heads, slightly higher for knickpoints, and notably higher for damaged ECS. These topographic thresholds were used with 10‐cm digital elevation models to create simple predictive models for gully extent and structure damage. The model predictions accounted for the observed gullies but there are also many false‐positives. Purely topographical models are probably inadequate at this scale and application, but models that also parameterize the variable soil properties across sites would be useful for predicting erosion problems and ECS failure. Copyright © 2006 John Wiley & Sons, Ltd.

Ecological roles of roadside fern ( Dicranopteris curranii) on logging road recovery in Peninsular Malaysia: Preliminary results

The role of common roadside fern ( Dicranopteris curranii) on the recovery of logging roads in headwater catchments of Peninsular Malaysia was examined. Hortonian overland flow from a 51.5 m road section was monitored during the 1-year study period along with sporadic measurements of sediment export before and during fern growth. Furthermore, the event-based influence of fern cover on rainfall input and flux of DOC, K, Mg, and Ca were estimated using bulk rainfall collectors (176 cm 2) and compared with those in the adjacent forest canopy for six storms. Air temperatures measured beneath fern cover and forest canopy were compared with those on the open road. Road-generated runoff was reduced by 4.8 mm after fern growth during storms with <30 mm rainfall; decreased sediment production rate from the road section (−86%) also coincided with fern growth. Interception loss of fern cover (19%) was as high as that of the forest canopy; throughfall under fern cover was enriched with K to the similar levels as beneath the forest canopy. With the fern cover (−57% of the road surface), net flux of K (+101%), Mg (+70%), and Ca (+26%) to the road increased significantly. Fern cover maintained daily maximum temperatures comparable to forested areas (<28 °C), whereas open road surfaces reached 40 °C. Our preliminary results suggest that roadside fern growth plays potentially important ecological roles in road recovery by reducing road runoff, mitigating splash and hydraulic surface erosion processes, trapping sediment where plant seeds can germinate, providing nutrient-enriched throughfall, and moderating harsh surface temperature environment.