Increasing Slope Length Research Articles

Modeling soil erosion dynamic processes along hillslopes with vegetation impact across different land uses on the Loess Plateau of China

Developing a process-based soil erosion model that comprehensively considers the effects of vegetation is complex but crucial for evaluating sediment reduction during vegetation restoration. Current understanding of the effect of vegetation on sediment reduction along hillslopes across different land uses is limited. In this paper, we developed a dynamic model of hillslope erosion that integrates the effects of vegetation on soil detachability and hydrodynamics (VED) based on the feedback mechanism between soil detachment and sediment transport. The VED was calibrated and validated with runoff plot data for woodland, grassland, and farmland in the Wuding, Yan, Jing and Wei Rivers of the Loess Plateau in China. Unified baseline parameters and decay coefficients of vegetation were obtained. The model validation results indicated that the coefficient of determination, Nash–Sutcliffe simulation efficiency and relative error ranged from 0.59 to 0.99, 0.56 to 0.90, and −45.99 % to 46.36 %, respectively. The decay coefficients for soil detachment capacity (Φ), sediment transport capacity (Tc), and the sediment reduction effect of vegetation exhibited the following order: woodland > grassland > farmland. Compared with VED, existing process-based soil erosion models failed to effectively characterize the influence of vegetation on Φ and Tc on the Loess Plateau, with differences of several orders of magnitude. The individual contributions of runoff, soil detachability, and hydrodynamics with vegetation impact on sediment yield were quantified by VED. The contribution rates of vegetation to sediment reduction decreased gradually with increasing slope length because the soil detachment capacity of bare slopes exceeded that of vegetated slopes, resulting in a faster increase in sediment yield. The contribution rates of vegetation to sediment reduction for woodland, grassland, and farmland with 20 %–60 % vegetation covers were 66.75 %–99.95 %, 57.43 %–99.63 %, and 27.91 %–88.63 % in the boundary conditions of this study, respectively. The results reveal the potential for integrating VED into other distributed watershed hydrological and sediment models to assess the effects of vegetation restoration on sediment reduction at a watershed scale.

A Field Study for the Effects of Grass Cover, Rainfall Intensity and Slope Length on Soil Erosion in the Loess Plateau, China

Slope length is an important topographic factor for controlling soil erosion. There exists limited knowledge of the interactions of slope length, vegetation restoration, and rainfall intensity on soil erosion. This study investigated the impact of the slope length on soil erosion for different grass coverages and different rainfall intensities via simulated rainfall experiments. The experiments included five rainfall intensity treatments (1, 1.5, 2, 2.5, and 3 mm min−1), four grass cover treatments (0%, 30%, 60%, and 90%), and five slope length treatments (2, 4, 6, 8, and 10 m). The change process of soil loss was significantly different (p < 0.05) for different slope lengths. The trend of soil loss changing with slope length is: under a grass cover of 0 or 30%, the soil erosion increased exponentially with increasing slope length. However, under a grass cover of 60%, the soil erosion rate peaked at a slope length of 8 m, and under a grass cover of 90%, the soil erosion rate peaked at a slope length of 6 m. At rainfall intensities of 1.5–2 mm min−1, the overall soil erosion amount was small. The soil loss increased drastically with slope length when the rainfall intensity exceeded 2 mm min−1. Compared with a slope length of 2 m, longer slope lengths increased the erosion rate by 225–930% under different grass coverages treatments. Regression analysis showed that grass cover and rainfall intensity change the trend of erosion with slope length, and the negative effect of slope length on erosion is strengthened with the increase of grass cover, while this negative effect gradually weakens with the increase of rainfall intensity.

Evaluation and modeling of factors influencing the depth of mixing layer in which soil solute releasing from soil to surface runoff

The depth of mixing layer is one of the important parameters which cannot be assigned a constant value affected by many factors in the slope runoff. The objective of this study was to investigate the effect of slope length and underground biomass on slope runoff, solute transport processes, as well as mixing layer depth. In this study, the experimental plots with the four slope lengths (5, 10, 15, and 20 m) and a width of 2 m were built on the slope with the gradient of 20°. In addition, the plots with the millet or wheat planting were built on the slope. The change of runoff and solute transport was analyzed through simulated rainfall experiments and then to estimate mixing layer depth. The results showed that the runoff rate decreased and more runoff seeped into the slope soil with increasing slope length. Increasing underground biomass also promoted greater rainfall infiltration into the soil. The increase in slope length increased the concentration of solute in runoff, but more underground biomass reduced the nutrients transported with runoff. The effective mixing depth increased with an increase in slope length, but effective mixing depth decreased with increased underground biomass. The modified expression of the equivalent mixing model under different slope lengths and underground biomass could accurately describe the solute transfer process in runoff when compared with complete mixing model and incomplete mixing model based on exponential functions. This research provided a reference for improving the application of mixing layer models in the slope management.

Distribution, morphology and influencing factors of rills under extreme rainfall conditions in main land uses on the Loess Plateau of China

Rill erosion is one of the main forms of soil erosion of the hillslopes on the Loess Plateau of China. Few studies have evaluated the effects of land use on the distribution and morphology of rills under extreme rainstorms. A total of 50 rill survey sites were selected in a typical watershed of the Loess Plateau of China to investigate the rill distribution and morphology of three land uses (fallow land, FL; slope farmland, SF; and annual grassland, AG) under extreme rainstorm conditions (called “7·26” rainstorm). Seven parameters (rill width, rill depth, width-depth ratio, cross-section area, rill density, degree of rill dissection and rill erosion modulus) were used to characterize rill morphology, and the basic soil and vegetation properties (bulk density, particle size distribution, and vegetation coverage) and topographic factors (slope length, gradient, direction) were measured. It was found that rills developed on three land uses, and those with widths ranging from 5 to 20 cm accounted for >80%. Rills developed on FL were deeper than those on SF and AG. Almost all rills on FL had a depth of 5–20 cm (90.3%), while the depth of rills on SF and AG were mainly smaller than 15 cm (91.5% and 91.9%, respectively). In addition to width, rill depth and cross-section area of SF and AG were also significantly less than those of FL. SF and FL had a relatively high width-depth ratio and dissection degree. However, land use had no significant effect on rill density. Crop plantations (SF) and grass restoration (AG) significantly reduced rill erosion modulus (28.70% and 45.07% reduction, respectively). Land use, topography, soil, and vegetation factors significantly affected rill morphology (p = 0.001) based on redundancy analysis (RDA) and can explain 48.0% of the total variability of rill morphology, with slope length, vegetation coverage and slope gradient having the greatest influences. Most of the rill morphological parameters significantly decreased as the vegetation coverage increased but they significantly increased with increasing slope length and slope gradient. This study clearly illustrates the severe rill erosion caused by this rainstorm, and indirectly indicates that crop harvesting (FL) aggravates rill erosion and that revegetation (AG) can effectively contain rill erosion. Therefore, rehabilitation efforts should be focused on revegetation (grassland and/or woodland) on steep slopes to control soil erosion.

Effects of minimum soil disturbance practices on controlling water erosion in China's slope farmland: A meta‐analysis

AbstractSlope farmlands are commonly found in China, and soil loss from slope farmland is the main source of river sediment. Minimum soil disturbance practices were considered effectively in controlling water erosion, especially on slope farmlands. Although their beneficial effects have been universally recognized, further research is still needed to quantify the extent of minimum soil disturbance practices in reducing runoff and sediment yield. We present a quantitative review based on a nationwide meta‐analysis, including 536 runoff and 615 sediment‐paired observations, to evaluate the effectiveness of minimum soil disturbance practices in controlling water erosion in China. Minimum soil disturbance practices in China reduced runoff and sediment yield by 36.09% and 51.69%, respectively. Collecting soil to form ridges with no‐tillage, contour tillage with hedgerow, and micro‐basins tillage demonstrated significantly greater (p < 0.05) runoff and sediment yield reductions than other three practices (no‐tillage, no‐tillage with mulch, and contour tillage). Practices that change in soil properties by microtopography management were superior (p < 0.05) to practices that change soil properties without tillage in reducing runoff and sediment yield. Minimum soil disturbance practices are more effective in reducing sediment yield than runoff due to its additional effects besides reducing runoff directly. No correlations between runoff or sediment yield reduction and slope gradient were observed, whereas the efficiency of water erosion control by minimum soil disturbance practices increased with the increasing slope length. As a soil and water conservation measure, minimum soil disturbance practices can be applied in slope farmland of China.

Optimizing vegetation pattern for the riparian buffer zone along the lower Yellow River based on slope hydrological connectivity

Riparian buffer zone is important ecological transitional region between river and upland. Restoring the degraded vegetation system is important for preventing soil erosion, improving ecological environment and helping to achieve the sustainable development of ecosystems. Based on the scenario simulation of vegetation pattern and flow length index, we analyzed the responses of hydrological connectivity to vegetation pattern under different vegetation coverages and slope gradients, and explored the optimal vegetation pattern of soil and water conservation in riparian buffer zone in the lower reaches of the Yellow River. The results showed that the midslope-coarsness-clustered distribution of vegetation configuration, which exhibited the shortest flow length and the weakest hydrological connectivity, being the optimal vegetation pattern for controlling slope runoff generation and flow concentration. For the optimal vegetation pattern, its flow length increased with increasing slope length, namely, the longer slope length the more significant difference of hydrological connectivity between different slope gradients. Meanwhile, flow length of the optimal vegetation pattern decreased with increasing vegetation coverage. The differences between different slope gradients were obvious under low vegetation coverage, while it was unobvious on slope with vegetation coverage of 45%. Compared with the irregular variation trend of flow length on the actual vegetation slope, there was a consistent trend of first increase and then decrease on the simulated slope with the optimal vegetation pattern. Within the pre-set slope gradient range (5°-20°), the optimal vegetation pattern changed the variation of flow length between different slope gradients in the process of coverage change, which highlighted the influence of riparian buffer zone vegetation pattern on hydrological connectivity.

Morphology and influencing factors of rills in the steep slope in Yuanmou Dry-Hot Valley (SW China)

It is very important to understand rill morphology in order to understand its dynamics. To reveal the morphology and influencing factors of rills in the steep slope (RSS) in the Yuanmou Dry-Hot Valley, the total station, photogrammetric technique and microtopographic profiler were used to obtain the parameters of the cross sections and longitudinal profiles of the RSS, and 12 parameters were used to characterize the rill morphology. The results show that the morphology of rills clearly varied between each layer; the topography, soil properties and ferruginous cemented layer all exerted important influences on the rill morphology. The average depth decreased with increasing slope length, and the depths dominated by silt and sand were larger than that dominated by clay. The average width and average width/depth ratio increased with increasing slope length, and the width and the width/depth ratio that was dominated by clay was larger than that dominated by silt and sand. The shape ratio of the rill cross section ranged from 0.50 to 0.58, and the shape of the cross-section gradually changed from narrow and deep to wide and shallow with increasing slope length. Concave–convex degree tended to be larger in the segment with ferruginous cement. In the longitudinal profile, the gradient gradually decreased with decreasing slope gradient and increasing slope length. The presence of the ferruginous cemented layer and topsoil crust resulted in abnormal changes in the shape of the rill. It is helpful to further understand the mechanisms of rill erosion under different conditions.

The effects of slope length and slope gradient on the size distributions of loess slides: Field observations and simulations

In this study, we characterize and consider the effects of slope length and slope gradient on the size distributions of loess slides. To carry out this study, we employ data on 275 loess slides within Zhidan County, Central Loess Plateau, China. These data were collected in the field and supplemented by the interpretation of remote sensing images. Both the field observations and slope stability analysis show that loess slide size increases with the slope length. Slide sizes is significantly correlated with slope length, showing a power law relationship in both cases. However, the simulation results show that slope gradient is not associated with loess slide size. The main part of the link between slope gradient and slide size seen in the observations is only apparent, as indicated by the strong connection between slope gradient and length. Statistical analysis of the field observations reveals that slope gradient decreases with increasing slope length, and this correlation interferes with the potential relationship between landslide sizes and slope gradient seen in the field observations. In addition, the probability densities of the areas of loess slides occurring on slopes of different slope lengths are determined using kernel density estimation. This analysis shows that slope length controls the rollover of the frequency-size distribution of loess slides. The scaling exponent increases with slope length.

Quantifying the effect of forests on frequency and intensity of rockfalls

Abstract. Forests serve as a natural means of protection against small rockfalls. Due to their barrier effect, they reduce the intensity and the propagation probability of falling rocks and thus reduce the occurrence frequency of a rockfall event for a given element at risk. However, despite established knowledge on the protective effect of forests, they are generally neglected in quantitative rockfall risk analyses. Their inclusion in quantitative rockfall risk assessment would, however, be necessary to express their efficiency in monetary terms and to allow comparison of forests with other protective measures, such as nets and dams. The goal of this study is to quantify the effect of forests on the occurrence frequency and intensity of rockfalls. We therefore defined an onset frequency of blocks based on a power-law magnitude–frequency distribution and determined their propagation probabilities on a virtual slope based on rockfall simulations. Simulations were run for different forest and non-forest scenarios under varying forest stand and terrain conditions. We analysed rockfall frequencies and intensities at five different distances from the release area. Based on two multivariate statistical prediction models, we investigated which of the terrain and forest characteristics predominantly drive the role of forest in reducing rockfall occurrence frequency and intensity and whether they are able to predict the effect of forest on rockfall risk. The rockfall occurrence frequency below forested slopes is reduced between approximately 10 and 90 % compared to non-forested slope conditions; whereas rockfall intensity is reduced by 10 to 70 %. This reduction increases with increasing slope length and decreases with decreasing tree density, tree diameter and increasing rock volume, as well as in cases of clustered or gappy forest structures. The statistical prediction models reveal that the cumulative basal area of trees, block volume and horizontal forest structure represent key variables for the prediction of the protective effect of forests. In order to validate these results, models have to be tested on real slopes with a wide variation of terrain and forest conditions.

The effect of slope length on sediment yield by rainfall impact under different land use types

To evaluate the impact of slope length on sediment yield under different rainfall intensities and land use types on low hill gentle slope, the characteristics of sediment yield process were analyzed based on the field artificial rainfall simulation. For the study, grassland and capsicum slope were taken from Anji county of Zhejiang province, China. Results indicated that rainfall intensity had stronger influence than slope length on sediment yield in south region. For capsicum slope, sediment yield increased quickly with increasing slope length when rainfall intensity greater than 90 mm h–1. The slope length had no significant effect on sediment yield when rainfall intensity less than or equaled to 90 mm h–1. For grassland, data from experiments indicated that sediment yield increased slowly with increasing slope length under rainfall intensity less than 120 mm h–1. There was a decreasing tendency of sediment yield at 6 m slope length under all rainfall events. It was concluded from particle size analysis of erosional sediment that silt and clay particles <0.02 mm were always preferentially transported on both capsicum slope (silt 47.1%, clay 40.9%) and grassland (silt 38.3%, clay 35.9%). We hope these results are useful for soil and water conservation and land management.

Selective organic carbon losses from soils by sheet erosion and main controls

AbstractAlthough the impact of sheet erosion on the selective transportation of mineral soil particles has been widely investigated, little is yet known about the specific mechanisms of organic carbon (OC) erosion, which constitutes an important link in the global carbon cycle. The present study was conducted to quantify the impact of sheet erosion on OC losses from soils. Erosion plots with the lengths of 1‐ and 5‐m were installed at different topographic positions along a hillslope in a mountainous South African region. A total of 32 rainfall events from a three years period (November 2010 up to February 2013), were studied and evaluated for runoff (R), particulate and dissolved organic carbon (POCL and DOCL). In comparison to the 0–0·05 m bulk soil, the sediments from the 1‐m plots were enriched in OC by a factor 2·6 and those from the 5‐m long plots by a factor of 2·2, respectively. These findings suggest a preferential erosion of OC. In addition, total organic carbon losses (TOCL) were incurred mainly in particulate form (~94%) and the increase in TOCL from 14·09 ± 0·68 g C m−1 yr−1 on 1‐m plots to 50·03 ± 2·89 g C m−1 yr−1 on 5‐m plots illustrated an increase in sheet erosion efficiency with increasing slope length. Both TOCL and sediment enrichment in OC correspondingly increased with a decrease in soil basal grass cover. The characteristics of rainstorms had no significant impact on the selectivity of OC erosion. The results accrued in this study investigating the links between sheet erosion and OC losses, are expected to be of future value in the generation of carbon specific erosion models, which can further help to inform and improve climate change mitigation measures. Copyright © 2016 John Wiley &amp; Sons, Ltd.

The role of roots in the stability of landfill clay covers under the effect of dry–wet cycles

This study investigates the role of vegetation roots in the stability of landfill clay covers subject to dry–wet cycles. Soil shear strength is tested at different depths of bare and vegetation covers before and after 1 year of seasonal dry–wet cycles. Results show that the effect of these cycles and of vegetation roots on the soil shear strength of covers mainly alters soil cohesion. By contrast, the effect on the internal friction angle of soil is inconspicuous. After 1 year of seasonal dry–wet cycles and grass root growth, the soil cohesion of the vegetation cover increases, whereas that of the bare cover decreases. The increment in the soil cohesions of two covers tends to decline exponentially with increasing cover depth. Thus, the cover stability model is established according to upper-bound solution theory based on the change rule of cover shear strength under the effect of dry–wet cycles and vegetation roots. The theoretical analysis results indicate that the increase in cover slope safety factor that was caused by vegetation roots is significant, compare that of bare cover under dry–wet cycles. Nonetheless, the effect of roots on cover slope safety factor decreases with increasing slope length and cover slope angle.

Remote sensing monitoring of gullies on a regional scale: A case study of Kebai region in Heilongjiang Province, China

Gully erosion is one of the major causes of land degradation in most areas and attracts increasing attention from researchers. We monitored gullies in the Kebai region in Heilongjiang Province of China by using remote sensing data and found that gully density increased with the increase in slope when the slope was less than 3°. Gully density in sunny slopes or windward slopes was greater than in shady slopes or leeward slopes because of the impacts of freezing and thawing, wind and solar radiation. Specifically, the gully density in northeast slope was the greatest and in southwest was the smallest. Gully density was reduced with increasing slope length and the longer the slope length, the less the gully density changed between 1965 and 2005. Affected by runoff, gullies most easily to occur in concave slopes and the critical elevation for gully erosion was 250–275 m. Moreover, hilly regions had the greatest gully density, followed by tableland regions, whereas the gully density in flatlands was the lowest. However, the gully density of these three types of landforms all increased between 1945 and 2000, and the portion of increase was 57.45% (hill), 52.91% (mesa) and 25.32% (plain), respectively.

Sediment concentrations in run‐off varying with spatial scale in an agricultural subwatershed of the Chinese Loess Plateau

AbstractInformation is scarce on the spatial‐scale effect on sediment concentrations in run‐off. This study addressed this issue within an agricultural subwatershed of the Chinese Loess Plateau, using data observed at a hilltop plot, three nested hillslope plots, two entire‐slope plots (a combination of hillslope and valley side slope) and the subwatershed outlet. Dominated by the splash and sheet erosions, the hilltop plot has a minimum Cae (mean sediment concentration for all recorded events) of 45 kg m−3. Unexpectedly, the high sediment concentrations at the hilltop do not occur at high rainfall intensities or large run‐off events because of the protection of surface soils by relatively thick sheet flows. Because of the emergence of rills, Cae is as high as 310 kg m−3 even on the most upper hillslope. Downslope, both Cae and ESC (extreme large values of recorded sediment concentrations) increase; such a slope length effect attenuates with increasing slope length and event magnitude as a result of insufficient sediment availability associated with rill development. Active mass wastings ensure sufficient sediment supply and thus a spatially invariant Cae (approximately 700 kg m−3) and ESC (approximately 1000 kg m−3) at the scale of the entire slope and subwatershed. Detailed examination shows that most small events experience a decrease in sediment concentrations when moving from the entire slope to the subwatershed, indicating that the spatially invariant sediment concentration is valid only for large run‐off events. This study highlights the control of the spatial scale, which determines the dominant erosional process, on erosional regime. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Impact of sheet erosion mechanisms on organic carbon losses from crusted soils in the Sahel

Soil surface crusting influences water infiltration and runoff but its impact on soil organic carbon (SOC) losses by sheet erosion is largely unknown. Because there are different mechanisms of sheet erosion, from raindrop detachment and transport by raindrops interacting with flow (RIFT), to detachment and transport by flow, that require a certain slope length to be operative, this study examined the impact of slope length on SOC and nutrient losses. Field experiments were conducted on crusted soils in the Sahel region of Africa. Three replicates of micro-plots (1m×1m), plots (10m long×5m width) and long plots (25m×6m) were installed for each crust type in the area (structural, STRU; desiccation, DES; gravel, GRAV; and erosion, ERO) and followed for each rainfall event in the 2012 rainy season. Sediment, SOC content in sediments and selected nutrients (NO3−; PO43−) in the runoff were analyzed to evaluate the annual losses by sheet erosion. SOC losses decreased significantly with increasing slope length from 0.24gCm−1 on micro-plots to 0.04gCm−1 on plots and to 0.01gCm−1 on long plots and similar trends were observed for NO3− and PO43− losses. This suggested a strong scale dependency of sheet erosion with the efficiency of transport by saltation and rolling by RIFT decreasing significantly with increasing slope length, by 6 folds in average between 1 and 10m, with values between 1.8 on DES crusts and 19 on STRU crusts. These results on the relationship between soil crusting and sheet erosion should be further used to mitigate against the loss of SOC through the implementation of improved soil conservation techniques, as well as to improve soil erosion and/or SOC models.

Validating and Improving Interrill Erosion Equations

Existing interrill erosion equations based on mini-plot experiments have largely ignored the effects of slope length and plot size on interrill erosion rate. This paper describes a series of simulated rainfall experiments which were conducted according to a randomized factorial design for five slope lengths (0.4, 0.8, 1.2, 1.6, and 2 m) at a width of 0.4 m, five slope gradients (17%, 27%, 36%, 47%, and 58%), and five rainfall intensities (48, 62.4, 102, 149, and 170 mm h−1) to perform a systematic validation of existing interrill erosion equations based on mini-plots. The results indicated that the existing interrill erosion equations do not adequately describe the relationships between interrill erosion rate and its influencing factors with increasing slope length and rainfall intensity. Univariate analysis of variance showed that runoff rate, rainfall intensity, slope gradient, and slope length had significant effects on interrill erosion rate and that their interactions were significant at p = 0.01. An improved interrill erosion equation was constructed by analyzing the relationships of sediment concentration with rainfall intensity, slope length, and slope gradient. In the improved interrill erosion equation, the runoff rate and slope factor are the same as in the interrill erosion equation in the Water Erosion Prediction Project (WEPP), with the weight of rainfall intensity adjusted by an exponent of 0.22 and a slope length term added with an exponent of −0.25. Using experimental data from WEPP cropland soil field interrill erodibility experiments, it has been shown that the improved interrill erosion equation describes the relationship between interrill erosion rate and runoff rate, rainfall intensity, slope gradient, and slope length reasonably well and better than existing interrill erosion equations.

Impact of fodder cover on runoff and soil erosion at plot scale in a cultivated catchment of North Vietnam

In Vietnam soil erosion is a major environmental problem with respect to soil fertility, water quality and downstream property damages and involves 40% of total land surface. Due to a continuous and persistent decrease of soil quality under annual crops, farmers gradually convert their fields to grazing lands and their crops to fodder cultures or tree plantations. Experimental 1-m2 field plots with three replicates each were monitored for two years (2006–2007) to evaluate the impact of three different fodder treatments (Paspalum atratum, Panicum maximum and Stylosanthes guianensis) on runoff and soil detachment in a cultivated catchment of North Vietnam. These experiments were designed to monitor at local scale the protective effect of vegetation cover against splash and rain-impacted erosion. The lowest runoffs (ca. 3.0–4.4%), sediment yields (ca. 14–19gm−2yr−1) and soil organic carbon losses (ca. 0.7gCm−2a−1) were obtained for P. maximum that provided the best soil protection with respect to the two other treatments. These values were low as compared to cultivated crops (cassava and rainfed rice). Soil surface characteristics (mainly biological activity and crusting) did apparently not play a key role, most likely because each plant cover provided, with its own efficiency, protection against rainfall erosivity and rapid plant regrowth wiped out traces of flow detachment. The extent of soil detachment and sediment export, mainly controlled by cut and carry operations of fodder management, was reduced by increasing slope length from 1 to 5m. The choice of dense fodders such as P. maximum appears to be, in terms of improved livelihood and environment sustainability, an interesting issue for uplands farmers.

Scale effects in Hortonian surface runoff on agricultural slopes in West Africa: Field data and models

This article provides an overview of both experimental and modeling research carried out over the past 15 years by the authors addressing scaling effects in Hortonian surface runoff. Hortonian surface runoff occurs when rainfall intensity exceeds infiltration capacity of the soil. At three sites in West Africa (Côte d’Ivoire, Ghana, and Burkina Faso) runoff was measured from plots of different lengths to assess scale effects. Consistently, longer plots showed much lower runoff percentages than shorter plots. There were large variations in runoff percentages from one rainstorm to the next but there were very good correlations between plots of equal length for each single event. This strongly suggests that temporal dynamics are the cause behind the observed scale effects. In the literature, spatial variability is often proffered as explanation for such scale effects without providing a mechanism that would cause consistent reduction in runoff percentages with increasing slope length. To further examine whether temporal dynamics can indeed provide the explanation, Hortonian runoff was simulated using models with increasing levels of complexity. The simplest model was already able to reproduce the observed scale effects. Also more complex models were used that accounted explicitly for spatial variability. The conclusions remained the same regarding the role of temporal dynamics. Finally, a dimensional analysis was developed that helps predict under which circumstances one can expect scale effects similar to the ones observed in West Africa.

The influence of rainfall distribution and morphological factors on runoff delivery from dryland catchments in SE Spain

A simplified theoretical model of storm runoff has been combined with analysis of rainfall records to improve understanding of dryland runoff processes in the Guadalentin catchment of SE Spain, and in particular the decrease in runoff fraction with increasing slope length or catchment area. Estimates of storm runoff volumes can be made using a simple runoff threshold, SCS Curve Numbers, or infiltration equations. A modified Green–Ampt equation is applied here to give storm runoff estimates for conditions of constant effective rainfall intensity. This can be seen as both an improvement on estimates made using the simple runoff threshold, although with no additional parameters, and a good approximation to the SCS Curve Number family of rainfall–runoff relationships, providing an improved theoretical basis for the Curve Number approach and an explicit means of linking it to existing soils data on infiltration and surface properties. The frequency distribution of intensities is examined to determine an appropriate effective intensity in relation to slope length, with a duration long enough to allow overland flow to reach the slope base. Hydrologically Similar Surfaces (HYSS) are defined by local runoff characteristics, and the linkage to infiltration allows an explicit comparison with routing over land areas of different area, shape and topography. This provides the basis for up-scaling from at-a-point runoff to runoff from larger areas which are mapped as belonging to the same HYSS class, explicitly allowing for their different topography and size, providing an essential step in up-scaling data on fine-scale surface characteristics to the response of catchment areas. Here we focus on the ways in which catchment area or slope length influence runoff production within a given HYSS class. Characteristics which drive catchment response include (1) variations in intensity over time through a storm (2) areal variations in storm volume. For measured rainfall in the upper Guadalentin catchment, SE Spain, the first of these factors is seen to be the most important for catchments of up to 500 km 2, and a procedure is suggested for deriving effective rainfalls from recorded intensity data. The rainfall data can then be combined with a HYSS analysis of the soil/land use response, and analyses of catchment network morphology and connectivity to forecast the heterogeneous distribution of ephemeral stream runoff.

WATER QUALITY MODEL OUTPUT UNCERTAINTY AS AFFECTED BY SPATIAL RESOLUTION OF INPUT DATA1

ABSTRACT: Resolution of the input GIS data used to parameterize distributed‐parameter hydrologic/water quality models may affect uncertainty in model outputs and impact the subsequent application of model results in watershed management. In this study we evaluated the impact of varying spatial resolutions of DEM, land use, and soil data (30 × 30 m, 100 × 100 m, 150 × 150 m, 200 × 200 m, 300 × 300 m, 500 × 500 m, and 1,000 × 1,000 m) on the uncertainty of SWAT predicted flow, sediment, NO3‐N, and TP transport. Inputs included measured hydrologic, meteorological, and watershed characteristics as well as water quality data from the Moores Creek watershed in Washington County, Arkansas. The SWAT model output was most affected by input DEM data resolution. A coarser DEM data resolution resulted in decreased representation of watershed area and slope and increased slope length. Distribution of pasture, forest, and urban areas within the watershed was significantly affected at coarser resolution of land use and resulted in significant uncertainty in predicted sediment, NO3‐N, and TP output. Soils data resolution had no significant effect on flow and NO3‐N predictions; however, sediment was overpredicted by 26 percent, and TP was underpredicted by 26 percent at 1,000 m resolution. This may be due to change in relative distribution of various hydrologic soils groups (HSGs) in the watershed. Minimum resolution for input GIS data to achieve less than 10 percent model output error depended upon the output variable of interest. For flow, sediment, NO3‐N, and TP predictions, minimum DEM data resolution should range from 30 to 300 m, whereas minimum land use and soils data resolution should range from 300 to 500 m.