RUNOFF GENERATION AND SOIL EROSION IN SMALL AGRICULTURAL CATCHMENTS WITH LOESS-DERIVED SOILS

Over a two-year period, rainfall, runoff and sediment output were measured in six small agricultural catchments (3–10 ha) in south Limburg (The Netherlands). These measurements were needed for validation of an erosion model for South Limburg (LISEM). In this paper, results of the measurements are presented and processes that determine surface runoff and sediment yield during winter and summer rainfall are identified. Before the start of the measurement programme, surface slaking and crust formation on the erodible loess soils were thought to be the main cause of overland flow and soil erosion in South Limburg. This was the starting point for soil conservation measures in the area. The measurement results discussed in this paper show that in some catchments much runoff occurred in winter and that soil moisture storage capacity may be just as important for runoff generation as infiltration capacity. Therefore, when modelling soil erosion and optimizing erosion control measures for South Limburg, runoff generation through Hortonian as well as through saturation overland flow must be considered.     

MEASUREMENT OF FROST-INDUCED SNOWMELT RUNOFF IN A FOREST SOIL

Snow interception in a coniferous stand leads to considerable short-range variability in snowcover depth, which in turn affects the water and heat regime of the soil. To study the coupling between snow accumulation, frost penetration, and hydrological response, plot-scale experiments were conducted in a subalpine spruce forest. The stony, sandy–loamy Spodosol was highly permeable and had an organic layer of 5–15 cm thickness. Within two plots, one underneath a tree crown and one in a canopy gap, we measured near-surface runoff, soil temperature, and liquid water content. Snow and frost depths varied more in space than between two winter periods at given locations. Frost penetration was greater near the trunk, where a higher portion of snowmelt water drained downslope close to the surface than in the gap due to frost-induced reduction of infiltration. In both years, the spring snowmelt occurred over two distinct periods. During the first snowmelt, the water percolated primarily through the frozen layer and part of it probably refroze within the frozen layer, thereby raising the total water and ice content. During the second event, near-surface runoff was more pronounced.     

SPATIAL AND TEMPORAL VARIABILITY OF SOIL SURFACE ROUGHNESS AND THE APPLICATION IN HYDROLOGICAL AND SOIL EROSION MODELLING

Accurate estimations of water retention and detention are needed to simulate surface runoff and soil erosion following a rainfall event in a catchment. Several equations to estimate the amount of surface depressional storage, the fraction of the soil surface covered by water and the amount of rainfall excess needed to start surface runoff have been developed by Onstad (1984). The random roughness and slope gradient are needed for those estimations. Surface micro-elevation data have been gathered by a photographic method. The random roughness was determined from those elevation measurements. Several factors which have an impact on the soil surface roughness were taken into account. The main sources of influence are the type of land use, the crop stage within the growing period and tillage direction. Analyses of variance indicated that the variation in the RR-index could be explained mainly by type of land use, orientation and field type. The temporal variation was relatively small. Gradient data have been determined from a digital elevation model, constructed by digitizing contours. Combining the random roughness and the steepness of slope, the amounts of surface water retention and detention could be estimated. Knowledge of water retention and detention will improve the estimations of runoff and soil erosion modelling in catchments, such as those made with the LISEM model. The agricultural systems examined in this study have similar random roughness values in summer. Different soil erosion rates for several types of land use can not therefore be explained by the random roughness.     

THE RELATIVE CONTRIBUTION OF SOIL TILLAGE AND OVERLAND FLOW EROSION TO SOIL REDISTRIBUTION ON AGRICULTURAL LAND

This study uses evidence for the long-term (35 years) pattern of soil redistribution within two agricultural fields in the UK to identify the relative importance of tillage and overland flow erosion. Spatially distributed long-term total soil redistribution data for the fields (Dalicott Farm and Rufford Forest Farm) were obtained using the caesium-137 (¹³⁷Cs) technique. These data were compared with predicted patterns of soil redistribution. Recent studies have demonstrated that the redistribution of soil by tillage may be described as a diffusive process. A two-component model was, therefore, developed which accounts for soil redistribution by both overland flow and diffusive processes. Comparison of the predicted patterns of overland flow erosion alone with the observed (¹³⁷Cs-derived) data indicated a poor agreement (r² = 0.17 and 0.11). In contrast, a good agreement exists between the predicted pattern of diffusive redistribution and the observed data (r² = 0.43 and 0.41). These results give a clear indication that diffusive processes are dominant in soil redistribution within these fields. Possible diffusive processes include splash erosion, soil creep and tillage. However, the magnitude of the diffusion coefficients for the optimum predicted pattern (c. 350–400 kg m⁻¹ a⁻¹) demonstrates that tillage is the only process capable of explaining the very significant soil redistribution which is indicated by the ¹³⁷Cs data. Consideration is given to the implications of these results for both soil erosion prediction and landscape interpretation.     

THE INFLUENCE OF TOPOGRAPHY ON TIME AND SPACE DISTRIBUTION OF SOIL SURFACE WATER CONTENT

The distribution of water content in time and space at the soil surface has been investigated on a small farmland catchment (1.3 km² ) from four field surveys corresponding to different moisture statuses. For each survey, about 400 samples were collected at the soil surface at a depth of 5 cm along ten axes parallel to the greatest slope. The relationship between the measurements and the topography has been analysed. The structure of the data is well explained by a topographic index referring to the downslope conditions and defined as the elevation difference between the sample point and the stream point corresponding to the outlet of the water pathway derived from the digital elevation model (DEM). This index can be considered as an hydraulic head, at least for saturated conditions. A threshold for this index allows two domains within the catchment to be distinguished; an upper domain where the water content is nearly constant and varies slowly, and a lower domain where moisture status increases and is highly variable. The spatial distribution of these two domains is well correlated to the spatial distribution of the soils. Thus, both topography and the spatial distribution of soil appear to control the spatial distribution of surface water content at the 1-km² scale. © 1997 by John Wiley & Sons Ltd.     

ESTABLISHING TEMPORALLY AND SPATIALLY VARIABLE SOIL HYDRAULIC DATA FOR USE IN A RUNOFF SIMULATION IN A LOESS REGION OF THE NETHERLANDS

Soil hydraulic functions for runoff simulation were collected in three catchments in a loess region of The Netherlands. To obtain these functions each soil horizon was sampled and water retention and hydraulic conductivity characteristics were determined. A simulation with the computer program SWMS_2D was used to quantify runoff generation during standard rain events. Based on the simulation outcome, soil horizons were merged. This resulted in a database of 25 soil hydraulic functions, each representing a soil horizon or a specific condition of the top layer. Maps showing the soil physical composition of the area were constructed using these soil physical building blocks. The maps can be used as input for soil and water erosion models to be applied on the catchment scale. Comparison of potential runoff figures with measured data showed that the soil physical schematization appeared to be appropriate. The soil physical schem-atization in the areas studied was based on structural rather than on textural differences of the top soil.     

SOIL ERODIBILITY PARAMETERS UNDER VARIOUS CROPPING SYSTEMS OF MAIZE

For four years, runoff and soil loss from seven cropping systems of fodder maize have been measured on experimental plots under natural and simulated rainfall. Besides runoff and soil loss, several variables have also been measured, including rainfall kinetic energy, degree of slaking, surface roughness, aggregate stability, soil moisture content, crop cover, shear strength and topsoil porosity. These variables explain a large part of the variance in measured runoff, soil loss and splash erosion under the various cropping systems. The following conclusions were drawn from the erosion measurements on the experimental plots (these conclusions apply to the spatial level at which the measurements were carried out). (1) Soil tillage after maize harvest strongly reduced surface runoff and soil loss during the winter; sowing of winter rye further reduced winter erosion, though the difference with a merely tilled soil is small. (2) During spring and the growing season, soil loss is reduced strongly if the soil surface is partly covered by plant residues; the presence of plant residue on the surface appeared to be essential in achieving erosion reduction in summer. (3) Soil loss reductions were much higher than runoff reductions; significant runoff reduction is only achieved by the ‘straw system’ having flat-lying, non-fixed plant residue on the soil surface; the other systems, though effective in reducing soil loss, were not effective in reducing runoff.     

MODELLING THE IMPACT OF LAND USE CHANGE ON WATER QUALITY IN AGRICULTURAL CATCHMENTS

Export coefficient modelling was used to model the impact of agriculture on nitrogen and phosphorus loading on the surface waters of two contrasting agricultural catchments. The model was originally developed for the Windrush catchment where the highly reactive Jurassic limestone aquifer underlying the catchment is well connected to the surface drainage network, allowing the system to be modelled using uniform export coefficients for each nutrient source in the catchment, regardless of proximity to the surface drainage network. In the Slapton catchment, the hydrological pathways are dominated by surface and lateral shallow subsurface flow, requiring modification of the export coefficient model to incorporate a distance–decay component in the export coefficients. The modified model was calibrated against observed total nitrogen and total phosphorus loads delivered to Slapton Ley from inflowing streams in its catchment. Sensitivity analysis was conducted to isolate the key controls on nutrient export in the modified model. The model was validated against long-term records of water quality, and was found to be accurate in its predictions and sensitive to both temporal and spatial changes in agricultural practice in the catchment. The model was then used to forecast the potential reduction in nutrient loading on Slapton Ley associated with a range of catchment management strategies. The best practicable environmental option (BPEO) was found to be spatial redistribution of high nutrient export risk sources to areas of the catchment with the greatest intrinsic nutrient retention capacity. © 1997 by John Wiley & Sons, Ltd.     

DISTRIBUTED NUMERICAL RAINFALL–RUNOFF MODELLING IN AN ARID REGION USING THEMATIC MAPPER DATA AND A GEOGRAPHICAL INFORMATION SYSTEM

The rainfall–runoff process consists of an excess rainfall process and a runoff concentration process. A transient one-dimensional finite difference model describing the partitioning of precipitation between surface runoff, soil moisture storage and deep percolation, through the coupling of saturated–unsaturated zones, has been implemented in a geographical information system including data on vegetation cover derived from the Landsat Thematic Mapper. The model has been used to simulate both the rainfall excess and the resultant outflow hydrographs for a small arid zone drainage basin in the Andean region of Argentina. The overall hydrograph shape, peak discharge, runoff volume and flow duration are predicted within a relative squared error of 13.2%. The spatial input data and the model structure are discussed and suggestions for applications to larger complex basins and for future refinements in the technique are presented.     

MATHEMATICAL MODEL OF OVERLAND FLOW AND MECHANISM OF SOIL CONSERVATION FOR FORESTED STEEP HILLSLOPE－－（II）Mechanism of Soil Conservation Over Steep Hillslope

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SOIL SCOUR RESISTANCE AND ARRANGEMENT OF SOIL AND WATER CONSERVATION SYSTEM IN THE CONTIGUOUS AREA OF SHANXI, SHAANXI AND INNER MONGOLIA 1

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SEDIMENTATION AND CONSOLIDATION OF A HIGH WATER CONTENT SOIL

I INTRODUCTIONHigh water coment soils are often at the centre of delicate industrial and environmental problems,particularly concerning the behaviour of port or estuary mud, and deposits at sea.In soil mechanics, an understanding of the behaviour of soils is based upon the concept of effectivestress proposed by Terzaghi (1936) and defined by the difference betWeen the total stress and the porepressure. Theoretical studies (Aleds et al., 1992) highlight the effective stress as the main co…     

PREFERENTIAL WATER FLOW IN A FROZEN SOIL — A TWO-DOMAIN MODEL APPROACH

Earlier modelling studies have shown the difficulty of accurately simulating snowmelt infiltration into frozen soil using the hydraulic model approach. Comparison of model outputs and field measurements have inferred the occurrence of rapid flow even during periods when the soil is still partly frozen. A one-dimensional, physically based soil water and heat model (SOIL) has been complemented with a new two-domain approach option to simulate preferential flow through frozen layers. The ice is assumed to be first formed at the largest water filled pore upon freezing. Infiltrating water may be conducted rapidly through previously air-filled pores which are not occupied by ice. A minor fraction of water is slowly transferred within the liquid water domain, which is absorbed by the solid particles. A model validation with field measurements at a location in the middle-east of Sweden indicated that the two-domain approach was suitable for improving the prediction of drainage during snowmelting. In particular, the correlation between simulated and observed onset of drainage in spring was improved. The validation also showed that the effect of the high flow domain was highly sensitive to the degree of saturation in the topsoil during freezing, as well as to the hydraulic properties at the lower frost boundary regulating the upward water flow to the frozen soil and ice formation.     

Rapid assessment of field erosion and sediment transport pathways in cultivated catchments after heavy rainfall events

This paper explores a scale‐adapted erosion mapping method which aims at a rapid assessment of field erosion and sediment transport pathways in catchments up to several square kilometres and compares the results with the output of a well‐known erosion model (LISEM). The mapping method is based on an event‐defined classification scheme of erosion intensity (zero, weak, moderate and strong) that is applied to arable fields, in combination with incision measurements of erosion features for each erosion intensity class on a small sample of fields. Sediment deposition is classified on the basis of quantity indicators and abundance. In addition, relevant conditions and erosion factors are determined for each field. The method was applied to an agricultural catchment (4·2 km²) in the Sundgau (Alsace), after a short but violent thunderstorm in May 2001, to illustrate its potential use and its limitations. The rainfall event led to strong erosion on the arable fields and a muddy flow that caused significant damage in the built‐up area. On the basis of the analyses of the incision measurements in combination with the mapping of erosion intensity classes, total erosion for the catchment was estimated as 15 000 t (an average of about 36 t[sol ]ha). Sediment deposition was found to occur in three major locations: (1) in thalwegs at the interface between maize and downslope winter wheat fields, (2) in downslope headlands where the flow direction suddenly changed due to oriented tillage structures in the perpendicular direction, and (3) the lowest corners of fields which collect all the runoff from the field. Preliminary data analyses suggest that erosion intensity is related to field size and[sol ]or tillage direction and to slope morphology. Model output (LISEM) appeared to depend more strongly on slope gradient than the results obtained with the mapping method. The method yields a database, which can be used as a foundation for conservation strategies in small regions with similar land use and geomorphology. The mapping and modelling methods are compared, and their complementary aspects are highlighted. Copyright © 2005 John Wiley & Sons, Ltd.     

SUSPENDED SEDIMENT AND PARTICULATE PHOSPHORUS TRANSPORT AND DELIVERY PATHWAYS IN AN ARABLE CATCHMENT,GELBÆK STREAM,DENMARK

Reliable quantification of suspended sediment (SS) and particulate phosphorus (PP) transport, and identification of the various delivery pathways at the catchment level, is an important and necessary aid to appropriate catchment management. In this study we measured storm event, seasonal and annual losses of SS and PP from a Danish arable catchment, Gelbæk Stream, using a multisampling strategy. SS losses for the study years May 1993–April 1994 and May 1994–April 1995 ranged from 71 to 88 kg ha⁻¹, while PP losses ranged from 0·32 to 0·36 kg P ha⁻¹. In both cases losses mainly occurred during infrequent storm events. In comparison with intensive storm sampling, infrequent (fortnightly) sampling underestimated annual transport during the two study years by −24 and −331%, respectively, for SS, and by −8·6 and −151%, respectively, for PP. Reliable estimation of the transport of sediment and sediment-associated nutrients and other substances thus necessitates the use of an intensive monitoring approach. Turbidimeters proved to be a good substitute for direct measurement of SS, especially during storm events, although careful calibration is needed at the seasonal and storm event levels. Experience shows that in artificially drained and geologically complex catchments such as Gelbæk, simultaneous comparative monitoring of different sources (e.g. subsurface drainage water) is an important means of reliably discriminating between the various diffuse sources of sediment and phosphorus. Subsurface drainage water was found to account for 11–15% of the annual SS export from the catchment; the corresponding figure for PP being 11–18%. Surface runoff was only a source of SS and PP during the first study year, when it accounted for 19% of SS and 7% of PP catchment export. Stream bank/bed erosion must therefore have been the major diffuse source of SS and PP in both study years. The study also revealed that analysis of the trace element content (e.g. ¹³⁷Cs, ²¹⁰Pb) of the SS transported in subsurface drainage water and stream water during storm events is a useful means of discriminating between diffuse losses of SS delivered from topsoil and subsoil compartments. © 1997 by John Wiley & Sons, Ltd.     

INTERRELATIONSHIP BETWEEN WATER QUALITY AND GROUNDWATER FLOW DYNAMICS IN A SMALL WETLAND SYSTEM ALONG A SANDY HILL RIDGE

Detailed modelling of the hydrological setting of fen meadows appears to be possible provided that detailed information on geomorphology, hydrochemistry and piezometric heads is available for a number of years. In the Laegieskamp, a small wetland reserve located in the central part of The Netherlands, a piezometric monitoring network was sampled for water quality analysis and piezometric heads between 1986 and 1992. Average yearly discharge and recharge periods were used for FLOWNET calculations. First, the models were used to determine, with the help of information on water quality, the hydrological systems in the study area. Secondly, they were used to define the present and past hydrological setting of a fen meadow in the reserve. The hydrological systems and water quality in the study area have changed considerably over the past 65 years. At present the fen meadow is mainly fed by precipitation. The mineral-rich conditions favouring the fen meadow vegetation are thought to be maintained thanks to a clayey peat layer and an oscillating shallow water body that prevents rapid leaching of minerals. The sulphate content in the fen exhibits a pattern of temporal variation, which is related to the severity of the annual drought. Our study showed that groundwater flow is mainly lateral, instead of the assumed vertical infiltration of groundwater in previous regional studies. This led us to the conclusion that conservation and restoration perspectives are much better than previously expected. The polluted middle, deep groundwater is not a major threat to this fen at the moment. © 1997 John Wiley & Sons, Ltd.     

声波与表层土壤水分含量变化的实验研究

为研究地面表层土壤中水分含量变化与震前记录到微震和极微震的关系，进行了声波与表层土壤含水量变化的实验．实验的目的是探讨震前微破裂引起的声波能否改变土壤表层的水分含量，从而建立起遥感图像中的热异常信息与孕震关系．通过实验发现，声波对土壤作用使得原本自然蒸发呈降低趋势的湿度值呈上升趋势，而土壤表面的温度呈下降趋势．      

Influence of soil‐surface types on the overall runoff of the Tabernas badlands (south‐east Spain): field data and model approaches

The Tabernas desert, an extensive badlands area in Almeria province (south‐east Spain), is characterized by a high variability in soil surface cover and soil properties along with important topographical contrasts giving rise to a wide range of hydrological behaviour. A double approach through field monitoring and modelling has been used to ascertain the influence of soil‐surface variability on the overall hydrological response. Small plots were monitored for 3 years to assess runoff from the different surface types. Data provided by the long‐term monitoring of three small catchments formed by different soil surfaces were used to find out the specific contribution of each soil surface to the catchment runoff. A simple spatially distributed model was built to predict runoff generation based on the infiltration rate of each soil‐surface type (defined as terrain units with the same cover, the same soil type and on the same landform). Plot results prove that the soil surface units within the study area behave differently in terms of hydrological response to natural rainfall. These responses are explained by the types of cover, topographical characteristics and soil properties. When runoff events are simple (with one or two runoff peaks), the modelled hydrographs reproduce the hydrographs observed reasonably well, but in complex events (with several runoff peaks) the adjustment is not as good. The model also shows the influence of the spatial distribution of soil surfaces on the overall runoff, aiding exploration of the spatial hydrological relationships among different landscape units. Copyright © 2002 John Wiley & Sons, Ltd.