Initial Soil Moisture Content Research Articles

Estimativa do escoamento superficial em uma bacia hidrográfica com base em modelagem dinâmica e distribuída

A estimativa do escoamento superficial em bacias hidrográficas é de suma importância para conservação dos recursos naturais; entretanto, esse é um processo complexo e dinâmico, principalmente no contexto de sua variabilidade espacial. Dessa forma, torna-se adequada a aplicação dos Sistemas de Informações Geográficas (SIG) usando pequenas células de informação, pois assim é possível considerar o comportamento espacial das variáveis associadas à origem do escoamento superficial. Este trabalho teve como objetivo implementar os modelos hidrológicos Curva Número (CN-SCS) e Curva-Número Modificado (CN-MMS), com base na linguagem de programação do SIG PCRaster e em uma base de dados reduzida, de forma distribuída e dinâmica, com o intuito de estimar as lâminas de escoamento superficial geradas numa bacia hidrográfica de Latossolos, localizada no município de Nazareno, região dos Campos das Vertentes, Minas Gerais. Para aplicação do modelo CN-SCS foi preciso desenvolver um mapa com valores de CN no formato do PCRaster, enquanto para o modelo CN-MMS foram necessários os seguintes mapas: umidade volumétrica de saturação do solo, umidade volumétrica inicial do solo e profundidade de solo. Para simulação e avaliação de ambos os modelos, foram aplicados 18 eventos de chuva natural que provocaram escoamento superficial, durante o ano hidrológico 2004-2005, e suas respectivas lâminas de escoamento observadas. A análise do desempenho dos modelos foi feita aplicando-se análise de sensibilidade baseada no erro médio e na Raiz do Erro Quadrático (REQ). Tendo-se como referência essas estatísticas de precisão, pôde-se constatar que o modelo CN-MMS apresentou melhor calibração quando comparado ao modelo CN-SCS, devido à consideração direta da umidade inicial do solo. Contudo, a estruturação dos modelos no SIG PCRaster possibilitou o desenvolvimento de uma ferramenta computacional eficaz e útil para simulação do escoamento superficial, visto que propicia estruturação de rotinas computacionais considerando os problemas associados à variabilidade espacial dos dados de entrada dos modelos.

The impact of various sprinkler irrigation patterns on spatial soil moisture variation in Vertisols

The objective of this research was to assess the effect of soil cracks on soil moisture distribution under various sprinkler irrigation applications and to identify the optimal irrigation strategy that enhances soil moisture distribution and reduces water drainage for the upper soil layer 0–250 mm. The assessment was made for six irrigation events: the first two were for 10 and 46 mm water applications using a hand shift-set sprinkler system. The second set was for 43 and 19 mm water applications using the lateral move system with fixed sprayer heads and the third pair of events were for 43 and 32 mm water applications using the lateral move system with rotating sprinklers. The experiments were conducted on two adjacent fields at the University of Queensland, Gatton, Australia. Each field was divided into 2 m × 2 m grids that covered 62 sampling locations. For each event, the initial soil moisture content (SMC) was measured at each sampling location before irrigation. After irrigation, catch can readings were recorded for each sampling location. After 12 h overnight, the second set of soil moisture measurements was taken at each location. The area1 distribution of SMC for the studied applications was quantified. An attempt was made to identify the relationship between the applied water uniformity using catch cans and the soil moisture uniformity using gravimetric water content measurements. The study also took into consideration variables that could affect the soil physical and hydrological properties including the field slope, the soil texture, the infiltration rate, the salt content and the soil organic matter content of the two fields. Since the soils were cracking clay Vertisols, further analyses were conducted on the crack dynamics, size and distribution using image analysis techniques. The research findings demonstrated that the cracks were the main contributors to water drainage below 250 mm soil depth due to the micro-run off from the crust surface to the cracks. The cracks ranged from a few millimeters to more than 40 mm in width. It was observed that the cracks which were wider than 15 mm remained open after irrigation for the specified application rates. Improving the irrigation system application uniformity did not always result in higher uniformity of the surface SMC (0–250 mm). The event that best enhanced soil moisture distribution and thus improved soil moisture recharging was observed after the sixth irrigation event when the field received 32 mm water application. The soil was at a relatively high initial SMC of 25%, (which represented 43.3% of the plant available water range) and the sprinkler water uniformity was rather high above 87% Christiansen coefficient of uniformity (CUc). At this SMC, the extent of soil cracking is limited.

Effect of the initial soil moisture content on the spatial distribution of the water retention

The initial soil moisture content affects the water flow and solute transport through the vadose zone, but researchers are in disagreement about the extent and nature of its effects. Better understanding of the initial moisture effect on the water movement will help to prevent groundwater contamination and increase crop production by improving the efficiency of water use in irrigation practice. Therefore, in this study, the effect of the initial moisture content on the spatial distribution of the water retention was investigated in the field. A total of 4 cm of water was applied to duplicate plots with each of three initial moisture conditions within 2 h using a rainfall simulator. Following the application and a 2-h redistribution period, 100 soil samples were taken from different depths of each plot using a grid sampling system to be analyzed for their gravimetric water content in the laboratory. Statistical and geostatistical analyses were performed to analyze the spatial structure of the collected data. The results showed that the preferential flow was more evident in the case of the dry initial soil water content than for the two wetter initial conditions. Both the classical and geostatistical analyses supported that the overall water retention was uniformly distributed throughout the profile except at 20–30 cm, where the coefficient of variation and the percent nugget to total semivariance ratio were high, indicating some degree of preferential flow through large pores (macropores). These results suggest that similar studies should be conducted on different field soils under more different initial moisture conditions so that the effect of the macropores on the water flow and chemical transport can be better understood.

Relationship between precipitation and the infiltration depth over the middle and lower reaches of the Yellow River and Yangtze-Huaihe River Valley

The purpose of the paper is to extract and make the best use of rainfall information contained in the Chinese historical archives of Qing Dynasty, and reconstruct the precipitation series during 1736–1911. The study followed the Yu-Fen-Cun (rainfall infiltration depth) observation method in Qing Dynasty, to conduct soil infiltration experiment under the natural rainfall conditions during 2004–2006. The related parameters, such as precipitation, intensity, initial soil moisture content and infiltration depth have been measured at 10 stations (east of 105°E, 30°–40°N) with representative climatic conditions and soil physical texture in the middle and lower reaches of the Yellow River and Yangtze-Huaihe River Valley. The statistical results indicate that precipitation is the primary influencing factor to infiltration depth and is positively correlated with the infiltration depth. Additionally, the effects of every factor on infiltration depth are significantly different between the middle and lower reaches of the Yellow River and Yangtze-Huaihe River Valley. Finally, the relationship equations of precipitation and infiltration depth are established for each site, most of which can pass over 60% of the variance explanations and provide quantitative models to reconstruct the precipitation series using Yu-Fen-Cun records in Qing Dynasty.

Environmental Controls on the Fate of Escherichia coli in Soil

An improved understanding of factors that influence the survival and/or growth of Escherichia coli (E. coli) in soil is essential to allow the formation of land management practices to control the spread of the pathogenic strains of the bacteria, whose transmission to fresh produce is a threat to food safety. Persistence of E. coli in soils held at different water potentials and with carbon additions then subjected to post-freezing incubation temperatures and in the presence of Klebsiella terrigena (K. terrigena) were investigated. Soil samples adjusted to different water potentials (−0.03, −0.1 and −1.5 MPa) were inoculated with a multi-antibiotic resistant strain of E. coli (E. coli 2+), which allowed recovery of the organism from soil samples. In addition to manipulation of water content, different C levels were added and samples were frozen for varying lengths of time, thawed and incubated. In freezing studies, initial soil moisture content significantly affected E. coil 2+ survival in soils following thawing, resulting in lower survival rate (k) at water potential of −0.03 than at −0.1 and −1.5 MPa. The effect of length of freezing time was significant only at −0.03 MPa. Glucose addition at 1.25 mg C g−1 improved survival rate versus glucose at 0.125. The low level glucose increased die-off rate versus no addition, suggesting that unless amendments provide C above a certain threshold level, they might facilitate the death of the bacteria. E. coli 2+ survival improved in the presence of K. terrigena at 6°C but not at 23°C. Persistence of E. coli under the interactive influence of various environmental factors highlights the urgency and importance of understanding its potential for transmission to fresh produce and water bodies.

Automation and Use of Mini Disk Infiltrometers

Measurement replication and objectivity of field soil hydraulic properties can be increased through automation. The goals of this study were to test two automated mini disk infiltrometers (AMDI). Both devices were fitted with differential pressure transducers connected to compact data loggers. Instrument design, method of calculation, and soil moisture condition all affected measured unsaturated hydraulic conductivity [K(h)] and sorptivity (S) at pressure head h = −2.0 cm. We found that the type of AMDI with a capillary tube head control can be operated at inclination angles up to 25° and returned the least variance in K(h) and S if data were not partitioned for calculation. Changing the initial soil moisture content from 0.07 to 0.26 m3 m−3, however, was found to influence K(h) calculation by up to 50% for a silt loam soil. The K(h) measured by the type of AMDI with a bubble chamber head control and larger disk diameter was less dependent on soil moisture content, but more sensitive to inclination.

Development and testing of a simple physically-based distributed rainfall-runoff model for storm runoff simulation in humid forested basins

A distributed rainfall-runoff model was developed to predict storm runoff from humid forested catchments. The model is physically based and takes into account the saturation excess overland flow mechanism and preferential subsurface flow. The watershed is discretized into a number of square grids, which then are classified into overland flow and channel flow elements based on water flow properties. On the overland elements, Infiltration, overland flow and lateral subsurface flow are estimated, while on channel flow elements river flow routing is performed. Lateral subsurface flow is calculated using Darcy’s law and the continuity equation, whereas overland flow and channel flow are modeled using a one dimensional kinematic wave approximation to the St. Venant equations. The model governing equations are solved by an implicit finite difference scheme. While using process-based equations and physically meaningful parameters, the model still maintains a relatively simple structure. Most of the model parameters can be derived from digital elevation models (DEMs), digital soil and land use data, and the remainder of the parameters that are comparatively sensitive can be determined by model calibration. The model is tested using nine storm events in the Jiaokou watershed, a sub-basin of Yongjiang River in Zhejiang Province, China. Of these storms, one storm is used for calibrating the model parameters and the remaining eight storms are used to verify the model. When judged by the model efficiency coefficient (R2), volume conversation index (VCI), absolute error of the time to peak (ΔT), and relative error of the peak flow rate (δPmax), acceptable results are achieved. Sensitivity analysis shows that the model is sensitive to saturated hydraulic conductivity (Ks), Manning’s roughness coefficients (n) and the initial soil moisture content.

Estimation of Kostiakov Infiltration Parameters Using Initial Soil Moisture

The Kostiakov equation was extensively used to estimate furrovv Infiltration but it is unadjusted for different field conditions. The Kostiakov infiltration parameters (k and a) were therefore developed as a function of initial soil moisture content and irrigation events in current study. The field experiments comprised two types trials. The first was infiltration measuring at 56 sites över the field for the first irrigation event. The second measurements were at the furrovvs along the same field for five successive irrigation events. İnfiltration measurements were taken by blocked furrovv infiltrometers. Results showed that the averages of k ranged from 272.84 to 733.991 mlm'1min'a and for a were from 0.614 to 0.815. The parameter a was described as a non-linear function of initial soil moisture content and as a logarithmic function of k. The parameter k was developed as a non-linear function of initial soil moisture and irrigation events. Cumulative infiltration from five successive irrigation events were correlated to each others and according to the results cumulative infiltration from each irrigation event can be predicted from the other previous irrigation events.

A modified Richards equation, its adjoint, and a new perspective on land data assimilation

Richards equation, which is the key relationship for addressing basic soil hydrological processes within the rhizosphere and constitutes the basis for multiple-layer soil hydrology models for meteorological studies, is modified to include two vegetation processes: the transpiration (daytime) and hydraulic lift (nighttime). The analytical form of the adjoint system is derived and can serve as the guideline for discrete forms, as implemented in a variety of multiple layer land surface models. The modified equation and its adjoint model may be used to address at least two types of inverse problems: estimation of the initial soil moisture contents from sparse observations of related quantities; and retrieval of critical soil/vegetation parameters with observations of soil moisture contents (or other related quantities) distributed over a certain period of time. The former may be of more interest for numerical weather prediction studies, whereas the latter may be more valuable for longer time integration of climate models.

Effect of Soil Moisture and Irrigation on Propargyl Bromide Volatilization and Movement in Soil

Propargyl bromide (3‐bromo‐propyne, 3BP) is a potential replacement for the soil fumigant methyl bromide. Since little is known about its movement in soil, a study was conducted to compare the volatilization and movement of 3BP in the soil profile for different irrigation treatments. A rectangular soil column was used to simulate a bed–furrow system. The surface of the bed was covered with high‐density polyethylene (HDPE) plastic (i.e., a tarp). The furrow was left uncovered. Multiple volatilization chambers were used to measure emissions from the furrows, the slopes of the bed, and the bed. The soil was fumigated by injecting 1.0 mL of 3BP to the center of the column. Three treatments were studied, no irrigation, a single 5‐h surface irrigation 24 h after fumigation, and a 2‐h daily surface irrigation. Volatilization was about three times greater from nonirrigated soil. Irrigation and higher initial soil moisture content were more effective in controlling 3BP volatilization than the use of a HDPE tarp. Volatilization and degradation were similar for both irrigation treatments, but the 2‐h irrigation had the advantage of requiring one‐third less water. Volatilization rates from the slopes of the bed were lower than from the bed surface. To obtain accurate total mass, volatilization chambers should cover the whole bed–furrow system. Short advective gas and liquid fluxes created by the irrigation had pronounced and prolonged effect on 3BP distribution and degradation. Henry's Law could not be used to predict the 3BP distribution pattern in the liquid phase even long after the irrigation ceased.

Effect of Soil Moisture and Irrigation on Propargyl Bromide Volatilization and Movement in Soil

Propargyl bromide (3-bromo-propyne, 3BP) is a potential replacement for the soil fumigant methyl bromide. Since little is known about its movement in soil, a study was conducted to compare the volatilization and movement of 3BP in the soil profile for different irrigation treatments. A rectangular soil column was used to simulate a bed–furrow system. The surface of the bed was covered with high-density polyethylene (HDPE) plastic (i.e., a tarp). The furrow was left uncovered. Multiple volatilization chambers were used to measure emissions from the furrows, the slopes of the bed, and the bed. The soil was fumigated by injecting 1.0 mL of 3BP to the center of the column. Three treatments were studied, no irrigation, a single 5-h surface irrigation 24 h after fumigation, and a 2-h daily surface irrigation. Volatilization was about three times greater from nonirrigated soil. Irrigation and higher initial soil moisture content were more effective in controlling 3BP volatilization than the use of a HDPE tarp. Volatilization and degradation were similar for both irrigation treatments, but the 2-h irrigation had the advantage of requiring one-third less water. Volatilization rates from the slopes of the bed were lower than from the bed surface. To obtain accurate total mass, volatilization chambers should cover the whole bed–furrow system. Short advective gas and liquid fluxes created by the irrigation had pronounced and prolonged effect on 3BP distribution and degradation. Henry9s Law could not be used to predict the 3BP distribution pattern in the liquid phase even long after the irrigation ceased.

Perdas de nitrogênio por volatilização da amônia em três solos argilosos tratados com uréia

The soil nitrogen (N) losses in gaseous form depend on their intrinsic characteristics and also on the environmental conditions. In soils with the same texture class, the mineralogy of the clay size fraction and the amount of organic matter are the main factors that may affect the N dynamics. In the State of Paraná northerner region, variations in these characteristics are found in the toposequence of soils derived from basalts where, in the landscape upper part, highly weathered soils, such as the Latosols, are found and, in the other sections, younger soils, such as Neosols and eventually Vertisols, are found. The aim of the present work was to evaluate the N volatilization losses from ammonia, using three clayey soils (Red Dystroferric Latosol-LVdf, Litholic Neosol-RL and Ortic Hydromorphic Vertisol-VGo) submitted to two soil moisture contents (S8 and SS) and treated with urea. The ammonia volatilization of the applied urea was evaluated in 2000cm3 plastic recipients that received 500g of soil and 200mg of urea, for an accumulative period of 10 days. Paper filter collectors, previously humidified with H2SO4 measured the N-NH3 losses, which were placed 5cm above the soil surface. The results showed that the soils presented differences in the ammonia losses, related to the initial soil moisture contents and the mineralogy of the clay size fraction. In the lower moisture treatment (S8) the least weathered soils (RL and VGo) showed the lowest ammonia losses, due to the higher percentages of 2:1 clay type minerals. In the soil-saturated treatment the N-NH3 losses were not statistically significant among the soils

Dependence of unfrozen water content in unsaturated frozen clay soil on initial soil moisture content

The unfrozen water content in an unsaturated frozen light clay soil having a range of initial soil moisture contents was measured independently using TDR and NMR in order to investigate the relationship between the unfrozen water content within a frozen soil and the initial soil moisture content. Unfrozen water contents measured by TDR and NMR were highly correlated and it was evident that the amount of unfrozen water in contact with ice at a given temperature was in proportion to the initial soil moisture content significantly. Since the effect of solutes in soil on the osmotic potential was negligible, a plausible interpretation for the dependence of the unfrozen water content on the initial soil moisture content was assumed to be the matric potential of unfrozen water being affected by both soil particles and ice.

Preferential Transport of Soil Colloidal Particles: Physicochemical Effects on Particle Mobilization

The quantification of particle transport through soil is of great importance for estimating the potential risk of adsorbing contaminants leaching into groundwater. In the present study, we investigated the mobilization of natural soil particles in an undisturbed soil column (diameter = 0.3 m, height = 0.66 m). We tested the effects of physicochemical properties of soil and infiltrating water on the mobilization and transport of soil particles. A square pulse of water was applied at the top of the column. Water was allowed to drain freely at the bottom of the column. We tested two rainfall intensities (11 and 23 mm h−1), three ionic strengths (10−5, 10−3, and 10−1 M), and two initial moisture contents (0.34 and 0.38 m3 m−3). For the whole set of infiltration experiments, the concentration of eluted particles was correlated with the drainage flow intensity, particularly during transient flow. Particle leaching during steady flow varied with the boundary and initial conditions. The highest mobilization of particles was observed for deionized water, the highest infiltration rate and the highest initial soil moisture content. Particle mobilization was limited for high ionic strength associated with the divalent cation Mg2+ During transient flow, mechanical detachment by hydrodynamic shear could lead to particle mobilization. During steady flow, the ionic strength of the incoming solution may alter the energy potential at the soil–water interface, and thus have an effect on the mobilization rate as well.

Water dynamics in soil-plant systems under grain farming in Northern Kazakhstan

The water dynamics and budget in soil-plant ecosystems under dry farming in northern Kazakhstan were investigated for two consecutive years from autumn in 1998 to the end of the cropping season in 2000. In total, 12 plots were established at the experimental farm of Barayev Kazakh Research and Production Center of Grain Farming, Shortandy, and the soil moisture content up to the 90 cm depth was measured several times throughout the period. In spite of snow management during the winter time, in which parallel snow rows were developed in order to accumulate additional snowfall between the rows, the increase in the soil moisture content at the time of thawing widely ranged from -40 to 74 mm in 1999 and from -6 to 84 mm in 2000, respectively. Monitoring of the soil temperature revealed that, in the plots after fallow, a higher moisture content in the frozen subsurface soil layer was responsible for the slow thawing there, resulting in slower water percolation from the overlying layers of the profile and 0n water loss through enhanced evaporation and possible surface runoff. After thawing, the soil moisture content decreased throughout the cropping season, except during several rainfall events. The evapotranspiration was estimated to range between 194 and 259 mm during the cropping season. The bNonmass and yield of wheat at harvest time were linearly correlated with the estimated evapotranspiration, indicating that crop production here was mostly determined by the amounts of available water. The initial soil moisture content accounted for 27 to 52% of the total evapotranspiratiou. In the summer fallow plots, 39 to 104 mm more water accumulated in 1999 and 100 to 119 mm in 2000 than in the cropped plots, respectively. Comparison of the water budgets during the pre-cropping and cropping seasons in the plots under fallow and cropping revealed that both summer fallow and snow management could increase the soil moisture content up to approximately 100 mm, but that the benefit of snow management would be occasionally canceled by the effect of the summer fallow. Given the possibly adverse effects of the summer fallow on enhanced decomposition of soil organic matter, we recommend that snow management should be the main approach for capturing water in the studied plots rather than the summer fallow practice. Further studies should be carried out to determine whether soil and /or topographical conditions are more effective for individual water-capturlng management and also are more suitable from economic and environmental viewpoints.

Preferential Transport of Soil Colloidal Particles: Physicochemical Effects on Particle Mobilization

The quantification of particle transport through soil is of great importance for estimating the potential risk of adsorbing contaminants leaching into groundwater. In the present study, we investigated the mobilization of natural soil particles in an undisturbed soil column (diameter = 0.3 m, height = 0.66 m). We tested the effects of physicochemical properties of soil and infiltrating water on the mobilization and transport of soil particles. A square pulse of water was applied at the top of the column. Water was allowed to drain freely at the bottom of the column. We tested two rainfall intensities (11 and 23 mm h−1), three ionic strengths (10−5, 10−3, and 10−1 M), and two initial moisture contents (0.34 and 0.38 m3 m−3). For the whole set of infiltration experiments, the concentration of eluted particles was correlated with the drainage flow intensity, particularly during transient flow. Particle leaching during steady flow varied with the boundary and initial conditions. The highest mobilization of particles was observed for deionized water, the highest infiltration rate and the highest initial soil moisture content. Particle mobilization was limited for high ionic strength associated with the divalent cation Mg2+ During transient flow, mechanical detachment by hydrodynamic shear could lead to particle mobilization. During steady flow, the ionic strength of the incoming solution may alter the energy potential at the soil–water interface, and thus have an effect on the mobilization rate as well.

Analysis and Modeling of an Extremely Dense Fog Event in Southern Ontario

In this study, a dense fog episode that occurred near Windsor, Ontario, Canada, on 3 September 1999 is investigated. The fog patch, with a spatial scale of several kilometers, reduced visibility on a major highway to a few meters and led to a series of collisions and loss of life. Satellite imagery and surface observations are used to analyze the physics of the event, and several hypotheses on the origin of the fog are presented. A series of simulations of the event with the fifth-generation Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model (MM5) suggest that the fog formed because of convergence of land breezes developing along the shores of a lake and subsequent advection of moisture over the site of the accident. Tests indicate that the small scale of the modeled event contributes to sensitivity of the results to a broad range of factors. Sensitivity to the initial and boundary conditions, including initial soil moisture content and parameterization of turbulence, is discussed.

Preferential Transport of Soil Colloidal Particles

The quantification of particle transport through soil is of great importance for estimating the potential risk of adsorbing contaminants leaching into groundwater. In the present study, we investigated the mobilization of natural soil particles in an undisturbed soil column (diameter = 0.3 m, height = 0.66 m). We tested the effects of physicochemical properties of soil and infiltrating water on the mobilization and transport of soil particles. A square pulse of water was applied at the top of the column. Water was allowed to drain freely at the bottom of the column. We tested two rainfall intensities (11 and 23 mm h−1), three ionic strengths (10−5, 10−3, and 10−1 M), and two initial moisture contents (0.34 and 0.38 m3 m−3). For the whole set of infiltration experiments, the concentration of eluted particles was correlated with the drainage flow intensity, particularly during transient flow. Particle leaching during steady flow varied with the boundary and initial conditions. The highest mobilization of particles was observed for deionized water, the highest infiltration rate and the highest initial soil moisture content. Particle mobilization was limited for high ionic strength associated with the divalent cation Mg2+ During transient flow, mechanical detachment by hydrodynamic shear could lead to particle mobilization. During steady flow, the ionic strength of the incoming solution may alter the energy potential at the soil–water interface, and thus have an effect on the mobilization rate as well.

A review of two strongly contrasting geomorphological systems within the context of scale

AbstractScale issues are often addressed in contemporary geo‐ecological studies and form one of the major challenges in the fields of physical geography, hydrology and ecology. In this paper the application of hierarchy theory and response units is proposed as an approach towards scale‐transcending environmental studies on degradation and geomorphological development.Goals of the research were to establish which processes are important at what spatio‐temporal scale, how hydro‐geomorphological response is influenced by biological processes and whether hierarchy theory and the response unit approach can be used as an up‐scaling methodology.Results from two climatologically and geomorphological different regions are discussed, one dominated by water shortage (SE Spain) and the other by water surplus (Luxembourg). In both cases detailed process research was carried out at scales ranging from the micro‐plot to the catchment. Process research was concentrated on understanding and quantifying sediment and water transfer through the geo‐ecosystems studied.Outcomes showed that in both cases the role of biological processes was important in the hydrological and degradation response of both areas. This was not only true for the finest scale levels but also had its impact on the emerging properties and response at the hillslope and catchment level.Connectivity of runoff‐generating and runoff‐absorbing areas was important on all scale levels. Connectivity is dominated by both the rainfall magnitude–frequency–duration characteristics and physically and biologically controlled thresholds, which range from initial soil moisture contents, vegetation patterns or soil biological activity, to the presence of water harvesting structures.The complex interrelationships of the processes involved showed that linear up‐scaling from fine to broad scale is impossible, as many thresholds and non‐linear processes are involved at specific scales. The identified response units are used to integrate these complex relationships in a relatively manageable way, and may provide a useful framework for up‐scaling, and for understanding catchment hydro‐geomorphological response and development. Copyright © 2002 John Wiley & Sons, Ltd.

Spatial and temporal variations in resistance of loess‐derived soils to ephemeral gully erosion

SummaryPlanners would like a simple means of describing spatial and temporal variations in soil erodibility accurately. We have done a series of concentrated flow detachment experiments to investigate the feasibility. Four different soil horizons, typical of loess‐derived soils in Belgium, were sampled seven times during one year, so that a representative range of initial soil moisture contents was obtained. Undisturbed soil samples were subjected to five different combinations of slope gradient and concentrated flow discharge. Results showed that for a given soil horizon, variations in detachment rate could be related well to temporal variations in initial soil moisture content. For a given initial soil moisture content the ploughed topsoil horizon (Ap) and the underlying clay‐enriched horizon (Bt) had only one fifth of the erodibility of the loess horizon whether decalcified (C1) or still calcareous (C2). Combining knowledge on spatial distribution of soil profiles and initial soil moisture content allowed us to explain observed spatial and temporal variations in resistance to ephemeral gully erosion for soils in loess. Also, differences in ephemeral gully morphology (cross‐sections) could be explained from differences in initial soil moisture content and soil horizon. In the short term these results have important implications for spatial and temporal variations in erosion, while in the medium or long term information on spatial distribution of soil profiles is crucial when predicting the volumes and patterns of (ephemeral gully) erosion. Finally, the importance of combining the effect of water and tillage erosion with respect to soil profile evolution and consequent erosion risk is stressed.