Antecedent Moisture Content Research Articles

Development of Unstable Flow and Reduced Hydraulic Conductivity due to Water Repellence and Restricted Drainage

The effect of water repellence and antecedent soil moisture on wetting front stability and infiltration rate are reported for a seasonally water repellent topsoil. The effect of water repellence on infiltration was determined by comparing the in situ infiltration of water to that of a 7M ethanol solution. Wetting front stability was measured during infiltration of water into repacked, wettable and water repellent soils, within a Hele‐Shaw chamber. Water repellence restricted in situ movement of water through large macropores (>500 μm), which decreased intrinsic permeability by 1 to 2 orders of magnitude. In repacked soils, water repellence caused the development of unstable wetting fronts and reduced infiltration from 240 mm h−1 to 101.7 mm h−1. Infiltration into wettable soils at moisture contents near field capacity was expected to result in rapid infiltration and stable wetting fronts. However in repacked soils, wetting front instability developed, and infiltration rates were 190% lower when air and/or water movement through the base of the chamber was restricted. Infiltration into in situ soil was also slower at high antecedent soil moisture. The hydraulic conductivity of the 7M ethanol solution decreased significantly from 112.3 mm h−1 in dry water repellent conditions, to 35.6 mm h−1 in wettable soils at high antecedent moisture contents. Consequently the previously reported development of wetting front instability and reduced infiltration into in situ wettable soils at high moisture contents were confirmed and attributed to difficulty displacing existing soil water during infiltration of new water.

Determining the frequency, depth and velocity of preferential flow by high frequency soil moisture monitoring

Preferential flow in agricultural soils has been demonstrated to result in agrochemical mobilisation to shallow ground water. Land managers and environmental regulators need simple cost effective techniques for identifying soil - land use combinations in which preferential flow occurs. Existing techniques for identifying preferential flow have a range of limitations including; often being destructive, non in situ, small sampling volumes, or are subject to artificial boundary conditions. This study demonstrated that high frequency soil moisture monitoring using a multi-sensory capacitance probe mounted within a vertically rammed access tube, was able to determine the occurrence, depth, and wetting front velocity of preferential flow events following rainfall. Occurrence of preferential flow was not related to either rainfall intensity or rainfall amount, rather preferential flow occurred when antecedent soil moisture content was below 226 mm soil moisture storage (0-70 cm). Results indicate that high temporal frequency soil moisture monitoring may be used to identify soil type - land use combinations in which the presence of preferential flow increases the risk of shallow groundwater contamination by rapid transport of agrochemicals through the soil profile. However use of high frequency based soil moisture monitoring to determine agrochemical mobilisation risk may be limited by, inability to determine the volume of preferential flow, difficulty observing macropore flow at high antecedent soil moisture content, and creation of artificial voids during installation of access tubes in stony soils.

Soil erosion and hydrology of the western Mediterranean badlands throughout rainfall simulation experiments: A review

Abstract Rainfall simulation experiments are widely used in geomorphological research in badland areas. This technique contributed significantly to our understanding of badland geomorphology in the Mediterranean belt. Due to the different types of simulated rainfall applied and the variable size of the plots a review of the State-of-the-Art is necessary. This study confirmed that rainfall simulations are well suited to (i) analyse runoff-infiltration processes and sediment detachment within badlands, and (ii) to establish the factors determining the hydrological and erosion response at interrill scale in badland areas. The hydrological response of badlands is characterized by rapid responses, low to moderate infiltration rates (ranging from close to 0 to 55 mm h − 1 ) and high runoff coefficients (i.e. > 60%). Positive relationships are observed between antecedent soil moisture content and rainfall intensity on the one hand, and runoff coefficients on the other. Sediment concentration and soil detachment rates are among the highest measured on the Mediterranean region (i.e. up to 84.8 g l − 1 and 4722.4 g m − 2 h − 1 ). Results show that rainfall intensity, runoff coefficient, and slope angle have a positive influence on sediment concentration and sediment detachment; in the case of rock fragment cover, its influence is variable according to the soil cover percentage. Rainfall simulations increase our understanding of the temporal and spatial variability of the soil erosion processes. However, rainfall simulation provides mostly qualitative rather than quantitative information because of the small plot size and the simulated rainfall characteristics due to facility for repeating experiments and recording a high quantity of data in short time. However, this method allows the comparison amongst different environments and soil conditions as the rainfall properties can be reproduced.

Effect of rainfall intensity, slope and antecedent moisture content on sediment concentration and sediment enrichment ratio

Abstract The transportability of sediment by runoff and its potential for subsequent deposition is dependent on the size and distribution of the transported material and the sediment concentration. The size distribution of the eroded sediment is also expected to vary depending on which erosion process is predominant. This study discusses the results of laboratory analysis to evaluate the effect of rainfall intensity, slope steepness, soil types and antecedent moisture content on sediment concentration, runoff coefficients, and sediment enrichment ratios. To achieve the objectives, laboratory experiments were conducted using FEL-3 rainfall simulator and erosion pan. Rainfall intensity of 120, 70, and 55 mm/h was applied sequentially at 9, 25 and 45% slope for three soil types found in Alemaya watershed, eastern Ethiopia Alemaya Black soil (Soil A), Regosols (Soil B), and Cambisols (Soil C) varied from clay to sandy clay loam in texture with wet and dry antecedent moisture contents. The rate of sediment concentration significantly varied with moisture contents. For Soil A, sediment concentrations (24.3 g/l) from an initially air-dry surface and (18.7 g/l) from an initially wet surface were observed. Wetting decreased sediment concentration of Soil A, B and C by 23, 45.7 and 1.3%, respectively. Sediment concentration didn't show any trend with rainfall intensities. For Soil B, the highest average silt enrichment ratio of 2.15 was observed followed by average clay enrichment ratio of 1.61. For Soil A and Soil C, average clay enrichment ratios of 1.13 and 1.91 were observed, respectively. Initial moisture contents had effect on the enrichment ratio, however, the actual effect varied with soil type and the size of the particle. Results of the study indicated that the actual effect of slope as well as rainfall intensity on sediment concentration and sediment yield vary with soil types and moisture contents.

Infiltration models for soil profiles bounded by a water table

Approximate solutions of Richards' equation are developed for rational forms of the soil hydraulic conductivity and moisture retention functions using a perturbation expansion method. A novel approach is devised in the perturbation technique whereby a priori introduced parameters are estimated in an optimal context to ensure the accuracy of the perturbation solution at all times and for all flow conditions. The perturbation model describes the one‐dimensional nonlinear nonsteady infiltration for arbitrary initial conditions under either a time‐dependent surface flux or a constant head boundary condition into either a semi‐infinite or a finite medium bounded from below by a shallow water table. The optimal perturbation approach proved to be an effective method by which closed‐form linear solutions can be adapted to nonlinear soils through proper scaling factors. The results show that the first‐order linear term of the optimal perturbation solution is sufficiently accurate for practical applications, especially for shallow water table conditions. They offer the means to analyze the effect of nonuniform initial conditions and the influence of the water table on the infiltration process. Practical results in terms of well‐defined soil water parameters and the antecedent moisture content include algebraic expressions for the surface moisture content variation under variable rainfall conditions, time‐to‐ponding expressions and infiltration equations for deep and shallow water table conditions, and a prediction formula for the water table recharge rate.

Rainfall-induced landslides in the residual soil of andesitic terrain, western Japan

Rainfall triggered landslides are frequent problems in the residual soil of andesitic terrain in western Japan. Characteristics of residual soils over bronzite andesite, procedure of in situ permeability measurement, matric suction and soil moisture content change and stability analyses considering unsaturated-saturated soils as integral system are presented in this paper. The paper highlights two landslides of small andesitic hillock of western Japan and describes modelling of rainwater seepage, slope stability analysis and contributing parameters for landsliding in andesitic terrain. For both landslides, results of geomorphological and geotechnical analyses were used as a direct input to the numerical modelling. For transient conditions, a finite element analysis was used to model the fluctuations in pore water pressure during the rainfall, with the computed hourly rainfall rate as the surface boundary condition. This was then followed by the slope stability analysis using the temporal pore water pressure distributions derived from the seepage analysis. Obtained trend for the factor of safety indicates that the most critical time step for failure was a few hours following the antecedent moisture content of previously day peak rainfall. Time of failure estimated by modelling has shown good match with time declared by eyewitnesses.

Quantitative assessment of the piping erosion susceptibility of loess-derived soil horizons using the pinhole test

The pinhole test is an empirical test based on the qualitative evaluation of the dispersivity (colloidal erodibility) of compacted fined-grained soils. This paper evaluates the pinhole test device for the quantitative assessment of the susceptibility of soil horizons to piping, assesses the effects of hydraulic head, water quality, antecedent soil moisture content and soil disturbance on the hydrological and erosion response during pipeflow, and formulates recommendations for the application of the pinhole test to study piping erosion processes. In total, 255 pinhole test experiments were conducted. The results obtained from disturbed soil samples (A p horizon) indicate: (i) a significant positive linear increase in pipeflow discharge ( Q w) and sediment discharge ( Q s) with increasing hydraulic head, (ii) no relationship between Q w and antecedent soil moisture content ( ASM), but significant and negative relationship between Q s and ASM, and (iii) significantly higher Q s for distilled water than for tap water. The use of disturbed soil samples resulted in significantly more rapid flow responses and lower sediment discharges than for undisturbed soil samples. The pinhole test experiments performed on different loess-derived soil horizons show that the plowed topsoil (A p) and the underlying clay-enriched horizon (B t) are at least two times less susceptible to piping than the decalcified (C 1) and the calcareous horizons (C 2). This confirms field observations indicating that loess-derived soil horizons are very susceptible for piping erosion, especially when the C horizon crops out. This study demonstrates that the pinhole test is suitable for assessing the susceptibility of soil horizons to piping in a quantitative way (i.e. Q w, Q s, the time to outflow and the cross-section of the pipe after the test). We recommend the use of: (i) two different hydraulic heads (i.e. 180 and 1020 mm), (ii) distilled water, (iii) contrasting antecedent soil moisture contents of the soil samples to be tested, and (iv) different soil sampling methods to investigate the effect of soil disturbance, and particularly to obtain a better understanding of the sediment response during pipeflow.

Infiltration models for semi‐infinite soil profiles

Infiltration models for hydrological applications are developed for rational forms of the soil hydraulic conductivity and moisture retention functions. The models are based on the traveling and kinematic wave solutions of the governing equation that proved to be highly accurate in specific flow conditions. The approximate solutions describe the one‐dimensional infiltration into a semi‐infinite medium for uniform initial conditions under either a time‐dependent flux or a constant head boundary condition. They capture the profile development from early to late time accurately due to the proper derivation of the wetting front speed in terms of the time‐varying surface boundary potential and flux rate. Algebraic expressions of theoretical and practical importance are derived in terms of well‐defined soil water parameters and include the effect of the antecedent moisture content in a specific fashion. Practical results include simple expressions for the surface moisture variation for variable rainfall patterns, time‐to‐ponding expressions and infiltration equations that account explicitly for the initial condition, formulas for the depth of the wetting zone, and algebraic equations for parameter estimation of the soil hydraulic parameters by inverse analysis.

Identifying controls of the rainfall–runoff response of small catchments in the tropical Andes (Ecuador)

Summary Tropical mountain regions are characterized by strong spatial climate gradients which together with the limited amount of data and knowledge of the underlying processes hinder the management of the water resources. Especially for regional-scale prediction it is important to identify the dominant factors controlling the rainfall–runoff response and link those to known spatial patterns of climate, soils, and vegetation. This study analyzes the rainfall–runoff relation of 13 intensively monitored micro-catchments in the Andes of southern Ecuador. The results of this study show that streamflow in the southern cordillera of the Ecuadorian Andes, above 2500 m a.s.l., primarily consists of subsurface flow. The yearly amount of streamflow is controlled by the annual rainfall depth, whereas the temporal distribution is mainly governed by the lateral saturated hydraulic conductivity, the soil water retention and the antecedent soil moisture content. Anthropogenic effects were found insignificant, with the exception in one of the studied micro-catchment. Effect of land use changes in most of the micro-catchments did not reflect in the shape of the flow duration curve because either the spatial extent of human impact was small and/or the overall basin slope was less than 20%.

The effect of slope steepness and antecedent moisture content on interrill erosion, runoff and sediment size distribution in the highlands of Ethiopia

Abstract. Soil erosion is a two-phase process consisting of the detachment of individual particles and their transport by the flowing water. This study discusses the results of laboratory experiments in which for three soils, the runoff depth, sediment yield, splash erosion and sediment size were measured. Rainfall intensity, slope and antecedent moisture contents were varied in the experiment. The soil types ranged from clay to sandy clay loam (Alemaya Black soil, Regosols and Cambisols). Rainfall was applied for six sequential 15-min periods with rainfall intensities varying between 55 and 120 mm h−1. The three slopes tested were 9, 25, and 45 %. Results show that as slope increased from 9 to 25 %, splash erosion and sediment yield increased. An increase in slope from 25 to 45 % generally decreases in splash erosion. Sediment yield for one soil increased and one soil decreased with slope and for the third soil the trend was different between the two initial moisture contents. Sediment yield was correlated (r = 0.66) with runoff amounts but not with splash erosion. Interrill erosion models that were based on the flowing water and rainfall intensity fitted the data better than when based on rainfall intensity solely. Models that assume a positive linear relationship between erosion and slope may overestimate sediment yield.

Reducing Herbicides and Veterinary Antibiotics Losses from Agroecosystems Using Vegetative Buffers

Multiple species vegetative buffer strips (VBSs) have been recommended as a cost-effective approach to mitigate agrochemical transport in surface runoff derived from agronomic operations, while at the same time offering a broader range of long-term ecological and environmental benefits. However, the effect of VBS designs and species composition on reducing herbicide and veterinary antibiotic transport has not been well documented. An experiment consisting of three VBS designs and one continuous cultivated fallow control replicated in triplicate was conducted to assess effectiveness in reducing herbicide and antibiotic transport for claypan soils. The three VBS designs include (i) tall fescue, (ii) tall fescue with a switchgrass hedge barrier, and (iii) native vegetation (largely eastern gamagrass). Rainfall simulation was used to create uniform antecedent soil moisture content in the plots and to generate runoff. Our results suggested that all VBS significantly reduced the transport of dissolved and sediment-bound atrazine, metolachlor, and glyphosate in surface runoff by 58 to 72%. Four to 8 m of any tested VBS reduced dissolved sulfamethazine transport in the surface runoff by more than 70%. The tall fescue VBS was overall most effective at reducing dissolved tylosin and enrofloxacin transport in the runoff (>75%). The developed exponential regression models can be used to predict expected field-scale results and provide design criteria for effective field implementation of grass buffers. Our study has demonstrated that an optimized VBS design may achieve desired agrochemical reductions and minimize acreage removed from crop production.

Phosphorus in runoff from two highly weathered soils of the tropics

Ramírez-Ávila, J. R., Sotomayor-Ramírez, D., Martínez-Rodríguez, G. A. and Pérez-Alegría, L. R. 2011. Phosphorus in runoff from two highly weathered soils of the tropics. Can. J. Soil Sci. 91: 267–277. Agricultural fields with high soil phosphorus (P) content are important contributors to surface water degradation. Two consecutive simulated rainfall events were conducted on two Ultisols previously amended with inorganic P fertilizer or broiler litter. Soil test P (Bray 1 and Olsen) levels evaluated ranged from 1 to 350 mg kg−1. Surface runoff concentrations of total P (TP) and dissolved P (DP) generated by a 30-min runoff event were quantified. Runoff DP concentrations ranged from 0.08 to 3.98 mg L−1in fertilizer P-amended soils and from 0.08 to 4.93 mg L−1in broiler litter-amended soils. A single exponential model adequately described the relationships between soil test P and DP concentrations in runoff. For each soil, the soil test P-DP concentration relationships were positively influenced by soil organic matter and negatively influenced by antecedent soil moisture (P<0.05). For both soils, the soil test P-DP concentration relationships were positively influenced by groundcover percentage and negatively influenced by slope. Environmental soil test P critical levels corresponding to a runoff threshold of 1 mg L−1DP, ranged between 176 and 296 mg kg−1(Olsen) and 143 to 276 mg kg−1(Bray 1) in soils amended with fertilizer-P. In broiler litter-amended soils, threshold values were 88 and 111 mg kg−1using Olsen and Bray 1, respectively. Differences in surface runoff-P concentrations due to amendment sources, antecedent soil moisture content, soil organic matter, groundcover and slope suggest that these factors need to be considered in P management decisions at the farm level.

Effects of rainfall intensity and antecedent soil water content on soil infiltrability under rainfall conditions using the run off-on-out method

Summary Soil infiltrability, or infiltration capacity, is the rate at which water penetrates into the soil at a rate directly controlled by soil factors alone. The infiltrability is of great importance in understanding and managing hydrological processes, crop water supply, irrigation, and soil erosion. The infiltration dynamics measured with the run off-on-out (ROOO) method follows the changes in infiltrability during the infiltration process under rainfall conditions. In this study, the ROOO method was used to quantitatively measure the soil infiltrability under three rainfall intensities (20, 40 and 60 mm/h) and three antecedent soil moisture contents (2.6%, 10.4% and 19.5%, equivalent to 7.5% (air-dry), 30% and 60% of field capacity, respectively, of a clay loam soil. The soil infiltrability determined by the ROOO method decreased with increase in initial soil water content, due to the lower hydraulic gradient at the wetting front. Surface seal formation due to raindrop impact had a significant influence on soil infiltrability and was related to the soil water content. The faster wetting rate of drier soil under higher rainfall intensities, or when using the double-ring infiltrometer for comparison, caused severe aggregate breakdown to promote surface sealing, and significant decrease in soil infiltrability. Cumulative infiltration increased rapidly at the beginning of the infiltration process and then increased approximately linearly with time as the infiltration rate approached to constant. The rate of increase in cumulative infiltration was less under higher initial soil water contents, especially in the initial rainfall stage. Moreover, lower rainfall intensity resulted in higher infiltration rates and greater cumulative infiltration. The soil infiltrability processes measured with the ROOO method were fitted better by Kostiakov, Horton and Philip infiltration models than those measured by the double-ring infiltrometer method. The ROOO method provided reliable data for the entire infiltration process without the limitations of conventional rainfall simulation during the initial phase or of the drawbacks of the double-ring infiltrometer method. The results will have introductory meanings to further studies along this line.

Comparative study of neural network, fuzzy logic and linear transfer function techniques in daily rainfall‐runoff modelling under different input domains

AbstractThis paper compares artificial neural network (ANN), fuzzy logic (FL) and linear transfer function (LTF)‐based approaches for daily rainfall‐runoff modelling. This study also investigates the potential of Takagi‐Sugeno (TS) fuzzy model and the impact of antecedent soil moisture conditions in the performance of the daily rainfall‐runoff models. Eleven different input vectors under four classes, i.e. (i) rainfall, (ii) rainfall and antecedent moisture content, (iii) rainfall and runoff and (iv) rainfall, runoff and antecedent moisture content are considered for examining the effects of input data vector on rainfall‐runoff modelling. Using the rainfall‐runoff data of the upper Narmada basin, Central India, a suitable modelling technique with appropriate model input structure is suggested on the basis of various model performance indices. The results show that the fuzzy modelling approach is uniformly outperforming the LTF and also always superior to the ANN‐based models. Copyright © 2010 John Wiley & Sons, Ltd.

IN SITU MEASUREMENT OF THE SOIL MOISTURE CONTENT PROFILE AT A DECOMPOSED GRANITE CUT-SLOPE

Two selected sites at a decomposed granite cut-slope were instrumented with a rain gauge and a series of ThetaProbes installed at depths up to 90 cm from the slope surface. The objective of this study was to evaluate the significance of antecedent soil moisture conditions in controlling the hydrologic responses of the soils at each site. The antecedent moisture content of the soil in Site #1, in general, increases with depth. The response of the soil to saturation process is more or less confined to the shallow depth. On the other hand, the measurement data of the soil in Site #2 showed that the near surface soil exhibits higher antecedent moisture condition than the deeper soil. The ingress of the wetting front could reach more than 90 cm depth during long period of rainfall. Thus, these results suggest that the hydrological response of the soils is spatially variable and is influenced by the antecedent soil moisture conditions within the soil profile. The presence of soil layers, the distribution of macro cracks within the soil profile, and additional lateral source of moisture may also have significant influences on soil response to saturation process.

Effects of rainfall change on water erosion processes in terrestrial ecosystems: a review

Water erosion is the most destructive erosion type worldwide, causing serious land degradation and environmental deterioration. Against a background of climate change and accelerated human activities, changes in natural rainfall regimes have taken place and will be expected to become more pronounced in future decades. Long-term shifts may challenge the existing cultivation systems worldwide and eventually alter the spatiotemporal patterns of land use and topography. Meanwhile, specific features of soil crusting/sealing, plant litter and its decomposition, and antecedent soil moisture content (ASMC) will accompany rainfall variability. All these changes will increase pressures on soil erosion and hydrological processes, making accurate erosion prediction and control more difficult. An improved knowledge and understanding of this issue, therefore, is essential for dealing with the forthcoming challenges regarding soil and water conservation practices. In this paper, the characteristics of changes in natural rainfall, its role on terrestrial ecosystems, the challenges, and its effect on surface water erosion dynamics are elaborated and discussed. The major priorities for future research are also highlighted, and it is hoped that this will promote a better understanding of water erosion processes and related hydrological issues.

Clay, Organic Matter, and Wetting Effects on Splash Detachment and Aggregate Breakdown under Intense Rainfall

The binding and cementing potentials of clay and organic matter (OM) and the weakening effect of wetting are important in the stability of soil aggregates. They can therefore influence aggregate breakdown (AB) and splash detachment (SD), which are initial steps in the erosion process. We investigated the interactive effects of clay and OM on AB and SD under various wetting rates (WR) and antecedent moisture contents (AMC) using six agricultural soils from Trinidad. The selection criteria for the six soils were based on three levels of clay; low (<20%), medium (20–45%) and high (>45%) and two levels of OM; low (≤3%), and high (>3%). Samples were prewetted with mist at slow (7.5 mm h−1) and fast (75 mm h−1) WR to AMC of 0.5 of field capacity (FC) and FC and exposed to intense simulated rainfall of 120 mm h−1 The sensitivity of a sample to disruption under varying wetting conditions and intense rainfall depended on the level of combination of clay and OM in the sample. Increase in clay beyond the medium level without raising OM to high level strengthened disruptive forces and increased the proportion of microaggregates. The AB and SD of the medium clay‐high organic matter (McHom) soils were significantly lower than their high clay‐low organic matter (HcLom) counterparts irrespective of WR and AMC. This implies that a threshold clay content exists beyond which an accompanying increase in OM is required to mitigate detachment mechanisms and erosion under intense rainfall.

A simple model to predict soil moisture: Bridging Event and Continuous Hydrological (BEACH) modelling

This paper introduces a simple two-layer soil water balance model developed to Bridge Event And Continuous Hydrological (BEACH) modelling. BEACH is a spatially distributed daily basis hydrological model formulated to predict the initial condition of soil moisture for event-based soil erosion and rainfall–runoff models. The latter models usually require the spatially distributed values of antecedent soil moisture content and other input parameters at the onset of an event. BEACH uses daily meteorological records, soil physical properties, basic crop characteristics and topographical data. The basic processes incorporated in the model are precipitation, infiltration, transpiration, evaporation, lateral flow, vertical flow and plant growth. The principal advantage of this model lies in its ability to provide timely information on the spatially distributed soil moisture content over a given area without the need for repeated field visits. The application of this model to the CATSOP experimental catchment showed that it has the capability to estimate soil moisture content with acceptable accuracy. The root mean squared error of the predicted soil moisture content for 6 monitored locations within the catchment ranged from 0.011 to 0.065 cm 3 cm −3. The predicted daily discharge at the outlet of the study area agreed well with the observed data. The coefficient of determination and Nash–Sutcliffe efficiency of the predicted discharge were 0.824 and 0.786, respectively. BEACH has been developed within freely available GIS and programming language, PCRaster. It is a useful teaching tool for learning about distributed water balance modelling and land use scenario analysis.

Lateral flow thresholds for aspen forested hillslopes on the Western Boreal Plain, Alberta, Canada

AbstractTo predict the long‐term sustainability of water resources on the Boreal Plain region of northern Alberta, it is critical to understand when hillslopes generate runoff and connect with surface waters. The sub‐humid climate (P ≤ ET) and deep glacial sediments of this region result in large available soil storage capacity relative to moisture surpluses or deficits, leading to threshold‐dependent rainfall‐runoff relationships. Rainfall simulation experiments were conducted using large magnitude and high intensity applications to examine the thresholds in precipitation and soil moisture that are necessary to generate lateral flow from hillslope runoff plots representative of Luvisolic soils and an aspen canopy. Two adjacent plots (areas of 2·95 and 3·4 m2) of contrasting antecedent moisture conditions were examined; one had tree root uptake excluded for two months to increase soil moisture content, while the second plot allowed tree uptake over the growing season resulting in drier soils. Vertical flow as drainage and soil moisture storage dominated the water balances of both plots. Greater lateral flow occurred from the plot with higher antecedent moisture content. Results indicate that a minimum of 15–20 mm of rainfall is required to generate lateral flow, and only after the soils have been wetted to a depth of 0·75 m (C‐horizon). The depth and intensity of rainfall events that generated runoff > 1 mm have return periods of 25 years or greater and, when combined with the need for wet antecendent conditions, indicate that lateral flow generation on these hillslopes will occur infrequently. Copyright © 2008 John Wiley & Sons, Ltd.

Laboratory-Scale Simulation of Runoff Response from Pervious-Impervious Systems

We posit that a more complete understanding of runoff response from urban catchments requires an assessment of the hydrologic behavior of composite impervious-pervious patches. We therefore examined how the factors of impervious extent, connectiv- ity, and antecedent moisture content of pervious areas might affect mechanisms of runoff production at small spatial scales in a laboratory setting. We used rainfall simulation with a storm comprised of 20, 30, 40 mm hr �1 rainfall rates for 48, 24, and 24 min, respectively to observe surface runoff from 0.6 m 2 boxes impervious or pervious-soil 0.2 m deep that were connected together in series to produce different arrangements of impervious and pervious surfaces 0, 25, 50% impervious with different connectivity to the outlet discon- nected, connected, and under two different antecedent moisture conditions for pervious areas drier, wetter. In general, an increase in percent impervious area led to fewer opportunities for infiltration, and a quicker onset of runoff, which was intensified by wetter antecedent moisture conditions and connectivity to the outlet. Runoff rate ratios were strongly affected by antecedent moisture condition and somewhat less significantly by an interaction between impervious area extent and its connectivity status. In each impervious treatment, we observed a decreased time to runoff initiation and higher final runoff rate ratio for wetter than drier treatments. Interestingly, we found that the connectivity of 25% impervious area accounted for differences in runoff rate ratio only early in the simulation. The patterns in runoff from connected and disconnected 25% treatments eventually converged, leaving antecedent moisture conditions the only relevant factor. As impervious area was increased to 50%, we noted a precipitous decline in infiltration rates due to a reduction in infiltration opportunities and infiltration behavior of the pervious surfaces in these treatments. Evidence of return flow in the 50% disconnected treatment is presented and discussed in the context of saturation-excess runoff mechanisms. These experimental results are then discussed in terms of their potential extension and application to better understand aspects of urban hydrology and models thereof.