Previous Rainfall Events Research Articles

Alternative streamflow-based approach to estimate catchment response time in medium to large catchments: case study in Primary Drainage Region X, South Africa

Event-based estimates of the design flood in ungauged catchments are normally based on a single catchment response time parameter expressed as either the time of concentration (TC), lag time (TL) and/or time to peak (TP). In small, gauged catchments, a simplified convolution process between a single observed hyetograph and hydrograph is generally used to estimate these time parameters. In medium to large heterogeneous, gauged catchments, such a simplification is neither practical nor applicable, given that the variable antecedent soil moisture status resulting from previous rainfall events and spatially non-uniform rainfall hyetographs can result in multi-peaked hydrographs. In ungauged catchments, time parameters are estimated using either empirical or hydraulic methods. In South Africa (SA), unfortunately, the majority of the empirical methods recommended for general use were developed and verified in catchments ≤ 0.45 km² without using any local data. This paper presents the further development and verification of the streamflow-based approach developed by Gericke (2016) to estimate observed TP values and to derive a regional empirical TP equation in Primary Drainage Region X, SA. A semi-automated hydrograph analysis tool was developed to extract and analyse complete hydrographs for time parameter estimation using primary streamflow data from 51 flow-gauging sites. The observed TP values were estimated using three methods: (i) duration of total net rise of a multi-peaked hydrograph, (ii) triangular-shaped direct runoff hydrograph approximations, and (iii) linear catchment response functions. The combined use of these methods incorporated the high variability of event-based time parameters, and Method (iii), in conjunction with an ensemble-event approach sampled from the time parameter distributions, should replace the event-based approaches to enable the improved calibration of empirical time parameter equations. The conceptual approach used to derive the regional empirical TP equation should also be adopted when regional equations need to be derived at a national scale in SA.

A probabilistic approach to stormwater runoff control through permeable pavements beneath urban trees

One of the most current and urgent challenges is making cities sustainable and resilient to climate change. From this perspective, Nature-Based Solutions (NBSs) are well-recognized strategies for stormwater control and water cycle restoration. Urban trees are an example of NBS. However, the high degree of soil sealing typically found in urban environments limits natural processes such as infiltration and hinders the water and nutrient supply for proper root development, which weakens tree stability. Permeable pavements at the base of urban trees, on the one hand, facilitate infiltration, which helps runoff control, and on the other hand, improve stormwater retention and soil humidity, which enhance root feeding.This paper proposes an analytical-probabilistic approach to estimate the contribution of permeable pavements to stormwater management. The equations developed in this study relate the runoff probability to the storage volume, the infiltration rate into the underlying soil, and the average values of the hydrological variables in the input. The model allows us to select different runoff thresholds and considers the possibility that residual volume from previous rainfall events prefills the storage capacity.An application to a case study in Sao Paulo (Brazil) has been presented. It investigates the influence of the different parameters used in the model on the results. The comparison of the outcomes obtained using the developed equations with those obtained from the continuous simulation of measured data confirmed the effectiveness of the proposed analytical-probabilistic approach and the suitability of using permeable pavements at the base of urban trees for improving stormwater retention.

Seasonal dynamics of marly badlands illustrated by field records of hillslope regolith properties, Draix–Bléone Critical Zone Observatory, South‐East France

AbstractSparsely vegetated badlands are loci of intense erosion that is sufficiently rapid to have observable effects on human timescales. Characterizing and understanding the physical weathering processes in these settings are key to predicting the temporal variability of regolith production and sediment flux, as well as their evolution under changing climate conditions. Here, we study intra‐annual changes of hillslope properties and explore the relationship between sediment production and transport in steep marly badland catchments of the Draix–Bléone Critical Zone Observatory (SE France), where decades‐long monitoring records show rapid morphological changes. There is evidence for seasonal dynamics of these badlands, but characterization and quantification of physical weathering processes have been lacking up to now. We explore this gap by monitoring key regolith parameters including grain‐size distribution (characterized by D50), surface resistance and water content in the regolith layer (surface to ∼10 cm depth) at different locations, through repeated field surveys over a 2.5‐year period. While water content appears to be directly controlled by the last previous rainfall event, the cyclic annual pattern in observed D50 suggests that loose and finely fragmented regolith is mainly produced and accumulates during the winter season, whereas sediment transport is dominant during spring–summer. This dynamic reduces regolith thickness and induces coarsening of hillslope surface material between early spring and autumn. Principal component analysis (PCA) highlights the strong correlation between resistance and D50. We therefore suggest that D50 provides the best proxy of regolith weathering in these marls. The spatial variability of the regolith was analysed through a geophysical profile, highlighting distinct behaviour depending on slope aspect. However, the distribution of slope angles is independent of aspect at the catchment scale. These results corroborate the strong annual dynamics of these catchments, where hillslopes and gullies are drained during spring and early summer high‐intensity precipitation events, inducing high sediment yields.

Influence of climatic parameters on the probabilistic design of green roofs

Green roofs are effective tools for stormwater control in highly urbanized areas since they allow the reduction of peak runoffs and volumes discharged in sewer systems. Their design is quite standardized, except for the thickness of the growing medium layer, which is strictly related to vegetation type and rainfall regime. The paper proposes an analytical probabilistic approach that relates the climatic variables, the growing medium thickness, and the water content in the condition of fulfilled field capacity to the probability that runoff from green roofs exceeds a fixed threshold. The developed equations also consider the possibility of a reduced retention capacity due to previous rainfall events, that strongly influence the performance of these green infrastructures, especially when short dry periods and/or low evapotranspiration rates occur. This feature, neglected by the traditional design storm approach, and only partially considered by previous analytical probabilistic models, represent a great potentiality of the proposed equations that are also more user-friendly and less time-consuming than continuous simulation analysis. The focus of the paper is on the influence of climatic parameters on runoff probability. To this aim to perform the monthly analysis is fundamental, especially when there is a strong variability of the climatic parameters throughout the year.The model was tested in a case study in Milano, Italy. The application presented a good agreement between the results obtained from the proposed equations and those obtained from the continuous simulation of recorded data. The results also highlighted the importance of performing analysis on a monthly scale.

The Mobility of Landslides in Pumice: Insights from a Flume Experiment

Risk of landslide hazards strongly depends on how far landslide sediment travels, known as landslide mobility. Previous studies mentioned enhanced mobility of earthquake-induced landslides in volcanic deposits compared to those from other geologic/soil settings. A flume apparatus constructed at a 1:300 scale was used to examine the mobility of landslides with pumice. Four pumice samples were collected from landslides induced by the 2018 Eastern Iburi earthquake, Hokkaido, Japan. Laboratory tests confirmed the unique low specific gravity of the pumice (1.29–1.33), indicating numerous voids within pumice particles. These voids allowed pumice to absorb a substantial amount of water (95–143%), about 9–15 times higher than other coarse-grained soils. Our flume experiments using various saturation levels (0–1) confirmed the influence of this inner-particle water absorption on pumice mobility. Because a low value of specific gravity indicates a low strength of soil, grain crushing may occur on the pumice layer, causing water from the internal voids to discharge and fluidize the transported landslide mass. Our findings indicate that such earthquake-induced landslides can be as mobile as those induced by rainfall, depending on the initial water content of the pumice layers. These conditions might be associated with water accumulation from previous rainfall events and the water-holding capability on pumice layers.

Improved calibration of the Green–Ampt infiltration module in the EROSION-2D/3D model using a rainfall-runoff experiment database

Abstract. Soil infiltration is one of the key factors that has an influence on soil erosion caused by rainfall. Therefore, a well-represented infiltration process is a necessary precondition for successful soil erosion modelling. Complex natural conditions do not allow the full mathematical description of the infiltration process, and additional calibration parameters are required. The Green–Ampt-based infiltration module in the EROSION-2D/3D model introduces a calibration parameter “skinfactor” to adjust saturated hydraulic conductivity. Previous studies provide skinfactor values for several combinations of soil and vegetation conditions. However, their accuracies are questionable, and estimating the skinfactors for other than the measured conditions yields significant uncertainties in the model results. This study brings together an extensive database of rainfall simulation experiments, the state-of-the-art model parametrisation method and linear mixed-effect models to statistically analyse relationships between soil and vegetation conditions and the model calibration parameter skinfactor. New empirically based transfer functions for skinfactor estimation significantly improving the accuracy of the infiltration module and thus the overall EROSION-2D/3D model performance are provided in this study. Soil moisture and bulk density were identified as the most significant predictors explaining 82 % of the skinfactor variability, followed by the soil texture, vegetation cover and impact of previous rainfall events. The median absolute percentage error of the skinfactor prediction was improved from 71 % using the currently available method to 30 %–34 % using the presented transfer functions, which led to significant decrease in error propagation into the model results compared to the present method. The strong logarithmic relationship observed between the calibration parameter and soil moisture however indicates high overestimation of infiltration for dry soils by the algorithms implemented in EROSION-2D/3D and puts the state-of-the-art parametrisation method in question. An alternative parameter optimisation method including calibration of two Green–Ampt parameters' saturated hydraulic conductivity and water potential at the wetting front was tested and compared with the state-of-the-art method, which paves a new direction for future EROSION-2D/3D model parametrisation.

Reliability analysis of an existing slope at a specific site considering rainfall triggering mechanism and its past performance records

Accurate estimation of landslide probability or slope failure probability when there is a rainstorm event is crucial for assessment and mitigation of rainfall-induced landslide risk. Slope reliability methods provide a rigorous way of estimating landslide probability based on slope failure mechanism and probability theory. However, it is well recognized in literature that the landslide probability estimated from existing slope reliability analysis methods are often much larger than the observed landslide frequency. To improve accuracy of the slope failure probability, this study proposes a slope reliability analysis method for an existing slope at a specific site, which considers both rainfall triggering mechanism and the slope's performance records during previous rainfall events. It was found that the fact that the slope survived from previous rainfall events could be utilized to effectively reduce uncertainties in soil parameters and might reduce the estimated landslide probability by one to two orders of magnitudes. In addition, the proposed method provides a real-time slope failure probability for a given rainfall event, and the estimated slope failure probability varies as the considered rainfall event evolves with time. • A method is proposed to estimate real-time slope failure probability under a target rainfall. • Bayesian updating of uncertainties is performed using slope performance records from past rainfall events. • Uncertainties in soil and other parameters are significantly reduced by Bayesian updating. • The challenge of overestimating slope failure probability is tackled.

Second-order Markov chain models of rainfall in Ibadan, Southwest Nigeria

Understanding the dynamics of rainfall is important for maximizing the agricultural potential, environmental planning, as well as policy development. In this study, a two-state second-order Markov chain model was developed for the annual and daily occurrence of different rainfall sequences in Ibadan, southwestern Nigeria. The annual variation was adapted to a sine series model. The model was able to account for more than 30% of the observed variation. Markov chain approach to modelling rainfall in tropical region is unique because it considers the “memory” – previous rainfall events, which is lacking in other methods. The daily occurrences showed patterns consistent with tropical regions. Results from this study is significant for economic, health and environmental planning in the location.

Performance of Green Roofs for Rainwater Control

Green roofs can be an effective tool for sustainable urban drainage, since they reduce and retain runoff by delaying its peak. Most studies analysing the retention capacity of green roofs are usually referred to a specific place and roof condition and do not consider the possibility that the roof could be partially pre-filled from previous rainfalls at the beginning of the given event. The aim of this paper is to develop an analytical probabilistic approach to evaluate green roof performance for stormwater control in terms of runoff that could be applied for different sites and climate conditions. To this end, the possibility that the green roof retention capacity could not be completely available owing to pre-filling from previous rainfall events has been considered and equations for an optimum green roof design, relating the runoff average return interval to the water retention capacity, have been proposed. The influence of parameters affecting the runoff process has been examined in depth and a case study to test the goodness of fit of the resulting equations has been developed.

Impact of rainfall regime on methane flux from a cool temperate fen depends on vegetation cover

Climate change projections forecast an intensification of the precipitation regime for many regions of the globe, including central North America, with fewer, larger events interspersed between longer periods devoid of rain or snow. This shift has the potential to affect the carbon cycling of peatland ecosystems, including the flux of methane from the peat. We conducted a field manipulation experiment where irrigation treatments were used to simulate different seasonal rainfall regimes. The treatments were designed such that total seasonal rainfall was held constant but discrete event frequency and magnitude were altered between treatments in a poor fen in southern Ontario, Canada. The rainfall regime was controlled over three vegetation types: Sphagnum capillifolium (moss); Carex oligoperma (sedge); and Chamaedaphne calyculata (shrub). Decreasing rainfall frequency from thrice-weekly to bi-monthly [coupled with 6X increase in event intensity] led to significantly greater CH4 flux from the moss and sedge communities in the latter third of the growing season. The shrub communities were unaffected by the changing rainfall regime. A companion lab mesocosm experiment revealed the control the fluctuating water table had on the CH4 fluxes from the vegetation community, particularly from the moss communities. Overall, there were significantly greater CH4 fluxes from all communities with increasing days since the previous rainfall event. As precipitation frequency decreases results of this study demonstrate the potential for increased CH4 flux to the atmosphere from peatland areas dominated by Sphagnum and herbaceous species. Wetland restoration and creation projects should consider these effects on peatland carbon cycling and function.

Direct estimation of catchment response time parameters in medium to large catchments using observed streamflow data

AbstractIn single‐event deterministic design flood estimation methods, estimates of the peak discharge are based on a single and representative catchment response time parameter. In small catchments, a simplified convolution process between a single‐observed hyetograph and hydrograph is generally used to estimate time parameters such as the time to peak (TP), time of concentration (TC), and lag time (TL) to reflect the “observed” catchment response time. However, such simplification is neither practical nor applicable in medium to large heterogeneous catchments, where antecedent moisture from previous rainfall events and spatially non‐uniform rainfall hyetographs can result in multi‐peaked hydrographs. In addition, the paucity of rainfall data at sub‐daily timescales further limits the reliable estimation of catchment responses using observed hyetographs and hydrographs at these catchment scales. This paper presents the development of a new and consistent approach to estimate catchment response times, expressed as the time to peak (TPx) obtained directly from observed streamflow data. The relationships between catchment response time parameters and conceptualised triangular‐shaped hydrograph approximations and linear catchment response functions are investigated in four climatologically regions of South Africa. Flood event characteristics using primary streamflow data from 74 flow‐gauging stations were extracted and analysed to derive unique relationships between peak discharge, baseflow, direct runoff, and catchment response time in terms of TPx. The TPx parameters are estimated from observed streamflow data using three different methods: (a) duration of total net rise of a multipeaked hydrograph, (b) triangular‐shaped direct runoff hydrograph approximations, and (c) linear catchment response functions. The results show that for design hydrology and for the derivation of empirical equations to estimate catchment response times in ungauged catchments, the catchment TPx should be estimated from both the use of an average catchment TPx value computed using either Methods (a) or (b) and a linear catchment response function as used in Method (c). The use of the different methods in combination is not only practical but is also objective and has consistent results.

Probabilistic modeling of the efficiency of a stormwater detention facility

In modern urban drainage systems, stormwater detention facilities are important tools to reduce flood effects and discharges of uncontrolled pollutants into water receivers. For their design, simplified methods and continuous simulations of observed data are generally used. One of the main targets is to limit the frequency and the entity of overflows. In this article, a probabilistic approach for the derivation of the distribution functions of spilled volumes to evaluate the efficiency of a stormwater detention facility is proposed. The possibility of pre-filling of the storage capacity from a previous rainfall event, which can increase the probability of overflows from the stormwater detention facility, has been considered. The effects of the simplified assumptions have been deeply investigated and discussed. Final expressions have been applied to a case study, and results have been compared to those obtained from the continuous simulation.

An improved and consistent approach to estimate catchment response time parameters: case study in the C5 drainage region, South Africa

AbstractLarge errors in estimates of peak discharge in medium to large catchments in South Africa can be largely ascribed to significant errors in the estimation of the catchment response time, mainly as a consequence of the use of inappropriate time variables, the inadequate use of a simplified convolution process between rainfall–run‐off time variables, and the lack of locally developed empirical methods to estimate catchment response time parameters. Furthermore, the use of a typical convolution process between a single hyetograph and hydrograph to estimate observed time parameters at large catchment scales is regarded as not practical, as such simplification is not applicable in real, large heterogeneous catchments where antecedent moisture from previous rainfall events and spatially non‐uniform rainfall hyetographs can result in multipeaked hydrographs. This paper presents the development and evaluation of an alternative, improved and consistent approach to estimate catchment response time expressed as the time to peak (TP) in the C5 secondary drainage region in South Africa, while the interrelationship, similarity and proportionality ratios between TP and the conceptual time of concentration (TC) and lag time (TL) are also investigated.

Persistent after-effects of heavy rain on concentrations of ice nuclei and rainfall suggest a biological cause

Abstract. Rainfall is one of the most important aspects of climate, but the extent to which atmospheric ice nuclei (IN) influence its formation, quantity, frequency, and location is not clear. Microorganisms and other biological particles are released following rainfall and have been shown to serve as efficient IN, in turn impacting cloud and precipitation formation. Here we investigated potential long-term effects of IN on rainfall frequency and quantity. Differences in IN concentrations and rainfall after and before days of large rainfall accumulation (i.e., key days) were calculated for measurements made over the past century in southeastern and southwestern Australia. Cumulative differences in IN concentrations and daily rainfall quantity and frequency as a function of days from a key day demonstrated statistically significant increasing logarithmic trends (R2 > 0.97). Based on observations that cumulative effects of rainfall persisted for about 20 days, we calculated cumulative differences for the entire sequence of key days at each site to create a historical record of how the differences changed with time. Comparison of pre-1960 and post-1960 sequences most commonly showed smaller rainfall totals in the post-1960 sequences, particularly in regions downwind from coal-fired power stations. This led us to explore the hypothesis that the increased leaf surface populations of IN-active bacteria due to rain led to a sustained but slowly diminishing increase in atmospheric concentrations of IN that could potentially initiate or augment rainfall. This hypothesis is supported by previous research showing that leaf surface populations of the ice-nucleating bacterium Pseudomonas syringae increased by orders of magnitude after heavy rain and that microorganisms become airborne during and after rain in a forest ecosystem. At the sites studied in this work, aerosols that could have initiated rain from sources unrelated to previous rainfall events (such as power stations) would automatically have reduced the influences on rainfall of those whose concentrations were related to previous rain, thereby leading to inhibition of feedback. The analytical methods described here provide means to map and delimit regions where rainfall feedback mediated by microorganisms is suspected to occur or has occurred historically, thereby providing rational means to establish experimental set-ups for verification.

Effect of sampling duration on the performance evaluation of a stormwater wetland.

In this study, the effect of sampling duration on the performance estimate for a stormwater wetland over both rainy and dry days was evaluated for the appropriate design of sampling duration. As the cumulative percentage volume (Vp), the ratio of cumulative stormwater volume concerning time to the total stormwater volume, varied between 60 and 100%, generally, the inflow total suspended solids, turbidity and total chemical oxygen demand (TCOD) event mean concentrations (EMCs) did not vary significantly, whereas the total nitrogen (TN) and total phosphorus (TP) EMCs were relatively stable. Compared to the inflow, the corresponding outflow EMCs changed much less as Vp changed. And these variations both from inflow and outflow EMCs did not result in significant changes in the removal efficiencies. The investigation during the dry days between two consecutive storm events showed that outflow pollutants did not change to a considerable extent after 1 day of the previous rainfall event. This study identifies the possibility of shortening the rainy sampling duration, because the performance of stormwater wetlands is usually estimated based on removal efficiencies rather than pollutant concentrations. Also, the sampling during dry days should be performed at least 1 day after a rainfall event.

A novel conceptual framework for long‐term leaching of autochthonous soil particles during transient flow

SummaryTo date, most of the experimental and modelling research on the mobilization mechanisms of autochthonous colloidal‐sized soil particles has focused on single rainfall events. However, in the field, natural events interrupted by rainless periods of various durations follow one another. Some studies have shown that the amount of leached particles is much affected by the chronology of rainfall events. In this paper, we propose a model to compute the amount of mobilized particles during the transient flow regime of long series of successive rainfall events. Particles that can be mobilized are assumed to be located at the surface of preferential flow paths in contact with flowing water. As water passes, particles are mobilized, thus uncovering new particles and renewing the stock of mobilizable particles. We also report experimental data on the impact of long series of rainfall events on particle mobilization in undisturbed calcareous soil columns under controlled conditions. The model provides a framework to understand the variation of mobilization observed during the transient flow regime of these rainfall events. These variations resulted in part from the interplay between macropore water content at the onset of rain and the chronology of previous rainfall events. Additionally, the model provides a feedback loop between particle mobilization and minute macropore structure modifications. Once coupled with a particle transport model this feature may be useful to model soil structure changes during long series of successive rainfall events.

Herbicide transport via surface runoff during intermittent artificial rainfall: A laboratory plot scale study

Surface runoff is highly relevant for herbicide transport into water bodies. Herbicide losses and the influence of physicochemical properties of flufenacet, metazachlor and terbuthylazine were studied on a 0.2 m 2 microplot of bare soil and slight slope (6%). Rainfall events were simulated with 13 mm h − 1 (± 3 mm h − 1 ) precipitation intensity 4, 11, 18, 25, 32 and 39 days after herbicide application. The runoff was sampled at 10-minute intervals during the simulated events. Herbicide concentrations and sediment amounts in the water phase were measured. The maximum concentrations in the water phase of the runoff ranged from 3184 μg l − 1 for metazachlor, 661 μg l − 1 for terbuthylazine and 207 μg l − 1 for flufenacet. The highest concentrations were detected in the first runoff samples which decreased exponentially with further rainfall. Highest loads occurred during the second rainfall event and after the first time surface runoff was generated. The artificial setup of intermittent rainfall revealed that with every new rainfall event after a rainless period, a higher amount of herbicides was released from the plot than at the last interval of the previous rainfall event. This effect was caused by alterations to the soil surface that occurred during the rainless periods. Furthermore, the results show that the herbicide loss relates to herbicide adsorption to organic carbon (K OC ) and herbicide solubility in water. Therefore, the highest losses for terbuthylazine were 6.6% of the pesticide application, followed by flufenacet (3.8%) and metazachlor (2.5%).

Hydrological behaviour of sub-Mediterranean alpine headwater streams in a badlands environment

Summary Investigations of flash floods generation processes are very few compared with studies in humid areas. The rapidity and intensity of these processes make difficult any data acquisition on the field. The present study aims to improve our knowledge on these environments by using the facilities of the Draix ERBs (Southern French Alps) where well-documented black marls show a typical badlands morphology with V-shaped gullies. Time-varying components of total flow and flow sources have been investigated from a long-term isotopic and hydrochemical survey of rainwaters, overland waters, soil waters and streamwaters. We present in greater detail two flood events which occurred in three adjacent basins and had contrasting antecedent soil moisture conditions. Isotopic data were used to apply time-based hydrograph separations. Due to the very short transfer time of waters, convolution of the rainfall isotopic signal with a two-parameter transfer function (dispersive transfer function) proved the best way to assess the event isotopic concentration. End Members Mixing Analysis (EMMA) has been carried out using non reactive chemical species to get source areas of runoff. New water participation by direct surface transfer was the main mechanism at the peak discharge. This contribution was found to dominate irrespective of whether the antecedent soil moisture was dry or wet. Nevertheless, up to 20% of subsurface water has been observed at peak discharge for dry pre-event conditions. During the flow recession phases, this subsurface water contribution was greater for wet antecedent conditions. In dry initial conditions, subsurface water was mostly new water, whereas in wet conditions pre-event water could contribute to the flow. A deep groundwater reservoir may take part in the pre-event component in the larger basin but in the smaller basins pre-event water was mostly made of short residence time water (from previous rainfall events) originating from sporadic, highly transmissive shallow hillslope reservoirs (loose detrital cover and upper regolith). Differences regarding the hierarchy of the processes were not found to be simply proportional to the basins’ area, nor were they clearly related to the mean slope gradients or to the percentage of vegetal cover. It is suggested that controls on streamflow generation occur at local scale and consist in the spatial organisation and the gradual accumulation downwards of first order basins with their respective physical specificities (mostly orientation and slope gradient).

Downward water flow through sloping layers in the vadose zone: time-dependence and effect of slope length

We develop a 2-D numerical model to simulate both steady-state and transient downward water flow through sloping layers in the vadose zone with a deep water table. The horizontal flow component obtained with the 2-D numerical model varies with position along the slope, but was nearly the same as the 1-D analytical solution once beyond a few meters from blocked vertical boundaries at the upper or lower end of the slope. Under time-variable precipitation, both the time delay and the peak magnitude of the response deep within the profile are not simply proportional or in one-to-one correspondence to the strength of each rainfall event, but rather depend on the initial moisture status in the profile as well as the previous rainfall events. An interesting phenomenon for the transient case is that the horizontal flow component reversed direction to upslope during the time of heavy rain. This is due to the flow pattern near the upper surface boundary (a sloping wetting front), and is a consequence of the sloping geometry. Previous studies (Warrick et al., 1996) have shown that multidimensional steady-state water flow through a sloping interface can essentially be solved by a one-dimensional analytical solution for an infinite long slope. The simulations suggest that a 1-D numerical analysis should also be valid for 2-D transient flow cases, if the slope is of sufficient length and the point of evaluation is sufficiently far from the beginning or end of the slope.

Evaluation of rainfall infiltration characteristics in a volcanic ash soil by time domain reflectometry method

Abstract. Time domain reflectometry (TDR) was used to monitor soil water conditions and to evaluate infiltration characteristics associated with rainfall into a volcanic-ash soil (Hydric Hapludand) with a low bulk density. Four 1 m TDR probes were installed vertically along a 6 m line in a bare field. Three 30 cm and one 60 cm probes were installed between the 1 m probes. Soil water content was measured every half or every hour throughout the year. TDR enabled prediction of the soil water content precisely even though the empirical equation developed by Topp et al. (1980) underestimated the water content. Field capacity, defined as the amount of water stored to a depth of 1 m on the day following heavy rainfall, was 640 mm. There was approximately 100 mm difference in the amount of water stored between field capacity and the driest period. Infiltration characteristics of rainfall were investigated for 36 rainfall events exceeding 10 mm with a total amount of rain of 969 mm out of an annual rainfall of 1192 mm. In the case of 25 low intensity rainfall events with less than 10 mm h-1 on to dry soils, the increase in the amount of water stored to a depth of 1 m was equal to the cumulative rainfall. For rain intensity in excess of 10 mm h-1, non-uniform infiltration occurred. The increase in the amount of water stored at lower elevation locations was 1.4 to 1.6 times larger than at higher elevation locations even though the difference in ground height among the 1 m probes was 6 cm. In the two instances when rainfall exceeded 100 mm, including the amount of rain in a previous rainfall event, the increase in the amount of water stored to a depth of 1 m was 65 mm lower than the total quantity of rain on the two occasions (220 mm); this indicated that 65 mm of water or 5.5% of the annual rainfall had flowed away either by surface runoff or bypass flow. Hence, approximately 95% of the annual rainfall was absorbed by the soil matrix but it is not possible to simulate soil water movement by Darcy's law over a long period at farm level due to the local differences in rainfall intensity.