Function Of Rainfall Intensity Research Articles

Constructing rainfall thresholds for debris flow initiation based on critical discharge and S-hydrograph

Debris flows caused by channel bed erosion present major hazards affecting life, livelihoods, and the built environment in mountainous regions. An efficient way to decrease hazard impact is through reliable hazard forecasts and appropriate early-warning strategies. Rainfall thresholds are fundamental in achieving reliable hazard forecasts. However, a lack of rainfall records often impedes the empirical establishment of such thresholds. This paper constructs rainfall intensity-duration thresholds based on process-based critical runoff discharge for the initiation of debris flows and a mathematical approximation among peak discharge, rainfall intensity and duration. Simulations of conditions that triggered debris flow and non-debris flow events allowed determination of the lower and upper limits of critical discharge for debris flow initiation. In turn, these critical discharge limits are compared with four estimates derived from process-based approaches to test which approach best delimit the critical conditions. Hydrological simulations derive S-hydrographs for recorded rainfall events. Further analysis of the S-hydrographs results in a mathematical approximation of peak discharge as a function of rainfall intensity and duration and the establishment of the minimum rainfall required to produce a particular peak discharge. The minimum rainfall threshold to trigger an event can be calculated by setting the process-based critical discharge as the peak dis charge. In turn, this enables the establishment of a conventional rainfall I-D threshold for debris flow initiation. This process-based approach enables the construction of valley-specific I-D thresholds in data-poor areas and provides a promising pathway to improve the reliability of debris flow hazard forecasts and early warnings.

Evaluation of Grid-Based Rainfall Products and Water Balances over the Mekong River Basin

Gridded precipitation products (GPPs) with wide spatial coverage and easy accessibility are well recognized as a supplement to ground-based observations for various hydrological applications. The error properties of satellite rainfall products vary as a function of rainfall intensity, climate region, altitude, and land surface conditions—all factors that must be addressed prior to any application. Therefore, this study aims to evaluate four commonly used GPPs: the Climate Prediction Center (CPC) Unified Gauge-Based Analysis of Global Daily Precipitation, the Climate Prediction Center Morphing (CMORPH) technique, the Tropical Rainfall Measuring Mission (TRMM) 3B42, and the Global Satellite Mapping of Precipitation (GSMaP), using data collected in the period 1998–2006 at different spatial and temporal scales. Furthermore, this study investigates the hydrological performance of these products against the 175 rain gauges placed across the whole Mekong River Basin (MRB) using a set of statistical indicators, along with the Soil and Water Assessment Tool (SWAT) model. The results from the analysis indicate that TRMM has the best performance at the annual, seasonal, and monthly scales, but at the daily scale, CPC and GSMaP are revealed to be the more accurate option for the Upper MRB. The hydrological evaluation results at the daily scale further suggest that the TRMM is the more accurate option for hydrological performance in the Lower MRB, and CPC shows the best performance in the Upper MRB. Our study is the first attempt to use distinct suggested GPPs for each individual sub-region to evaluate the water balance components in order to provide better references for the assessment and management of basin water resources in data-scarce regions, suggesting strong capabilities for utilizing publicly available GPPs in hydrological applications.

Soil sodicity originating from marginal groundwater

AbstractSoil salinity and sodicity are among the oldest soil and groundwater pollution problems and are widespread across the globe. Where salinity affects crop water uptake and yield, sodicity may additionally cause poorly reversible soil structure degradation and a severely reduced hydraulic conductivity. We use the model HYDRUS‐1D to simulate sodicity development in soils with shallow, Na‐rich groundwater under a normal weather regime with distinct dry seasons. Attention is given to the impact of a sudden fresh water input on the formation of a sodic layer. The complex interplay between soil chemistry, soil physics, soil mechanics (as far as swell–shrink behavior is concerned), and fluctuating atmospheric conditions results in a remarkably regular relation between depth, location, and severity of a sodic layer that forms within the soil as a function of rainfall intensity. A threshold behavior is observed: sodic layer formation is absent at rainfall intensities below this threshold, whereas sodic layer thickness and hydraulic conductivity reduction increase rapidly with intensities exceeding this threshold. This is the case even for different soil types and groundwater depths. Field observations agree with our simulations: the properties of the layer with sodicity‐induced structure degradation are more strongly developed, as this layer is situated at a shallower depth. The implementation of hydraulic conductivity reduction as a function of exchangeable Na percentage and ionic strength in HYDRUS‐1D can be improved towards a smooth reduction function, changing soil physical parameters due to swelling and dispersion of clay and reconsideration of the reversibility of sodicity development.

Investigation of Tipping-Bucket Rain Gauges Using Digital Photographic Technology

AbstractWhen studying the tipping-bucket rain gauge (TBR), it is rather difficult to make an objective and sophisticated measurement of the duration of bucket rotation. From the perspective of digital photographic technology, however, the problem can be easily solved. The primary interest of this research has been to use digital photographic technology to study the TBR under laboratory conditions. In this study, the interframe difference algorithm and a camera recording device were used. Based on three types of JDZ TBRs, the time variation characteristics of bucket rotation were obtained. The time from the beginning of a tip to the time that the bucket is horizontal T1 and the time for a complete tip T2 were analyzed in detail. The results showed that T1 and T2 were functions of rainfall intensity, and T1 and T2 decrease as the rain intensity increases significantly (P < 0.001). Moreover, excellent evidence shows that the averages of T1 and T2 were positively correlated with bucket mass. It took more time for the bucket to tip as the mass of the bucket increased. Furthermore, the error of each TBR was calculated by the new proposed error calculation formula, and the new method was compared with the traditional method. The results from the two methods were very close, which demonstrates the correctness and feasibility of the new formula. However, the traditional calibration cannot acquire the variation characteristics of the tipping time, but the proposed approach can achieve this.

Quantifying and modeling sediment yields from interrill erosion under armouring

Surface soil properties can change as a result of soil disturbances, erosion, or deposition. When soils contain rock, surface soil properties can also change over time as a result of the process of soil armouring, which is the selective removal of finer particles by erosion, leaving an armoured layer of coarser particles that may reduce further soil loss. Rapid armouring is typically reported in steep and bare slopes on mine sites, construction sites, road embankments, and also rangelands. Changes in surface soil properties over time induced by armouring are not accounted for in current erosion models such as WEPP or RUSLE2 because little is known about rates of armouring over time as a function of rainfall intensity, rock content, slopes, and other factors. In this paper we simulate soil armouring induced by interrill erosion in two sets of experiments and propose ways to account for the process in WEPP and RUSLE2 without modifying the science behind the models. The first set of experiments was conducted to demonstrate and quantify the effect of armouring on sediment yields under varying rainfall intensities. Rainfall with intensities ranging from 22 to 80 mm h−1 was simulated on 0.56 m2 plots at slopes of 18 degrees (32.5%) using topsoil with high rock content from a mine restoration site. Results showed a clear relationship between rainfall intensity and armouring. There was an over 75% reduction in total soil loss under 22 mm h−1 rainfall between freshly applied soils and highly armoured soils at the same slope. A second set of experiments was conducted to understand the relationships between soil rock content, rate of surface rock cover change, slope change and sediment yields. Sediment yields and surface rock cover were quantified for non-cohesive soils consisting of glass beads with a diameter range of 45 to 90 μm and 0, 20 and 40% rock content in 0.22 m2 plots at a 15° (26.8%) slope under 80 mm h−1 simulated rainfall. Linear relationships were observed between cumulative sediment yields and rock cover, and exponential relationships between total runoff and rock cover. The armouring process was modeled at the experimental scale with WEPP and RUSLE2 by iteratively altering the rock cover and the slope over time. WEPP event based simulations at this scale resulted in reasonable predictions of sediment yields throughout the armouring process, and RUSLE2 required modification of soil erodibility values to account for high rock content and changes in runoff over time. Automation of this process, however, would require modifications of the models to expose rock cover and change slope over time. A method for doing this is discussed for interrill processes, but interactions between rill erosion and armouring need further study. The effect of long-slope high-energy rill flows on armouring also needs further investigation through field scale experiments, as this study did not examine the extent to which rock is uncovered and/or moved on long slopes differently from the residues already modeled in WEPP and RUSLE2.

Effect of Slope Length and Rainfall Intensity on Runoff and Erosion Conversion from Laboratory to Field

Predictions of soil and water loss at large extents often relies on data obtained from laboratory flume experiments. It is necessary to have a reliable approach to extrapolate from laboratory collected data to larger field areas. In this study, a series of experiments were designed using rainfall simulator on laboratory flumes with varying surface areas (ranging from 0.5 to 2.5 m2) as well as field plots ranging in surface areas between 4 and 20 m2. Both the flumes and field plots had the same slope gradient (20°), surface trait (bare slope) and soil type (red soil). We varied rainfall intensities from 30 to 150 mm/h, and measured runoff and erosion. Results confirmed that actual erosion in field cannot be simply extrapolated from laboratory data, rather we showed that erosion modulus directly relates to surface area and rainfall intensity in both the laboratory and field experiments. However, the effect of surface area on runoff is more complicated. Compared to surface area, rainfall intensity showed more pronounced influence on runoff. Based on our experimental results, a conversion calculation method was investigated and a conversion coefficient model, which is a function of rainfall intensity and ratio of field area to laboratory area, was introduced. The model provides a reference for laboratory to field conversions and allows for soil erosion prediction at larger extents in the field.

Accounting for variation in rainfall intensity and surface slope in wash-off model calibration and prediction within the Bayesian framework

Exponential wash-off models are the most widely used method to predict sediment wash-off from urban surfaces. In spite of many studies, there is still a lack of knowledge on the effect of external drivers such as rainfall intensity and surface slope on wash-off predictions. In this study, a more physically realistic “structure” is added to the original exponential wash-off model (OEM) by replacing the invariant parameters with functions of rainfall intensity and catchment surface slope, so that the model can better represent catchment and rainfall conditions without the need for lookup tables and interpolation/extrapolation. In the proposed new exponential model (NEM), two such functions are introduced. One function describes the maximum fraction of the initial load that can be washed off by a rainfall event for a given slope and the other function describes the wash-off rate during a rainfall event for a given slope. The parameters of these functions are estimated using data collected from a series of laboratory experiments carried out using an artificial rainfall generator, a 1 m2 bituminous road surface and a continuous wash-off measuring system. These experimental data contain high temporal resolution measurements of wash-off fractions for combinations of five rainfall intensities ranging from 33 to 155 mm/h and three catchment slopes ranging from 2 to 8%. Bayesian inference, which allows the incorporation of prior knowledge, is implemented to estimate parameter values. Explicitly accounting for model bias and measurement errors, a likelihood function representative of the wash-off process is formulated, and the uncertainty in the prediction of the NEM is quantified. The results of this study show: 1) even when the OEM is calibrated for every experimental condition, the NEM's performance, with parameter values defined by functions, is comparable to the OEM. 2) Verification indices for estimates of uncertainty associated with the NEM suggest that the error model used in this study is able to capture the uncertainty well.

Characteristics of rainfall intensity, duration, and kinetic energy for landslide triggering in Taiwan

Rainfall intensity-duration (I-D) characteristics are widely used to study landslide triggering. Recent studies also propose that rainfall kinetic energy (ek) can be used to quantify the rate of soil erosion induced by strong precipitation events. In the present study, the Joss-Waldvogel Disdrometers (JWD) was utilized to measure ek data, and the relationship between rainfall kinetic energy and rainfall intensity was determined as the regression results of ekN=32.19×(1–0.725e(−0.029I)) for northern Taiwan and ekS=32.23×(1–0.643e(−0.022I)) for southern Taiwan. Additionally, by examining data from 76 landslide events from the landslide inventory for the period of 2006–2012, the study established an empirical power law I-D relationship, which is crucial for determining the rainfall threshold needed in forecasting landslides. Two rainfall thresholds, i.e., functions of rainfall intensity (I) and rainfall duration (D), were established for study areas in northern and southern Taiwan to be IN=13.81 D−0.31 and IS=66.44 D−0.57 respectively. Landslides in northern Taiwan usually occurred when D was over 10h and such events in southern Taiwan took place in two different D groups whose gap was between 16h and 20h. Two mechanisms were operative in triggering landslides in the two areas—water infiltration and high rainfall intensity. However, ek also proved to be an important rainfall parameter, and the present study further established that not only rainfall intensity-duration but also kinetic energy can provoke landslide triggering, thus allowing the compiling of more comprehensive information for more accurate landslide forecasting.

Simulated precipitation diurnal cycles over East Asia using different CAPE-based convective closure schemes in WRF model

Closure assumption in convection parameterization is critical for reasonably modeling the precipitation diurnal variation in climate models. This study evaluates the precipitation diurnal cycles over East Asia during the summer of 2008 simulated with three convective available potential energy (CAPE) based closure assumptions, i.e. CAPE-relaxing (CR), quasi-equilibrium (QE), and free-troposphere QE (FTQE) and investigates the impacts of planetary boundary layer (PBL) mixing, advection, and radiation on the simulation by using the weather research and forecasting model. The sensitivity of precipitation diurnal cycle to PBL vertical resolution is also examined. Results show that the precipitation diurnal cycles simulated with different closures all exhibit large biases over land and the simulation with FTQE closure agrees best with observation. In the simulation with QE closure, the intensified PBL mixing after sunrise is responsible for the late-morning peak of convective precipitation, while in the simulation with FTQE closure, convective precipitation is mainly controlled by advection cooling. The relative contributions of different processes to precipitation formation are functions of rainfall intensity. In the simulation with CR closure, the dynamical equilibrium in the free troposphere still can be reached, implying the complex cause-effect relationship between atmospheric motion and convection. For simulations in which total CAPE is consumed for the closures, daytime precipitation decreases with increased PBL resolution because thinner model layer produces lower convection starting layer, leading to stronger downdraft cooling and CAPE consumption. The sensitivity of the diurnal peak time of precipitation to closure assumption can also be modulated by changes in PBL vertical resolution. The results of this study help us better understand the impacts of various processes on the precipitation diurnal cycle simulation.

Scale‐dependency of effective hydraulic conductivity on fire‐affected hillslopes

AbstractEffective hydraulic conductivity (Ke) for Hortonian overland flow modeling has been defined as a function of rainfall intensity and runon infiltration assuming a distribution of saturated hydraulic conductivities (Ks). But surface boundary condition during infiltration and its interactions with the distribution of Ks are not well represented in models. As a result, the mean value of the Ks distribution ( ), which is the central parameter for Ke, varies between scales. Here we quantify this discrepancy with a large infiltration data set comprising four different methods and scales from fire‐affected hillslopes in SE Australia using a relatively simple yet widely used conceptual model of Ke. Ponded disk (0.002 m2) and ring infiltrometers (0.07 m2) were used at the small scales and rainfall simulations (3 m2) and small catchments (ca 3000 m2) at the larger scales. We compared between methods measured at the same time and place. Disk and ring infiltrometer measurements had on average 4.8 times higher values of than rainfall simulations and catchment‐scale estimates. Furthermore, the distribution of Ks was not clearly log‐normal and scale‐independent, as supposed in the conceptual model. In our interpretation, water repellency and preferential flow paths increase the variance of the measured distribution of Ks and bias ponding toward areas of very low Ks during rainfall simulations and small catchment runoff events while areas with high preferential flow capacity remain water supply‐limited more than the conceptual model of Ke predicts. The study highlights problems in the current theory of scaling runoff generation.

Is groundwater response timing in a pre‐alpine catchment controlled more by topography or by rainfall?

AbstractGroundwater levels in steep headwater catchments typically respond quickly to rainfall, but the timing of the response may vary spatially across the catchment. In this study, we investigated the topographic controls and the effects of rainfall and antecedent conditions on the groundwater response timing for 51 groundwater monitoring sites in a 20‐ha pre‐alpine catchment with low permeability soils. The median time to rise and median duration of recession for the 133 rainfall events were highly correlated to the topographic characteristics of the site and its upslope contributing area. The median time to rise depended more on the topographic characteristics than on the rainfall characteristics or antecedent soil wetness conditions. The median time to rise decreased with Topographic Wetness Index (TWI) for sites with TWI < 6 and was almost constant for sites with a higher TWI. The slope of this relation was a function of rainfall intensity. The rainfall threshold for groundwater initiation was also a function of TWI and allowed extrapolation of point measurements to the catchment scale. The median lag time between the rainfall centroid and the groundwater peak was 75 min. The groundwater level peaked before peak streamflow at the catchment outlet for half of the groundwater monitoring sites, but only by 15 to 25 min. The stronger correlations between topographic indices and groundwater response timing in this study compared to previous studies suggest that surface topography affects the groundwater response timing in catchments with low permeability soils more than in catchments with more transmissive soils. Copyright © 2015 John Wiley & Sons, Ltd.

Is the connectivity function a good indicator of soil infiltrability distribution and runoff flow dimension?

AbstractAmong the studies on runoff connectivity of soils with heterogeneous properties, the need to understand the relationships between soil heterogeneity and the associated runoff organization and amount is frequently mentioned. In this study, we simulate the stationary runoff–runon process on bi‐dimensional (2D) flat slopes for five infiltrability distributions, one of them correlated, as a function of rainfall intensity and flow dimension. We define flow dimension by 1 + ε, where ε is the outflow fraction transferred from one pixel to each of the two lateral downslope pixels. Our aim is to assess the effect of ε and soil heterogeneity on the connectivity function compared to the mean runoff flow rate, the wet area and the number of runoff patterns. The analysis of connectivity is carried within the percolation framework. The results show that the integral connectivity scale is more sensitive to the flow dimension and soil heterogeneity compared to the other variables. The wet area fraction does not depend on ε. Unlike previous studies, we find that increased runoff production is not necessarily related to increased connectivity. The use of the connectivity function within the percolation framework appears to be a valuable method for assessing the impact of soil heterogeneity and flow dimension on the runoff organization during a rainfall event. Copyright © 2014 John Wiley & Sons, Ltd.

Experimental rainfall–runoff data: Reconsidering the concept of infiltration capacity

► Combination of a variable rainfall intensity field infiltrometer and photography. ► Final infiltration rates and inundated area increase with rainfall intensity. ► Residues, sealing, microtopography interact with rainfall intensity and inundation. ► We argue for a dynamic rather than constant effective hydraulic conductivity ( K e ). Many infiltration models rely on an effective hydraulic conductivity parameter ( K e ) which is often determined in the field from rainfall simulation experiments on small plots. K e can be defined as the spatially averaged infiltration capacity when the soil is ‘field-saturated’ and steady state is reached. Then it equals the infiltration rate ( f ), provided ponding occurs. When a homogeneous surface is assumed, with negligible ponding depth, K e is constant and does not vary with rainfall intensity ( r ). We developed a drop infiltrometer that allows measuring K e on small plots under simulated rainfall intensities that vary between experiments. Infiltration experiments were conducted on a winter wheat field in the Belgian Loess Belt and various surface and soil properties were measured. Furthermore, photos were taken of the soil surface during the infiltration experiments for the determination of the inundated surface fraction. The results of the experiments show that K e is strongly dependent on rainfall intensity. In a statistical approach a dynamic K e could be estimated with a function of rainfall intensity, tillage treatment, percentage residue cover and bulk density. Observations indicate that microtopography, surface fraction covered by a sedimentary seal and macroporosity interact with rainfall intensity, surface ponding and infiltration. We propose that K e in physically based infiltration models should either be made dependent on dynamic state variables in a mechanistic way, such as ponding depth and water content or made dependent on rainfall intensity using an empirical relationship. With such adaptations, both surface runoff and erosion models might have more potential to deal with scale effects in runoff generation.

Aero- and hydroacoustics of the impact for a droplet freely falling onto the water surface

Studies of processes arising in the case of the impact of a droplet falling vertically onto a quiescent water surface, which began in the nineteenth century [1], nowadays, as previously, attract attention of scien� tists, both theoreticians and experimentalists [2]. This is associated with the importance of these processes for practical applications and the complexity of the mathematical description of relevant flows. Among the most interesting phenomena accompanying these processes, we can indicate the evolution of the shape of the arising cavern, as well as the splash and the cas� cade of vortices propagating into the bulk of the liquid. The objects of the studies are rapid processes (forma� tion of a cavern or of a splash and droplets ejected by basic liquid), which are illustrated well by the mono� graphs [3, 4]. However, slow process (e.g., droplet transformation to the vortex ring [1] and the cascade� vortex generation [5]) are also extremely interesting. The appearance in the beginning of the last century of sensitive microphones has complemented hydrody� namic studies by investigations of sound propagating from the region of the impact of a droplet with water in the perturbed medium [6]. The hydroacoustics of the droplet impact with water began to develop during World War II when problems of remote location and secrecy of submarines became urgent. The potentiali� ties of the equipment existing that time made it possi� ble to find only qualitative regularities, in particular, the power and the spectrum of sound as functions of rainfall intensity [7]. More recently, studies of sound emitted into the bulk of the liquid and associated with the impacts of individual droplets have begun [8].

Impacts of Polarimetric Radar Observations on Hydrologic Simulation

Abstract Rainfall estimated from the polarimetric prototype of the Weather Surveillance Radar-1988 Doppler [WSR-88D (KOUN)] was evaluated using a dense Micronet rain gauge network for nine events on the Ft. Cobb research watershed in Oklahoma. The operation of KOUN and its upgrade to dual polarization was completed by the National Severe Storms Laboratory. Storm events included an extreme rainfall case from Tropical Storm Erin that had a 100-yr return interval. Comparisons with collocated Micronet rain gauge measurements indicated all six rainfall algorithms that used polarimetric observations had lower root-mean-squared errors and higher Pearson correlation coefficients than the conventional algorithm that used reflectivity factor alone when considering all events combined. The reflectivity based relation R(Z) was the least biased with an event-combined normalized bias of −9%. The bias for R(Z), however, was found to vary significantly from case to case and as a function of rainfall intensity. This variability was attributed to different drop size distributions (DSDs) and the presence of hail. The synthetic polarimetric algorithm R(syn) had a large normalized bias of −31%, but this bias was found to be stationary. To evaluate whether polarimetric radar observations improve discharge simulation, recent advances in Markov Chain Monte Carlo simulation using the Hydrology Laboratory Research Distributed Hydrologic Model (HL-RDHM) were used. This Bayesian approach infers the posterior probability density function of model parameters and output predictions, which allows us to quantify HL-RDHM uncertainty. Hydrologic simulations were compared to observed streamflow and also to simulations forced by rain gauge inputs. The hydrologic evaluation indicated that all polarimetric rainfall estimators outperformed the conventional R(Z) algorithm, but only after their long-term biases were identified and corrected.

Neighborhood Verification: A Strategy for Rewarding Close Forecasts

Abstract High-resolution forecasts may be quite useful even when they do not match the observations exactly. Neighborhood verification is a strategy for evaluating the “closeness” of the forecast to the observations within space–time neighborhoods rather than at the grid scale. Various properties of the forecast within a neighborhood can be assessed for similarity to the observations, including the mean value, fractional coverage, occurrence of a forecast event sufficiently near an observed event, and so on. By varying the sizes of the neighborhoods, it is possible to determine the scales for which the forecast has sufficient skill for a particular application. Several neighborhood verification methods have been proposed in the literature in the last decade. This paper examines four such methods in detail for idealized and real high-resolution precipitation forecasts, highlighting what can be learned from each of the methods. When applied to idealized and real precipitation forecasts from the Spatial Verification Methods Intercomparison Project, all four methods showed improved forecast performance for neighborhood sizes larger than grid scale, with the optimal scale for each method varying as a function of rainfall intensity.

A New Splash and Sheet Erosion Equation for Rangelands

Soil loss rates predicted from erosion models for rangelands have the potential to be important quantitative indicators for rangeland health and for assessing conservation practices. Splash and sheet erosion processes on rangelands differ from croplands, where the process is conceptualized in part as interrill erosion. Previous interrill equations were developed from cropland soils where interrill erosion was conceptualized and modeled for small plots, which are not generally large enough to encompass the relative high spatial heterogeneity of rangelands. Also, interrill erosion is usually modeled as a function of rainfall intensity (I) and runoff rate (q) such that I and q are independent of each other. Splash and sheet erosion is the dominant type of erosion on most undisturbed rangeland hillslopes where there is adequate vegetation, and these important erosion processes need to be addressed to develop an appropriate rangeland erosion model. In this study, we developed a new equation for calculating the combined rate of splash and sheet erosion (Dss) using a large set of rainfall simulation data from the western United States. The equation we propose: Dss = KssI1.052q0.592, where Kss is a splash and sheet erosion coefficient, takes into account a key interrelationship between I and q revealed in the data. This equation was successfully evaluated using independent sets of multiple‐intensity experimental data. The new equation should enable improved estimation of water erosion on rangelands in the western United States and in other parts of the world.

Quantifying rainfall–runoff relationships on the Dera Calcic Fluvic Regosol ecotope in Ethiopia

Droughts, resulting in low crop yields, are common in the semi-arid areas of Ethiopia and adversely influence the well-being of many people. The objective of this study was to assess the benefit that in-field rainwater harvesting (IRWH) would have, compared to conventional tillage, on maize yields on a semi-arid ecotope at Dera situated on the eastern part of the Rift Valley. Rainfall–runoff measurements were made during 2003 and 2004 on 2 m × 2 m plots provided with a runoff measuring system and replicated three times for each treatment. There were two treatments: conventional tillage (CT) and no-till (NT), the latter with a flat surface that promotes runoff and therefore IRWH. Rainfall intensity was measured at 1 min intervals with an automatic tipping bucket instrument, and runoff was measured after each rain event. Measured runoff as a function of rainfall intensity and duration from half the rainfall–runoff events was used to determine the critical parameters of a appropriate runoff model. The calibrated model was found to be capable of predicting runoff in a satisfactory way. Rainfall–runoff measurements were made during the rain seasons in 2003 and 2004 during which there were 25 rain events with >9 mm of rain. There was no statistical difference between the runoff on the two treatments. The measured runoff ( R) for the two rain seasons, expressed as a fraction of the rainfall during the measuring period ( P), i.e. R/ P, gave values of 0.46 and 0.39 for the NT and CT treatments, respectively. Results from 7 years of field experiments with IRWH at Glen in South Africa were used to estimate the yield benefit of NT for Dera compared to CT. The results were 696 and 494 kg ha −1 for 2003 and 2004, respectively. Based on the estimated average long-term maize yield of 2000 kg ha −1 at Dera, this was an estimated yield increase ranging from 25% to 35%.

Drainage hydraulics of permeable friction courses

This paper describes solutions to the hydraulic equations that govern flow in permeable friction courses (PFC). PFC is a layer of porous asphalt approximately 50 mm thick that is placed as an overlay on top of an existing conventional concrete or asphalt road surface to help control splash and hydroplaning, reduce noise, and enhance quality of storm water runoff. The primary objective of this manuscript is to present an analytical system of equations that can be used in design and analysis of PFC systems. The primary assumptions used in this analysis are that the flow can be modeled as one‐dimensional, steady state Darcy‐type flow and that slopes are sufficiently small so that the Dupuit‐Forchheimer assumptions apply. Solutions are derived for cases where storm water drainage is confined to the PFC bed and for conditions where the PFC drainage capacity is exceeded and ponded sheet flow occurs across the pavement surface. The mathematical solutions provide the drainage characteristics (depth and residence time) as a function of rainfall intensity, PFC hydraulic conductivity, pavement slope, and maximum drainage path length.

Discussion of "Response of a residual soil slope to rainfall"

Londono 983 The advance of the wetting front in unsaturated soils has motivated continuous laboratory and field research. Rahardjo et al. (2005) presented an interesting field study of the response of residual slopes to rainfall, monitoring the runoff generation and rapid advance of infiltration within soils. A separate study by A.C. Londono2 in 2004 of the instability of the lower Licking River banks (Fig. 1) found similar results for riverbanks under flooding conditions. The research focused on (a) the mechanical characteristics of the material forming the banks, (b) the processes responsible for the instability of the banks, (c) a sensitivity analysis of soil parameters in the stability computations, and (d) the role that pore-water pressure played in affecting the strength of the cohesive materials. The following discussion will report only the pore-water pressure behavior portion of the study. Floodplains have been centers of human occupation since the dawn of civilization because of their fertility and the accessibility to water for agriculture and industry. There are, however, problems with floodplains: they flood, and their banks are often unstable and prone to erosion. The banks of the Licking River in northern Kentucky display continued instability, making them ideally suited for analyzing the processes initiating mass movement and ways they may be mitigated. In 1997, a major flood occurred in portions of southern Ohio, Indiana, and northern Kentucky. Floodplains in these areas were covered by 10 m (32.8 ft) of water and remained inundated for 2 weeks. During this period, water percolated into the silty-clay soil, changing its hydraulic conditions from partially saturated to saturated with positive pore pressure, thereby decreasing the effective stress. When the flood receded, the banks were fully saturated under undrained conditions without lateral confinement. Elevated pore pressures combined with the loss of soil strength triggered several rotational slumps, causing damage to adjacent property. The resulting slope failures motivated the study of slope stability conducted by A.C. Londono2 in 2004. Recent studies (Simon et al. 2000; Darby et al. 2000; Simon et al. 2002) have also correlated bank instability with increased pore-water pressure resulting from flooding events. During periods of flooding or high water level, soils become saturated. When the river level drops with sufficient rapidity, the soil remains saturated and significant seepage forces develop, thereby increasing lateral shear stresses, which may cause the banks to fail. In natural conditions, the soil is subjected to changes in moisture content along its profile owing to seasonal variations. The uppermost layer of the profile is affected by daily evaporation and incipient infiltration during low-intensity rainfalls. As depth increases, the changes in moisture are controlled by the rate of infiltration and by the fluctuations in the position of the water table, thus enabling rapid changes in the soil from unsaturated to saturated conditions. Different approaches can be taken to approximate the infiltration of water into the soil. Field experiments with simulated rainfall conducted by Londono (1995) considered infiltration as a function of rainfall intensity and slope by variation of the resistivity. That study found the wetting front rapidly advanced into the soil (Fig. 2), increasing its moisture content hence decreasing its resistivity. The same values of specific resistance were found in the deepest sections of the soil after a rainfall event. This was interpreted in two ways: (i) all infiltrated water was incorporated into the soil in the upper levels, or (ii) the moisture content did not change because the soil had reached saturation. Rahardjo et al. (2005) found that the recession limb of the rainfall hydrograph is influenced by slope topography and rainfall characteristics. They suggested that the content of water in the soil was greater at the toe of the slope than at the crest. This influence of slope topography was also found by Londono (1995) to be an important factor in the advance of the wetting front. Tests of simulated rainfall in slopes of similar soil characteristics and different inclination indicated that there is a gradual decrease in the depth the infiltration reaches with increase in slope inclination. During dry periods, soils are partially saturated and have relatively high shear strength, which allows them to remain stable. During wet periods, however, soils lose strength. Although many studies have correlated landsliding with highintensity rain events, several studies have found just one