Low-intensity Rainfall Research Articles

Effects of erosion on the microaggregate organic carbon dynamics in a small catchment of the Loess Plateau, China

Soil erosion significantly affects the dynamics of the terrestrial carbon (C) cycle. Erosion removes C-rich topsoil and deposits it in lower-elevation areas and exposes deeper C-poor horizons at the surface. However, the mechanisms responsible for the mobilization and deposition of microaggregate soil organic carbon (SOC) due to soil erosion at the catchment scale remain unclear. The main objective of this study is to illustrate the effects of different erosion processes on the distribution and deposition of microaggregate SOC. The activity of 137Cs and extreme rainfall events were used as dating methods Based on the microaggregate size distribution and the K-means clustering approach, 75 flood couplets were identified in an 11.3-m sediment deposit profile of the check dam in the Nianyangou catchment and classified into three couplet types. In each flood couplet, the SOC concentrations and amounts in three microaggregate fractions (250–50, 50–20 and <20μm) were quantified. The following results were obtained. (1) 137Cs activity and a record of extreme rainfall events can be used to characterize the sediment deposition process. (2) Sediments with fine microaggregate fractions (Couplet Type I) contained abundant SOC and were mainly associated with low-intensity rainfall conditions, whereas sediments with coarse microaggregate fractions (Couplet Type III) had higher SOC concentrations and were mainly associated with high-intensity rainfall conditions. (3) Couplet Type I sediments were most likely related to sheet and interrill erosion, whereas Couplet Type III sediments were likely associated with rill and gully erosion. Our study contributes to the understanding of the effects of soil erosion on microaggregate SOC dynamics at the catchment scale and further illustrates the carbon sequestration mechanism active in check dams of the Loess Plateau.

Statistical modeling of CMIP5 projected changes in extreme wet spells over China in the late 21st century

The observed intensity, frequency, and duration (IFD) of summer wet spells, defined here as extreme events with one or more consecutive days in which daily precipitation exceeds a given threshold (the 95th percentile), and their future changes in RCP4.5 and RCP8.5 in the late 21st century over China, are investigated by using the wet spell model (WSM) and by extending the point process approach to extreme value analysis. Wet spell intensity is modeled by a conditional generalized Pareto distribution, frequency by a Poisson distribution, and duration by a geometric distribution, respectively. The WSM is able to realistically model summer extreme rainfall spells during 1961–2005, as verified with observations at 553 stations throughout China. To minimize the impact of systematic biases over China in the global climate models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5), five best GCMs are selected based on their performance to reproduce observed wet spell IFD and average precipitation during the historical period. Furthermore, a quantile–quantile scaling correction procedure is proposed and applied to produce ensemble projections of wet spell IFD and corresponding probability distributions. The results show that in the late 21st century, most of China will experience more extreme rainfall and less low-intensity rainfall. The intensity and frequency of wet spells are projected to increase considerably, while the duration of wet spells will increase but to a much less extent. The IFD changes in RCP8.5 are in general much larger than those in RCP4.5.

An experimental study on the influences of wind erosion on water erosion

In semi-arid regions, complex erosion resulted from a combination of wind and water actions has led to a massive soil loss and a comprehensive understanding of its mechanism is the first step toward prevention of the erosion. However, the mutual influences between wind erosion and water erosion have not been fully understood. This research used a wind tunnel and two rainfall simulators and simulated two rounds of alternations between wind erosion and water erosion (i.e., 1st wind erosion–1st water erosion and 2nd wind erosion–2nd water erosion) on three slopes (5°, 10°, and 15°) with six wind speeds (0, 9, 11, 13, 15, and 20 m/s) and five rainfall intensities (0, 30, 45, 60, and 75 mm/h). The objective was to analyze the influences of wind erosion on succeeding water erosion. Results showed that the effects of wind erosion on water erosion were not the same in the two rounds of tests. In the 1st round of tests, wind erosion first restrained and then intensified water erosion mostly because the blocking effect of wind-sculpted micro-topography on surface flow was weakened with the increase in slope. In the 2nd round of tests, wind erosion intensified water erosion on beds with no rills at gentle slopes and low rainfall intensities or with large-size rills at steep slopes and high rainfall intensities. Wind erosion restrained water erosion on beds with small rills at moderate slopes and moderate rainfall intensities. The effects were mainly related to the fine grain layer, rills and slope of the original bed in the 2nd round of tests. The findings can deepen our understanding of complex erosion resulted from a combination of wind and water actions and provide scientific references to regional soil and water conservation.

Characterising urban zinc generation to identify surface pollutant hotspots in a low intensity rainfall climate.

Characterising stormwater runoff quality provides useful insights into the dynamics of pollutant generation and wash off rates. These can be used to prioritise stormwater management strategies. This study examined the effects of a low intensity rainfall climate on zinc contributions from different impermeable urban surface types. First flush (FF) and steady state samples were collected from seven different surfaces for characterisation, and the data were also used to calibrate an event-based pollutant load model to predict individual 'hotspot' surfaces across the catchment. Unpainted galvanised roofs generated very high concentrations of zinc, primarily in the more biologically available dissolved form. An older, unpainted galvanised roof had FF concentrations averaging 32,338 μg/L, while the new unpainted roof averaged 4,782 μg/L. Roads and carparks also had elevated zinc, but FF concentrations averaged only 822-1,584 μg/L. Modelling and mapping expected zinc loads from individual impermeable surfaces across the catchment identified specific commercial roof surfaces to be targeted for zinc management. The results validate a policy strategy to replace old galvanised roof materials and avoid unpainted galvanised roofing in future urban development for better urban water quality outcomes. In the interim, readily-implemented treatment options are required to help mitigate chronic zinc impacts on receiving waterways.

Simulation and feasibility of biological and structural BMPs for stormwater control in the urbanizing watersheds

The development and increasing of construction activities results in the permeability reduction of surfaces and consequently improper impacts on the transport and convey of surface runoff in the urban areas. Regarding the development of urban areas, it is crucial to select and implement the best management practices (BMPs), the functional and practical methods for reducing and controlling runoff volume. In this research, BMPs are implemented in one of the developing areas of Tehran, Iran, with 30 sub-watersheds and modeled in stormwater management model program. Then, the effects of implementing BMPs are investigated on the reduction of runoff volume in the main manholes of collecting and transporting runoff network. For this purpose, the plan maps of BMPs are prepared for different hydrological conditions, regarding their types and implementation surface areas. Besides, the design, construction and maintenance of BMPS are evaluated economically. According to the results obtained in this research, runoff volume is reduced for 33–70 and 22–54%, respectively, in the low and high intensity rainfalls with the return periods of 2–10 years. These results can provide a practical and strategic approach to properly understanding the effects of BMPs implementation in reducing and controlling the surface runoff volume in the urban areas. Moreover, they can be considered as an important step to promote the implementation of BMPs for surface runoff management in the developing urban areas.

Usability of the BLRP model for hydrological applications in arid and semi-arid regions with limited precipitation data

In this study, Hydrological Simulation Program-FORTRAN (HSPF) is used to investigate rainfall-runoff process in Taleghan watershed, northern Iran. Despite the high accuracy of the model, the lack of rainfall data at short time scales (hour and less than hour) restricted implementation of the model especially for long time simulations. Some studies use simple division for daily rainfall disaggregation into the hourly values to provide data requirements of HSPF model. In simple division, each rainfall event is divided into 24 pulse stochastically and the peak flows may not properly being simulated due to the lower rainfall intensities. In this study, random parameter Bartlett–Lewis rectangular pulse (BLRP) model was implemented to disaggregate daily rainfall time series into the hourly values and the results compared with that of simple division. In BLRP model, parameters of the model calibrated against the 1, 24 and 48 h mean, variance, lag1 auto covariance and proportion dry of observed rainfall. The calibrated model was then implemented to disaggregate daily rainfall data into the hourly values. To compare two disaggregation approaches, daily stream flow simulation by HSPF model is initialized in 2 scenarios by applying the hourly rainfall data resulted from two disaggregation methods. The results indicated that while using the simple division method leads to the underestimation of peak flows, using the BLRP model improved peak flow simulations. This study indicated usability of the BLRP model for rainfall disaggregation in arid and semi-arid regions with limited fine scale precipitation data availability.

Laboratory Studies on the Influence of Rainfall Pattern on Rill Erosion and Its Runoff and Sediment Characteristics

AbstractRill is a major type of erosion on upland slopes. Continuous rainfall is commonly used in laboratory studies on rill erosion despite the fact the rainfall was often discontinuous in the field; this is particularly true in the Chinese Loess Plateau. This study compares rill erosion under continuous and intermittent rainfalls by using laboratory experiments. The experiments include two rainfall‐intensity treatments (90 and 120 mm h−1) and two rainfall‐pattern treatments (continuous and intermittent). The results indicate that rill formation had a significant effect on runoff and sediment concentration. For continuous and intermittent rainfall at the rainfall intensity of 90 mm h−1, the mean sediment concentrations were 1·91 and 1·73 times after rill initiation than those before rill initiation, respectively, and the rill erosion accounted for 75·5% and 77·7% of runoff duration, respectively. For continuous and intermittent rainfall at the rainfall intensity of 120 mm h−1, the mean sediment concentrations after rill initiation were 1·38 and 1·32 times that those before rill initiation, respectively, and the rill erosion represented 88·7% and 78·8% of the total runoff duration, respectively. We observed sediment sorting under all treatments; however, the low rainfall intensity boosted but the high rainfall intensity lowered the clay fraction; in contrast, the sorting remained roughly the same between the rainfall‐pattern treatments. The runoff velocity also affected the sediment sorting. Our empirical results indicated the important significance of the rainfall intermittence in predicting rill erosion. Copyright © 2017 John Wiley &amp; Sons, Ltd.

The potential of urban rainfall monitoring with crowdsourced automatic weather stations in Amsterdam

Abstract. The high density of built-up areas and resulting imperviousness of the land surface makes urban areas vulnerable to extreme rainfall, which can lead to considerable damage. In order to design and manage cities to be able to deal with the growing number of extreme rainfall events, rainfall data are required at higher temporal and spatial resolutions than those needed for rural catchments. However, the density of operational rainfall monitoring networks managed by local or national authorities is typically low in urban areas. A growing number of automatic personal weather stations (PWSs) link rainfall measurements to online platforms. Here, we examine the potential of such crowdsourced datasets for obtaining the desired resolution and quality of rainfall measurements for the capital of the Netherlands. Data from 63 stations in Amsterdam (∼ 575 km2) that measure rainfall over at least 4 months in a 17-month period are evaluated. In addition, a detailed assessment is made of three Netatmo stations, the largest contributor to this dataset, in an experimental setup. The sensor performance in the experimental setup and the density of the PWS network are promising. However, features in the online platforms, like rounding and thresholds, cause changes from the original time series, resulting in considerable errors in the datasets obtained. These errors are especially large during low-intensity rainfall, although they can be reduced by accumulating rainfall over longer intervals. Accumulation improves the correlation coefficient with gauge-adjusted radar data from 0.48 at 5 min intervals to 0.60 at hourly intervals. Spatial rainfall correlation functions derived from PWS data show much more small-scale variability than those based on gauge-adjusted radar data and those found in similar research using dedicated rain gauge networks. This can largely be attributed to the noise in the PWS data resulting from both the measurement setup and the processes occurring in the data transfer to the online PWS platform. A double mass comparison with gauge-adjusted radar data shows that the median of the stations resembles the rainfall reference better than the real-time (unadjusted) radar product. Averaging nearby raw PWS measurements further improves the match with gauge-adjusted radar data in that area. These results confirm that the growing number of internet-connected PWSs could successfully be used for urban rainfall monitoring.

First flush characteristics of rainfall runoff from a paddy field in the Taihu Lake watershed, China.

Nonpoint storm runoff remains a major threat to surface water quality in China. As a paddy matures, numerous fertilizers are needed, especially in the rainy seasons; the concentration of nitrogen and phosphorus in rainfall runoff from farmland is much higher than at other times, and this poses a great threat to water bodies and is the main reason for water eutrophication, especially in high concentration drainages. To date, most studies regarding the characteristics of pollutants in rainfall runoff have mainly been concentrated on urban runoff and watershed runoff; therefore, it is particularly important to investigate the characteristics of nitrogen and phosphorus loss in rainfall runoff from paddy fields. To study the characteristics of nitrogen and phosphorus loss and whether the first flush effect exists, continuous monitoring of the rainfall runoff process of six rainfall events was conducted in 2013, of which four rainfall events during storm, high, middle, and low intensity rainfalls were analyzed, and runoff and quality parameters, such as suspended solids (SS), total nitrogen (TN), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), total phosphorus (TP), and phosphate (PO43--P), were analyzed to determine the relationship between runoff and water quality. The paddy field is located north of Wuxi Lake Basin along the Hejia River upstream in Zhoutie town, Yixing city. An analysis of the load distribution during rainfall runoff was conducted. Event mean concentration (EMC) was used to evaluate the pollution situation of the paddy field's rainfall runoff. A curve of the dimensionless normalized cumulative load (L) vs. normalized cumulative flow (F) (L-F curve), the probability of the mass first flush (MFFn), and the pollutants carried by the initial 25% of runoff (FF25) were used to analyze the first flush effect of the paddy field runoff, and different contaminants show different results: the concentration of nitrogen and phosphorus fluctuate and follow a similar trend as runoff changes, NO3--N concentration is lower in the early part of runoff and higher in the later, and TP mainly occurs in the particle state in storm runoff and mainly in the dissolved state when the rainfall intensity is smaller. Nitrogen and phosphorus losses from paddy fields are closely related to the average rainfall intensity and the max rainfall intensity, and the runoff loss of nitrogen and phosphorus is more severe when the rainfall intensity is large. Based on an analysis of multiple methodologies, TN and NH4+-N show a certain degree of a first flush effect, whereas the first flush effect of TP is not obvious. The first flush effect of SS is obvious in larger intensity rainfall and shows a slight secondary flush effect in smaller rainfall events.

Contribution of raindrop impact to the change of soil physical properties and water erosion under semi-arid rainfalls.

Soil erosion by water is a three-phase process that consists of detachment of soil particles from the soil mass, transportation of detached particles either by raindrop impact or surface water flow, and sedimentation. Detachment by raindrops is a key component of the soil erosion process. However, little information is available on the role of raindrop impact on soil losses in the semi-arid regions where vegetation cover is often poor and does not protect the soil from rainfall. The objective of this study is to determine the contribution of raindrop impact to changes in soil physical properties and soil losses in a semiarid weakly-aggregated agricultural soil. Soil losses were measured under simulated rainfalls of 10, 20, 30, 40, 50, 60 and 70mmh-1, and under two conditions: i) with raindrop impact; and, ii) without raindrop impact. Three replications at each rainfall intensity and condition resulted in a total of 42 microplots of 1m×1.4m installed on a 10% slope according to a randomized complete block design. The contribution of raindrop impact to soil loss was computed using the difference between soil loss with raindrop impact and without raindrop impact at each rainfall intensity. Soil physical properties (aggregate size, bulk density and infiltration rate) were strongly damaged by raindrop impact as rainfall intensity increased. Soil loss was significantly affected by rainfall intensity under both soil surface conditions. The contribution of raindrop impact to soil loss decreased steadily with increasing rainfall intensity. At the lower rainfall intensities (20-30mmh-1), raindrop impact was the dominant factor controlling soil loss from the plots (68%) while at the higher rainfall intensities (40-70mmh-1) soil loss was mostly affected by increasing runoff discharge. At higher rainfall intensities the sheet flow protected the soil from raindrop impact.

Determination of Optimal Parameters for Wind Driven Rain CFD Simulation for Building Design in the Tropics

The study of wind driven rain is an important design consideration in the built environment. Numerical prediction of this phenomenon requires knowledge of wind speed, rain drop sizes and rain fall intensity among others. The wind speed and rain drop sizes are usually calculated from field measurements. Rainfall intensity is usually assumed to follow a generalized Pareto distribution. Studies have indicated that the rain drop sizes are a function of the rainfall intensity. From a design optimization perspective, it is unclear what value of rainfall intensity should be considered in the process. This paper seeks to examine that in detail. Rain over a simplified building is chosen as the representative test case. The results suggest that it is the lower rainfall intensities that are more critical to be considered in the design process. A combination of using both the median as well as the mean rainfall intensities is proposed for the same.

A catastrophic debris flow in the Wenchuan Earthquake area, July 2013: characteristics, formation, and risk reduction

In the Wenchuan Earthquake area, many co-seismic landslides formed blocking-dams in debris flow channels. This blocking and bursting of landslide dams amplifies the debris flow scale and results in severe catastrophes. The catastrophic debris flow that occurred in Qipan gully (Wenchuan, Southwest China) on July 11, 2013 was caused by intense rainfall and upstream cascading bursting of landslide dams. To gain an understanding of the processes of dam bursting and subsequent debris flow scale amplification effect, we attempted to estimate the bursting debris flow peak discharges along the main gully and analyzed the scale amplification process. The results showed that the antecedent and triggering rainfalls for 11 July debris flow event were 88.0 mm and 21.6 mm, respectively. The event highlights the fact that lower rainfall intensity can trigger debris flows after the earthquake. Calculations of the debris flow peak discharge showed that the peak discharges after the dams-bursting were 1.17–1.69 times greater than the upstream peak discharge. The peak discharge at the gully outlet reached 2553 m3/s which was amplified by 4.76 times in comparison with the initial peak discharge in the upstream. To mitigate debris flow disasters, a new drainage channel with a trapezoidal V-shaped cross section was proposed. The characteristic lengths (h1 and h2) under optimal hydraulic conditions were calculated as 4.50 m and 0.90 m, respectively.

Urban rainwater runoff quantity and quality – A potential endogenous resource in cities?

Rainwater harvesting might help to achieve self-sufficiency, but it must comply with health standards. We studied the runoff quantity and quality harvested from seven urban surfaces in a university campus in Barcelona according to their use (pedestrian or motorized mobility) and materials (concrete, asphalt and slabs). An experimental rainwater harvesting system was used to collect the runoff resulting from a set of rainfall events. We estimated the runoff coefficient and initial abstraction of each surface and analyzed the physicochemical and microbiological properties, and hydrocarbon and metal content of the samples. Rainfall intensity, surface material and state of conservation were essential parameters. Because of low rainfall intensity and surface degradation, the runoff coefficient was variable, with a minimum of 0.41. Concrete had the best quality, whereas weathering and particulate matter deposition led to worse quality in asphalt areas. Physicochemical runoff quality was outstanding when compared to superficial and underground water. Microorganisms were identified in the samples (>1 CFU/100 mL) and treatment is required to meet human consumption standards. Motorized traffic mostly affects the presence of metals such as zinc (31.7 μg/L). In the future, sustainable mobility patterns might result in improved rainwater quality standards.

Evaluation of the relative roles of a vegetative filter strip and a biofiltration swale in a treatment train for road runoff.

In order to determine the relative importance of a vegetative filter strip and a biofiltration swale in a treatment train for road runoff, US EPA Storm Water Management Model was used to model infiltration and runoff from the filter strip. The model consisted of a series of subcatchments representing the road, the filter strip and the side-slopes of the swale. Simulations were carried out for different rain scenarios representing a variety of climatic conditions. In addition, a sensitivity analysis was conducted for the model's different parameters (soil characteristics and initial humidity, roughness, geometry, etc.). This exercise showed that for the system studied, the majority of road runoff is treated by the filter strip rather than the biofiltration swale, an effect observed especially during periods of low-intensity rainfall. Additionally, it was observed that the combination of infiltration of road runoff in the filter strip and direct rainfall on the system leads to a significant and variable dilution of the runoff reaching the swale. This result has important implications for evaluating the treatment efficiency of the system.

Characteristics of rainfall and runoff in urban drainage based on the SWMM model.

The characteristics of 235 rainfall and surface runoff events, from 2009 to 2011 in a typical urban drainage area in Shanghai were analyzed by using SWMM model. The results showed that the rainfall events in the region with high occurrence frequency were characterized by small rainfall amount and low intensity. The most probably occurred rainfall had total amount less than 10 mm, or mean intensity less than 5 mm·h-1,or peak intensity less than 10 mm·h-1, accounting for 66.4%, 88.8% and 79.6% of the total rainfall events, respectively. The study was of great significance to apply low-impact development to reduce runoff and non-point source pollution under condition of less rainfall amount or low mean rainfall intensity in the area. The runoff generally increased with the increase of rainfall. The threshold of regional occurring runoff was controlled by not only rainfall amount, but also mean rainfall intensity and rainfall duration. In general, there was no surface runoff when the rainfall amount was less than 2 mm. When the rainfall amount was between 2 to 4 mm and the mean rainfall intensity was below 1.6 mm·h-1, the runoff was less than 1 mm. When the rainfall exceeded 4 mm and the mean rainfall intensity was larger than 1.6 mm·h-1, the runoff would occur generally. Based on the results of the SWMM simulation, three regression equations that were applicable to regional runoff amount and rainfall factors were established. The adjustment R2 of the three equations were greater than 0.97. This indicated that the equations could reflect well the relationship between runoff and rainfall variables. The results provided the basis of calculations to plan low impact development and better reduce overflow pollution in local drainage area. It also could serve as a useful reference for runoff study in similar drainage areas.

Measurement and modeling of rainfall interception by two differently aged secondary forests in upland eastern Madagascar

Secondary forests occupy a larger area than old-growth rain forests in many tropical regions but their hydrological functioning is still poorly understood. In particular, little is known about the various components of evapotranspiration in these possibly vigorously regenerating forests. This paper reports on a comparison of measured and modeled canopy interception losses (I) from a semi-mature (ca. 20years) and a young (5–7years) secondary forest in the lower montane rain forest zone of eastern Madagascar. Measurements of gross rainfall (P), throughfall (Tf), and stemflow (Sf) were made in both forests for one year (October 2014–September 2015) and the revised analytical model of Gash et al. (1995) was tested for the first time in a tropical secondary forest setting. Overall measured Tf, Sf and derived I in the semi-mature forest were 71.0%, 1.7% and 27.3% of incident P, respectively. Corresponding values for the young forest were 75.8%, 6.2% and 18.0%. The high Sf for the young forest reflects the strongly upward thrusting habit of the branches of the dominant species (Psiadia altissima), which favours funneling of P. The value of I for the semi-mature forest is similar to values reported for old-growth tropical lower montane rain forests elsewhere but I for the younger forest is higher than reported for similarly aged tropical lowland forests. These findings can be explained largely by the prevailing low rainfall intensities and the frequent occurrence of small rainfall events. The revised analytical model was able to reproduce measured cumulative I at the two sites accurately and succeeded in capturing the variability in I associated with the seasonal variability in rainfall intensity, provided that Tf-based values for the average wet-canopy evaporation rates were used instead of values derived with the Penman-Monteith equation.

Performance evaluation of TMPA version 7 estimates for precipitation and its extremes in Circum-Bohai-Sea region, China

Precipitation and its extremes are of significance for drought and flood warning and monitoring. This study evaluates the capability of Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) 3B42 V7 to detect rainfall events, especially extreme precipitation events, using gauge observations for the period 1998–2012 over Circum-Bohai-Sea region, a mid-altitude and semi-humid monsoon area. The results show that 3B42 V7 performs better at monthly and annual scales than at a daily scale. Spatially or seasonally, the rainfall pattern is more effectively captured by 3B42 V7 for the wet region or season than for the dry region or season. 3B42 V7 displays a positive relative bias in most areas, and the largest is situated in high latitude region, while negative relative bias is found at coastal regions. 3B42 V7 tends to overestimate at low and middle rainfall intensity (RI) ranges (RI <50 mm/day) but underestimate at high RI range (RI ≥50 mm/day). Overall, the total rainfall amount (PRETOT) and extreme precipitation amount (EPRETOT, above 95th percentile of daily rainfall) are slightly overestimated by 3B42 V7, while EPRETOT exhibits a lower correlation with observations than PRETOT does. The relative root mean square error (RMSE) are higher than 50 % relative to rain gauges for eight extreme precipitation indices except the maximum number of consecutive dry days (CDD), demonstrating that extreme precipitation estimates of 3B42 V7 are generally unreliable. The improvement of 3B42 V7 in capturing extreme precipitation events is anticipated through extensive efforts for its wide range of climate and hydrological applications. Overall, this study provides an evaluation of the quality of TMPA 3B42 V7 in estimating precipitation and its extremes in a mid-altitude and semi-humid monsoon region.

An approach to analysing plot scale infiltration and runoff responses to rainfall of fluctuating intensity

AbstractSimulated rainfall of fluctuating intensity was applied to runoff plots on bare dryland soils in order to explore a new method for analysing the non‐steady‐state responses of infiltration and overland flow. The rainfall events all averaged 10 mm/h but included intensity bursts of up to 70 mm/h and lasting 5–15 min, as well as periods of low intensity and intermittency of up to 25 min. Results were compared with traditional steady‐state estimates of infiltrability made under simulated rainfall sustained at a fixed intensity of 10 mm/h. Mean event infiltration rate averaged 13.6% higher under fluctuating intensities, while runoff ratios averaged only 63% of those seen under constant intensity. In order to understand the changing soil infiltrability, up to three affine Horton infiltration equations were fitted to segments of each experiment. All equations had the same final infiltrability fc, but adjusted values for coefficients f0 (initial infiltrability) and Kf (exponential decay constant) were fitted for periods of rainfall that followed significant hiatuses in rainfall, during which subsurface redistribution allowed near‐surface soil suction to recover. According to the fitted Horton equations, soil infiltrability recovered by up 10–24 mm/h during intra‐event rainfall hiatuses of 15 to 20‐min duration, contributing to higher overall event infiltration rates and to reduced runoff ratios. The recovery of infiltrability also reduced the size of runoff peaks following periods of low intensity rainfall, compared with the predictions based on single Horton infiltration equations, and in some cases, no runoff at all was recorded from late intensity peaks. The principal finding of this study is that, using a set of affine equations, the intra‐event time variation of soil infiltrability can be tracked through multiple intensity bursts and hiatuses, despite the lack of steady‐state conditions. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Effect of rainfall infiltration on the hydraulic response and failure mechanisms of sandy slope models

This paper presents experimental results obtained from silty sand slope models subjected to an artificial rainfall. Four models were constructed to evaluate the effect of initial water content and rainfall intensity on the hydraulic behavior and failure mechanisms of the slopes. The models were instrumented with volumetric water content sensors to monitor the advance of the water front, and inclinometers to measure lateral movements of the slope. The models were subjected to rainfall intensities ranging from 25 to 50 mm/h, and durations from 19 to 152 minutes. The influence of low intensity rainfall events before a high intensity rainfall is discussed herein. The results showed that the time the slope models required to reach failure was influenced by the soil initial water content, being shorter at high initial water contents. These results are useful to understand the behavior of unsaturated natural slopes and embankments exposed to rainfall infiltration, and to complement the existing laboratory database existing in this subject.

Short term climate trend and variability around Woliso, Oromia Region, Central Ethiopia

Based on the meteorological data of Woliso for the last decade (2004-2013), short-term climate variability was assessed. Computation were made to identify mean monthly, seasonal and annual temperature patterns and deviations, linear trends and R 2 values, rainfall coefficient, rainfall intensity, water surplus and deficit, and moisture region of Woliso area. The findings showed the presence of almost uniform pattern of both temperature and rainfall. The temperatures showed significant decrement in its minimum and increment in its maximum, and mean annual temperature by about -0.3, +1.28, and +0.49 0 C, respectively over the last decade. Similarly, seasonal variability were identified with increment in the winter, summer and autumn by 1.13, 2.78 and 0.9 0 C, respectively while it showed decrement in the spring by -0.5 0 C. About 60% of the years (2004, 2005, 2008, 2009 and 2011) experienced low rainfall intensity while 40% of them (2006, 2007, 2010 and 2013) had medium rainfall intensity. The only year which experienced extreme rainfall intensity was in 2012. Six months (ONDJFM) were dry (low RC <0.6), whereas May and September received big rains with moderate concentration (rainfall coefficient =1.0-1.9) and the summer (JJA) received big rainfall with high concentration (rainfall coefficient=2.0-2.9). The climate type of Woliso was under Dry sub- humid (C1) that also requires further water development schemes to supplement the life and livelihood especially during the dry months. However, such findings should be scrutinized as the current report used only a decade’s data. K ey w o r d s: Climate Pattern, Variability, Rainfall, Temperature, Trend