Rainfall Intensity Values Research Articles

Projected increases in intensity and frequency of rainfall extremes through a regional climate modeling approach

AbstractGlobal warming is changing the hydrological cycle in multiple ways such as increased cloudiness, latent heat fluxes, and intense precipitation events. How extreme rainfall events will be influenced by the changing climate is becoming one of the most important problems for hydrological risk analysis and engineering design. In this study, a regional climate modeling approach based on the Providing REgional Climates for Impacts Studies modeling system is proposed for investigating the potential impacts of climate change induced by increased greenhouse gases on the intensity and frequency of extreme rainfall events in the context of Ontario, Canada. An ensemble of high‐resolution climate projections is first developed under both current and future forcing conditions. Validation of the ensemble simulations is then conducted through comparing the simulated rainfall annual extremes for 1960–1990 to the observed ones. Following that, the rainfall projections for future periods are used to develop projected intensity‐duration‐frequency curves and their plausible changes in 2030s, 2050s, and 2080s for the City of Toronto. The results suggest that intensities of rainfall extreme events versus various durations with different return periods are all likely to increase over time: [5, 17]% in 2030s, [11, 22]% in 2050s, and [25, 50]% in 2080s. Such a consistent increase would lead to an overall uplift in the exceedance values of rainfall intensity of extreme events, implying that intense rainfall events are likely to occur more frequently in the future. In addition, more significant changes in the rainfall intensity are projected for extreme events with longer return periods at all given durations.

Effects of land use change on soil splash erosion in the semi-arid region of Iran

Land use change may escalate the process of splash erosion as the primary mechanism causing water erosion. The objective of this study was to investigate the impacts of different management practices and land uses on splash erosion in a semiarid region in Iran. The major land uses in the area were pasture, degraded pasture, dry land farming, and irrigated farming. For the purposes of this study, soil properties including organic matter, CaCO3, surface shear strength (SSS), particle size distribution, mean weight diameter (MWD), and the topographic attributes were measured. Soil splash erosion was measured at 80 different locations under the following four conditions comprising different values of slope (S:%) and rainfall intensity (RI:mm·h−1): 5–50, 5–80, 15–50, and 15–80, respectively, using the multiple splash sets (MSS) especially designed and tailored for the purposes of this study. A completely randomized design was used in which soil texture and the land use systems were independently analyzed. The fuzzy linear regression (FLR) was used and compared with the multiple-linear regression (MLR) analysis. It was found that the splash erosion in the study region was mainly influenced by landuse and soil management practices rather than by intrinsic soil properties like tested textures. The average splash erosion values among landuse types are: degraded pasture>cultivated farming>pasture; this is claimed to be associated with the lower organic matter content and shear strength due to overgrazing and untimely grazing. The FLR models outperformed the MLR ones (p>0.01). MWD and SSS attributes were the most effective variables in estimating soil splash, indicating the structural susceptibility of the soils to management practices. Based on the results obtained, MWD and SSS may be regarded as important indices of splash erosion.

Funcionamiento hidrológico de una pequeña cuenca de montaña con morfologías acarcavadas en el Pirineo Central

The Araguás catchment has been monitored since 2004 to study weathering, erosion and the hydrological and sediment response in order to understand the dynamics of a badland area in a relatively humid environment. This paper studies the main hydrological characteristics of the Araguás catchment, a representative badland area in a subhumid mountain environment. 79 floods have been recorded between October 2005 and April 2007. The results obtained confirm the temporal variability in the hydrological response within a small catchment with badland areas, and its high capacity of reaction against precipitation. Storm-flow mainly depends on the combination between rainfall volume and antecedent baseflow. On the other hand, a significant correlation was observed between maximum rainfall intensity and peak flow values. Moreover, seasonal differences were observed: during the dry season, significant correlations were observed between maximum rainfall intensity and peak flow; during the wet season this significant correlation disappeared and relationships with antecedent moisture conditions improved. Finally, the shape of the different hydrographs suggests two different hydrological patterns, due to runoff generation mechanisms.

Integrating intensity–duration-based rainfall threshold and antecedent rainfall-based probability estimate towards generating early warning for rainfall-induced landslides in parts of the Garhwal Himalaya, India

In order to generate early warning for landslides, it is necessary to address the spatial and temporal aspects of slope failure. The present study deals with the temporal dimension of slope failures taking into account the most widespread and frequent triggering factor, i.e. rainfall, along the National Highway-58 from Rishikesh to Mana in the Garhwal Himalaya, India. Using the post-processed three-hourly rainfall intensity and duration values from the Tropical Rainfall Measuring Mission-based Multi-satellite Precipitation Analysis and the time-tagged landslide records along this route, an intensity–duration (I–D)-based threshold has been derived as I = 58.7D−1.12 for the rainfall-triggered landslides. The validation of the I–D threshold has shown 81.6 % accuracy for landslides which occurred in 2005 and 2006. From this result, it can be inferred that landslides in the study area can be initiated by continuous rainfall of over 12 h with about 4-mm/h intensity. Using the mean annual precipitation, a normalized intensity–duration relation of NI = 0.0612D−1.17 has also been derived. In order to account for the influence of the antecedent rainfall in slope failure initiation, the daily, 3-day cumulative, and 15- and 30-day antecedent rainfall values associated with landslides had been subjected to binary logistic regression using landslide as the dichotomous dependent variable. The logistic regression retained the daily, 3-day cumulative and 30-day antecedent rainfall values as significant predictors influencing slope failure. This model has been validated through receiver operating characteristic curve analysis using a set of samples which had not been used in the model building; an accuracy of 95.1 % has been obtained. Cross-validation of I–D-based thresholding and antecedent rainfall-based probability estimation with slope failure initiation shows 81.9 % conformity between the two in correctly predicting slope stability. Using the I–D-based threshold and the antecedent rainfall-based regression model, early warning can be generated for moderate to high landslide-susceptible areas (which can be delineated using spatial integration of preconditioning factors). Temporal predictions where both the methods converge indicate higher chances of slope failures for areas predisposed to instability due to unfavourable geo-environmental and topographic parameters and qualify for enhanced slope failure warning. This method can be verified for further rainfall seasons and can also be refined progressively with finer resolutions (spatial and temporal) of rainfall intensity and multiple rain gauge stations covering a larger spatial extent.

Special Issue on Radar Rainfall Data Analyses and Applications

Special Issue on Radar Rainfall Data Analyses and Applications

Introducing leaky-well concept for stormwater quantity control in Dhaka, Bangladesh

Dhaka, the capital city of Bangladesh with rapid and unplanned urbanization, is subjected to annual average rainfall of 2,076 mm. The intensity of rainfall during 10 years recurrence interval and 1 h duration of the city is 98 mm/h. The stormwater drainage systems of the city are often unable to manage peak runoff volume and hence urban flooding is common after medium to heavy rainfall events. A proposal to introduce leaky-wells using water sensitive urban design (WSUD) principles was investigated for Dhaka’s drainage network to transfer the present unsatisfactory situation into one which is sustainable. The regime in balance strategy was considered to control the stormwater for 100 years recurrence interval. We applied scaling theory to 57 years (1953–2009) daily rainfall data for the estimation of sub-daily rainfall intensity values. It was found that two leaky-wells; each with depth H = 2.0 m and diameter D = 2.0 m, in 500 m2 allotment can improve the situation. The emptying (drain) time of the proposed device is around 1.25 days, which meets the standard criterion. Groundwater table, soil hydraulic conductivity and topographic slope of Dhaka also support for installations of leaky-wells.

Spatial analysis of daily rainfall intensity and concentration index in Peninsular Malaysia

This study presents the spatial analysis of daily rainfall intensity and concentration index over Peninsular Malaysia. Daily rainfall data from 50 rainfall stations are used in this study. Due to the limited number of stations, the geostatistical method of ordinary kriging is used to compute the values of daily rainfall concentration and intensity and to map their spatial distribution. The resultant analysis of rainfall concentration indicated that the distribution of daily rainfall is more regular over the west, northwest and southwest regions compared to the east. Large areas of the eastern Peninsula display an irregularity in distribution of daily rainfall. In terms of number of rainy days, analysis of daily rainfall confirms that a large number of rainy days across the Peninsula arise from low-intensity events but only contribute a small percentage of total rain. On the other hand, a low frequency of rainy days with high-intensity events contributes the largest percentage of total rain. The results indicated that the total rain in eastern areas is mainly contributed by the high-intensity events. This finding explains the occurrence of a large number of floods and soil erosions in these areas. Therefore, precautionary measures should be taken earlier to prevent any massive destruction of property and loss of life due to the hazards. These research findings are of considerable importance in providing enough information to water resource management, climatologists and agriculturists as well as hydrologists for planning their activities and modelling processes.

Synthesis of Rainfall Characteristics for Predicting the Erosivity Pattern of Wakavali Region in Maharashtra, India

AbstractThe Wakavali region of the Maharashtra state in India is characterized by high-intensity rainfall over a short duration. Precipitation in Wakavali occurs mainly from May to October. Rain gauge charts of self-recording rain gauge stations were used for rainfall characteristics analysis. Intensity of storms were recorded along with the depth of individual storms. It was found that the variation in magnitude of total kinetic energy was more attributable to unit increase in magnitude of rainfall depth than unit increase in rainfall intensity by keeping other variables constant. Values of 30-min maximum rainfall intensity (I30) were found to be very high, reaching up to 140 mm/h. Most of higher value of I30 occurred in June and early July. Rainfall intensity of 8 mm/h was treated as threshold rainfall intensity for the Wakavali region, above which the erosion process begins. There was significant correlation between erosivity indexes and rainfall parameters, namely, I30 and daily precipitation for th...

Comparison of recorded rainfall with quantitative precipitation forecast in a rainfall-runoff simulation for the Langat River Basin, Malaysia

AbstractObserved rainfall is used for runoff modeling in flood forecasting where possible, however in cases where the response time of the watershed is too short for flood warning activities, a deterministic quantitative precipitation forecast (QPF) can be used. This is based on a limited-area meteorological model and can provide a forecasting horizon in the order of six hours or less. This study applies the results of a previously developed QPF based on a 1D cloud model using hourly NOAA-AVHRR (Advanced Very High Resolution Radiometer) and GMS (Geostationary Meteorological Satellite) datasets. Rainfall intensity values in the range of 3–12 mm/hr were extracted from these datasets based on the relation between cloud top temperature (CTT), cloud reflectance (CTR) and cloud height (CTH) using defined thresholds. The QPF, prepared for the rainstorm event of 27 September to 8 October 2000 was tested for rainfall runoff on the Langat River Basin, Malaysia, using a suitable NAM rainfall-runoff model. The response of the basin both to the rainfall-runoff simulation using the QPF estimate and the recorded observed rainfall is compared here, based on their corresponding discharge hydrographs. The comparison of the QPF and recorded rainfall showed R2 = 0.9028 for the entire basin. The runoff hydrograph for the recorded rainfall in the Kajang sub-catchment showed R2 = 0.9263 between the observed and the simulated, while that of the QPF rainfall was R2 = 0.819. This similarity in runoff suggests there is a high level of accuracy shown in the improved QPF, and that significant improvement of flood forecasting can be achieved through ‘Nowcasting’, thus increasing the response time for flood early warnings.

Rainfall variability and hydrological and erosive response of an olive tree microcatchment under no‐tillage with a spontaneous grass cover in Spain

AbstractMost studies on runoff and soil loss from olive orchards were performed on plots, despite the fact that measurements that examine a range of erosive processes on different scales are essential to evaluate the suitability of the use and soil management of this type of land. The main environmental limitations of much of the land used for olive orchards in the Mediterranean are the steep slopes and the shallow soil depth – and this was the case in the study area. Soil erosion and runoff over two hydrological years (2005–2006 and 2006–2007) were monitored in an olive orchard microcatchment of 6·1 ha under no‐tillage with spontaneous grass in order to evaluate its hydrological and erosive behaviour. Moreover, soil parameters such as organic matter (%OM), bulk density (BD) and hydraulic saturated conductivity (Ks) were also examined in the microcatchment to describe management effects on hydrological balance and on erosive processes.In the study period, the results showed runoff coefficients of 6·0% in the first year and 0·9% in the second. The differences respond to the impact of two or three yearly maximum events which were decisive in the annual balances. On the event scale, although maximum rainfall intensity values had a big influence on peak flows and runoff, its importance on mean sediment concentrations and sediment discharges was difficult to interpret due to the likely control of grass cover on volume runoff and on soil protection. In the case of annual soil erosion, they were measured as 1·0 Mg ha−1 yr−1 and 0·3 Mg ha−1 yr−1. Both are lower than the tolerance values evaluated in Andalusia (Spain). These results support the implementation of no‐tillage with spontaneous grass cover for sloping land, although the reduced infiltration conditions determined by Ks in the first horizon suggest grass should be allowed to grow not only in spring but also in autumn. In addition, specific measurements to control gullies, which have formed in the terraced area in the catchment, should be included since it is expected that they could be the main sources of sediments. Copyright © 2010 John Wiley & Sons, Ltd.

Slope failure and related processes in the Mt. Rocciamelone area (Cenischia Valley, Western Italian Alps)

The present article aims at studying slope failure processes and debris flows in the Mt. Rocciamelone area in the Cenischia Valley of the Western Italian Alps (Graian Alps). Debris flow events in the study area since 1993 have been investigated through instrumental records, on-site surveys and data analysis related to the triggering causes, debris flow dynamics and rainfall. The first rockfall was witnessed on the western slopes of Mt. Rocciamelone on 26 December 2006, heading from the Marderello catchment above Novalesa Village in the Torino Province. The inception zone was recognized just below the crest line around 3200 m a.s.l., which connects the Cà d'Asti landslide to the highest peak (3538 m a.s.l.). Such events occurred during the winter and from June 2007 to early September when the frequency of major rotational slides and rockfalls increased. The whole process of instability affected at least 4 × 10 4 m 2 of surface area and dismantled about 4 × 10 5 m 3 of weathered rock. Stony debris appears to have been inscribed in a larger deep-seated gravitational slope failure that comprises the whole valley system surrounding Novalesa Village. The catchment characteristics of the study area have been investigated through intensive field surveys since 1991. Meteorological and rainfall data of different elevations have also been collected since 1991 by the monitoring equipment set up in the field to investigate the environmental conditions and meteorological factors responsible for triggering debris flows. Rainfall intensity values for the onset conditions of a given event have been identified. The values appeared surprisingly low compared to usual values reported for soil slip/debris flow events worldwide. A continuous sediment supply from rockfalls has been recorded which is associated with a larger, deep-seated gravitational spreading system.

ESTIMATION OF WATER BALANCE ELEMENTS OF RAINFALL SIMULATION EXPERIMENTS

Rainfall simulation is an easy and relatively cheap method to get remarkable amount of hydrological data within short period of time instead of depending on a very rare natural rainfall particularly in arid regions. This technique was largely utilized in different parts of the worlds especially in arid and semiarid regions. However, research and applications of rainfall simulators are very rare in the Arab World. This study is probably one of the earliest applications of rainfall simulators in this part of the world. The study location is an experimental plot within the main stream of Namman basin located in the south east of Makkah city in western Saudi Arabia. In this study, six rainfall simulation experiments are conducted using rainfall simulator which has been developed from locally available materials. The experiments provided values of runoff and infiltration volumes resulted from simulated rainfall events which have varied characteristics. The results of the study indicated that values of runoff volume mainly depend on values of rainfall intensity and then on values of rainfall duration. The results of the study are promising and it is recommended to conduct more rainfall simulation experiments on the study region and on other semi arid regions in the Arab World.

Spatial–temporal mesoscale modeling of rainfall intensity using gage and radar data

Gridded estimated rainfall intensity values at very high spatial and temporal resolution levels are needed as main inputs for weather prediction models to obtain accurate precipitation forecasts, and to verify the performance of precipitation forecast models. These gridded rainfall fields are also the main driver for hydrological models that forecast flash floods, and they are essential for disaster prediction associated with heavy rain. Rainfall information can be obtained from rain gages that provide relatively accurate estimates of the actual rainfall values at point-referenced locations, but they do not characterize well enough the spatial and temporal structure of the rainfall fields. Doppler radar data offer better spatial and temporal coverage, but Doppler radar measures effective radar reflectivity (Ze) rather than rainfall rate (R). Thus, rainfall estimates from radar data suffer from various uncertainties due to their measuring principle and the conversion from Ze to R. We introduce a framework to combine radar reflectivity and gage data, by writing the different sources of rainfall information in terms of an underlying unobservable spatial temporal process with the true rainfall values. We use spatial logistic regression to model the probability of rain for both sources of data in terms of the latent true rainfall process. We characterize the different sources of bias and error in the gage and radar data and we estimate the true rainfall intensity with its posterior predictive distribution, conditioning on the observed data. Our model allows for nonstationary and asymmetry in the spatio-temporal dependency structure of the rainfall process, and allows the temporal evolution of the rainfall process to depend on the motions of rain fields, and the spatial correlation to depend on geographic features. We apply our methods to estimate rainfall intensity every 10 minutes, in a subdomain over South Korea with a spatial resolution of 1 km by 1 km.

Temporal variability in hydrological response within a small catchment with badland areas, central Pyrenees

The lower Araguás catchment, central Pyrenees, is characterized by extensive badlands (25% of the total catchment), whereas the upper catchment is covered by dense plantation forest. The catchment (45 ha) has been monitored since October 2005 with the aim of studying its hydrological response. The 44 floods recorded over this period were analysed to identify the factors that control the rainfall—runoff relationship. The first relevant feature of the catchment was its responsiveness. The catchment reacted to all rainfall events, but the irregular nature of the hydrological response was the most characteristic feature of the response. No single variable could explain the response of the Araguás catchment. It was found that stormflow coefficients mainly depend on the combination of rainfall volume and antecedent baseflow. A significant correlation was observed between maximum rainfall intensity and peak flow values. The shapes of the different hydrographs are very similar, regardless of the peak flow magnitude; they show a short time lag, relatively narrow peak flow, and steep recession limb. This indicates a large contribution by overland flow, resulting mainly from the generation of infiltration excess runoff in badland areas.

Identifying erosive periods by using RUSLE factors in mountain fields of the Central Spanish Pyrenees

Abstract. The Mediterranean environment is characterized by strong temporal variations in rainfall volume and intensity, soil moisture and vegetation cover along the year. These factors play a key role on soil erosion. The aim of this work is to identify different erosive periods in function of the temporal changes in rainfall and runoff characteristics (erosivity, maximum intensity and number of erosive events), soil properties (soil erodibility in relation to freeze-thaw processes and soil moisture content) and current tillage practices in a set of agricultural fields in a mountainous area of the Central Pyrenees in NE Spain. To this purpose the rainfall and runoff erosivity (R), the soil erodibility (K) and the cover-management (C) factors of the empirical RUSLE soil loss model were used. The R, K and C factors were calculated at monthly scale. The first erosive period extends from July to October and presents the highest values of erosivity (87.8 MJ mm ha−1 h−1), maximum rainfall intensity (22.3 mm h−1) and monthly soil erosion (0.25 Mg ha−1 month−1) with the minimum values of duration of erosive storms, freeze-thaw cycles, soil moisture content and soil erodibility (0.007 Mg h MJ−1 mm−1). This period includes the harvesting and the plowing tillage practices. The second erosive period has a duration of two months, from May to June, and presents the lowest total and monthly soil losses (0.10 Mg ha−1 month−1) that correspond to the maximum protection of the soil by the crop-cover ($C$ factor = 0.05) due to the maximum stage of the growing season and intermediate values of rainfall and runoff erosivity, maximum rainfall intensity and soil erodibility. The third erosive period extends from November to April and has the minimum values of rainfall erosivity (17.5 MJ mm ha−1 h−1) and maximum rainfall intensity (6.0 mm h−1) with the highest number of freeze-thaw cycles, soil moisture content and soil erodibility (0.021 Mg h MJ−1 mm−1) that explain the high value of monthly soil loss (0.24 Mg ha−1 month−1). The interactions between the rainfall erosivity, soil erodibility, and cover-management factors explain the similar predicted soil losses for the first and the third erosive periods in spite of the strong temporal differences in the values of the three RUSLE factors. The estimated value of annual soil loss with the RUSLE model (3.34 Mg ha−1 yr−1) was lower than the measured value with 137Cs (5.38 Mg ha−1 yr−1) due to the low values of precipitation recorded during the studied period. To optimize agricultural practices and to promote sustainable strategies for the preservation of fragile Mediterranean agrosystems it is necessary to delay plowing till October, especially in dryland agriculture regions. Thus, the protective role of the crop residues will extend until September when the greatest rainfall occurs together with the highest runoff erosivity and soil losses.

A Review of Climate Loads Relevant to Assessing the Watertightness Performance of Walls, Windows, and Wall-Window Interfaces

Abstract When assessing a wall assembly's ability to manage rainwater and control rain penetration, the two key climatic elements to consider are wind speed and rainfall intensity. However, of significance to rain penetration is the effect of wind-driven rain on the building cladding — that is wind coincident with rainfall. When water is present at openings in the cladding, water is driven into the layers of the assembly by the action of wind. Paths providing a direct line from openings in the cladding to inside the assembly offer particularly vulnerable points for water entry. Performance testing helps determine the location of vulnerable locations in a wall assembly and the test loads at which penetration occurs, and it possibly relates the amount of entry to specific details and simulated climate effects. Undertaking watertightness performance tests requires knowledge of extremes in wind-driven rain or specifically the occurrence and level of extreme rainfall events for locations of interest. A review of climate information on wind-driven rain is provided, and its relevance to assessing the watertightness performance of walls, windows, and wall-window interfaces is discussed. Values of rainfall intensity, duration, and frequency or occurrence are given, emphasizing the level of significance of these variables to different North American climates.

Suspended solids discharge from a small forested basin in the humid tropics

AbstractSuspended solids (SS) discharge from a tropical rain forest was observed at the Bukit Tarek Experimental Watershed in Peninsular Malaysia in order to elucidate mechanisms of SS production and transport. Peaks of water discharge and electrical conductivity (EC) lagged further behind rainfall peaks than did dissolved oxygen (DO), indicating that the discharge in the early stage of a storm is mainly formed by rain water with high DO. Stream water showing a high value of EC originating from subsurface water formed the main storm flow and lagged behind the rainfall. SS concentrations rose to a peak quickly and, like DO, the peak preceded that of water discharge. A clockwise hysteresis loop in the relationship between SS and water discharge exists, and the magnitude of hysteresis loop is in proportion to storm size. The values of SS concentration correlate positively with the values of rainfall intensity on logarithmic axes. The time intervals between peaks of rainfall and SS concentrations are assumed to be a delivery term expressing distance from sediment source to measuring point. Immediate transport of SS from the source to the sampling site, the short time gap between the peaks of rainfall and SS concentrations, and the high rate of infiltration on the hillslope suggest that the sources of high SS concentrations are located close to the stream. The calculated source area is located at a gentler part of the stream, where wet riparian areas exist. The strong relationship between water discharge and SS concentration during the small storm proves that the source areas of SS and water were the same. In contrast, the source area of SS disappeared when rainfall ceased, whereas the source area of water discharge was still expanding in the larger storm. These phenomena may produce clockwise loops in the SS concentration–water discharge relationship. Copyright © 2004 John Wiley & Sons, Ltd.

Probability of Failure Occurrence and Recurrence in Rock Slopes Stability Analysis

The paper illustrates the procedures for computing the probability of failure occurrence that represents the potential that a slope can undergo failure due to both the spatial variability of the structural and geomechanical aspects of the joint sets and the existing boundary conditions. The presented approach to a global probabilistic assessment of the rock slopes stability conditions is based on the analysis of the spatial variability of structural aspects of the rock mass, of the uncertainty in geomechanical properties, and of the occurrence probabilities of certain values of seismic forces and rainfall intensities. Finally, the probability of failure recurrence is computed taking into account occurrences, in time, of the external acting forces, so providing an evaluation of the rock slope landslide hazard, that is the probability that an instability event with a given intensity may occur within a reference time period.

RAINFALL POINT INTENSITIES IN AN AIR MASS THUNDERSTORM ENVIRONMENT: WALNUT GULCH, ARIZONA1

ABSTRACT: Point rainfall intensities for a given return period are often used to formulate design storms for rainfall/runoff models to simulate design floods. These design floods are in turn used to design bridges, culverts, and a variety of drainage and flood control structures. The projected rapid growth in the southwestern United States will require very substantial monetary investments in drainage infrastructure. Accurate estimates of point rainfall intensities are critical to ensure both safe designs while not wasting dollars in overdesign. Rainfall point intensities (accumulated rainfall depth over a specified duration) for 5‐, 15‐, 30‐, and 60‐minute durations for the 2‐, 5‐, 10‐, 25‐, 50‐, and 100‐year return periods were determined for southeast Arizona. Thirty‐five years of rainfall record (1961 to 1995) were used in this study. The records came from 20 stations that were grouped into five sets of four independent stations to extend the rainfall records. The stations are in the USDA‐ARS Walnut Gulch Experimental Watershed (WGEW), which is representative of large portions of the Southwest whose runoff generation is dominated by air‐mass thunderstorms. The 5‐, 15‐, 30‐, and 60‐minute maximum intensities per year followed log‐normal distributions. The mean point rainfall intensities of the five sets of gages are very close (between 0 and 11 percent) to the NOAA values of the 5‐, 15‐, 30‐, and 60‐minute durations for all return periods. Much larger differences between the mean point rainfall intensities for all durations were found when these results were compared to those of a previous study done with a shorter rainfall record (between 14 and 33 percent for the 25‐, 50‐, and 100‐year return‐periods). The difference between the largest and the smallest values of point rainfall intensities recorded by each group, for all durations, usually increases as the return period increases.

Estimating maximum expected short-duration rainfall intensities from extreme convective storms

Certain hydrological applications require estimates of maximum rainfall intensity values for short time durations, as it is the case of urban hydrology design among others. Short duration rainfall intensities are affected by a large uncertainty when durations under 10 or 5 minutes are considered, especially when they are produced during extreme convective rainfall events.In this paper, 408 rainfall events have been statistically analyzed for the period 1925–1992 in Alicante (Spain). Maximum intensities for durations ranging from 2 minutes up to 240 minutes were extracted from the series, and described in terms of a set of GEV-three parameter extreme value distributions. The resulting quantiles were plotted, and nine frequently used empirical functions imax(t) were fitted, including 2-parameter and 3-parameter functions.Considerable differences are found in the behaviour of the empirical functions for short durations (t < 10 minutes). A systematic comparison is performed in all cases, with return periods ranging from 5 to 500 hundred years, assessing reliability of the estimates of maximum rainfall intensities for short durations. The 3-parameter functions together with the Talbot function provide satisfactory goodness-of-fit, although in most cases, underestimation of maximum rainfall intensities for very short durations is observed. On the other hand, the postulated empirical Intensity-Duration-Frequency (idf) relationships are more suitable when data for larger durations (i.e., t<60 min) are considered. This result is consistent with the consideration of essentially different dynamics of the maximum rainfall intensities producing mechanisms at the different time scales.