Rainy Events Research Articles

Impact of fertilisation practices on nitrogen leaching under irrigation

Field experiments were carried out over a 2-year period on a loamy soil plot under corn in Montpellier (south-east France). The effectiveness of improved irrigation practices in reducing the adverse impact of irrigation on the environment was assessed. Different irrigation and fertiliser treatments were applied to identify the best irrigation and fertilisation strategy for each technique (furrow and sprinkler) to ensure both good yields and lower NO3 – leaching. No significant differences in corn yield and NO3 – leaching were found for the climatic scenario of 1999 between sprinkler and furrow irrigation during the irrigation season. Following the rainy events occurring after plant maturity (and the irrigation season), differences in N leaching were observed between the treatments. The study shows that both the fertiliser method, consisting of applying a fertiliser just before ridging the furrows, and the two-dimensional (2D) infiltration process, greatly influence the N distribution in the soil. N distribution seems to have a beneficial impact on both yield and N leaching under heavy irrigation rates during the cropping season. But, under rainy events (particularly those occurring after harvesting), the N, stored in the upper part of the ridge and not previously taken up by plants, can be released into the deeper soil layers in a furrow-irrigated plot. In contrast, the 1D infiltration process occurring during sprinkler irrigation events affects the entire soil surface in the same way. As a result the same irrigation rate would probably increase N leaching under sprinkler irrigation to a greater extent than under furrow-irrigation during an irrigation period. In order to assess the robustness of these interpretations derived from soil N-profile analysis, a modelling approach was used to test the irrigation and fertilisation strategies under heavy irrigation rates such as those occurring at the downstream part of closed-end furrows. The RAIEOPT and STICS models were used to simulate water application depths, crop yield and NO3 – leaching on three measurement sites located along the central furrow of each treatment. The use of a 2D water- and solute-transport model such as HYDRUS-2D enabled us to strengthen the conclusions derived from the observations made on the N distribution under a cross-section of furrow. This model helped to illustrate the risk of over-estimation of N leaching when using a simplified 1D solute-transport model such as STICS.

Fault detection on a sewer network by a combination of a Kalman filter and a binary sequential probability ratio test

One of the aims of sewer networks is the protection of population against floods and the reduction of pollution rejected to the receiving water during rainy events. To meet these goals, managers have to equip the sewer networks with and to set up real-time control systems. Unfortunately, a component fault (leading to intolerable behaviour of the system) or sensor fault (deteriorating the process view and disturbing the local automatism) makes the sewer network supervision delicate. In order to ensure an adequate flow management during rainy events it is essential to set up procedures capable of detecting and diagnosing these anomalies. This article introduces a real-time fault detection method, applicable to sewer networks, for the follow-up of rainy events. This method consists in comparing the sensor response with a forecast of this response. This forecast is provided by a model and more precisely by a state estimator: a Kalman filter. This Kalman filter provides not only a flow estimate but also an entity called ‘innovation’. In order to detect abnormal operations within the network, this innovation is analysed with the binary sequential probability ratio test of Wald. Moreover, by crossing available information on several nodes of the network, a diagnosis of the detected anomalies is carried out. This method provided encouraging results during the analysis of several rains, on the sewer network of Seine-Saint-Denis County, France.

PAH fluctuations in rivers in the Paris region (France) : Impact of floods and rainy events

PAHs produced by motor vehicle combustion were determined in the dusts from the substructures of a parisian car park. The high levels of benzo(ghi)perylene (bgp) and pyrene (pyr) expressed as ratios to fluoranthene (flu), were 1.73 and 1.23, respectively, which was typical of these exhausts. From these findings, bgp and pyr may be used as tracers for estimating the contamination fluctuations and transfer processes in rain waters and river waters, in a highly industrialized and urbanized area. During flood events, the pollution related to atmospheric scavenging alone was negligible as compared to that due to urban surface leaching, contrary to steady period events. During that last period, when the river contamination was low, about 40 ng L−1, and when mean bulk precipitation concentrations were about 350 ng L−1, minor precipitations might contribute less than 2% to river contamination, whereas heavy showers might induce a quite important pollution flush: PAH concentrations were twice higher and 26% of the inputs came from rain and 74% from runoff in the river Seine.

Méthode d'analyse des pluies pour l'évaluation de l'impact de la pollution générée par le ruissellement urbain

A method of rainfall data analysis was defined to study the impact of the pollution due to urban stormwater runoff on receiving waters. The available pluviographic records were analysed to draw up an inventory of the rainy events over the observation period. An automatic classification was then used to obtain a typology of the events. At last a synthetic year made of rainfalls generating runoff was produced on the basis of the dijferent classes previously defined.

Rainfall monitoring during HAPEX-Sahel. 2. Point and areal estimation at the event and seasonal scales

The water and energy balance models that will be used in HAPEX-Sahel to analyze the data collected during the experiment are strongly conditioned by the rainfall estimation accuracy over the areas of interest. Following the description of the rainfall conditions that prevailed during HAPEX-Sahel and the computation of statistics characterising the point rainfall processfor the Sahel, presented in a companion paper, it is examined here how accurate are the areal rainfall estimates provided by the EPSAT-Niger network at the event and seasonal scales. Using a geostatistical framework, it is shown that it is possible to infer a climatological variogram that represents the average variability of the event rainfields, within the limits imposed by the resolution and sampling window of the network. Average event rainfall estimation errors are derived. The density of the EPSAT-Niger mesoscale network (one station for 150 km 2) allows the estimation of the event areal rainfall over the 1° × 1° square with an average uncertainty around 5%. On the supersites (100–750 km 2) the average uncertainty can be as large as 20%, depending on the site and rainfall considered. This uncertainty is reduced by two-thirds, or more, when using the denser network designed specifically to cover the supersites. It is then demonstrated how the variability within the event rainfields and the space-distribution of the rainy events combine to determine the spatial structure of a rainfield resulting from the accumulation of several event rainfelds. Over the HAPEX-Sahel study area, the within-the-event-rainfield variability is dominant over the variability resulting from the distribution of the events in space. Consequently the correlation length does not vary much when shifting from the event to the seasonal scale. At an unsampled location the seasonal rainfall error magnitude is on the order of the average error at the event scale, divided by the square root of the number of events recorded during the season.

Rainfall climatology of the HAPEX-Sahel region during the years 1950–1990

In the Sahel, rainfall is the single most important factor conditioning the hydrology and the climate, but comprehensive statistical analyses of the rainfall climatology in the region are rare. Yet, even though in the Sahel rainfall data are scarce by the standards of the temperate countries, it is shown here that it is possible to obtain a reasonably good idea of what the rainfall has been over Sahelian Niger for the past 40 years, both in terms of interannual variability and spatial distribution. To that aim a statistical model is used, which decomposes the space-time fluctuations of long-term rainfall averages into the fluctuations of the mean event rainfall on the one hand, and of the mean number of rainfall events over any period of accumulation, on the other hand. This model is first applied to the analysis of monthly rainfall data over the whole of Niger. It is shown that the lasting drought which has affected Niger for more than 20 years is associated with a decrease in the number of rainy events, rather than to a decrease of the mean event rainfall, and that this decrease is more pronounced for the core of the rainy season. Because these fluctuations are not homogeneous over Niger, a 5° × 4° zone centred on the HAPEX-Sahel 1° × 1° square is selected in order to characterise more accurately the rainfall climatology of the HAPEX-Sahel area between 1950 and 1990. In comparison with what it was between 1950 and 1970, the average length of the rainy season has not changed significantly during the dry period 1970–1990. Rather, it is the decrease of rainfall in July and August that explains most of the diminution of the total annual rainfall over this part of the Sahel since 1970. The average number of rainy events in August was reduced by about 30%, while the mean event rainfall remained roughly constant. Finally, the analysis of the daily rainfall series for Niamey (which constitutes the longes record available in Niger, starting in 1905) enables the comparison of four periods of 20 years between 1910 and 1990. The period 1970–1989 appears to be by far the longest and most severe dry spell of the past century. Almost 90% of the annual rainfall decrease over this period is explained by the decrease of the mean number of rainfall events during July and August, while both the length of the rainy season and the mean event rainfall remained stable.

Rainfall monitoring during HAPEX-Sahel. 1. General rainfall conditions and climatology

The HAPEX-Sahel experiment took place in the midst of the most severe drought that has ever plagued the region since rainfall records have been available in the Sahel. The aim of this paper is to describe the rainfall conditions that were observed during HAPEX-Sahel by a network of 100 recording raingauges, and to analyse them in the perspective of the long-term statistics at the Niamey station. Globally it is found that the average rainfall over the HAPEX-Sahel study area, as well as over Niger as a whole, during the experiment has been moderately (1991 and 1992) to markedly dry (1990 and 1993), indicating that the drought of the past 20 years has not ended. More detailed statistics point to the high intermittency of the Sahelian rainfall, both in space (each year the ratio between the maximum and minimum recorded seasonal rainfall was of the order of 2) and in time (half the annual rain falls in 5 h). Particular attention is paid to the climatology of the rainy events. The point event rainfall is nearly exponentially distributed with a mean of about 14 mm, while the average areal event rainfall over the HAPEX-Sahel 1° × 1° square is of the order of 10.5 mm. The probability of zero rainfall is thus close to 1 4 . Rain rates are often heavy, with half the annual rain falling at rain rates higher than 35 mm h −1 and one third of it falling at rain rates higher than 50 mm h −1. The year to year statistics of both the event rainfall and the rain rates vary little, indicating that most of the inter-annual rainfall variability is due to the variation of the number of rainfall events, rather than to variations in rainfall intensities or in the mean event rainfall.

Mineralogy and granulometry of settled dust in a dust-dune-loess-liquid waste system in the northern Negev Desert, Israel

Settled dust mineralogy and granulometry of the Negev semi-arid desert were examined in samples collected at 1 and 3 m above ground level over a period of 5 years. Evaporation ponds which are designed to collect industrial waste, overflow and contaminate the loess in which they are built and the dust that settled around them. Some of this dust is incorporated in new loess, and some dust continues its transport to new locations, carrying the contaminants with it. Kaolinite and calcite particles in the dust deposits were found to serve as hosts to grains of quarts, pollen, and stamen that were much smaller in diameter and were tightly attached to the surfaces of the particles. The median diameters of the particles at both levels varied between 4 and 50 μm, and the coarsest diameters approached 150 μm. Most of the finest five percentile fraction, during 4 of the 5 years, were below 4 μm in diameter at both levels. These particles are small enough to enter the alveoli of our lungs. Modal triangle analyses show that the loess is better sorted in relation to the local dust, and it is coarser grained. It is suggested that the local winds may supply coarse particles at ground level, and light rainy events may remove, by suspension, some of the finer particles, thus converting the constitution of the local dust deposits into the highly porous structure of loess; thus creating a potential trap for a large volume of liquid industrial waste. Lognormal distribution of the same samples shows variation from non-lognormalcy in the dust, to ∼ 100% lognormalcy in the loess. Two major constituents, quartz and calcite, dominate in all the samples, but finer grained dust populations are richer in calcite vs. quartz than coarser populations. Dust east of the monitoring station is different from that west of it, as is the geology in the corresponding areas. The several industrial plants east of the dust monitoring station also contribute to the elemental dust composition.

A stochastic model of dimensionless hyetograph

A very simple statistical model is proposed to describe the time distribution of rainfall within a rainy event. The modeled process is the dimensionless hyetograph H(tj) that represents the fraction of rainfall accumulated over the time interval (0, tj), where tj is a fraction of the total duration. In any rainy episode lasting n+1 units of time, the sequence H(t1), …, H(tn) is described as an ordered sample from the beta distribution. Dimensionless rainfall from hourly data at three stations in Southern Spain show little dependence on rainfall duration, amount of precipitation, or period of the year. These results, though not essential to the model structure, simplify its formulation, reducing the number of parameters. Other features of the historical records, such as correlations, maximum dimensionless hourly precipitation, or frequency curves, are reasonably well reproduced by the model.

Obstacles orographiques et champ des précipitations sur le Massif Vosgien

Relief Barriers and Rainfall Distribution over the Vosges Mountains. The spatial dissymetry in rainy episodes over the Vosges mountains (France) opposing wetter slopes in the Lorraine region to dryer slopes in the Alsace region, doesn't systematicaly occure during every rainy event. The following study propose on the basis of some given meteorological events to demonstrate the variety of relief barriers in the Vosges region.

Relationships between meteorological situations and acid rain in Spanish Basque country

With an automatic sequential precipitation collector, 52 rainy events were sampled at Vitoria (Spanish Basque country) in 1986. The chemical composition of each rainy event was studied and classified, using statistical techniques of multivariate data analysis (linear correlation + principal component analysis) and an automatic classification (length χ 2) of the rainy events. These techniques enabled us to show, taking into account the influence of the meteorological conditions in which the precipitations formed, the presence, in Spanish Basque country, of acid rain episodes with an anthropogenic origin. These episodes are certainly reinforced by the trans-border transport from Southern France. Moreover, rainy episodes were shown to have a basic character due to a local source of terrigenous particles.