Rain | EPFL Graph Search

Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems. The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to th

Variability of Rainfall in a Semi-Arid Catchment in Burkina Faso

Christine Wiedmann

The climate in the Sahel is marked by the spatial and temporal variability of rainfall. As the agriculture is mainly rainfed in Burkina Faso, the information about rainfall is crucial to help the farmers in their decisions. A network of wireless sensorscope stations is installed in the catchment of the Singou River (in the Gourma region of Burkina Faso) and monitor at a time step of 5 minutes meteorological parameters such as rain.The raindata of 2010 was evaluated and unreliable stations were excluded from the dataset. The rain events for the period of May to August were identified. More rain is measured in the month of August, which is rather due to thr higher frequency of rain events than to the intensity of the events. The accuracy of the Davis and précis rain gauges was evaluated through outdoor and laboratory experiments. At low intensities, the proposed error range for the Davis raingauges is +-15% and for the précis rain gauge data a correction factor of 2 (multiplication) should be applied. Indigenous knowledge was integrated in the thesis through a survey on the perception of rain in Tambarga. The most important aspects of rain are the quantity, the regularity of rain events and wind (which may destroy houses). The conditions that favour the development of a storm are analyzed through principle components analysis (PCA). The analysis showed that only few events are clearly influenced by one single parameter (soil moisture, latent heat flux, sensible heat flux, antecedent rain). The persistence of rainfall patterns was also subjected to analysis. A certain persistence of the rainfall patterns was detected for the month of August during which the rain events succeed closely each other. Finally, the validity of the formula of time of ponding based on Richard’s equation was evaluated (using the data from the infiltration experiments). The formula is valid for the catchment only in the case when continous flow in the river is observed without the contribution of rain. The results of the different analyses presented in the thesis give different insights on the characteristics of the rain events. Some information can already be used, but in order to obtain more meaningful results the analyses can be repeated with data that covers several years.

2012

Modelling the potential of Natural Flood Management practices in an upland peatland catchment with the distributed TOPMODEL

Hélène Amélie Madeleine Boisgontier

In order to reduce flood risk, there is a growing interest in more nature-based solutions, such as Natural Flood Management (NFM), rather than hard-engineering ones. To help build the evidence towards these solutions, we analysed the influence of different land cover change and land management in an upland peatland catchment in the Yorkshire Dales, UK. Using the distributed TOPMODEL from Gao et al., we first modelled the impact of revegetation, grazing and prescribed burning on the flood hydrograph. Then, tree planting as woodland and hedgerow, was implemented in the model and its influence on the flood peak was assessed. Revegetation of the whole catchment, from bare peat to \textit{Eriophorum} grasses, reduced the flood peak by 47% and delayed it by an hour and twenty minutes for an 18mm/hour rainfall. Taking a baseline \textit{Eriophorum} cover of the catchment, prescribed burning did not have a significant impact and grazing gave a 45-minute earlier and 31% higher flood peak. Woodland and hedgerow planting on grazed areas both had a positive influence on the flood peak. Planting hedgerows on every field edges in the catchment reduced the flood peak by 14% and delayed it by 15 minutes for the same above rainfall. Different scenarios of woodland planting were analysed: existing, floodplain, riparian and wider catchment woodlands. The most efficient scenarios were floodplain and riparian planting which gave a similar 3-hour late and 59% lower flood peak. The existing woodland scenario was the least efficient (12.5% reduction and 6-minute delay) and the wider catchment scenario was in between (40% reduction and 1-hour delay). Location of planting in the catchment was found to be very important and planting is near-stream zones is more efficient than in the headwaters or midslope of the catchment. Tree type and age at the catchment scale did not have a strong influence. Efficiency of tree planting on flood peak reduction and delay decreases with increasing rainfall intensity and storm duration. Floodplain woodland was found to be the most interesting scenario as flood risk was significantly reduced while only changing the land cover of a small portion of the catchment area (3%).

2018

Fonctionnement hydrique de différents types de placages sableux dans le sahel burkinabè

Dial Niang

The study was performed in the Sahelian part of Burkina Faso. The country is subject to difficult climatic conditions, a strong demographic growth and a continuous decrease in soil fertility. The resulting degradation affects the processes which govern ecological systems and leads, on the long run, to a modification of the ecosystems. This is particularly the case of the eolian formations, the only ecological units susceptible of being useful in the area, since they present good infiltration capacity and support the main part of the vegetation. Field measurements (rainfall, runoff, saturated hydraulic conductivity, bulk density, hydraulic conductivity function, soil water content and pressure head) have been done to asses the impacts of the environmental degradation on the hydrodynamic behavior of these formations. Within this context, an experimental device made of seven measurement sites has been installed in three contrasting zones. The results show significant differences in the hydrodynamic behavior of the soils, according to their surface properties. On one hand, the sites located on an erosion crust as well as on a drying crust in transition towards an erosion crust, are characterized by a low infiltration capacity which favors runoff. The quantity of infiltrated water is very low when compared to total precipitation. Water is stored near the soil surface (in the first 30 cm) favoring evaporation in the days following rainfall events. Thus, little water is available for plants. The resulting water stress causes a weak density, or even complete absence of plants which have difficulties to colonize such surfaces. Surface runoff is favored with runoff coefficients ranging from 50 to 80 %. On the other hand, the sites situated on drying crusts are characterized by good hydraulic conductivity and infiltration capacity. The storage of water in the root zone is more important. Water flows deeper into the soil and, occasionally, drains at the reference depth of 50 cm. The runoff coefficients were found to be lower than in the other sites (30 to 40 %). This differentiated behavior is one of the main factors affecting the formation of different landscapes in the area through the existence of a relationship between soil surface properties, the type of landscape and the ratio runoff/infiltration. The sites located on an erosion crust and those in transition towards an erosion crust behave like impluviums, while those situated on drying crusts are more favorable to infiltration. Observation of the evolution of the hydraulic properties of the soil surface shows a progressive alteration that frequently leads to strongly degraded mediums unfavorable to plants. Control and rehabilitation methods considered in the study, namely "restoration" and "mise en défens" have only partial and time-limited effects; they do not make it possible to stop durably degradation or to favor rehabilitation. Among the other measures assessed on the INERA plots, only the restitution to the soil of the beforehand mown plants causes an improvement of the soil surface properties; the restitution causes the formation of an important macroporal system favoring water infiltration and circulation in the soil. The effects of those different measures occur mainly in the first years after their introduction and concern especially vegetation regeneration, soil protection against degradation factors, improvement of hydrodynamic soil properties, reduction of runoff and erosion. They make it possible to slow down soil degradation, but not to stop it definitively. Within this context, proposals of further studies focussed on other measures to control and rehabilitate degraded areas have been suggested, taking into account the specificity of the sahelian zone, the endogenous knowledge of the agro-pastors, as well as socio-economic and ecological conditions.

EPFL2006