Path-average rainfall estimation from optical extinction measurements using a large-aperture scintillometer

The potential of a near-infrared large-aperture boundary layer scintillometer as path-average rain gauge is investigated. The instrument was installed over a 2.4-km path in Benin as part of the African Monsoon Multidisciplinary Analysis (AMMA) Enhanced Observation Period during 2006 and 2007. Measurements of the one-minute-average received signal intensity were collected for 6 rainfall events during the dry season and 16 events during the rainy season. Using estimates of the signal base level just before the onset of the rainfall events, the optical extinction coefficient is estimated from the path-integrated attenuation for each minute. The corresponding path-average rain rates are computed using a power-law relation between the optical extinction coefficient and rain rate obtained from measurements of raindrop size distributions with an optical spectropluviometer and a scaling-law formalism for describing raindrop size distribution variations. Comparisons of five-minute rainfall estimates with measurements from two nearby rain gauges show that the temporal dynamics are generally captured well by the scintillometer. However, the instrument has a tendency to underestimate rain rates and event total rain amounts with respect to the gauges. It is shown that this underestimation can be explained partly by systematic differences between the actual and the employed mean power-law relation between rain rate and specific attenuation, partly by unresolved spatial and temporal rainfall variations along the scintillometer path. Occasionally, the signal may even be lost completely. It is demonstrated that if these effects are properly accounted for by employing appropriate relations between rain rate and specific attenuation and by adapting the pathlength to the local rainfall climatology, scintillometer-based rainfall estimates can be within 20% of those estimated using rain gauges. These results demonstrate the potential of large-aperture scintillometers to estimate path-average rain rates at hydrologically relevant scales

Combinaison d'un échantillonnage par hyper-cubes latins et de la méthode du recuit simulé pour optimiser un modèle hydrologique à base physique

International audiencePhysically based hydrological models involve a large amount of parameters and data. Any of them is associated with uncertainties because of indirect measurements of some characteristics or because of spatial or temporal variability of others, ... Then, even if lots of data are measured in the field or in the laboratory, ignorance and uncertainty about data persist and a large degree of freedom remains for modeling. Moreover the choice for physical parameterization also induces uncertainties and errors in model behavior and simulation results. To address this problem, sensitivity analyses are useful. They allow the determination of the influence of each parameter on modeling results and allow the adjustment of an optimal parameter set by minimizing a cost function. However, the larger the number of parameters, the more expensive the computational costs to explore the whole parameter space. In this context, we carried out an original approach in the hydrology domain to perform this sensitivity analysis using a 1D Soil - Vegetation - Atmosphere Transfer model. The chosen method is a global method. It focuses on the output data variability due to the input parameter uncertainties. The latin hypercube sampling is adopted to sample the analyzed input parameter space. This method has the advantage to reduce the computational cost. The method is applied using the SiSPAT (Simple Soil Vegetation Atmosphere Transfer) model over a complete year period with observations collected in a small catchments in Benin, within the AMMA project. It involves sensitivity to 30 parameters sampled in 40 intervals. The quality of the modeled results is evaluated by calculating several criteria: the bias, the root mean square error and the Nash-Sutcliffe efficiency coefficient between modeled and observed time series of net radiation, heat fluxes, soil temperatures and volumetric water contents.... To hierarchize the influence of the various input parameters on the results, the study of correlations between several criteria (cost functions) and each parameter is necessary. Correlation coefficients can be chosen as sensitivity factors. The advantage is that they are independent of the chosen range of each parameter (unlike the regression coefficients). Nonetheless, as all parameters are varying simultaneously, total correlation coefficients are not providing the expected information, whereas partial correlation coefficients indicate parameter influence considering the other parameters. The multiple correlation coefficients between a result and parameters indicates the quality of the multiple linear regression. To optimize the parameter set, we use the simulated annealing method. Thus, it is possible to look for a global extremum (while most of other methods allow to find only a local extremum) as it includes a random part to explore a large scale of possibilities. It is its main advantage. The optimal parameter set allows a significant improve of the cost functions. Despite this enhancement, a certain degree of uncertainty still remains on the optimal parameter set

Impact of Deforestation on Water Budget in Sudanian Climate (Benin).

International audienceIn West Africa, surface atmosphere exchanges have been found to impact both regional and local features of the Monsoon. At local scale the spatial patterns of evaporative fraction can drive the trajectories of mesoscale convective systems. Within Sudanian climate, ~80% of the precipitation returns to atmosphere through evapotranspiration. However, this amount and its seasonal dynamic may vary with the vegetation cover. Consequently, one might expect that any land use or climate changes could lead to the modification of the surface feedbacks, and, thus on both the atmospheric and the continental water cycle. The sudanian region of West Africa is submitted to a 3% demographical increase per year, which induces a drastic expansion of crops areas. In the Upper Oueme basin, the natural forest cover reduces from 70% to 25% in 40 years. This study aims at quantifying the changes in evapotranspiration regime caused by such a land use change under sudanian climate. The AMMA-CATCH observatory documents evapotranspiration flux in West Africa since 2007. A pluri-annual energy budget of a forest and a cropland area are analysed. It is shown that sudanian forest evapo-transpirated always more than cropland areas because of agricultural practice and water availability for trees. Thus, during the dry season, the cropland areas are bare while the forests do not completely lose their leaves. Their deep root systems allow the trees to get access to water. Observed evapotranspiration is significant over forests. During the rainy season, vegetation is fully developed and well-watered. Nevertheless, lower but significant differences in evaporative fraction are also observed. At annual scale these differences lead to a 13% to 30% reduction of evapotranspiration with deforestation

Stratégie pour documenter l'hétérogénéité des propriétés des sols et impact sur les transferts d'eau de l'échelle du versant à celle du bassin versant

International audienceHeterogeneity in soil properties has been identified to impact water transfers at different scales from vertical column, hillslopes to watershed. Thus Distributed physically based hydrological models require distributed hydraulic characteristics to quantify these impacts. To characterize soil properties and their heterogeneity, a multi-scale sampling strategy was proposed based on distributed information including electromagnetic survey maps, topography and land use coverage. Each identified units are characterized by there hydraulic properties including in situ infiltration tests. This strategy was applied over the Ara Catchment (12km2) in northern Benin. It has been instrumented in the framework of the AMMA-Catch experimental network in West Africa, to better determine water resources and to investigate possible hydrological retro-action on monsoon cycle. From hydrological point of view, distributed soil hydraulic properties are supposed to impact water transfers and watershed dynamics all along the monsoon cycle. To document this heterogeneity, an electrical conductivity map and geological survey was used as starting points to identify the ground structures which align with the north-south direction with a dip angle of 20° east. A total of 20 pits have been opened to document the 0-2m horizons, and 2 more for the 0-5m horizons. 3 pits were digged within each geological structure areas at the surface. In each pit, the retention and hydraulic conductivity curves of each pedological horizon were characterized with three replicates. This database is used to document the variability of these properties and to produce soil hydraulic property maps. Using the variability information, we tested their impact with the Parflow-CLM 3D distributed model. It was run in an homogeneous configuration and compared with a data controlled heterogeneous configuration. The latest is prepared using a turning band algorithm to distribute soil hydraulic properties

Comment les structures de subsurface affectent-elles les bilans d'énergie: un cas d'étude en Afrique soudannienne

Fractured bedrock areas are still challenging for hydrological modeling because of their complex underground property distributions. The heterogeneity in soil hydraulic properties, for example, can control the subsurface water fluxes and create surface soil moisture pattern which becomes preferential areas for runoff production or evapotranspiration. This study aimed to evaluate the impact of a bedrock topography, including outcropping, on subsurface water fluxes and the induced energy budget patterns at the surface. To deal with these ground water/surface water interactions, we run the Parflow-CLM distributed coupled land surface and groundwater model over the 12km2 Ara watershed (Northern Benin) for different bedrock configurations. The Ara catchment is submitted to a sudanian climate with 1200mm total rainfall per year. It is part of the AMMA-Catch project in which 3 meso sites have been documented along a south to north transect in West Africa. The geology of the Ara catchment is composed of metamorphic rocks. The main orientation of the geological structures (and of the gneiss foliation) is roughly north-south and the dip angle is 20° east. These structure create patterns in effective porosity distribution which is supposed to induce subsurface flow perpendicular to surface slope direction. Controlled Parflow-CLM simulation results are compared with energy budget data, including 3 net radiation measurements, eddy covariance station, scintillometric measurements to estimate evapotranspiration at different scales. The experimental device also include ground measurements like distributed surface soil moisture profile and piezometers. Parflow-CLM simulations are in good agreement with energy budget observations if observed Leaf Area Index time series are take into account. Then different hydraulic property distributions (effective porosity, hydraulic transmissivity, water retention curves) are evaluated through watershed dynamic differences

Impact of deforestation on surface water and energy feedbacks in sudanian region of West Africa.

International audienceIn West Africa, surface atmosphere exchanges have been found to impact both regional and local features of the Monsoon. At local scale the spatial patterns of evaporative fraction can drive the trajectories of mesoscale convective systems. Within Sudanian climate, ~80% of the precipitation returns to atmosphere through evapotranspiration. However, this amount and its seasonal dynamic may vary with the vegetation cover. Consequently, one might expect that any land use or climate changes could lead to the modification of the surface water and energy feedbacks, and, thus both the atmospheric and the regional water cycle. Finally, the sudanian region of West Africa is submitted to a 3% demographical increase per year, which induces a drastic expansion of crops areas. This study aims at quantifying the changes in evapotranspiration and sensible heat flux regimes caused by such a land use change under sudanian climate.The AMMA-CATCH observatory documents evapotranspiration flux in West Africa since 2007. A pluri-annual energy budget term of a clear forest and a cropland area are analysed. It is shown that sudanian forest evapo-transpirated always more than crop areas because of agricultural practice, which cleaned to bare the crops areas with bush fires and water availability for trees. Thus, during the dry season, the cultivated areas remain bare. At the same time, more than 1mm per day of evapotranspiration rate is measured above the forest area despite the lack of precipitations. Deep rooting systems allow the clear forest to get access to water from deep soil layers for transpiration throughout the dry season. During the rainy season, low but significant differences in evaporative fraction are also observed. These differences will lead to a large deficit of the water vapour that returns to the atmosphere, and, thus, will change significantly the continental water cycle when forests will be replaced by crops. In the future, agroforestry that combines local crops (yam, manioc, millet, etc.) and sparse trees could mitigate surface feedbacks. Finally, the selected trees could provide extra agricultural products taking benefit from deep water availability, but also strengthen social equilibrium between men and women

Soil moisture mapping over West Africa with a 30-min temporal resolution using AMSR-E observations and a satellite-based rainfall product

An original and simple method to map surface soil moisture over large areas has been developed to obtain data with a high temporal and spatial resolution for the study of possible feedback mechanisms between soil moisture and convection in West Africa. A rainfall estimation product based on Meteosat geostationary satellite measurements is first used together with a simple Antecedent Precipitation Index (API) model to produce soil moisture maps at a spatial resolution of 10x10 km(2) and a temporal resolution of 30-min. However, given the uncertainty of the satellite-based rainfall estimation product, the resulting soil moisture maps are not sufficiently accurate. For this reason, a technique based on assimilating AMSR-E C-band measurements into a microwave emission model was developed in which the estimated rainfall rates between two successive AMSR-E brightness temperature (TB) measurements are adjusted by multiplying them by a factor between 0 and 7 that minimizes the difference between simulated and observed TBs. Ground-based soil moisture measurements obtained at three sites in Niger, Mali and Benin were used to assess the method which was found to improve the soil moisture estimates on all three sites

Evaluation des termes du bilan hydrologique sur le bassin versant de la Donga par mesure et modélisation

L'Observatoire Hydrologique de la Haute Vallée de l'Ouémé (OHHVO, Bénin) d'une superficie de 14 600 km2 est le plus méridional (1200 mm/an) des 3 sites d'observation de l'O.R.E. AMMA-CATCH, répartis selon un gradient pluviométrique décroissant du Golfe de Guinée au Sahara. La longue période de sécheresse qui a frappé l'Afrique de l'Ouest (1970-1990) a diminué significativement la pluviométrie en climat soudanien (-11%) principalement en réduisant le nombre d'événements pluvieux (Le lay & Galle, 2005b). Ceci se traduit par une diminution des débits bien plus importante (environ 35%) sur l'Ouémé supérieur (Le lay &Galle, 2005a). L'état actuel des connaissances ne permet pas de transposer ce résultat à d'autres bassins de taille ou de couverture végétale différente. Pour identifier, quantifier les processus entrant en jeu dans les différents termes du bilan hydrologiques, le bassin de la Donga (586 km2) situé dans le cours supérieur du Haut Ouémé a été instrumenté. Les objectifs sont de: ·quantifier les différents processus (pluie, ruissellement, infiltration écoulements souterrains, évapotranspiration), ·identifier leurs temps caractéristiques, ·préciser leur rôle dans la variabilité intra et inter-annuelle en prenant en compte la dynamique de la végétation.Sous un climat soudanien, l'écoulement de la Donga est saisonnier et soutenu par le drainage des nappes. Chaque année, on observe un démarrage retardé des écoulements par rapport au début de la saison des pluies. Les questions sous-jacentes à l'origine non exclusivement superficielle des écoulements portent notamment sur : ·les parts du ruissellement direct et des nappes dans les écoulements en rivières, leurs évolutions intra-saisonnières, ·l'existence d'écoulement souterrain et rapide de sub-surface dans les tous premiers mètres du sol, ·l'existence d'une mémoire inter-annuelle liée au stock souterrain modulant le démarrage retardé des écoulements. Hormis l'origine des écoulements, l'autre inconnue majeure est le rôle de la végétation dans le cycle hydrologique à travers la transpiration. Les questions reliées à la végétation sont notamment: ·La végétation en début de cycle par ses prélèvements contribue-t-elle à retarder les écoulements ? ·Le défrichement des forêts au profit d'une mosaïque culture-jachère a-t-il un impact sur les écoulements