Exercice de scénarisation hydrologique en Afrique de l’Ouest—Bassin du Bani

Résumé

Les pays sub-sahariens basent principalement leur économie sur l’agriculture pluviale. Les projections démographiques à moyen ou long terme montrent que la pression démographique va s’accroître très fortement. A l’avenir, il faudra soutenir encore plus fortement le développement de stratégies agricoles de ces pays : cela nécessite une meilleure connaissance de leurs ressources en eau futures. Cette connaissance de la ressource en eau dans le futur passe par l’élaboration de scenarios climatiques et hydrologiques qui font intervenir différents acteurs, compétents dans des activités très variées, qu’ils soient scientifiques de divers domaines (climatologues, hydrologues, sociologues, …) mais aussi financiers, politiques, acteurs sociaux et décideurs. Ce processus d’élaboration de scenarios s’accompagne de nombreuses incertitudes, plus ou moins bien maitrisées, avec lesquelles doivent s’accommoder les gestionnaires d’ouvrages et planificateurs d’aménagements régionaux. Illustré par l’exemple du bassin du Bani, affluent du fleuve Niger, ce travail propose une réflexion sur la « scénarisation hydrologique » et la prédétermination des paramètres d’un modèle hydrologique dans un contexte global non stationnaire. Le choix de jeux de paramètres a de nombreux impacts sur les résultats de la scénarisation comme l’estimation de la ressource et la variabilité de cette ressource. Ces deux éléments sont fondamentaux pour qui doit conduire un processus de décision en réponse à une demande de stratégie d’aménagement ou à une demande de stratégie de développement. Nous proposons une méthodologie de détermination des jeux de paramètres se basant sur la construction de plusieurs jeux de paramètres issus de calages glissants, qui permet d’aller dans le sens de ce que de plus en plus de gestionnaires préconisent : ne pas essayer de prévoir le futur mais plutôt des futurs envisageables qui forceront des modèles d’impact, ce qui leur permettra de fournir des éléments de décision afin de s’adapter ou d’adapter des solutions d’aménagement aux conditions de ces futurs.

Editeur Z.W. Kundzewicz; Editeur associé C. Perrin

Citation Paturel, J.-E., 2014. Exercice de scénarisation hydrologique en Afrique de l’Ouest—Bassin du Bani. Hydrological Sciences Journal, 59 (6), 1135–1153. http://dx.doi.org/10.1080/02626667.2013.834340     

Analysis of hydrological seasonality across northern catchments using monthly precipitation–runoff polygon metrics

Abstract

Seasonality is an important hydrological signature for catchment comparison. Here, the relevance of monthly precipitation–runoff polygons (defined as scatter points of 12 monthly average precipitation–runoff value pairs connected in the chronological monthly sequence) for characterizing seasonality patterns was investigated to describe the hydrological behaviour of 10 catchments spanning a climatic gradient across the northern temperate region. Specifically, the research objectives were to: (a) discuss the extent to which monthly precipitation–runoff polygons can be used to infer active hydrological processes in contrasting catchments; (b) test the ability of quantitative metrics describing the shape, orientation and surface area of monthly precipitation–runoff polygons to discriminate between different seasonality patterns; and (c) examine the value of precipitation–runoff polygons as a basis for catchment grouping and comparison. This study showed that some polygon metrics were as effective as monthly average runoff coefficients for illustrating differences between the 10 catchments. The use of precipitation–runoff polygons was especially helpful to look at the dynamics prevailing in specific months and better assess the coupling between precipitation and runoff and their relative degree of seasonality. This polygon methodology, linked with a range of quantitative metrics, could therefore provide a new simple tool for understanding and comparing seasonality among catchments.

Editor Z.W. Kundzewicz; Associate editor K. Heal

Citation Ali, G., Tetzlaff, D., Kruitbos, L., Soulsby, C., Carey, S., McDonnell, J., Buttle, J., Laudon, H., Seibert, J., McGuire, K., and Shanley, J., 2013. Analysis of hydrological seasonality across northern catchments using monthly precipitation–runoff polygon metrics. Hydrological Sciences Journal, 59 (1), 56–72.     

Comparison of the Hargreaves and Samani equation and the Thornthwaite equation for estimating dekadal evapotranspiration in the Free State Province,South Africa

Reference evapotranspiration (ET) is an important parameter that needs to be estimated accurately to enhance its utility in numerous applications. Although the widely recommended procedure for calculating this index involves using the FAO Penman–Monteith equation (ETo), the latter’s effectiveness is constrained by its considerable data requirements. To overcome this constraint, alternative methods using the limited data available have to be explored. In this study the ability of the Hargreaves and Samani (ET_HS) and Thornthwaite (ET_T) equations to estimate ET was investigated using multi-year data (1999–2008) from eight weather stations in the semi-arid Free State Province of South Africa. Results for non-calibrated equations are closely correlated, with ET_HS tending to underestimate ET for the July to December period while ET_T underestimates ET for all months of the calendar year. Although estimates from calibrated equations are also closely correlated, they have smaller deviations compared to the original equations with the calibrated Hargreaves and Samani equation (ET_CHS) estimating reference evapotranspiration better than its calibrated Thornthwaite (ET_CT) counterpart. The former’s better performance suggests that in data-scarce areas, the Hargreaves and Samani model is capable of giving results within acceptable ranges of accuracy.     

Diachronic metamorphic and structural evolution of the Connecticut Valley–Gaspé trough,Northern Appalachians

The Connecticut Valley–Gaspé (CVG) trough represents a major, orogen-scale Silurian–Devonian basin of the Northern Appalachians. From Gaspé Peninsula to southern New England, the CVG trough has experienced a contrasting metamorphic and structural evolution during the Acadian orogeny. Along its strike, the CVG trough is characterized by increasing strain and polyphase structures, and by variations in the intensity of regional metamorphism and contrasting abundance of c. 390–370 Ma granitic intrusions. In southern Quebec and northern Vermont, a series of NW–SE transects across the CVG trough have been studied in order to better understand these along-strike variations. Detailed structural analyses, combined with phase equilibria modelling, Raman spectrometry, and muscovite ⁴⁰Ar/³⁹Ar dating highlight a progressive and incremental deformation involving south–north variation in the timing of metamorphism. Deformation evolves from a D₁ crustal thickening event which originates in Vermont and progresses to southern Québec where it peaked at 0.6 GPa/380°C at c. 375 Ma. This was followed by a D₂ event associated with continuous burial in Vermont from 378 to 355 Ma, which produced peak metamorphic conditions of 0.85 GPa/380°C and exhumation in Quebec from 368 to 360 Ma. The D₃ compressional exhumation event also evolved from south to north from 345 to 335 Ma. D₁ to D₃ deformation events form part of a continuum with an along-strike propagation rate of ~50 km/Ma During D₁, the burial depth varied by more than 15 km between southern Quebec and Vermont, and this can be attributed to the occurrence of a major crustal indenter, the Bronson Hill Arc massif, in the New England segment of the Acadian collision zone.     

Spatio-temporal patterns of the interaction between groundwater and surface water in plains

The study of the dynamics of anthropic disturbances that have an effect on the hydrological systems in plains requires integral simulation tools for their diagnosis. The objective of this article is, first, to analyse and reproduce the spatio-temporal interactions between groundwater (GW) and surface water, net recharge, GW level, surface run-off, and evapotranspiration in the upper creek basin of Del Azul, which is located in the centre of the province of Buenos Aires, Argentina, and second, to obtain insights to apply the methodology to other similar situations. For this purpose, a model coupling the semidistributed hydrological model (Soil and Water Assessment Tool [SWAT]) and the hydrogeological model (MODFLOW) has been used. A simulation was carried out for a period of 13 years (2003–2015) on a daily scale. The application of the SWAT–MODFLOW coupling gave good results based on the adjustment between the calculated flows and levels, reaching a Nash–Sutcliffe of 0.6 and R²0.6 at the Seminario hydrometric station located at the watershed outlet point. According to the annual average balance, out of the total rainfall, evapotranspiration accounts for 85%, recharge accounts for 10.2%, and surface run-off accounts for 4.8%. Annual and monthly trends of the stream–aquifer interaction were determined, obtaining on average an annual GW discharge of 34 mm and an annual average recharge of the stream to the aquifer of 1.4 mm. Monthly GW discharges are higher in winter–spring (July to December with an average of 3.3 mm) and lower in summer–autumn (January to June with an average of 2.8 mm). The monthly average recharge of the stream towards the aquifer varies from 0.02 to 0.36 mm and is higher in March, May, and August, when water excess is produced in the basin. Through the analysis of coupled modelling, it is possible to analyse and reproduce the spatio-temporal transitions of flow existing between the stream, the hyporheic zone, and the aquifer.     

Seeing the trees for the (urban) forest: more-than-human geographies and urban greening

ABSTRACT

Urban spaces have long been places to think through human relationships with nature. The recent shift in thinking from urban green space as outcome to urban greening as a process provides an opportunity to consider more explicitly how we engage with more-than-human worlds in urban spaces, in more differentiated ways, and for what ends. In this paper we contribute to growing interest in improved urban sustainability and well-being by bringing human geography perspectives on more-than-human worlds into conversation with the literature on urban greening. Drawing on key examples oriented around urban trees, we consider two main themes: sensibilities and belonging. We argue for an understanding of urban places as collective achievements that not only involve knowing and living with diverse humans and non-humans but also involve the re/making of sensibilities and belongings. Through this paper, we aim to open dialogue about how more-than-human geographies might help us to differently understand urban trees, contemporary urban greening, and people–plant relations.     

Optimal design of snow stake networks to estimate snow depth in an alpine mountain range

Monitoring and estimation of snow depth in alpine catchments is needed for a proper assessment of management alternatives for water supply in these water resources systems. The distribution of snowpack thickness is usually approached by using field data that come from snow samples collected at a given number of locations that constitute the monitoring network. Optimal design of this network is required to obtain the best possible estimates. Assuming that there is an existing monitoring network, its optimization may imply the selection of an optimal network as a subset of the existing one (if there are no funds to maintain them) or enlarging the existing network by one or more stations (optimal augmentation problem). We propose an optimization procedure that minimizes the total variance in the estimate of snowpack thickness. The novelty of this work is to treat, for the first time, the problem of snow observation network optimization for an entire mountain range rather than for small catchments as done in the previous studies. Taking into account the reduced data available, which is a common problem in many mountain ranges, the importance of a proper design of these observation networks is even larger. Snowpack thickness is estimated by combining regression models to approach the effect of the explanatory variables and kriging techniques to consider the influence of the stakes location. We solve the optimization problems under different hypotheses, studying the impacts of augmentation and reduction, both, one by one and in pairs. We also analyse the sensitivity of results to nonsnow measurements deduced from satellite information. Finally, we design a new optimal network by combining the reduction and augmentation methods. The methodology has been applied to the Sierra Nevada mountain range (southern Spain), where very limited resources are employed to monitor snowfall and where an optimal snow network design could prove critical. An optimal snow observation network is defined by relocating some observation points. It would reduce the estimation variance by around 600 cm² (15%).     

Coupling of P–T–t–D histories of eclogite and metagreywacke—Insights to late Ordovician–Silurian crustal folding events recorded in the Beishan Orogen (NW China)

The Beishan complex is composed of orthogneiss and metagreywacke that both enclose bodies of eclogite and serves as a unique example for comparative petrological study of all these lithologies. The rocks show the earliest regional steep N-S striking fabric (S2) preserved in low strain domains that are reworked by ubiquitous steep N-NE dipping cleavage (S3). The eclogite shows an almost isotropic fabric defined by an M1 assemblage of Grt–Cpx–Amp–Qz–Rt–Ilm that is locally retrogressed to M2-3 amphibolite facies mineral assemblages, with P–T peak at 20–21 kbar and 750–775°C and retrogression to 2–3kbar and 530–550°C. The typical mineral assemblage of the host metagreywackes is Bt–Ms–Pl–Qz−Chl–Ilm±Grt. Rare Al-rich metagreywacke layers are composed of Grt–Ky–St±Sil−And–Bt–Ms–Pl–Qz±Chl±Rt–Ilm giving a P–T path with peak at 8–8.5kbar and ~670°C correlated with the S2 fabric and retrogression to ~2.5kbar and 525–550°C correlated with the S3 foliation. In two eclogite samples, the garnet-whole rock-clinopyroxene Lu–Hf isochrons give ages of 461.9±1.6 Ma and 462.0±6.2 Ma interpreted as reflecting average age of garnet formation, and Sm–Nd isochrons give ages of 453.6±2.7 Ma and 452.8±3.0 Ma interpreted as dating near-peak metamorphism. In metagreywacke, in-situ U–Pb dating of monazite gives two groups of ages of 445–440 Ma (Mnz cores) and 436–429 Ma (Mnz rims), interpreted as reflecting the metamorphic peak and retrogression. Our results show that eclogite was formed during Ordovician by subduction of a continental crust (D1). Eclogite and metagreywacke underwent partly decoupled P–T–t–D paths until their juxtaposition at mid-crustal levels during a first late Ordovician–early Silurian D2 shortening. Coupling of their P–T–t–D paths occurred during exhumation in the Silurian and a second and orthogonal D3 shortening event. The data from the Beishan Orogen are consistent with a collisional intra-Gondwanan orogen located south of the Central Asian Orogenic Belt.     

Relative pollen productivities of the major plant taxa of subtropical evergreen–deciduous mixed woodland in China

Quantifying the relationship between pollen and vegetation is an essential step in the pollen-based quantitative reconstruction of past vegetation cover. In this study, we use the Extended R-Value (ERV) model and a modern dataset of pollen (collected from moss polsters) and related vegetation from 50 sites in the Daba Mountains (subtropical China) to (i) estimate the relevant source area of pollen (RSAP) of the moss samples and the relative pollen productivities (RPPs) of nine major plant taxa-characteristic of the region, and (ii) evaluate the obtained RPPs. The RSAP estimates of moss polsters vary between 225 and 610 m depending on the ERV submodels and models of pollen dispersal and deposition used. The RPP estimates are different from values published in previous studies from temperate and subtropical China. This may be explained by differences in methodology, climate and vegetation (species composition and spatial distribution), of which vegetation is probably the most important factor. The ranking of the RPP estimates for the nine taxa is Pinus > Juglandaceae > D − Quercus (deciduous Quercus) > Poaceae > Rosaceae > Cyperaceae > Anacardiaceae > Castanea > Fabaceae. We use a ‘leave-one-out’ cross-validation strategy and the Landscape Reconstruction Algorithm (LRA) for pollen-based reconstruction of regional and local plant cover to evaluate the ERV model-based RPP estimates. Both the REVEALS (Regional Estimates of VEgetation Abundance from Large Sites)-based and the LOVE (LOcal Vegetation Estimates)-based plant cover using the RPP estimates are closer to the modern vegetation composition than pollen percentages, thus confirming the applicability of the ERV model and the LRA approach in subtropical China.