Climate Normalized Spatial Patterns of Evapotranspiration Enhance the Calibration of a Hydrological Model

Spatial pattern-oriented evaluations of distributed hydrological models have contributed towards an improved realism of hydrological simulations. This advancement has been supported by the broad range of readily available satellite-based datasets of key hydrological variables, such as evapotranspiration (ET). At larger scale, spatial patterns of ET are often driven by underlying climate gradients, and with this study, we argue that gradient dominated patterns may hamper the potential of spatial pattern-oriented evaluation frameworks. We hypothesize that the climate control of spatial patterns of ET overshadows the effect model parameters have on the simulated patterns. To address this, we propose a climate normalization strategy. This is demonstrated for the Senegal River basin as a modeling case study, where the dominant north-south precipitation gradient is the main driver of the observed hydrological variability. We apply the mesoscale Hydrological Model (mHM) to model the hydrological cycle of the Senegal River basin. Two multi-objective calibration experiments investigate the effect of climate normalization. Both calibrations utilize observed discharge (Q) in combination with remote sensing ET data, where one is based on the original ET pattern and the other utilizes the normalized ET pattern. As objective functions we applied the Kling-Gupta-Efficiency (KGE) for Q and the Spatial Efficiency (SPAEF) for ET. We identify parameter sets that balance the tradeoffs between the two independent observations and find that the calibration using the normalized ET pattern does not compromise the spatial pattern performance of the original pattern. However, vice versa, this is not necessarily the case, since the calibration using the original ET pattern showed a poorer performance for the normalized pattern, i.e., a 30% decrease in SPAEF. Both calibrations reached comparable performance of Q, i.e., KGE around 0.7. With this study, we identified a general shortcoming of spatial pattern-oriented model evaluations using ET in basins dominated by a climate gradient, but we argue that this also applies to other variables such as, soil moisture or land surface temperature.

1970s Drought, State Crisis, and Opportunities for Transnational River Development in the Senegal Basin

During the early 1970s, the Sahel suffered from drought and famine. Previous research has emphasized how these factors weakened West African states. The drought, however, provided an opportunity for a transnational river organization in the Senegal River basin (the OMVS) to obtain financing for an integrated development program. Wall shows how the OMVS leveraged concern about famine to obtain funding. She uses digital text analysis to demonstrate institutional priorities shifting to focus on agriculture. This combination of document analysis with digital methods demonstrates how famine strengthened a multi-state organization, requiring a revision of how this event affected African political capacity.

Exploitation de l’espece Anadara Senilis (Arches) dans le Parc National de la Langue de Barbarie (PNLB), Gandiol (Senegal)

Dans l’espace du Gandiol, les activités économiques traditionnelles étaient la pêche et l’agriculture avec lesquelles la population a une longue tradition. En 2003, les fortes pluies enregistrées dans le Haut bassin du fleuve Sénégal expliquent l’augmentation massive du débit du fleuve consécutivement à l’addition des ondes de crue. Pour empêcher des inondations dans la ville de Saint-Louis, une brèche a été ouverte. Cependant, cette brèche a eu des effets négatifs sur l’agriculture et beaucoup d’espèces de poissons. Elle a, aussi, entrainé des modifications écologiques se manifestant par l’apparition de certaines espèces aquatiques : Anadara Senilis, Carasostrea Gasar. Dès lors, l’étude de l’exploitation de l’espèce Anadara senilis dans le parc national de la langue de Barbarie (commune de Ndiebène Gandiol) fait l’objet de cette contribution. La méthodologie adoptée intègre les enquêtes de terrain sous forme de questionnaires et de guides d’entretien et le traitement des données quantitatives et qualitatives. Les résultats montrent que la collecte des coquillages est progressivement devenue la principale activité des femmes. Cette activité est pratiquée dans un but essentiellement lucratif et a eu des impacts considérables tant au niveau socioéconomique qu’environnemental. C’est pourquoi, l’encadrement des exploitants est souhaitable pour une gestion durable de cette ressource renouvelable. In the Gandiol area, the traditional economic activities were fishing and agriculture, with which the population has a long tradition. In 2003, the heavy rains recorded in the upper Senegal River basin explain the massive increase in the river's flow following the addition of flood waves. To prevent flooding in the city of Saint-Louis, a breach was opened. However, this breach had negative effects on agriculture and many fish species. It has also caused ecological changes manifested by the appearance of certain aquatic species: Anadara Senilis, Carasostrea Gasar. Therefore, the study of the exploitation of the species Anadara senilis in the National Park of the Langue de Barbarie (municipality of Ndiebène Gandiol) is the subject of this contribution. The methodology adopted integrates field surveys in the form of questionnaires and interview guides and the processing of quantitative and qualitative data. The results show that shellfish collection has gradually become the main activity of women. This activity is practiced essentially for profit and has had considerable impacts at both the socio-economic and environmental levels. This is why the supervision of the operators is desirable for a sustainable management of this renewable resource.

Changements historiques à long terme du débit écoulé dans le Haut bassin du fleuve Sénégal

In this study, the flow trends on either side of the Manantali dam, in the upper Senegal River basin, were assessed. The assessment was carried out for the period 1970 to 2019 using the Sen slope and the Mann-Kendall test. The Mann-Kendall trend test was used to indicate the direction of the trends, while the Sen slope was used to assess the magnitude of the change. The results of the study showed that there was an upward trend in the flow of the Senegal River both annually and for every month upstream (Bafing-Makana station) and downstream (Bakel station) from the barrage. The strongest upward trend is noted in October on the upstream station with 4.12 m3/month and in September on the downstream station with 10.99 m3/month. In return, the weakest upward trend is observed in May on the upstream station with 0.07 m3/month and in August on the downstream station with 0.67 m3/month. The annual flow also showed a significant increase upstream (with a rate of 0.35 m3/year) and downstream (with a rate of 4.53 m3/year) with a confidence level of 99 %. The results of the study also showed that an increase in precipitation causes an increase in runoff, despite the construction of hydraulic structures.

Bioenergy Potential of Crop Residues in the Senegal River Basin: A Cropland–Energy–Water-Environment Nexus Approach

Access to energy services is a priority for sustainable economic development, especially in rural areas, where small- and medium-sized enterprises have many difficulties in accessing reliable and affordable electricity. Western African countries are highly dependent on biomass resources; therefore, understanding the potential of bioenergy from crop residues is crucial to designing effective land-management practices. The assessment of the capability to use crop residues for electricity production is particularly important in those regions where agriculture is the dominant productive sector and where electrification through grid extension might be challenging. The objective of this work was to guide the development of sustainable strategies for rural areas that support energy development by simultaneously favouring food self-sufficiency capacity and environmental benefits. These complex interlinkages have been jointly assessed in the Senegal river basin by an integrated optimization system using a cropland–energy–water-environment nexus approach. The use of the nexus approach, which integrates various environmental factors, is instrumental to identify optimal land-energy strategies and provide decision makers with greater knowledge of the potential multiple benefits while minimizing trade-offs of the new solutions such as those connected to farmers’ needs, local energy demand, and food and land aspects. By a context-specific analysis, we estimated that, in 2016, 7 million tons of crop residues were generated, resulting in an electricity potential of 4.4 million MWh/year. Several sustainable land-energy management strategies were explored and compared with the current management strategy. Our results indicate that bioenergy production from crop residues can increase with significant variability from 5% to +50% depending on the strategy constraints considered. An example analysis of alternative irrigation in the Guinea region clearly illustrates the existing conflict between water, energy, and food: strategies optimizing bioenergy achieved increases both for energy and food production (+6%) but at the expense of increasing water demand by a factor of nine. The same water demand increase can be used to boost food production (+10%) if a modest decrease in bioenergy production is accepted (−13%).

Trend and Sensitivity Analysis of Reference Evapotranspiration in the Senegal River Basin Using NASA Meteorological Data

Understanding evapotranspiration and its long-term trends is essential for water cycle studies, modeling and for water uses. Spatial and temporal analysis of evapotranspiration is therefore important for the management of water resources, particularly in the context of climate change. The objective of this study is to analyze the trend of reference evapotranspiration (ET0) as well as its sensitivity to climatic variables in the Senegal River basin. Mann-Kendall’s test and Sen’s slope were used to detect trends and amplitude changes in ET0 and climatic variables that most influence ET0. Results show a significant increase in annual ET0 for 32% of the watershed area over the 1984–2017 period. A significant decrease in annual ET0 is observed for less than 1% of the basin area, mainly in the Sahelian zone. On a seasonal scale, ET0 increases significantly for 32% of the basin area during the dry season and decreases significantly for 4% of the basin during the rainy season. Annual maximum, minimum temperatures and relative humidity increase significantly for 68%, 81% and 37% of the basin, respectively. However, a significant decrease in wind speed is noted in the Sahelian part of the basin. The wind speed decrease and relative humidity increase lead to the decrease in ET0 and highlight a “paradox of evaporation” in the Sahelian part of the Senegal River basin. Sensitivity analysis reveals that, in the Senegal River basin, ET0 is more sensitive to relative humidity, maximum temperature and solar radiation.

Evaluation and Calibration of Alternative Methods for Estimating Reference Evapotranspiration in the Senegal River Basin

Reference evapotranspiration (ET0) is a key element of the water cycle in tropical areas for the planning and management of water resources, hydrological modeling, and irrigation management. The objective of this research is to assess twenty methods in computing ET0 in the Senegal River Basin and to calibrate and validate the best methods that integrate fewer climate variables. The performance of alternative methods compared to the Penman Monteith (FAO56-PM) model is evaluated using the coefficient of determination (R2), normalized root mean square error (NRMSE), percentage of bias (PBIAS), and the Kling–Gupta Efficiency (KGE). The most robust methods integrating fewer climate variables were calibrated and validated and the results show that Trabert, Valiantzas 2, Valiantzas 3, and Hargreaves and Samani models are, respectively, the most robust for ET0 estimation. The calibration improves the estimates of reference evapotranspiration compared to original models. It improved the performance of these models with an increase in KGE values of 45%, 32%, 29%, and 19% for Trabert, Valiantzas 2, Valiantzas 3, and Hargreaves and Samani models, respectively. From a spatial point of view, the calibrated models of Trabert and Valiantzas 2 are robust in all the climatic zones of the Senegal River Basin, whereas, those of Valiantzas 3 and Hargreaves and Samani are more efficient in the Guinean zone. This study provides information on the choice of a model for estimating evapotranspiration in the Senegal River Basin.

Trade-off, vulnerability and power asymmetry in the Senegal River basin

<p>The development of Senegal River basin involves trading-off competing objectives in an uncertain environment. Through a stochastic analysis, the trade-off discovery can be enriched to identify vulnerabilities; that is, the sensitivity of those losses with respect to changing natural and anthropogenic factors. In the Senegal River basin, the availability of water at a particular point in space and time is directly linked to both the hydrologic processes and the level of development of the water resources system. Our analysis of the trade-off relationships reveals the existence of two coalitions of objectives: traditional food production (agriculture and floodplain fisheries) versus hydropower-navigation. In terms of vulnerability, the examination of probabilistic trade-offs also shows that of the two main coalitions of objectives, the one dealing with traditional food production is much more vulnerable to changes in both hydro-climatic conditions and allocation policies. Of interest is the fact that the first coalition mostly concerns downstream riparian countries while hydropower, and to a less extent, navigation concern upstream countries. The result is a reinforced power asymmetry where vulnerable downstream riverine communities compete for water with politically and/or economically more powerful upstream water users like power companies.</p>