Hydrological functioning of western African inland valleys explored with a critical zone model

Inland valleys are seasonally waterlogged headwater wetlands, widespread across western Africa. Their role in the hydrological cycle in the humid, hard-rock-dominated Sudanian savanna is not yet well understood. Thus, while in the region recurrent floods are a major issue, and hydropower has been recognized as an important development pathway, the scientific community lacks precise knowledge of streamflow (Q) generation processes and how they could be affected by the presence of inland valleys. Furthermore, inland valleys carry an important agronomic potential, and with the strong demographic rates of the region, they are highly subject to undergoing land cover changes. We address both the questions of the hydrological functioning of inland valleys in the Sudanian savanna of western Africa and the impact of land cover changes on these systems through deterministic sensitivity experiments using a physically based critical zone model (ParFlow-CLM) applied to a virtual generic catchment which comprises an inland valley. Model forcings are based on 20 years of data from the AMMA-CATCH observation service and parameters are evaluated against multiple field data (Q, evapotranspiration – ET –, soil moisture, water table levels, and water storage) acquired on a pilot elementary catchment. The hydrological model applied to the conceptual lithological/pedological model proposed in this study reproduces the main behaviours observed, which allowed those virtual experiments to be conducted. We found that yearly water budgets were highly sensitive to the vegetation distribution: average yearly ET for a tree-covered catchment (944&thinsp;mm) exceeds that of herbaceous cover (791&thinsp;mm). ET differences between the two covers vary between 12&thinsp;% and 24&thinsp;% of the precipitation of the year for the wettest and driest years, respectively. Consequently, the tree-covered catchment produces a yearly Q amount of 28&thinsp;% lower on average as compared to a herbaceous-covered catchment, ranging from 20&thinsp;% for the wettest year to 47&thinsp;% for a dry year. Trees also buffer interannual variability in ET by 26&thinsp;% (with respect to herbaceous). On the other hand, pedological features (presence – or absence – of the low-permeability layer commonly found below inland valleys, upstream and lateral contributive areas) had limited impact on yearly water budgets but marked consequences for intraseasonal hydrological processes (sustained/non-sustained baseflow in the dry season, catchment water storage redistribution). Therefore, subsurface features and vegetation cover of inland valleys have potentially significant impacts on downstream water-dependent ecosystems and water uses as hydropower generation, and should focus our attention.</p

A new parameterisation scheme of ground heat flux for land surface flux retrieval from remote sensing information

The objective of the study was to assess the performance of a new parameterisation scheme of ground heat flux (G) for retrieving surface fluxes from remote sensing data (MODIS-Terra). Formulae that are based on empirical relationships relating G to net radiation, Rn (G = αRn, α being a function of a vegetation index, VI) are currently used, but presented drawbacks, especially in bare or sparse vegetation areas because of the poor adequacy of VI-based relationships to account for changes in soil moisture. In this study, we proposed to link α to the evaporative fraction, EF. In a first step, using a non-dimensional form of the surface energy balance, we demonstrated that α is functionally related to EF and to the ratio γ = G/H (H = sensible heat flux). In a second step, we proposed an EF-based parameterisation of α, using ground fluxes data sets collected throughout the years 2005, 2006 and 2007 at four flux-tower sites in West African countries (Mali, Benin, Niger) that differ in surface conditions and Monsoon influence. The analysis indicated that the average site-specific values of α and EF were well described by a linear relationship of the type α = a EF + b, with a = −0.22 and b = 0.23. In a third stage, we investigated whether ET-retrieval from remote sensing information (MODIS-Terra) using the new parameterisation of α perform better than the classical formulation through VI-based relationships. We found that the retrieved values of H using the new parameterisation supplied the best agreement with the observed ground data and significant improvement with respect to estimates from α–VI relationships. Advantages and limitations of the proposed parameterisation scheme were discussed

Comparison of three sensible heat flux measurement techniques over homogeneous asphalt in simulated rain

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The contribution of MRS and resistivity methods to the interpretation of actual evapo-transpiration measurements: a case study in metamorphic context in north Benin

International audienceA quantitative budget estimate of actual evapo-transpiration is a key issue for enhanced hydrological modelling in northern Bénin. Actual evapo-transpiration is estimated using large aperture scintillometer equipment, devoted to sensible heat flux measurements. However, a previous study reported that the actual evapo-transpiration cycle is not fully understood. Indeed, the actual evapo-transpiration depends strongly on several factors such as climate, vegetation pattern, soil water storage and human activities. The respective contributions of the aquifer and vadose zone to the actual evapo-transpiration budget are not known. When using piezometric variations of the water table, the aquifer contribution is not easy to quantify since the specific yield may vary in the investigated area, located in a metamorphic rock environment. In the present study, we investigate whether significant differences in the aquifer's specific yield could exist within the large aperture scintillometer measurement area, leading to different actual evapo-transpiration water losses. We use joint frequency electromagnetic resistivity mapping, geological surveys and magnetic resonance sounding (MRS) to delineate the effective porosity of the regolith around the scintillometre measurement area. Thirteen MRS soundings implemented in key areas reveal a clear classification of the main geological units on the basis of their water content. The MRS water content varies between 1.5-3% for amphibolite and micaschists formations to more than 12% for quartzitic fractured formations, whereas the MRS relaxation time T1 is less discriminating (150-250 ms), indicating a small variation in pore size. Then, as a first modelling exercise, we assumed that the MRS water content (the effective porosity) maximizes the specific yield. The actual evapo-transpiration budget given by a previous study (Guyot et al. 2009) is then re-interpreted using geophysical data: we found that a) the measured water table depletion can explain the actual evapo-transpiration value providing enough water for the transpiration process and b) the significant discrepancies in actual evapo-transpiration signals observed between the eastern and western parts of the watershed can be explained by the respective effective porosity of the geological units. Even if further research is needed to link MRS water content to the specific yield and to evaluate a possible role of the deep vadose zone, the hydrogeophysical mapping presented in this study highlights the role of the MRS method for providing relevant information to understand hydrological processes in this complicated geological context of north Bénin

Surface response to rain events throughout the West African monsoon

This study analyses the response of the continental surface to rain events, taking advantage of the long-term near-surface measurements over different vegetation types at different latitudes, acquired during the African Monsoon Multidisciplinary Analysis (AMMA) by the AMMA-CATCH observing system. The simulated surface response by nine land surface models involved in AMMA Land Model Intercomparison Project (ALMIP), is compared to the observations. The surface response, described via the evaporative fraction (EF), evolves in two steps: the immediate surface response (corresponding to an increase of EF occurring immediately after the rain) and the surface recovery (characterized by a decrease of EF over several days after the rain). It is shown that, for all the experimental sites, the immediate surface response is mainly dependent on the soil moisture content and the recovery period follows an exponential relationship whose rate is strongly dependent on the vegetation type (from 1 day over bare soil to 70 days over forest) and plant functional type (below and above 10 days for annual and perennial plants, respectively). The ALMIP model ensemble depicts a broad range of relationships between EF and soil moisture, with the worst results for the drier sites (high latitudes). The land surface models tend to simulate a realistic surface recovery for vegetated sites, but a slower and more variable EF decrease is simulated over bare soil than observed

Riparian forest and permanent groundwater: a key coupling for balancing the hillslope water budget in Sudanian West Africa

International audience(Com., 7 juin 2011, n° 10-18.108

Evaluating Evapotranspiration of a crop mosaic using microwave scintillometry

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