Christophe Peugeot

Water balance in a banded vegetation pattern: A case study of tiger bush in western Niger

Abstract The tiger bush is a patterned woodland with alternating bare area and vegetated stripes. In Niger, it covers one third of the Sahelian zone. These natural forests are of considerable economical interest since they are the main source of livestock forage and domestic energy. Its sustainable exploitation needs improved understanding of its dynamics. The redistribution of water between thicket and intervening bare areas is decisive for the water supply of the vegetation. Tiger bush patterning replicates an elementary unit composed of a bare area, the upslope border, the core and the downslope margin of the thicket. (Each zone of tiger bush is characterised by specific soil crusting associated with vegetation). Both water storage and runoff have been monitored after each rain, over a period of 4 yr, including contrasting rainy seasons, on the different zones composing the tiger bush. On the three crusted zones, runoff has a piecewise linear relationship with rain: on closed plots, runoff yield vs. annual rainfall ratio reaches 54% on bare soil, 2% on upslope border and 18% on downslope border. The measured infiltration confirms these rates on independent plots. In the core of the thicket, measured infiltration corresponds with the sum of the contributions of upslope zones, weighted by their relative lengths. This model predicts that bare area contributes up to 62% of the thicket supply, while direct rain is 27%, the senescence zone is 10% and the upslope border contribution is negligible (1%). The average water infiltration in the thicket is equal to 4× the incident rainfall, but water redistribution is not homogeneous within the core of the thicket. By the most favourable location, infiltration depth is measured to be about 8× the rainfall. The important runoff, mainly generated on the impervious bare area crosses the upslope border of the thicket without infiltrating, and entirely benefit to the core. Nothing is left to the downslope border, only rainfed. The upslope border, often described as favourable location for young plants is only rainfed most part of the year. By the end of the season, its increasing porosity, due to vegetation and termite activity let it benefit of the last rains. The simple water balance model based on runoff measurement is satisfactorily validated by independent observed infiltration.

Hydrologic process simulation of a semiarid, endoreic catchment in Sahelian West Niger. 1. Model-aided data analysis and screening

The drought in the African Sahel, persisting since 1968, emphasizes the vulnerability of local water resources to climatic variations and land use changes in this semiarid environment. Following the Hapex–Sahel experiment (1992–1994), research aims at understanding and modeling the impact on the hydrological cycle of these changing climatic and environmental conditions over a 1500-km2 area east of Niamey, Niger. The hydrological landscape consists of a myriad of small endoreic catchments feeding temporary or permanent pools from which water percolates to an unconfined aquifer. As the first step in a local-to-regional scale approach to the water resource recharge in the area, a physically based rainfall–runoff model is developed for a typical, 1.9-km2 endoreic system named Wankama, which is monitored continuously since 1992. This first of two companion papers describes the key elements of the hydrological landscape, the experimental setup with which the Wankama catchment and pool are monitored, the r.water.fea distributed hydrological model and its application to the construction of the Wankama catchment model. Because of the difficulties of long-term field experimentation in the region, not all observed events can be considered equally well recorded; in particular, detection of exogenous water inflow occurrences that may alter Wankama runoff estimates is needed. The paper describes the careful, model-aided data analysis performed in order to select the observation set with which the model can most reliably be tuned and operated. In this rainfall–runoff data analysis phase, the model is used prior to any calibration. Model calibration and verification, including output uncertainty, are the subject of the second paper [J. Hydrol. (2003)]. All steps of analysis and modeling were performed on the 1992–1998 data only, before the 1999–2000 data became available, and are tested a posteriori against the latter.

Hydrologic process simulation of a semiarid, endoreic catchment in Sahelian West Niger. 2. Model calibration and uncertainty characterization

The Wankama endoreic system in the region of Niamey (Niger), monitored over the period 1992–2000, is studied with r.water.fea, a physically-based, spatially-distributed rainfall–runoff model. Catchment characteristics and data, together with model principles and construction, are described in Peugeot et al. [J. Hydrol., 2003], who used the uncalibrated model as one of several investigation tools for the screening of rainfall–runoff observations. This second paper focuses on model calibration and verification, namely the methods and criteria used to that end followed by the results thereby obtained. Based on a diagnostic function that combines errors in runoff volumes and in peak discharges, calibration is performed by exploring a 3D parameter space. A resampling-based cross-validation technique is used to investigate calibration stability with respect to data sample fluctuations, and to assess the predictive capabilities of the calibrated model. The issues of parameter uncertainty, sample representativeness, and presence of influential observations, are discussed. An empirical, non-parametric method is devised to characterize parameter uncertainty and to assign intervals to volume predictions. Model verification is performed against the data from the last two seasons. Internal catchment behavior, as produced by the model, appears qualitatively consistent with field information, including a weak upper-area contribution to catchment outflow due to large runoff abstraction by the conveying hydrographic network.

An Intercomparison of Simulated Rainfall and Evapotranspiration Associated with a Mesoscale Convective System over West Africa

Abstract An evaluation of precipitation and evapotranspiration simulated by mesoscale models is carried out within the African Monsoon Multidisciplinary Analysis (AMMA) program. Six models performed simulations of a mesoscale convective system (MCS) observed to cross part of West Africa in August 2005. Initial and boundary conditions are found to significantly control the locations of rainfall at synoptic scales as simulated with either mesoscale or global models. When initialized and forced at their boundaries by the same analysis, all models forecast a westward-moving rainfall structure, as observed by satellite products. However, rainfall is also forecast at other locations where none was observed, and the nighttime northward propagation of rainfall is not well reproduced. There is a wide spread in the rainfall rates across simulations, but also among satellite products. The range of simulated meridional fluctuations of evapotranspiration (E) appears reasonable, but E displays an overly strong zonal sy...

Evaluating LSM-Based Water Budgets over a West African Basin Assisted with a River Routing Scheme

AbstractWithin the framework of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project phase 2 (ALMIP-2), this study evaluates the water balance simulated by the Interactions between Soil, Biosphere, and Atmosphere (ISBA) over the upper Oueme River basin, in Benin, using a mesoscale river routing scheme (RRS). The RRS is based on the nonlinear Muskingum–Cunge method coupled with two linear reservoirs that simulate the time delay of both surface runoff and base flow that are produced by land surface models. On the basis of the evidence of a deep water-table recharge in that region, a reservoir representing the deep-water infiltration (DWI) is introduced. The hydrological processes of the basin are simulated for the 2005–08 AMMA field campaign period during which rainfall and streamflow data were intensively collected over the study area. Optimal RRS parameter sets were determined for three optimization experiments that were performed using daily streamflow at five ...

Dynamics of water vapor and energy exchanges above two contrasting Sudanian climate ecosystems in Northern Benin (West Africa)

Natural ecosystems in sub-Saharan Africa are experiencing intense changes that will probably modify land surface feedbacks and consequently the regional climate. In this study, we have analyzed water vapor (QLE) and sensible heat (QH) fluxes over a woodland (Bellefoungou, BE) and a cultivated area (Nalohou, NA) in the Sudanian climate of Northern Benin, using 2 years (from July 2008 to June 2010) of eddy covariancemeasurements. The evaporative fraction (EF) response to environmental and surface variables was investigated at seasonal scale. Soil moisturewas found to be themain environmental factor controlling energy partitioning.During thewet seasons, EFwas rather stablewith anaverageof 0.75 ± 0.07over thewoodlandand 0.70 ± 0.025 over the cultivated area. This means that 70–75% of the available energy was changed into actual evapotranspiration during the investigated wet seasons depending on the vegetation type. The cumulative annual actual evapotranspiration (AET) variedbetween730 ± 50mmyr 1 at theNAsite and1040 ± 70mmyr 1 at theBEsite.Withsimilarweatherconditionsat thetwosites, theBEsite showed30%higherAETvalues thanthe NA site. The sensible heat flux QH at the cultivated site was always higher than that of the woodland site, but observed differences weremuch less than those ofQLE. In a land surface conversion context, these differences are expected to impact both atmospheric dynamics and the hydrological cycle.

Streamflows over a West African Basin from the ALMIP2 Model Ensemble

Comparing streamflow simulations against observations has become a straightforward way to evaluate a land surface models (LSM) ability in simulating water budget within a catchment. Using a mesoscale river routing scheme (RRS), this study evaluates simulated streamflows over the upper Oueme River basin resulting from 14 LSMs within the framework of phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2). The ALMIP2 RRS (ARTS) has been used to route LSM outputs. ARTS is based on the nonlinear Muskingum-Cunge method and a simple deep water infiltration formulation representing water-table recharge as previously observed in that region. Simulations are performed for the 2005-08 period during which ground observations are largely available. Experiments are designed using different ground-based rainfall datasets derived from two interpolation methods: the Thiessen technique and a combined kriging-Lagrangian methodology. LSM-based total runoff (TR) averages vary from 0.07 to 1.97 mm day 21 , while optimal TR was estimated as ;0.65 mm day 21. This highly affected the RRS parameterization and streamflow simulations. Optimal Nash-Sutcliffe coefficients for LSM-averaged streamflows varied from 0.66 to 0.92, depending on the gauge station. However, individual LSM performances show a wider range. A more detailed rainfall distribution provided by the kriging-Lagrangian methodology resulted in overall better streamflow simulations. The early runoff generation related to reduced infiltration rates during early rainfall events features as one of the main reasons for poor LSM performances.

Modeling Surface Runoff and Water Fluxes over Contrasted Soils in the Pastoral Sahel: Evaluation of the ALMIP2 Land Surface Models over the Gourma Region in Mali

AbstractLand surface processes play an important role in the West African monsoon variability. In addition, the evolution of hydrological systems in this region, and particularly the increase of surface water and runoff coefficients observed since the 1950s, has had a strong impact on water resources and on the occurrence of floods events. This study addresses results from phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2), carried out to evaluate the capability of different state-of-the-art land surface models to reproduce surface processes at the mesoscale. Evaluation of runoff and water fluxes over the Mali site is carried out through comparison with runoff estimations over endorheic watersheds as well as evapotranspiration (ET) measurements. Three remote-sensing-based ET products [ALEXI, MODIS, and Global Land Evaporation Amsterdam Model (GLEAM)] are also analyzed. It is found that, over deep sandy soils, surface runoff is generally ove...

Le cycle de l'eau dans le système de mousson d'Afrique de l'Ouest

Improving our knowledge of the water cycle in the West Africanmonsoon system and the way it is represented in numerical models is one of the major goals of the AMMA programme. The water cycle results from complex interactions between the atmosphere and the continent, with contrasted behaviour depending on the region and the space and time scales at which it is analysed. The properties of the surface, the soil and the sub-soil strongly drive water redistribution over the continent and towards the atmosphere, resulting in complex feed back loops that are still not fully understood. Process studies and water budgets computed from a mixture of observations and simulation products provided advances in the knowledge of thesemechanisms. Some of the processes still remain uncertain, such as the links between evapotranspiration, vegetation and ground water storage. Beyond a better knowledge of the monsoon system, model improvement, better numerical weather predictions and the development of tools for water resources assessment and management are among the main applications of these studies.