Basile Hector

Water storage changes as a marker for base flow generation processes in a tropical humid basement catchment (Benin): Insights from hybrid gravimetry

In basement catchments of subhumid West Africa, base flow is the main component of annual streamflow. However, the important heterogeneity of lithology hinders the understanding of base flow generation processes. Since these processes are linked with water storage changes (WSCs) across the catchment, we propose the use of hybrid gravity data in addition to neutron probe-derived water content and water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment (16 ha) in Benin. WSC behaviors are shown to provide insights into hydrological processes in terms of water redistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from time-lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting gravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) covering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (<2.5 lGal) on station gravity determinations (with respect to the network reference station). Empirical orthogonal function analyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the seasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau during the wet season), suggesting threshold-governed fast subsurface redistribution, are identified. This observed storage dynamics, together with geological structures investigated by electrical resistivity tomography and drill log analysis, make it possible to derive a conceptual model for the catchment hydrology.

pyGrav, a Python-based program for handling and processing relative gravity data

pyGrav is a Python-based open-source software dedicated to the complete processing of relative-gravity data. It is particularly suited for time-lapse gravity surveys where high precision is sought. Its purpose is to bind together single-task processing codes in a user-friendly interface for handy and fast treatment of raw gravity data from many stations of a network. The intuitive object-based implementation allows to easily integrate additional functions (reading/writing routines, processing schemes, data plots) related to the appropriate object (a station, a loop, or a survey). This makes pyGrav an evolving tool. Raw data can be corrected for tides and air pressure effects. The data selection step features a double table-plot graphical window with either manual or automatic selection according to specific thresholds on data channels (tilts, gravity values, gravity standard deviation, duration of measurements, etc.). Instrumental drifts and gravity residuals are obtained by least square analysis of the dataset. This first step leads to the gravity simple differences between a reference point and any point of the network. When different repetitions of the network are done, the software computes then the gravity double differences and associated errors. The program has been tested on two specific case studies: a large dataset acquired for the study of water storage changes on a small catchment in West Africa, and a dataset operated and processed by several different users for geothermal studies in northern Alsace, France. In both cases, pyGrav proved to be an efficient and easy-to-use solution for the effective processing of relative-gravity data. Display Omitted Aggregation of multiple processing steps in an open-source, user-friendly program.Object-oriented programming to handle and process structured relative-gravity data.Simple code structure which allows fast and easy implementation of new functions.Simple data-selection step through object-oriented GUI.

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.

Hybrid Gravimetry as a Tool to Monitor Surface and Underground Mass Changes

This paper is devoted to an overview of the use of hybrid gravimetry in Earth and Environmental Sciences. We first recall the concept of hybrid gravimetry which relies on the simultaneous use of different types of gravimeters either superconducting, absolute or relative spring gravimeters. This combination of instruments provides a complete tool for time-lapse gravimetry: while superconducting gravimeters and/or absolute gravimeters are used to obtain temporal gravity changes at one or several base stations, relative gravity surveys provide spatial differences with respect to these base stations, and allow to cover a much wider area than base stations only. Hybrid gravimetry therefore provides time-lapse gravity changes at a survey scale. We present here an overview of different published applications in hydrology, glaciology, volcanology and geothermics in order to point out that hybrid gravimetry is a powerful tool to monitor spatially and temporarily surface and underground mass changes.

Preliminary Results from the Superconducting Gravimeter SG-060 Installed in West Africa (Djougou, Benin)

A GWR superconducting gravimeter of the new generation (OSG-60) has been installed in July 2010 in sub-humid West Africa, at the Djougou station in Benin. This station is located in the AMMA-CATCH long term hydrological observing system. We present the first results in terms of instrumental drift as well as the calibration results using FG5 absolute gravity measurements. We show that geophysical contributions due to hydrological load can bias the initial drift estimate. The noise level is compared to the Strasbourg SG as well as to the reference New Low Noise Model (NLNM) used in seismology. We also investigate the gravity response to atmospheric pressure changes and show that, because of the presence of large thermal tides, the gravity response to mass changes in the atmosphere is more complex than in the simple case of a constant barometric admittance.