The GHYRAF (Gravity and Hydrology in Africa) experiment: Description and first results

Abstract

This paper is the first presentation of a project called GHYRAF (Gravity and Hydrology in Africa) devoted to the detailed comparison between models and multidisciplinary observations (ground and satellite gravity, geodesy, hydrology, meteorology) of the variations of water storage in Africa from the Sahara arid part to the monsoon equatorial part. We describe the various actions planned in this project. We first detail the actions planned in gravimetry which consist in two main surface gravity experiments: on the one hand the periodic repetition of absolute gravity measurements along a north–south monsoonal gradient of rainfall in West Africa, going from Tamanrasset (20 mm/year) in southern Algeria to Djougou (1200 mm/year) in central Benin; on the other hand the continuous measurements at Djougou (Benin) with a superconducting gravimeter to monitor with a higher sampling rate the gravity changes related to an extreme hydrological cycle. Another section describes the actions planned in GPS which will maintain and develop the present-day existing network in West Africa. The third type of actions deals with hydrology and we review the three sites that will be investigated in this joint hydrogeophysics project namely Wankama (near Niamey) and Bagara (near Diffa) in the Niger Sahelian zone and Nalohou (near Djougou) in the Benin monsoon zone. We also address the question of the ground truth of satellite-derived missions; in this context the GHYRAF project will lead to test the hydrology models by comparison both with in situ and satellite data such as GRACE, as well as to an important increase of our knowledge of the seasonal water cycle in Africa. We finally present preliminary results in GPS based on the analysis of the vertical motion of the Djougou site. The resulting absolute gravity changes related to the 2008 monsoon are finally given.

Introduction

GHYRAF (Gravity and Hydrology in Africa) is a multidisciplinary project that aims to better understand the water cycle in West Africa from a combination of geodetic (GPS), gravity (surface and satellite-derived), hydrogeophysics (Magnetic Resonance Sounding and resistivity methods) and hydrology experiments. We will focus on four specific sites with very different hydrological conditions: the first one is located in the Sahara desert zone (Tamanrasset, Algeria); two of them sample the Sahelian band (Niamey and Diffa in Niger) and the last one the monsoon zone (Djougou, Benin). Djougou is the location of an existing hydrometeorological environmental Observatory of the AMMA (African Multidisciplinary Monsoon Analysis) program (http://amma.mediasfrance.org/) with a very dense network of hydrological, geophysical and meteorological data. This is why we plan to install there a superconducting gravimeter in order to monitor the time-variable surface gravity resulting from changes in water storage in a region submitted to heavy precipitations during the monsoon.

The teams involved in this federating project, namely IPGS (Strasbourg), Géosciences (Montpellier), IPGP (Paris), DTP/CNES (Toulouse), LMTG/IRD (Toulouse), have developed complementary expertises in fieldwork, ground and space data processing and geophysical interpretation. The multidisciplinary aspect of the project will benefit to the whole gravimetry community. LAREG/IGN (Marne la Vallée) is also partner of the project since we need to combine gravity data with precise positioning (GPS) at every measurement point; this will be done using permanent GPS receivers managed in West Africa by the AMMA geodesists and establishing new sites when needed (e.g. in Diffa, Niger).

The participation of hydrologists from IRD/Hydrosciences (Montpellier) and IRD/LTHE (Grenoble) further strengthens the multidisciplinary approach of the project. Their expertise in field measurements and hydrology modelling, as well as the numerous hydrological data they will provide in several sites, will be essential.

Climate changes impact water resources and a better knowledge of the present-day variations in water storage worldwide, and more specifically in the tropical monsoon zones where the variability is large, is needed as stressed by a recent IPCC report (Bates et al., 2008). A primary goal of the proposal is to set up new constraints to the problem of the monsoon cycle in Africa since our ground measurements as well as satellite ones are sensitive to the total variation of stored water (changes in the total water column i.e. surface water, water of the unsaturated zone and groundwater), which is an additional independent information on the internal state of an hydrosystem. The Niamey site is interesting because there is an apparent long-term increase in water storage despite a deficit in rainfall (Favreau et al., 2009) while our study in the Diffa region will allow to investigate the impact of the shrinkage of lake Chad in terms of water resources. The Benin site shows widespread pluri-metric water table seasonal fluctuations.

A useful by-product will be the ground validation of the time-variable solutions of the Earth's gravity field derived from satellite mission GRACE (Gravity Recovery and Climate Experiment) launched in 2002. As for any new type of measurement, it is necessary to test the quality of these gravity solutions with respect to independent data. Presently the satellite data are often compared to predictions of global models for the atmosphere, oceans and hydrology, whereas they are supposed to be used as constraints to improve these models. Therefore we propose to collect a set of new, independent and high quality ground data to test the quality of the satellite observations and to investigate how information from space and ground gravity can be combined and optimally used to assess and improve the quality of the hydrology models. Comparing GRACE and ground surface observations needs to understand in detail the local hydrological system at various sites. This will be done at our selected sites by a multidisciplinary approach including geophysical and hydrogeological observations. These sites have been chosen because they are representative of hydrological processes studied for a long time and hence will allow an efficient comparison of local water storage changes with larger scale models.

Finally, let us mention that this ambitious multidisciplinary project requires the most accurate ground instruments (FG5 absolute gravimeter and GWR superconducting gravimeter) with strong needs in terms of manpower and logistics. Our project has been designed to provide scientific outcome for the knowledge of the water cycle in Africa as well as for the confrontation of ground measurements with those from space gravity missions.