HAPEX-Sahel: a large-scale study of land-atmosphere interactions in the semi-arid tropics

Abstract

Abstract. The Hydrologic Atmospheric Pilot EXperiment in the Sahel (HAPEX-Sahel) was carried out in Niger, West Africa, during 1991 - 1992, with an intensive observation period (IOP) in August - October 1992. It aims at improving the parameterization of land surface atmosphere interactions at the Global Circulation Model (GCM) gridbox scale. The experiment combines remote sensing and ground based measurements with hydrological and meteorological modelling to develop aggregation techniques for use in large scale estimates of the hydrological and meteorological behaviour of large areas in the Sahel. The experimental strategy consisted of a period of intensive measurements during the transition period of the rainy to the dry season, backed up by a series of long term measurements in a 1° by 1° square in Niger. Three "supersites" were instrumented with a variety of hydrological and (micro) meteorological equipment to provide detailed information on the surface energy exchange at the local scale. Boundary layer measurements and aircraft measurements were used to provide information at scales of 100 - 500 km2. All relevant remote sensing images were obtained for this period. This programme of measurements is now being analyzed and an extensive modelling programme is under way to aggregate the information at all scales up to the GCM grid box scale. The experimental strategy and some preliminary results of the IOP are described.

Hydrological Land Surface Response in a Tropical Regime and a Midlatitudinal Regime

“…The evapotranspirative patterns associated with these two domains could also be different. In general, because of a lack of persistent cloudiness and the high solar zenith angle in the Tropics, radiation reaching the ground will be typically higher than in the midlatitudes (Arya 1988;Goutorbé et al 1994). As a result, there is higher surface energy available for evapotranspiration and water loss.…”

“…Keeping these features in perspective, additional differences such as vegetation phenology, effects of thermal stress, and rooting depth can be deduced for the surface-atmosphere exchanges. Our intent is to analyze the net direct and interactive surface feedback pathways using data from two specialized field experiments, one conducted in the semiarid Tropics [Hydrological Atmospheric Pilot Experiment (HAPEX)-Sahel; Goutorbé et al 1994;Prince et al 1995] and the other in the midlatitudes [First International Satellite Land Surface Climatology Project Field Experiment (FIFE); Sellers et al 1988;Sellers and Hall 1992] using a soil vegetation hydrological scheme [Simplified Simple Biosphere (SSiB) model; Xue et al 1991]. A series of statistical-dynamical experiments are performed using a combination of modeling and observational approach.…”

Microwave remote sensing of temporal variations of brightness temperature and near‐surface soil water content during a watershed‐scale field experiment, and its application to the estimation of soil physical properties

“…A few studies have used L band (i.e., 21 cm wavelength or 1.4 GHz frequency) microwave data to map spatial and temporal variation in soil water content [Engroan et al, 1989;Wang et al, 1989]. Recent projects such as Monsoon'90 [Schmugge et al, 1992], FIFE [Sellers et al, 1993], and HAPEX-Sahel [Goutorbe et al, 1994] have improved our ability to remotely monitor soil water content by taking vegetation effects into account. This encourages development of a This paper is not subject to U.S. copyright.…”

The effect of temperature differences between porometer head and leaf surface on stomatal conductance measurements

“…The data used in this paper are from field measurements of leaf conductances of a bush species (Guiera senegalensis) and two forb species (Jacquemontia tamnifolia and Mitracarpus scaher) as measured at a fallow savannah site during the HAPEX-Sahel campaign (Goutorbe et al 1994), This paper was initiated by the observation that the conductances of these savannah species measured by porometry differed considerably from other observations made at the same site and presented in Hanan & Prince (1997), Furthermore, values of canopy conductance, g^, calculated by scaling up porometry data ('bottom-up approach') were up to a factor of 2-3 higher than values obtained by inverting the Penman-Monteith equation using measured values of evaporation ('top-down approach') as obtained with the eddy correlation technique or sapflow measurements (Huntingford, Allen & Harding 1995;Verhoef 1995), Values of g^ as obtained with the two methods described above are often assumed to be identical, although it has been shown both theoretically and experimentally that these two measures of canopy conductance are not the same (Finnigan & Raupach 1987), The 'top-down' value of gc contains additional information relating to the net radiation balance, the aerodynamic resistances inside the canopy and the fact that soil evaporation is usually nonzero (Finnigan & Raupach 1987;Baldocchi et al 1991), However, the discrepancies found for this savannah vegetation were so large that possible errors in the resistance network had to be investigated. This paper aims to quantify the temperature-related uncertainty in conductance values obtained with a dynamic porometer and to point out possible implications when these data are upscaled and used in soil vegetation atmospheric transfer models.…”