Bruno Monteny

Sensible heat flux and radiometric surface temperature over sparse Sahelian vegetation. I. An experimental analysis of the kB−1 parameter

Estimating the sensible heat flux H over sparse vegetation from thermal infrared temperature requires an estimate of the excess resistance rr to the aerodynamic resistance. This excess resistance has been commonly expressed as a function of the dimensionless parameter kB−1. Experimental analysis of this parameter over diverse sparse vegetation in semi-arid areas showed a large and unexpected range of variation. The flux and ground-based thermal infrared data of the East Central Site of HAPEX-Sahel were used in this paper to study the behaviour of the kB−1 parameter and to compare these results with the conclusions of previous studies. At first, the global accuracy of the estimation of H as a function of the kB−1 value was analysed; it was found that overestimating kB−1 has less serious consequences than underestimating it. Then, optimal values of kB−1, obtained for each site (one fallow savannah site and two millet sites), ranged from 5.1 to 10.2, with significantly different values for the two millet sites. The corresponding Root Mean Square Error (RMSE) between estimated and measured H ranged from 43 to 67 W m−2. Finally, the instantaneous values of kB−1 proved to be highly variable and the parameterization of kB−1 proposed by Kustas et al. (1989) as a function of u(Tr − Ta) appeared valid only for high H values. For lower values, the slope of this relationship must be adjusted for each class of H. All the previous results confirmed that a constant value can neither be assumed for semi-arid areas nor for a given type of vegetation and that further studies are needed to understand its behaviour. Considering the difficulty in predicting the kB−1 values, a different approach based on the relationship observed between aerodynamic and radiometric temperatures was investigated. This approach gave more accurate estimate of H (RMSE between 35 and 48 W m−2), but remains at this stage purely empirical. Further investigation is now needed to predict the parameterization of this approach.

Estimating sensible heat flux from radiometric temperature over sparse millet

A two-layer model was developed and used to estimate sensible heat flux over a sparse millet crop from surface radiometric temperature. The millet crop was grown in farming conditions on the central site of the HAPEX-Sahel experiment in southern Niger. Surface temperature was measured with a nadir-looking radiometer. Measurements of the convective fluxes of sensible and latent heat were made simultaneously by means of the energy balance-Bowen ratio method. It is assumed that infra-red surface temperature can be represented by a weighted sum of foliage and soil surface temperatures, the weighting factors being the fractional areas of foliage and soil surface. With this assumption, the basic equations of two-layer models lead to an expression of sensible heat flux H close in form to the Ohms law type formulation obtained from a one-layer approach, but in which the temperature difference between the surface and the air Tr − Ta has to be corrected by a factor proportional to the temperature difference δT between the foliage and the substrate. δT being not available in our experiment, it was assumed that a statistical relationship linking δT to Tr − Ta of the type δT = a(Tr − Ta)m could be used. Using one part of the data set, m and a were statistically determined by adjusting H estimated by the model to H observed by the Bowen ratio method. The best adjustment gave m = 2 and a = 0.10. For the other part of the data set (different from the one employed to calibrate this relationship) it was found that H estimated using the two-layer model with this empirical relationship compared fairly well with the values of H observed.

Examination of the difference between radiative and aerodynamic surface temperatures over sparsely vegetated surfaces

Abstract A four-layer hydrologic model, coupled to a vegetation growth model, has been used to investigate the differences between aerodynamic surface temperature and radiative surface temperature over sparsely vegetated surface. The rationale for the coupling of the two models was to assess the dependency of these differences on changing surface conditions (i.e., growing vegetation). A simulation was carried out for a 3-month period corresponding to a typical growth seasonal cycle of an herbaceous canopy in the Sahel region of West Africa (Goutorbe et al., 1993). The results showed that the ratio of radiative-aerodynamic temperature difference to radiative-air temperature difference was constant for a given day. However, the seasonal trend of this ratio was changing with respect to the leaf area index (LAI). A parameterization involving radiative surface temperature, air temperature, and LAI was then developed to estimate aerodynamic-air temperature gradient, and thus sensible heat flux. This parameterization was validated using data collected over herbaceous site during the Hapex-Sahel experiment. This approach was further advanced by using a radiative transfer model in conjunction with the above models to simulate the temporal behavior of surface reflectances in the visible and the near-infrared spectral bands. The result showed that sensible heat flux can be fairly accurately estimated by combining remotely sensed surface temperature, air temperature, and spectral vegetation index. The result of this study may represent a great opportunity of using remotely sensed data to estimate spatiotemporal variabilities of surface fluxes in arid and semiarid regions.

Estimation of sensible heat flux over sparsely vegetated surfaces

The approach of using remote sensing of surface temperature to estimate spatially distributed surface energy balance components is very attractive. This approach has been applied successfully over surfaces with near full vegetation cover. However, large discrepancies between measured and simulated surface fluxes have been observed over surfaces with sparse vegetation cover. The reason for these discrepancies is that the assumption that radiative surface temperature can be equated to aerodynamic surface temperature is not correct over sparsely vegetated surfaces. In this study an empirical model, relating radiative-aerodynamic surface temperature difference to radiative-air temperature gradient and leaf area index, was used to estimate sensible heat flux over sparse shrub in the Central East supersite during the Hydrologic and Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) measurement campaign. The result shows that this parameterization leads to reasonable estimates of sensible heat flux; the root mean square error (RMSE) was about 50 W m−2. A second data set over sparse cotton in Arizona had a RMSE of about 20 W m−2. Although the results of this study are encouraging, one should be cautious, however, because there is a need for additional investigation of this procedure.

Effective parameters of surface energy balance in heterogeneous landscape

This paper addresses the problem of estimating surface fluxes at large scale over heterogeneous terrain, and the corresponding determination of effective surface parameters. Two kinds of formulation are used to calculate the fluxes of sensible and latent heat: the basic diffusion equations (Ohms law type) and the Penman-Monteith equations. The strategy explored is based upon the principle of flux conservation, which stipulates that the average flux over a large area is simply the area-weighted mean of the contributions from the different patches making up the area. We show that the application of this strategy leads to different averaging schemes for the surface parameters, depending on the type of flux (latent heat, sensible heat) and on the type of formulation used to express the flux. It appears that the effective value of a given parameter must be appraised for each individual application, because it is not unique, but differs according to the magnitude being conserved and the equation used to express this magnitude. Numerical simulations are carried out to test over contrasted areas the aggregation procedures obtained. The areal fluxes estimated from these effective parameters, together with the areal fluxes calculated by means of a simple areal averaging of the parameters, are compared to the “true’ average fluxes, calculated as area-weighted means of the elementary fluxes. The aggregation procedures obtained prove to be much more accurate for estimating areal fluxes and for closing the energy balance equation than those based upon simple areal averaging of the parameters.

The role of the Sahelian biosphere on the water and the CO2 cycle during the HAPEX-Sahel experiment

The HAPEX-Sahel experiment was organized to investigate the impact of water, energy and CO2 fluxes at the soil-vegetation-atmosphere interface on climate processes in the Sahelian region. Measurements of the energy balance components, CO2 flux and soil moisture were conducted over a savanna area at the East Central Supersite of the one degree square during a 3 month period in 1992. The aim of this particular investigation was to understand the role of surface conditions (i.e. vegetation and moisture) in the partitioning of available energy at the surface into sensible and latent heat flux. It also aimed to improve the understanding of how water and carbon cycles are affected by vegetation functioning, soil water availability and atmospheric demand. The analysis presented in this paper showed that the relative contribution of the soil and the vegetation to latent heat flux varies intimately with the temporal rainfall distribution and the growth of the savanna grass species, which is more sensitive to the distribution of precipitation than to its amount. Finally, semi-empirical parameterizations were developed to formulate (1) the daily evapotranspiration rate of the savanna in terms of available energy at the surface and soil water content, and (2) the instantaneous carbon uptake in terms of photosynthetically active radiation received at the surface and soil water availability.

Sensible heat flux and radiometric surface temperature over sparse Sahelian vegetation II. A model for the kB−1 parameter

Abstract To estimate sensible heat flux from radiometric surface temperature over sparse vegetation, it is necessary to add an excess resistance to the aerodynamic resistance calculated between the canopy source height and the reference height. This excess resistance is classically expressed as a function of the kB −1 parameter. By using the Shuttleworth-Wallace two-layer model, together with the linearity hypothesis on radiometric temperature, an analytical expression of the kB −1 parameter has been obtained. This expression and the numerical simulations show that kB −1 is not a constant parameter: it varies as a function of the structural characteristics of the vegetation, the level of water stress and the climatic conditions. The model predictions have been confronted with experimental data, obtained on a fallow savannah during HAPEX-Sahel. The model performs fairly well, with a slight under-estimation which has been explained. From these results, it emerges that the kB −1 parameter does not seem to be an appropriate and accurate tool to estimate sensible heat flux over sparse vegetation. Since it does not depend only on structural characteristics, it is not a constant parameter for a given vegetation, and thus, it can not be used in an operational way.

Unidimensional modelling of a fallow savannah during the HAPEX-Sahel experiment using the SiSPAT model

Abstract In the framework of the HAPEX-Sahel experiment, a data set was gathered on a fallow savannah site of the Central East Supersite. This includes 54 days of atmospheric forcing (air temperature and humidity, wind speed, solar and long-wave radiation and rainfall), net radiation, sensible, latent and soil heat fluxes and soil temperature series at a time step of 20 min. Furthermore, 17 soil moisture profiles, the evolution of the leaf area indices and some soil characteristics were available. The data set was used, at the field scale, to calibrate and validate the SiSPAT (simple soil plant atmosphere transfer) model, a 1D model of coupled heat and mass transfer in the soil-plant-atmosphere continuum. The objectives of the study were (i) to assess the performances of the model in the prediction of the diurnal cycle of net radiation, turbulent fluxes, soil temperatures and the evolution of soil water content over a period of 54 days (day of the year 239–292, 1992), characterized by early stage intense rainfall events and fast drying afterwards, (ii) to analyse the influence of soil surface crust on the water balance and (iii) to identify the 1D modelling limits when the surface area consists of two strates: a ground sparse herb layer, characterized by a large spatial variability of surface properties and water content with scattered bushes. The model was calibrated over a 2-week period and then run over the whole 54-day period. We were able to reproduce the main characteristics of the observed net radiation, turbulent fluxes, soil temperature and soil moisture for the intense rainfall events and for an elongated dry period. Nevertheless, when the crust was not taken into account, the rainfall-runoff-infiltration process and the evapotranspiration after rain were poorly predicted (overestimation of evapotranspiration of infiltration). When a crust was considered to model the water balance at the field scale, its influence was found to be substantial on the runoff generation and the infiltration, and consequently on the bare soil evaporation. However, runoff predictions were much larger than the observations. Indeed, at the field scale, no runoff was generally observed. Lateral redistribution of water between crusted and noncrusted zones was observed in the plot. However, this cannot be taken into account with the presented 1D deterministic modelling. Hence further model development is needed to yield a better representation of soil water fluxes at the field scale.

An approach to couple vegetation functioning and soil-vegetation-atmosphere-transfer models for semiarid grasslands during the HAPEX-Sahel experiment

Abstract This paper presents a model which has been developed to simulate the major land surface processes occurring in arid and semiarid grasslands. The model is composed of a hydrological submodel which describes the water and energy budgets, and a vegetation growth submodel which groups the processes associated with biomass production. Emphasis has been placed on developing a realistic representation of the interaction between these subprocesses taking account of the different time scales involved. The hydrological submodel couples the energy balance of the soil/canopy with the soil moisture and thermal dynamics. It interacts with the vegetation growth submodel by exchanging information needed to account for the influence of plant water status and canopy temperature on photosynthesis, and the influence of the vegetation canopy on the boundary layer within which transport processes are taking place. The model has been tested with meteorological, biomass and energy flux measurements made on a grassland site during the HAPEX-Sahel experiment, Niger, in 1992. Model simulations of biomass over the growing season are all found to be within a 15% error margin allowed on biomass measurements. Hourly values of net radiation, as well as latent and sensible heat fluxes, are simulated with an RMSE of less than 50 W m −2 . Given the relative simplicity of the model and the long period of uninterrupted simulation, these results are considered satisfactory. Overall, the results show that the model behaves consistently at different stages of vegetation growth, and satisfactorily reproduces the interdependence of vegetation growth with the physical processes giving rise to the water and energy balances.

The correction of soil heat flux measurements to derive an accurate surface energy balance by the Bowen ratio method

Abstract A method is presented for calculating conductive heat flux at the soil surface (G0) from measured soil heat flux (G) some centimetres beneath the ground surface. The method does not require estimation of thermal properties and is valid for inhomogeneous soils with regard to their thermal properties. Data from the central sub-site of the Eastern Super Site of the HAPEX-Sahel experiment are used to illustrate the method. Finally, the influence of using corrected values of surface soil heat flux G0, rather than measured values of G, in the energy budget with the Bowen ratio is evaluated. The corrections for G are small in the case of the highly diffusive soil of the Sahel. Errors in estimating latent heat flux with G instead of G0 are negligible. However, calculations show that these errors could be much more important for other soils with lower soil thermal diffusivity.