J.P. Lhomme

Estimation of surface sensible heat flux using dual angle observations of radiative surface temperature

Abstract In this study, dual angle observations of radiative surface temperature have been used in conjunction with a two-layer model to derive sensible heat flux over a sparsely vegetated surface. Data collected during the semi-arid-land-surface-atmosphere program (SALSA) over a semi-arid grassland in Mexico were used to assess the performance of the approach. The results showed that this approach led to reasonable estimates of the observed fluxes. The mean average percentage difference (MAPD) between observed and simulated fluxes was about 23%, which is not statistically different from the expected 20% scatter, when different flux measuring devices are compared over the same site. However, the sensitivity analysis indicated that the approach was rather sensitive to uncertainties in both measured radiative temperatures and aerodynamic characteristics of the vegetation. Finally, the issue of using dual angle observations of surface temperature for characterizing the difference between aerodynamic and nadir viewing radiative temperature has been examined. The results showed that this difference is linearly correlated with the difference between nadir and oblique radiative temperatures. Based on this finding, we expressed sensible heat flux in terms of the (nadir) radiative-air temperature gradient and a corrective term involving the nadir–oblique temperature differences. This formulation has been successfully tested. The resulting MAPD was about 33%.

Determination of sensible heat flux over Sahelian fallow savannah using infra-red thermometry

Abstract The estimation of the partitioning of available energy from remote sensing techniques is addressed for a Sahelian fallow savannah. It is a composite vegetation consisting of shrubs of Guiera senegalensis scattered above a stand of sparse grass. A two-layer model is employed to estimate sensible heat flux ( H ) from radiometric surface temperature, with data collected during the HAPEX-Sahel international experiment, carried out in Niger in 1992. The model, based upon the assumption that the radiometric surface temperature ( T r ) might be represented by the composite surface temperature (area-weighted mean of shrub and grass temperatures), leads to a simple formulation of H as a function of the temperature difference between the surface and the air ( T r − T a ) and the temperature difference between the grass and the shrubs δT . The estimates of the model compare fairly accurately with measurements obtained by the Bowen ratio-energy balance method, the root mean square error being about 52 W m −2 . Because δT is not easily measured from remote sensing systems, it has been shown that for the fallow savannah this temperature difference is linearly correlated to ( T r − T a ) with r 2 = 0.94. Therefore, it is possible to estimate sensible heat flux from ( T r − T a ) without additional component temperature measurements.

Non-steady-state modelling of water transfer in a Mediterranean evergreen canopy

A model simulating the diurnal pattern of water transfer within a Holm oak (Quercus ilex) canopy in Mediterranean conditions has been designed. It combines a non-steady-state hydraulic model with a transpiration model. The hydraulic model includes a reservoir represented by a capacitance, a soil‐plant hydraulic resistance and a storage hydraulic resistance connected to the capacitance. It simulates the diurnal variation of water uptake and storage flow from the diurnal course of transpiration used as input. The transpiration model is based upon the Penman‐Monteith equation and a Jarvis-type representation of the stomatal resistance (i.e., a minimum stomatal resistance multiplied by the product of independent stress functions). Simultaneous measurements of canopy evaporation by an eddy covariance system and water uptake from the soil by sap flow measurements have allowed one to calibrate and validate the model. The capacitance has been found to be equal to 0.17 mm MPa 1 (with a storage hydraulic resistance of about 2 MPa h mm 1 ), generating a time lag of about 1 h between the transpiration rate and the water uptake from the soil. The hydraulic model correctly represents the experimental data. The transpiration model provides reasonable estimates, but with a significant scatter. The combined model simulates the diurnal variation of water uptake, storage flow and transpiration rate directly from environmental variables, but in this latter case, the storage flow is estimated with a rather poor accuracy.

Theoretical relationship between stomatal resistance and surface temperatures in sparse vegetation

Abstract The relationship between stomatal resistance and foliage temperature in sparse vegetation has been the subject of previous papers [Smith, R.C.G., Barrs, H.D., Fischer, R.A., 1988. Agric. Forest. Meteorol. 42, 183–198; Shuttleworth, W.J., Gurney, R.J., 1990. Q. J. R. Meteorol. Soc. 116, 497–519], in which the modeling is based upon the one-dimensional two-layer approach of Shuttleworth and Wallace [Shuttleworth, W.J., Wallace, J.S., 1985. Q. J. R. Meteorol. Soc. 111, 839–855]. In both studies, however, a major assumption exists concerning the contribution of the substrate to the evaporation process. Using the same approach as these previous studies, an extended and upgraded model is presented in the sense that it relates stomatal resistance to foliage and substrate temperatures ( T f and T s ) without any assumption on substrate contribution. A comparison of stomatal resistances estimated from component temperatures ( T f and T s ) with values measured on fallow savannah during the HAPEX–Sahel experiment confirms the good performance of the model. Numerical simulations show the general behavior of the relationship between stomatal resistance and foliage temperature in several scenarios involving various weather conditions and canopy characteristics. The sensitivity of the calculated stomatal resistance to input variables and model parameters is investigated. It is shown that the calculation of stomatal resistance exhibits a significant sensitivity to foliage temperature and a much lesser one to substrate temperature. Uncertainties in leaf area index have a relatively weak impact on the calculated stomatal resistance. The sensitivity of stomatal resistance to the two main coefficients involved in the partitioning of available energy has also been investigated.

Modelling nocturnal heat dynamics and frost mitigation in Andean raised field systems

Abstract The raised fields system is an old technique of soil and water management dating back to prehispanic time. Very common in the Lake Titicaca region, it essentially consists of a series of earth platforms on which crops are grown, surrounded by water canals connected to inlet and outlet ditches. An important and widely recognised benefit of this system of management is its contribution to frost mitigation during the growing season. A physical process-based model is presented to explain the role played by the canals in the nocturnal heat dynamics and the cold mitigation process. Adapted from a two-layer transfer scheme (Shuttleworth–Wallace type) with a vegetation layer and a substrate layer representing the canals, the model shows that greater heat flux emanating from the canals and greater water condensation on the crop both contribute to the mitigation effect. Model outputs are compared with experimental data collected on a system of raised fields in the Lake Titicaca region. Crop temperature appears to be correctly estimated and more accurately than water temperature. When used in a predictive way, the model shows that wider canals or narrower platforms have a positive impact on the minimum crop temperature reached during the night. Increasing water depth also improves frost mitigation, but conversely, a deeper canal (with the same level of water) has a negative impact. Leaf area index (LAI) and crop height, both have a positive impact. Higher wind velocity or higher air relative humidity also enhances the frost mitigation effect.