Consistency between hydrological model, large aperture scintillometer and remote sensing based evapotranspiration estimates for a heterogeneous catchment

The catchment averaged actual evapotranspiration rate is a hydrologic model variable that is difficult to quantify. Evapotranspiration rates - up till present - cannot be continuously observed at the catchment scale. The objective of this paper is to estimate the evapotranspiration rates (or its energy equivalent, the latent heat fluxes LE) for a heterogeneous catchment of 102.3 km(2) in Belgium using three fundamentally different algorithms. One possible manner to observe this variable could be the continuous measurement of sensible heat fluxes (H) across large distances (in the order of kilometers) using a large aperture scintillometer (LAS), and converting these observations into evapotranspiration rates. Latent heat fluxes are obtained through the energy balance equation using a series of sensible heat fluxes measured with a LAS over a distance of 9.5 km in the catchment, and point measurements of net radiation (R-n) and ground heat flux (G) upscaled to catchment average through the use of TOPLATS, a physically based land surface model. The resulting LE-values are then compared to results from the remote sensing based surface energy balance algorithm ETLook and the land surface model. Firstly, the performance of ETLook for the energy balance terms has been assessed at the point scale and at the catchment scale. Secondly, consistency between daily evapotranspiration rates from ETLook, TOPLATS and LAS is shown

Continuous time series of catchment-averaged sensible heat flux from a large aperture scintillometer: efficient estimation of stability conditions and importance of fluxes under stable conditions

A large aperture scintillometer (LAS) observes the intensity of the atmospheric turbulence across large distances, which is related to the path-averaged sensible heat flux H. In this paper, two problems in the derivation of continuous series of H from LAS data are investigated and the importance of nighttime H fluxes is assessed. First, as an LAS is unable to determine the sign of H, the transition from unstable to stable conditions is evaluated in order to make continuous H series. Therefore, different algorithms to judge the atmospheric stability for an LAS installed over a distance of 9.5 km have been tested. The diurnal cycle of the refractive index structure parameter, C-N(2), results in the best suitable, operational algorithm. A second issue is the humidity correction for LAS data, which is performed by using the Bowen ratio (beta). As beta is taken from ground-based measurements with data gaps, the number of resulting H values is reduced. Not including this humidity correction results in a marginal error in H, but increases the completeness of the H series. Applying these conclusions to the 2-yr time series of the LAS results in an almost-continuous H time series. As the majority of the time steps has been found to be under stable conditions, there is a clear impact of H-stable on H-24h-the 24-h average of H. For stable conditions, H-stable values are mostly negative, and hence lower than the H - 0 W m(-2) assumption as mostly adopted. For months where stable conditions prevail (winter), H-24h is overestimated using this assumption, and calculation of H-stable is recommended

Estimation of catchment averaged sensible heat fluxes using a large aperture scintillometer

Evapotranspiration rates at the catchment scale are very difficult to quantify. One possible manner to continuously observe this variable could be the estimation of sensible heat fluxes (H) across large distances (in the order of kilometers) using a large aperture scintillometer (LAS), and inverting these observations into evapotranspiration rates, under the assumption that the LAS observations are representative for the entire catchment. The objective of this paper is to assess whether measured sensible heat fluxes from a LAS over a long distance (9.5 km) can be assumed to be valid for a 102.3 km2 heterogeneous catchment. Therefore, a fully process-based water and energy balance model with a spatial resolution of 50 m has been thoroughly calibrated and validated for the Bellebeek catchmentin Belgium. A footprint analysis has been performed. In general, the sensible heat fluxes from the LAS compared well with the modeled sensible heat fluxes within the footprint. Moreover, as the modeled Hwithin the footprint has been found to be almost equal to the modeled catchment averaged H, it can be concluded that the scintillometer measurements over a distance of 9.5 km and an effective heightof 68 m are representative for the entire catchment

Estimation of catchment averaged sensible heat fluxes using a large aperture scintillometer

Evapotranspiration rates at the catchment scale are very difficult to quantify. One possible manner to continuously observe this variable could be the estimation of sensible heat fluxes (H) across large distances (in the order of kilometers) using a large aperture scintillometer (LAS), and inverting these observations into evapotranspiration rates, under the assumption that the LAS observations are representative for the entire catchment. The objective of this paper is to assess whether measured sensible heat fluxes from a LAS over a long distance (9.5 km) can be assumed to be valid for a 102.3 km2 heterogeneous catchment. Therefore, a fully process-based water and energy balance model with a spatial resolution of 50 m has been thoroughly calibrated and validated for the Bellebeek catchmentin Belgium. A footprint analysis has been performed. In general, the sensible heat fluxes from the LAS compared well with the modeled sensible heat fluxes within the footprint. Moreover, as the modeled Hwithin the footprint has been found to be almost equal to the modeled catchment averaged H, it can be concluded that the scintillometer measurements over a distance of 9.5 km and an effective heightof 68 m are representative for the entire catchment

Estimation of catchment averaged sensible heat fluxes using a large aperture scintillometer

Evapotranspiration rates at the catchment scale are very difficult to quantify. One possible manner to continuously observe this variable could be the estimation of sensible heat fluxes (H) across large distances (in the order of kilometers) using a large aperture scintillometer (LAS), and inverting these observations into evapotranspiration rates, under the assumption that the LAS observations are representative for the entire catchment. The objective of this paper is to assess whether measured sensible heat fluxes from a LAS over a long distance (9.5 km) can be assumed to be valid for a 102.3 km2 heterogeneous catchment. Therefore, a fully process-based water and energy balance model with a spatial resolution of 50 m has been thoroughly calibrated and validated for the Bellebeek catchmentin Belgium. A footprint analysis has been performed. In general, the sensible heat fluxes from the LAS compared well with the modeled sensible heat fluxes within the footprint. Moreover, as the modeled Hwithin the footprint has been found to be almost equal to the modeled catchment averaged H, it can be concluded that the scintillometer measurements over a distance of 9.5 km and an effective heightof 68 m are representative for the entire catchment