F. Castellví

Assessment of reliability of Bowen ratio method for partitioning fluxes

The errors associated with the Bowen ratio-energy balance (BREB) method are analysed, to determine analytically the reliable values of the Bowen ratio (β) and of the latent and sensible heat fluxes. It is shown that, if advection is considered negligible, the BREB method is able to determine correctly the surface flux partitioning or the flux values when certain conditions, consistent with the flux-gradient relationship, are fulfilled. An analytical method to find the range of β around −1 that produce unacceptable flux calculations of latent and sensible heat is presented. It is based on an error analysis of the Bowen ratio and, rather than being fixed, this excluded region depends on the vapor pressure gradient measured in each averaging period and on the resolution limits of the sensors used.

Analysis of methods for estimating vapor pressure deficits and relative humidity

Vapor pressure deficit (VPD) is one of the critical variables that drives evapotranspiration (ET), and is of fundamental importance in crop models. The objectives of this work were to compare the methods used to calculate VPD, and to propose several different ways of calculating VPD at daily and diurnal time scales depending on the available data. The new estimation methods for VPD and relative humidity were tested and gave good results. Since these methods are not site specific, estimates of VPD can be used in several evapotranspiration estimation equations in order to estimate regional evapotranspiration patterns where only temperature, precipitation and insolation measurements are available.

A method based on high-frequency temperature measurements to estimate the sensible heat flux avoiding the height dependence

[1]xa0A new method to estimate the sensible heat flux H using high-frequency temperature data has been proposed. The new method proposes to scale the mean eddy vertical velocity responsible for the air parcels renewal using similarity formulae. It is shown that the empirical coefficient resulting from this scaling (apparently height dependant) is relatively constant with height when comparing estimated versus measured H over half hour periods. This work suggests that the coefficient is universal. This allows to us to propose a new method that provides useful advantages for field applications.

Estimating sensible heat flux using surface renewal analysis and the flux variance method : A case study over olive trees at Sástago (NE of Spain)

[1]xa0The eddy covariance technique for measuring surface fluxes is often not affordable outside experimental research institutes. Therefore knowledge of the performance of alternative methods for determining surface fluxes is valuable. The performance of surface renewal (SR) analysis and the flux variance (FV) method for estimating sensible heat flux has been evaluated in an experiment carried out over a heterogeneous canopy (olive orchard, 50% ground cover) at a semiarid climate in a windy area. Measurements were made at a single level close to the canopy top. SR analysis was accurate under both stable and unstable conditions. The FV method also showed a good performance under unstable conditions, but it was uncertain near neutral conditions and was not applicable under stable conditions.

Methods for estimating vapor pressure deficit at a regional scale depending on data availability

Abstract In this work, a study was carried out to develop and evaluate several methods for estimating the daily vapor pressure deficit (VPD) at thermo-pluviometric stations (weather stations in which only temperature and precipitation are measured) located in a region with a homogeneous climate where there is at least one complete meteorological station. Four estimation methods were compared with the method of calculation proposed by the FAO used in the equations for calculating evapotranspiration (ET), which was considered as a reference. From the analysis of the operability of each method depending on the availability of data on the results obtained, it is concluded that the method proposed for estimating the vapor pressure deficit in climatic zones where there is little meteorological information is quite reliable, and that it can, therefore, be used as an input in empirical equations that need to know the VPD for calculating ET.

Daytime sensible heat flux estimation over heterogeneous surfaces using multitemporal land-surface temperature observations

Equations based on surface renewal (SR) analysis to estimate the sensible heat flux (H) require as input the mean ramp amplitude and period observed in the ramp-like pattern of the air temperature measured at high frequency. A SR-based method to estimate sensible heat flux (HSR-LST) requiring only low-frequency measurements of the air temperature, horizontal mean wind speed, and land-surface temperature as input was derived and tested under unstable conditions over a heterogeneous canopy (olive grove). HSR-LST assumes that the mean ramp amplitude can be inferred from the difference between land-surface temperature and mean air temperature through a linear relationship and that the ramp frequency is related to a wind shear scale characteristic of the canopy flow. The land-surface temperature was retrieved by integrating in situ sensing measures of thermal infrared energy emitted by the surface. The performance of HSR-LST was analyzed against flux tower measurements collected at two heights (close to and well above the canopy top). Crucial parameters involved in HSR-LST, which define the above mentioned linear relationship, were explained using the canopy height and the land surface temperature observed at sunrise and sunset. Although the olive grove can behave as either an isothermal or anisothermal surface, HSR-LST performed close to H measured using the eddy covariance and the Bowen ratio energy balance methods. Root mean square differences between HSR-LST and measured H were of about 55 W m−2. Thus, by using multitemporal thermal acquisitions, HSR-LST appears to bypass inconsistency between land surface temperature and the mean aerodynamic temperature. The one-source bulk transfer formulation for estimating H performed reliable after calibration against the eddy covariance method. After calibration, the latter performed similar to the proposed SR-LST method.