Comparison of aerodynamic and radiometric surface temperature using precision weighing lysimeters

Abstract

Radiometric surface temperature (T s ) is commonly used as a surrogate for aerodynamic temperature (T o ) in computing the sensible heat flux term (H) in the energy balance. However, these temperatures may differ by several degrees, leading to possible errors (especially for large H) and their relationship is not well known. Previous researchers have established empirical and semi-empirical parameterizations of the radiometric roughness length (z or ) or some related form (e.g., kB r -1 = ln[z om /z or ], where z om is the momentum roughness length). In this paper, we estimated T o -T a (where T a is air temperature at 2 m height) and z or using large, precision weighing lysimeters planted with irrigated alfalfa, irrigated and dryland cotton, and dryland grain sorghum. T s was measured by infrared thermometers mounted over the lysimeters. No apparent relations were found between (T o -T a ) and (T s -T a ) or between z or (in the kB r -1 form) and meteorological variables or leaf area index (LAI). The kB r -1 parameter appeared to be most influenced by the different surface roughness of each crop type. Using constant kB r -1 values established for each type of surface, the energy balance model showed reasonable agreement with H and LE derived from lysimeter measurements.

Sensible and latent heat flux from radiometric surface temperatures at the regional scale: methodology and evaluation

“…The comparison of totally independent estimates of H and λE can in fact provide unprecedented evaluation at the proper regional scale. In this light, our study extends and reinforces previous validation exercises that compared remote-sensing based estimates of energy fluxes with point measurement made by eddy covariance and lysimeters (Colaizzi et al, 2004;Boegh et al, 2004;Roerink et al, 2000). The overall good match between H rae /λE rae and H EC /λE EC holds across the three land use types that have substantial differences in their properties and homogeneity.…”

“…Validations based either on independent measurements of latent heat fluxes by lysimeters (Colaizzi et al, 2004) or on direct measurements of sensible and latent heat fluxes by eddy covariance (Boegh et al, 2004) cannot in fact take into account the large spatial variability of the fluxes that occurs at the regional scale. However the recent advent of a new generation of small research aircraft capable of measuring concurrently surface radiometric temperature and sensible and latent heat fluxes over large areas (Crawford et al, 1996) offers a new tool for such validation.…”

“…However, such error may become large for partial crop cover and dry or waterstressed vegetation (Kalma and Jupp, 1990). A number of solutions have been proposed either to find a relationship between radiometric and aerodynamic temperatures or to account for the difference between them by introducing additional atmospheric resistance terms to heat transfer equations (Colaizzi et al, 2004). One of the most classical approaches involves the determination of the ratio between the roughness length for momentum and the roughness length for heat (often referred to as kB −1 ).…”

Estimating evapotranspiration fraction by modeling two-dimensional space of NDVI/albedo and day–night land surface temperature difference: A comparative study

“…Estimation of evapotranspiration is an important issue for proper water management both at the farm and the irrigation project level [1][2][3][8][9][10]15,19]. The evaporative fraction (EF) is critical to quantify the surface water deficit and balance the hydrological cycle [17,18,24,29].…”

ET mapping for agricultural water management: present status and challenges

“…Instead, the author found that the logarithmic diurnal average of the thermal roughness lengths (Z oh ) was a function of LAI. While on the other hand, Colaizzi et al (2004) compared (T s -T a ) with (T o -T a ) using weighing lysimeters planted with irrigated alfalfa, irrigated and dryland cotton, and dryland grain sorghum. They did not find a relationship between (T s -T a ) and (T o -T a ).…”