R.J. Reginato

Analysis of an empirical model for soil heat flux under a growing wheat crop for estimating evaporation by an infrared-temperature based energy balance equation

Abstract A net-radiation-based empirical model for soil heat flux (G) is analyzed for inclusion in a canopy-temperature-based energy balance equation to estimate evaporation from a growing wheat crop. The observed direct correlation between net radiation and soil heat flux for bare soils is extended to include the effect of growing vegetation by considering the canopy attenuation of net radiation. The parameters of the soil heat flux model are determined using observations of net radiation, evaporation and estimated sensible heat flux over Pavon wheat, while the empirical model is used to calculate and compare against the latent heat flux observations over Ciano wheat. Comparisons are done for 9 days of diurnal observations, which included clear and partially cloudy skies and the leaf area index varying from 0 (i.e., bare soil) to 4.7. The performance of the empirical model was not very satisfactory for 2 days of data over bare soil, primarily because of the phase difference in the diurnal variations of soil heat flux and net radiation. A linear regression analysis using the estimated and the observed latent heat fluxes for all 9 days gave a correlation coefficient of 0.97 and a standard error of 39 W m−2. The results of a sensitivity analysis show that errors in estimating G translate directly into a bias in estimating the latent heat flux, and the magnitude of this bias decreases as the vegetation leaf area index increases.

Determination of sensible heat flux over sparse canopy using thermal infrared data

Surface temperatures, Ts, were estimated for a natural vegetative surface in Owens Valley, California, with infrared thermometric observations collected from an aircraft. The region is quite arid and is composed primarily of bushes (∼30%) and bare soil (∼70%). Application of the bulk transfer equation for the estimation of sensible heat, H, gave unsatisfactory values when compared to Bowen ratio and eddy correlation methods over a particular site. This was attributed to the inability with existing data to properly evaluate the resistance to heat transfer, rah. To obtain appropriate rah-values the added resistance to heat transfer, kB−1, was allowed to vary although there is both theoretical and experimental evidence that kB−1 for vegetative surfaces can be treated as constant. The present data indicate that for partial canopy cover under arid conditions kB−1 may be a function of Ts measured radiometrically. The equation determining kB−1 was simplified and tested over another arid site with good results; however, this had a limited data set (i.e., 6 data points). The dimensionless kB−1 equation is simplified for use over full canopy cover and is shown to give satisfactory estimates of H over a fully-grown wheat crop.

An analysis of infrared temperature observations over wheat and calculation of latent heat flux

From half-hourly averaged observations of net radiation, latent and soil heat fluxes over wheat, the sensible heat flux is calculated as the residual component of the surface energy balance. Then, the aerodynamic surface temperature is obtained by solving iteratively the aerodynamic equation for sensible heat flux, taking into consideration the bluffbody (differences in roughness height for heat and momentum exchange) and the stability (differences in surface and air temperatures) corrections to the aerodynamic resistance. The aerodynamic temperatures are found to be lower (higher) than the infrared thermometric observations under stable (unstable) atmospheric conditions. However, when the infrared temperatures were used in a resistance-energy balance equation to estimate the latent heat flux, then the estimated fluxes showed a high linear correlation (r = 0.96) and a moderate standard error (47 W m−2) under regression analysis with the observed fluxes.

Evaluation of canopy temperature—evapotranspiration models over various crops

Canopy temperatures, when measured remotely, offer a method of estimating evapotranspiration with surface energy balance models. Equations which have been developed by others have been evaluated only at a limited number of locations and with a few crops. Our study was conducted at several locations with weighing lysimeters with a variety of crops around the United States: Brawley, CA; Temple, TX; Lincoln, NE; St. Paul, MN; Fargo, ND; Kimberly, ID; and Davis, CA, to evaluate evapotranspiration utilizing canopy temperature as an input into the surface energy balance. The results show that evapotranspiration calculated from the aerodynamic resistance form of the surface energy balance was well correlated with lysimeter measurements at all locations. The errors using the surface energy balance were less than 10% in all cases for full ground cover. The Bartholic—Namken—Wiegand method was more closely coupled to net radiation than canopy temperature. n nUnder partial canopy cover, differences between the two models were apparent. The Bartholic—Namken—Wiegand model overpredicted when the actual evapotranspiration was above 200 W m−2 because of its insensitivity to surface temperature. However, the surface energy balance model exhibited only a slight overprediction above 200 W m−2 when a weighed composite surface temperature (representative of bare soil and crop temperature) was used. This small overprediction could be overcome by considering the soil heat flux term. There was no location bias in the surface energy balance model, which shows that it should work well at other locations.

Interception and use efficiency of light in winter wheat under different nitrogen regimes

In an identical experiment conducted at Mandan (ND), Manhattan (KS) and Lubbock (TX), the influence of the environment and nitrogen (N) fertility upon light interception efficiency (ei) and light use efficiency (ec) of winter wheat (Triticum aestivum L.) were examined using remotely sensed canopy reflectance data to estimate ei. Treatments consisted of two cultivars, four levels of applied N and three levels of irrigation. Increased N application resulted in increased ei, with only secondary effects on ec. Whole season values of ec did not differ significantly between sites or between crops grown under different N regimes. However, ec did change through the season, increasing from an average of 1.5 during the double ridge-to-terminal spikelet stage to an average of 3.8 during the terminal spikelet-to-anthesis stage and finally decreasing to an average of 3.1 during the anthesis-to-soft dough stage. These changes in ec corresponded to changes in the mean temperatures for each growth period.

Leaf-area estimates from spectral measurements over various planting dates of wheat

Abstract Several vegetative indices were analysed for their sensitivity and stability to green-leaf-area index (LAI) changes over various planting dates and irrigation frequencies of wheat grown at Phoenix, Arizona, from 1978 to 1980. Seasonal patterns of greenness showed that values saturated at LAI values above 4·0 did not return to the pre-emergence bare-soil value at senescence, and were not uniquely related to LAI over the various planting dates. Regressions of individual MSS band reflectances against LAI also showed that there was not a unique relation between any of the bands and LAI. However, the near-infrared/red reflectance ratio was stable over all planting dates and could be used successfully over a number of years and locations

View angle effects in the radiometric measurement of plant canopy temperatures.

Abstract The thermal infrared sensor response from a wheat canopy was extremely non-Lambertian because of spatial variations in energy flow processes; the effective radiant temperature of the sensor varied as much as 13°C with changing view angle. This variation of sensor response was accurately quantified (root-mean-square of deviations between theoretical and measured responses reduced to 1.1°C) as a function of vegetation canopy geometry, vertical temperature distribution of canopy components, and sensor view angle. The results have important implications for optimizing sensor view angles for remote sensing missions.

Ground and aircraft infrared observations over a partially-vegetated area

Abstract Thermometric observations over a row crop (cotton, Gossypium hirsutum L.) with 20 per cent cover using hand-held radiometers were made during several clear days near Maricopa, a town in south central Arizona. A ground sampling routine developed for estimating a composite temperature for the cotton field compared favourably with surface temperatures taken for one day from an aircraft flying at an altitude of approximately 150 m. Both ground and airborne radiometric measurements were from nadir angle traversing the row crop. It was found that by considering an equation for the radiative balance at the surface, a composite surface temperature could be calculated which was within ± 1.5°C of the value given by the labour-intensive ground sampling method or the aircraft. The data requirements were temperatures of sunlit and shaded soil and the sunlit canopy temperature and their respective fractional areas. The fractional areas of the sunlit and shaded soil as a function of time and the vegetative cove...

Effects of panicles on infrared thermometer measurements of canopy temperature in wheat

Abstract A study was conducted in Phoenix, AZ on stressed and unstressed field plots of Anza wheat ( Triticum aestivum L.) on an Avondale loam soil (a fine, loamy, mixed calcareous hyperthermic Anthropic Torrifluvent) to determine effects of panicles on the apparent canopy temperature and their consequent impact on the estimation of crop stress. The panicles were removed from a 1.5 × 4-m sample of each plot by extracting the peduncle from the upper sheath. For each treatment canopy radiative temperature measurements were made from vertical and oblique angles (30° from the horizontal), using an 8° field-of-view (FOV) infrared thermometer, at half-hour intervals from sunrise to sunset on 20, 22, and 30 April. Complementary measurements included leaf water potential and leaf diffusive resistance. Apparent canopy temperatures obtained from the oblique view of the canopy with panicles and under well-watered conditions were 2°C warmer than those of the unstressed canopy without panicles. In the stressed plot the canopy with panicles was 1°C cooler than that without panicles, but this effect was only noticed around 1200 MST. The temperature difference between viewing angles was apparently caused by different percentages of panicle area viewed by the radiometer. In the vertical view panicles contributed to 3% of the total viewed area while at the 30° oblique view panicles comprised 40% of the area. Since energy balance calculations of a non-transpiring cylinder with dimensions similar to a typical wheat panicle showed its temperature would remain very close to that of the surrounding air, canopy temperatures were adjusted for the proportion of panicles viewed assuming they were in equilibrium with air temperature. Results showed the corrected canopy temperatures of the canopy with panicles were the same as those measured in the canopy without panicles. Such a correction is necessary to avoid an overestimate of the stress level and an underestimate of differences between treatments. Crops with non-transpiring and/or well-ventilated morphological structures above the foliage will require this correction if radiative canopy temperatures are to be used in irrigation management programs or stress detection studies.

Analysis of a resistance-energy balance method for estimating daily evaporation from wheat plots using one-time-of-day infrared temperature observations

Abstract Accurate estimates of evaporation over field-scale or larger areas are needed in hydrologic studies, irrigation scheduling, and meteorology. Remotely sensed surface temperature might be used in a model to calculate evaporation. A resistance-energy balance model, which combines an energy balance equation, the Penman-Monteith evaporation equation, and van den Honerts equation for water extraction by plant roots, is analyzed for estimating daily evaporation from wheat using post-noon canopy temperature measurements. Additional data requirements are half-hourly averages of solar radiation, air and dew point temperatures, and wind speed, along with reasonable estimates of canopy emissivity, albedo, height, and leaf area index. Evaporation fluxes were measured in the field by precision weighing lysimeters for well-watered and water-stressed wheat. Errors in computed daily evaporation were generally less than 10%, while errors in cumulative evaporation for 10 clear sky days were less than 5% for both well-watered and water-stressed wheat. Some results from sensitivity analysis of the model are also given.