Bhaskar J. Choudhury

Relations between evaporation coefficients and vegetation indices studied by model simulations

Abstract Calculations using a heat balance and a radiative transfer model have been done to study relations among evaporation coefficients and vegetation indices. The evaporation coefficients are the crop coefficient (defined as the ratio of total evaporation and reference crop evaporation) and the transpiration coefficient (defined as the ratio of unstressed transpiration and reference crop evaporation), while the vegetation indices considered in this study are the normalized difference, soil adjusted vegetation index, and transformed soil adjusted vegetation index. The reference crop evaporation has been calculated using the Priestley-Taylor equation. The observed variations of crop (wheat) height, leaf area index, and weather conditions for 30 days at Phoenix (Arizona), together with the reflectances of different types of soil in wet and dry states, are used in the simulation. The total evaporation calculated from the model compared well with lysimeter observations. Variations in soil evaporation can introduce considerable scatter in the relation between the crop coefficient and leaf area index, while this scatter is much less for the relation between transpiration coefficient and leaf area index. The simulation results for 30 days of crop and weather data and reflectances of 19 soil types in wet and dry conditions gave significant linear correlations between the transpiration coefficient and the vegetation indices, the explained variance (r 2 ) being highest for the soil adjusted vegetation index ( r 2 = 0.88) and lowest for the normalized difference ( r 2 = 0.81). A clump model is used to address the effect of spatial heterogeneity on the relationship between the transpiration coefficient and soil adjusted vegetation index. These simulated relationships between transpiration coefficient and vegetation indices for wheat are discussed in the context of the relationships derived from observations for several crops and grasses. The present analysis provides a theoretical basis for estimating transpiration from remotely sensed data.

Relationships between vegetation indices, radiation absorption, and net photosynthesis evaluated by a sensitivity analysis

Abstract A two-stream approximation to the radiative transfer equation is used to calculate the vegetation indices (simple ratio and normalized difference), the fraction of incident photosynthetically active radiation (PAR) absorbed by the canopy, and the daily mean canopy net photosynthesis under clear sky conditions. The model calculations are tested against field observations over wheat, cotton, corn, and soybean. The relationships between the vegetation indices and radiation absorption or net photosynthesis are generally found to be curvilinear, and changes in the soil reflectance affected these relationships. The curvilinearity of the relationship between normalized difference and PAR absorption decreases as the magnitude of soil reflectance increases. The vegetation indices might provide the fractional radiation absorption with some a priori knowledge about soil reflectance. The relationship between the vegetation indices and net photosynthesis must be distinguished for C 3 and C 4 crops. Effects of spatial heterogeneity are discussed.

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.

Spatial heterogeneity in vegetation canopies and remote sensing of absorbed photosynthetically active radiation: A modeling study☆

Abstract A large number of previous studies have investigated the possibility of estimating absorbed photosynthetically active radiation (PAR) from spectral reflectance of plant canopies. An important factor not considered in these studies is the case of horizontally heterogeneous plant stands where the ground cover is usually less than 100%. We investigated the relationship between spectral indices and fraction of absorbed PAR in horizontally heterogeneous vegetation canopies with the aid of a three-dimensional radiative transfer model. Canopy reflection at optical wavelengths and PAR absorption was simulated using this model. The errors incurred in using a 1D model for calculating the radiation regime of spatially heterogeneous canopies are shown to be significant. Our analysis indicates that the leaf area index of a canopy is less of an instructive parameter than ground cover and clump leaf area index for these canopies. The relationship between normalized difference vegetation index and fraction of absorbed PAR is found to be almost linear and independent of spatial heterogeneity. However, this relationship is sensitive to the reflectance of the soil or background.

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.

Satellite remote sensing of drought conditions

Multitemporal satellite data have application in the detection and quantification of drought through the ability of these data to estimate the photosynthetic capacity of the terrestrial surface and record microwave surface brightness at the 37 GHz frequency. With proper calibration and registration, comparisons can be made between and among years for specific months using the photosynthetic capacity and the 37 GHz microwave surface brightness for selected time periods or growing seasons. This technology has application in identifying and quantifying areas experiencing drought.

Analyzing the discharge regime of a large tropical river through remote sensing, ground‐based climatic data, and modeling

This study demonstrates the potential for applying passive microwave satellite sensor data to infer the discharge dynamics of large river systems using the main stem Amazon as a test case. The methodology combines (1) interpolated ground-based meteorological station data, (2) horizontally and vertically polarized temperature differences (HVPTD) from the 37-GHz scanning multichannel microwave radiometer (SMMR) aboard the Nimbus 7 satellite, and (3) a calibrated water balance/water transport model (WBM/WTM). Monthly HVPTD values at 0.25° (latitude by longitude) resolution were resampled spatially and temporally to produce an enhanced HVPTD time series at 0.5° resolution for the period May 1979 through February 1985. Enhanced HVPTD values were regressed against monthly discharge derived from the WBM/WTM for each of 40 grid cells along the main stem over a calibration period from May 1979 to February 1983 to provide a spatially contiguous estimate of time-varying discharge. HVPTD-estimated flows generated for a validation period from March 1983 to February 1985 were found to be in good agreement with both observed arid modeled discharges over a 1400-km section of the main stem Amazon. This span of river is bounded downstream by a region of tidal influence and upstream by low sensor response associated with dense forest canopy. Both the WBM/WTM and HVPTD-derived flow rates reflect the significant impact of the 1982–1983 El Nino-;Southern Oscillation (ENSO) event on water balances within the drainage basin.

Relative sensitivity of Normalized Difference Vegetation Index (NDVI) and Microwave Polarization Difference Index (MPDI) for vegetation and desertification monitoring

Abstract Given that a Microwave Polarization Difference Index (MPDI) is rather sensitive to the water content of plant, while the Normalized Difference Vegetation Index (NDVI) is sensitive to chlorophyll absorption, there should be a correlation between these two indices, depending upon the relationship between water content and chlorophyll absorption occuring in the plant itself. Assuming a simple form for this relationship, a simple equation relating MPDI and NDVI was derived. Comparing with actual data from Nimbus 7/SMMR at 37 GHz and NOAA/AVHRR, Channels 1 and 2, the proposed formula represents correctly the general tendency of data. It is then shown that there exists a limit characteristic of a particular type of cover which has to be analyzed, given by NDVI = 0.13 ± 0.02, for which both indices are equally sensitive to the variation of vegetation, and below which MPDI is more efficient than NDVI. The scatter of the data around the theoretical curve obtained is briefly analyzed. It is suggested that, despite the dispersion due to the processing itself, this spreading could give some insight into the relationship between water content and chlorophyll absorption at pixel size scales. In this respect, particular biologic cycles, which have to be confirmed, are observed and discussed.

A reexamination of the crop water stress index

SummaryHand-held infrared radiometers, developed during the past decade, have extended the measurement of plant canopy temperatures from individual leaves to entire plant canopies. Canopy temperatures are determined by the water status of the plants and by ambient meteorological conditions. The crop water stress index (CWSI) combines these factors and yields a measure of plant water stress. Two forms of the index have been proposed, an empirical approach as reported by Idso et al. (1981), and a theoretical approach reported by Jackson et al. (1981). Because it is simple and requires only three variables to be measured, the empirical approach has received much attention in the literature. It has, however received some criticism concerning its inability to account for temperature changes due to radiation and windspeed. The theoretical method is more complicated in that it requires these two additional variables to be measured, and the evaluation of an aerodynamic resistance, but it will account for differences in radiation and windspeed. This report reexamines the theoretical approach and proposes a method for estimating an aerodynamic resistance applicable to a plant canopy. A brief history of plant temperature measurements is given and the theoretical basis for the CWSI reviewed.