Robert J. Reginato

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.

The Dependence of Bare Soil Albedo on Soil Water Content

Abstract Simple albedo measurement may prove useful for sensing surface soil water content and as a research tool in the study of evaporation of water from soil. Intensive concurrent measurements of the albedo and soil water content of a drying bare soil indicate that albedo, normalized for sun zenith angle effects, is a linear function of the soil water content of a very thin surface layer (less than 0.2 cm thick) over a sizeable volumetric water content range (0.00 to 0.18 for an Avondale loam). Albedo is also well correlated with the average soil water content of greater soil thicknesses. Measurements to a depth of 10 cm indicate that the relation is relatively independent of season.

Remote-Sensing of Crop Yields

Our research efforts with durum wheat have led to the development of the SDD concept. Its application makes possible crop yield estimates from remotely acquired canopy temperatures and auxiliary air temperature measurements obtained during the period from head emergence to the cessation of head growth. Canopy albedo measurements appear adequate to delineate this critical period, making the technique potentially adaptable to predictions of crop yields by remote-sensing. The trifactor nomograms produced from combinations of the linear regression equations also suggest that the SDD concept may be used for scheduling irrigations by remote-sensing.

Estimation of Daily Evapotranspiration from one Time-of-Day Measurements

Jackson, R.D., Hatfield, J.L., Reginato, R.J., Idso, S.B. and Pinter, P.J., Jr., 1983. Estimation of daily evapotranspiration from one time-of-day measurements. Agric. Water Manage., 7: 351–362. The estimation of evapotranspiration (ET) on a regional basis requires remote sensing inputs. When obtained from air or space platforms, remotely sensed measurements are usually made at periodic intervals, and are essentially instantaneous in nature. A problem, then, is the estimation of daily values of ET from one time of day measurements. A technique is presented that allows the calculation of the coefficient necessary to convert one time of day measurements to daily totals. Input requirements are latitude, day of year, and time of day. This coefficient was applied to measured one time of day ET values and the results were compared to lysimetrically determined daily totals obtaind at five locations and for four crops. One time of day ET was also calculated using an ET model that requires remotely sensed surface temperatures. These values were converted to daily totals and compared with measured values. The results indicated that reliable estimates of daily total ET from one time of day measurements could be made for cloud free days. For cloudy days the results are less reliable, but they suggest that estimates may be improved by considering the amount and temporal distribution of cloud cover.

Estimation of soil heat flux from net radiation during the growth of alfalfa

Abstract Soil heat flux studies have indicated that the instantaneous daytime flux can be estimated as a fraction of the net radiation, the ratio ranging from 0.1 to 0.5, depending on the amount of vegetation present and on the time of day. Soil heat flux and net radiation were measured for an alfalfa crop over two regrowth cycles during the fall growing season. For both sparse alfalfa stubble and full vegetative canopy, the surface soil water content did not significantly affect the fraction of net radiation consumed as soil heat flux. The ratio of soil heat flux to net radiation around midday was found to be a linearly decreasing function of crop height only for heights up to 450 mm. As crop growth continued beyond this height, the ratio remained nearly constant at 0.1. The ratio data were also found to be well-fitted by a linearly decreasing function of a spectral vegetation index (near-IR to Red ratio) over both regrowth cycles. These results indicate that both crop height and spectral vegetation indices can be used to estimate soil heat flux from net radiation measurements.

Plant canopy information extraction from composite scene reflectance of row crops

As an aid in the interpretation of remotely sensed data from row crops with incomplete canopies, a model was developed that allowed the calculation of the fractions of sunlit soil, shaded soil, sunlit vegetation, and shaded vegetation for each resolution element in a scan of a remote sensor for a given set of conditions (plant cover, plant height/width ratio, row spacing, row orientation, time of day, day of year, latitude, and size of resolution element). Using measured representative reflectances of the four surfaces, composite reflectances were calculated as a function of view angle. Also, representative temperatures for each surface were used to simulate composite temperatures viewed by an IR scanner. With composite reflectances and temperatures known as a function of view angle, ways were explored to extract plant cover and plant temperature data from the composite data.

Evapotranspiration calculated from remote multispectral and ground station meteorological data

Abstract Evapotranspiration was evaluated by combining remotely sensed reflected solar radiation and surface temperatures with ground station meteorological data (incoming solar radiation, air temperature, windspeed, and vapor pressure) to calculate net radiation and sensible heat flux. Soil heat flux was estimated as a fraction of the net radiation. Instantaneous values of ET were calculated for 18 wheat plots for 44 cloudless days over a growing season. Three of the 18 plots contained lysimeters which provided data to compare against the instantaneous values. For the remaining plots, daily ET was estimated from the instantaneous data and compared with values calculated from soil water contents measured with a neutron moisture meter. For generally clear sky conditions, the comparisons indicated that ET could be adequately evaluated using a combination of remotely sensed and ground based meteorological data. The results suggest that ET maps of relatively large areas could be made using this method with data from airborne sensors. The extent of the area covered appears to be limited by the distance that air temperature and windspeed data can be extrapolated.

Plant stress detection by remote measurement of fluorescence

Chlorophyll fluorescence of mature lemon trees was measured with a Fraunhofer line discriminator (FLD). An increase in fluorescence was correlated with plant water stress as measured by stomatal resistance and twig water potential.

Net radiation calculated from remote multispectral and ground station meteorological data

Of the four terms that comprise net radiation, the incoming solar and the longwave radiation from the atmosphere are essentially independent of surface conditions (e.g., bare soil, vegetation). These terms can be measured at a single location using groundbased instruments and extrapolated over relatively large areas. The reflected solar and the emitted longwave terms are surface dependent, but are amenable to measurement by remote means. A method is described whereby net radiation is evaluated by combining ground-based meteorological and remote multispectral measurements. Net radiation values obtained using the remote method were compared to values obtained using minature net radiometers. The net radiometers were positioned near the center of 18 wheat plots and the multispectral measurements were made over a 6-m transect on each plot. Although the two methods used instruments having different fields-of-view, good agreement was obtained. The results imply that, by combining ground-based and remote measurements, net radiation maps of relatively large areas can be constructed at the level of detail determined by the resolution element of the multispectral radiometer.

Remote Detection of Biological Stresses in Plants with Infrared Thermometry

Green leaves of mature sugar beets infected with Pythium aphanidermatum and cotton infected with Phymatotrichum omnivorum had midday radiant leaf temperatures 3� to 5� warmer than adjacent plants with no sign of disease. The temperature difference persisted under varying conditions of soil moisture and could be used to detect biological stress imposed by these soilborne root-rotting fungi.