J.L. Hatfield

Discerning the forest from the trees: an essay on scaling canopy stomatal conductance

Abstract Stomata are major conduits for the diffusion of many trace gases between leaves and the atmosphere. The role of the stomata in controlling gas exchange between the terrestrial biosphere and the atmosphere at the landscape, meso- and global scales has only recently been appreciated. Further advances in modeling trace gas exchange will depend on our ability to provide realistic information on stomatal mechanics at the sub-grid scale of landscape and meso scale models; in other words, information is needed at the canopy scale. This paper describes two approaches for estimating canopy stomatal conductance. These are the ‘bottom-up’ and ‘top-down’ scaling methods. The bottom-up method entails computing canopy stomatal conductance by integrating the response of individual leaves to controling biotic and abiotic factors, which are determined by the micrometeorology of the canopy. The top-down approach entails inverting a descriptive stand-level, trace gas exchange model to estimate canopy stomatal conductance. The model is driven with abiotic variables measured at a reference point above the canopy. The strengths and weaknesses of these two approaches are discussed, and recommendations for future research are presented.

Remote sensing estimators of potential and actual crop yield

Abstract Spectral-reflectance patterns, vegetative indices of infrared/red, were utilized to signal the beginning and ending times for a stress-degree-day summation. The maximum vegetative index ( mss 7 5 ) corresponded to the beginning of head-emergence in grain sorghum and the boot-stage in wheat. When the vegetative index declined to 0.5 of the maximum, 90% of the reproductive dry matter had been accumulated, delineating the reproductive portion of the growth cycle. In both grain sorghum and wheat, a linear relationship was found between yield and stress-degree-days over the reproductive period defined by the vegetative index. However, in all years, the grain sorghum data did not fit the same linear relationship because of differences in cultural practices. It was found that the maximum vegetative index could be used to define the potential harvestable yield that was encountered during the reproductive stage. A linear relationship was then found between the yield harvested relative to this potential harvestable yield and the summation of the stress-degree-days. In these experiments, there was a wide range in ground covers and yield providing a rigorous test of the relationship. This technique provides a method for defining the interval of the stress-degree-day accumulation and has applicability as a generalized yield model.

Aerodynamic properties of partial canopies

Abstract Roughness lengths ( z o ) and displacement heights ( d ) are often assumed to be simple functions of canopy height. This study was designed to evaluate these canopy aerodynamic properties for cotton ( Gossypium hirsutum L.) having partial ground cover. Both z o and d were calculated from wind profile measurements for neutral conditions as defined with a Richardson number over irrigated and rainfed cotton in 1984 and 1985. Throughout the season z o started at 0.01 m and increased to a maximum of 0.20 m then declined to a value of 0.06 m when the canopy reached 70% ground cover. This change throughout the season was caused by the individual rows first acting as discrete bluff-bodies and then as a more uniform surface as the foliage elements overlapped as the canopy developed. Displacement heights increased linearly throughout the season as the canopy increased in height and leaf area.

Surface temperature variability patterns within irrigated fields

Abstract During 1981, an experiment was conducted to examine the spatial variability of surface temperature within differentially irrigated grain sorghum fields. The measurements were performed at the University of California-Davis in 85 m long transects measured with an infrared thermometer reading at 1.0 m intervals. These measurements were made repeatedly throughout the season. When the data were analyzed to evaluate the spatial variability along each transect, it was found that along the transects the readings at 1 m intervals were not correlated. This lack of structure or spatial dependence within a given field suggests that a random sampling in a field would be adequate for infrared thermometer measurements. During a drying cycle from one irrigation to the next, the variance of surface temperature along a transect increased. However, there was not a statistically significant relationship between the amount of water extracted from the upper 150 cm of the profile when more than 40% of the available water remained. When more than 60% of the available water had been removed, the variance increased above 0.5°C 2 but below this amount the variances did not exhibit any significant pattern. These data suggest that the variance of surface temperature within a field may not be a sensitive indicator of the soil water status. Utilizing the fact that the surface temperatures along the transects were random and normally distributed, we calculated that the number of samples needed to achieve a confidence limit of 1.0°C about the mean was 10 samples for a field with more than 40% available soil water and increased to 20 samples as the field dried below the 60% extraction level. These data suggest that the spatial variability techniques could be utilized to evaluate agricultural fields for their distribution efficiencies and problems. In application the samples required per field are a manageable number and could be analyzed with classical statistical methods.

Wheat yield estimation by albedo measurement

Abstract Experiments conducted in two different years at two different locations have shown that grain yield of a fall-planted spring wheat can be adequately estimted from the minimum albedo of the crop just prior to grain ripening. The predictive relationship appears to be independent of any other factor, as long as complete ground cover is achieved.

Grain Yield Prediction: Extending the Stress-Degree-Day Approach to Accommodate Climatic Variability

Abstract Combination of the stress-degree-day concept of plant water stress assessment with the growing degree-day concept of plant phenological development leads to a new approach to the prediction of grain crop yields that accounts for climatic effects of light, temperature, and water. The model requires knowledge of the dates of plant emergence and heading, maximum and minimum air temperatures through the growing season, and midafternoon canopy-air temperature differentials from the time of heading to physiological maturity. Its utility is illustrated by development of the predictive relationship for a specific cultivar of wheat grown at Phoenix, Arizona, and a test of its predictions for the same cultivar grown at Davis, California. Both final yield and the time of cessation of crop growth are adequately predicted.

Equivalence of airborne and ground-acquired wheat canopy temperatures

The relationship between airborne and ground-based measurements of soil and crop canopy temperatures is investigated for a partial crop canopy. Daily ground-based measurements using a wide-field-of-view radiometer oriented towards the nadir at a height of 1.5 m and airborne thermal imagery at two-week intervals were obtained throughout the entire growing season of a stand of wheat. When corrected for atmospheric effects, the airborne measurements were found to be virtually identical to ground-based measurements, with a regression line slope of 0.985, a standard deviation of 1.8 C and a correlation coefficient of 0.97.