Gaylon S. Campbell

On the relationship between incoming solar radiation and daily maximum and minimum temperature

Abstract A relationship between atmospheric transmittance and the daily range of air temperature is developed. The relationship is T t = A [1— exp (— BΔT c )] where T t is the daily total atmospheric transmittance, ΔT is the daily range of air temperature, and A, B, and C are empirical coefficients, determined for a particular location from measured solar radiation data. Tests on three data sets indicate that 70–90% of the variation in daily solar radiation can be accounted for by this simple model.

Simple equation to approximate the bidirectional reflectance from vegetative canopies and bare soil surfaces

A simple equation has been developed for describing the bidirectional reflectance of some vegetative canopies and bare soil surfaces. The equation describes directional reflectance as a function of zenith and azimuth view angles and solar azimuth angle. The equation works for simulated and field measured red and IR reflectance under clear sky conditions. Hemispherical reflectance can be calculated as a function of the simple equation coefficients by integrating the equation over the hemisphere of view angles. A single equation for estimating soil bidirectional reflectance was obtained using the relationships between solar zenith angles and the simple equation coefficients for medium and rough soil distributions. The equation has many useful applications such as providing a lower level boundary condition in complex plant canopy models and providing an additional tool for studying bidirectional effects on pointable sensors.

CropSyst, a cropping systems simulation model: water/nitrogen budgets and crop yield

Abstract In agriculture, water and nitrogen are two critical resources for growing a crop. However, their management cannot be analyzed independently of weather, soil characteristics, field hydrology, crop characteristics, crop rotation, and management factors. This paper describes the water, nitrogen, and crop growth components of CropSyst, a comprehensive cropping systems simulation model, and provides preliminary verification of these components. The water budget of the model properly describes crop water use. Predicted nitrogen contents throughout the soil profile did not exactly match the measured values from leaching experiments, but they did follow the general trends of the data. The agreement between simulated and observed biomass and yield of corn, winter wheat and spring wheat grown in two locations with a total of 77 data points was good as shown by several statistical indicators. Based on this preliminary validation, CropSyst appears promising as a tool to analyze management practices for water and nitrogen. Additional validation of model components, including a wide range of crops and conditions, should be conducted in the future.

Simulation of heat and moisture transfer through a surface residue—soil system

Abstract A simulation model suitable for describing the dynamic aspects of mass and energy transfer in a soil—residue—atmosphere system has been developed and used to determine soil heat and water budgets. The model has been programmed in BASIC and uses numerical methods suitable for microcomputer applications. It displays a fundamental coupling of the surface residue to the soil—atmosphere system, and uses network analysis to describe heat and moisture transfer. Short-wave and long-wave radiative transfer, changes in energy status, rainfall interception by the residue, infiltration, redistribution, evaporation, and drainage are all accounted for. Daily input requirements include global short-wave radiation, maximum and minimum air temperatures, average wind speed, precipitation, and temperature and water content deep within the soil. General site, residue, and soil characteristics are also needed. Surface residues are treated as a thin layer with uniform horizontal distribution and residue loading rate is 3000 kg ha−1. Application of the model to energy and water budget studies using environmental data recorded at Pullman, WA during 1981/82 is discussed. Simulation results indicate that for the Pullman conditions surface residues decreased evaporation by roughly 36% when compared with simulations of bare soil evaporation. Monthly trends, however, indicate that the effectiveness of residues in conserving soil moisture decreases as the length of rain-free periods increases.

Derivation of an angle density function for canopies with ellipsoidal leaf angle distributions

Abstract The leaf inclination angle density function is a fundamental property of plant canopy structure and is needed for computing distributions of leaf irradiance. These computations are important for predicting canopy photosynthesis and energy balance, and for estimating bidirectional reflectances of plant canopies in remote sensing applications. An analytical expression is derived for the inclination angle density function which is g(α)= 2χ 3 sin α λ( cos 2 α+χ 2 sin 2 α) 2 where α is the leaf inclination angle, X is the ratio of vertical to horizontal projections of canopy elements, and A is a normalized ellipse area, approximated by λ=χ+1.774(χ+1.182) −0.733 An empirical equation relating mean leaf angle and X is also given.

Low temperature or low water potential effects on the microbial decomposition of wheat residue

Abstract To predict the amount of crop residue remaining on a field at a given time, the rates of residue decomposition under conditions of low temperature or low water potentials need to be known. Laboratory experiments were made in which temperature and moisture were controlled. The amount of CO2 evolved from soils treated with wheat residue was used as a measure of decomposition. After 30 days at 20°C and −33 kPa, 38% of the surface-applied wheat residue was lost as CO2-C. Decomposition decreased as the temperature decreased, the 30 day CO2-C losses were 32.7, 28.6, 21.7 and 17.2% at 15, 10. 5 and 0°C, respectively. When water potential at 20°C was varied, decomposition decreased as the water potential decreased with 35.4, 25.1, 22.1, 17.0 and 10.1% of the residue decomposing at −150 kPa and −1.0, −1.5, −2.5 and −5.0 MPa, respectively. The results obtained with residue incorporated into soil were similar to those with surface-applied residue. Equations for the relationship between the amount of residue decomposition after 30 days and temperature or water potential are presented.

Reduction of transpiration through foliar application of chitosan

In this study, we investigate the potential of chitosan, a natural beta-1-4-linked glucosamine polymer, to reduce plant transpiration. Chitosan was applied foliarly to pepper plants and water use was monitored. Peppers were grown in pots in growth-chambers, where transpiration was measured by weighing pots. In an accompanying field study, water use was determined by monitoring soil moisture depletion with time domain reflectometry. An automated irrigation system replenished the water used each day. Plant biomass and yield were determined to calculate biomass-to-water ratios. Differences in canopy resistance between control and chitosan treated plants were analyzed with the aid of the Penman‐Monteith equation. Scanning electron microscopy (SEM) and histochemical analyses demonstrated that chitosan induced closure of the plant’s stomata, resulting in decreased transpiration. Foliar application of chitosan reduced water use of pepper plants by 26‐43% while maintaining biomass production and yield. We suggest that chitosan might be an effective antitranspirant to conserve water use in agriculture.

A dewpoint hygrometer for water potential measurement

Abstract An improved instrument for determining water potential by measuring dewpoint depression has been developed. Theoretical considerations show the instrument to have a sensitivity of 0.75 μV bar −1 and a change in sensitivity with temperature of 0.45% C −1 . Without compensation for changes in Peltier cooling, sensitivity, and heat dissipation with temperature, errors of ±6% result from changing the temperature of the sensor between 20 and 50°C. Performance of the instrument appears to agree well with the theory developed for the technique.

Improved heat balance method for determining sap flow rate

Abstract The heat balance method measures sap flow in plants by heating a small section of the stem and determining the amount of heat transported away from the heater by xylem water movement. Two methods have been available for measuring sap flow in herbaceous species, but both have limitations, due either to a failure to correctly measure all components of the stem energy budget or to the lack of control of heater temperature. Aspects of both methods were combined to provide temperature control of the heater on the stem and monitoring of all conductive heat losses. The new sap flow meter was used on potted maize, sunflower and potato, and was shown to give transpiration estimates which were within 8% of those from weight loss measurements. Measurements on field-grown potato showed good agreement with estimates from a Penman calculation. The magnitude of the conductive and convective heat loss components was measured, and errors were determined for several methods of estimating these losses. Construction details and control circuitry are shown. A numerical simulation of the method showed that inserting thermocouples into the stem to provide radial averaging of stem temperature resulted in a significant reduction in error of the flow measurement.

Measuring sap flow with the heat balance approach using constant and variable heat inputs

The performance of a heat balance-based sap flow gauge design, capable of alternating variable heat input (VHI) and constant heat input (CHI), was analyzed. The gauge employs differentially wired thermocouples to monitor differences in stem temperature and radial heat losses. The VHI method employs a control circuit to maintain a constant temperature difference between a thermocouple placed upstream from the gauge heater and a thermocouple at the heated portion of the plant stem. The CHI method applies a constant voltage to the gauge heater with no control circuit. Simultaneously, on the same plants, the effect of the placement of thermocouples, either inserted into the stem structure or placed on the surface, was also investigated. Sets of 15 min and daily integrated measurements were compared with gravimetric water losses determined by lysimetry. The evaluation included three plant species: sunflower (Helianthus), maize (Zea maize) and potato (Solanum tuberosum). The results showed that the VHI gauges tended to underestimate flows for both the 15 min and daily sap flows. The CHI gauges gave generally better results and were easier to implement and monitor. No consistent differences in the performance of the inserted and surface-mounted thermocouples were found; any differences that were noted were generally small and of no practical significance. An exception was the case of sunflower CHI at large flow rates, when the inserted thermocouples outperformed the surface-mounted ones. Important departures between gauge sap flows and lysimetric plant water losses at low flow rates were observed. Gauge performance was better for daily integrated flows (relative absolute error, RAE ≈ 10% for combined data) than for 15 min interval average flows (RAE = 32–36%v for combined data). However, the performance of the short-time-interval gauge improved dramatically when low flows (< 10 g h−1) were not included in the analysis.