Independent Field Measurements Research Articles

Evaluation of an algorithm for predicting albedo in heliotropic crops

Albedo (the fraction of reflected incoming solar radiation in the 400–3000 nm range) is of critical importance in crop simulation studies because it determines how much solar radiation is available for carbon uptake, crop water use, and ultimately productivity and yield. However, many crop simulation models do not place as much emphasis on albedo as is warranted given its importance in determining these crop processes. In addition, prediction of albedo in crop simulation models dealing with heliotropic crops (their leaves follow the sun) has received relatively little attention. An albedo algorithm based on a spherical leaf angle distribution was modified to predict albedo in two heliotropic crops, alfalfa ( Medicago sativa L.) and soybean ( Glycine max (L.) Merr.). Since no analytical expression exists for the heliotropic leaf angle distribution, linear relationships were developed to predict the albedo of these heliotropic crops from the albedo of a crop with a spherical leaf angle distribution. These relationships were developed from field data and tested on independent field measurements. Albedo was measured over soybean for a wide range of leaf area indices and leaf optical properties and between two cuttings in alfalfa. The normalized mean absolute error was used to compare predicted and measured values. There was good agreement between predicted and measured values except under foggy conditions when the direct beam fraction was zero and when water droplets on leaf surfaces may have influenced leaf optical properties.

A process‐based model for methane emission predictions from flooded rice paddies

Estimation and prediction of methane emission from flooded rice paddies is impaired by the large spatial and temporal variability in methane emissions and by the dynamic nonlinear relations between processes underlying methane emissions. This paper describes a process‐based model on methane emission prediction from flooded rice paddies that can be used for extrapolation. The model is divided into two compartments; rhizosphere, which is a function of root length density, and bulk soil. The production of carbon substrates drives methane emission and originates from soil organic matter mineralization, organic fertilizer decomposition, in both compartments, and root exudation and root decay, in the rhizosphere compartment only. It is assumed that the methanogens are completely outcompeted for acetate by nitrate and iron reducers but that competition takes place with sulfate reducers. Produced methane is transported to the root surface in the rhizosphere or the soil‐water interface in the bulk soil. Transport time coefficients are different for the two compartments. Part of the methane is oxidized, a constant fraction of produced methane in the bulk soil, whereas the oxidation fraction varies according to root activity dynamics in the rhizosphere. The remaining methane is emitted to the atmosphere. The model was validated with independent field measurements of methane emissions at sites in the Philippines, China, and Indonesia with only few generally available site‐specific input parameters. The model properly predicts methane emission dynamics and total seasonal methane emission for the sites in different seasons and under different inorganic and organic fertilizer conditions. A sensitivity analysis on model assumptions showed that the assumptions made in this model are reasonable and that the division into two compartments was necessary to obtain good results with this model. The combination of proper prediction and the necessity of few input parameters allow model application at regional and global scales.

Biodegradation modelling of a dissolved gasoline plume applying independent laboratory and field parameters

Biodegradation of organic contaminants in groundwater is a microscale process which is often observed on scales of 100s of metres or larger. Unfortunately, there are no known equivalent parameters for characterizing the biodegradation process at the macroscale as there are, for example, in the case of hydrodynamic dispersion. Zero- and first-order degradation rates estimated at the laboratory scale by model fitting generally overpredict the rate of biodegradation when applied to the field scale because limited electron acceptor availability and microbial growth are not considered. On the other hand, field-estimated zero- and first-order rates are often not suitable for predicting plume development because they may oversimplify or neglect several key field scale processes, phenomena and characteristics. This study uses the numerical model BIO3D to link the laboratory and field scales by applying laboratory-derived Monod kinetic degradation parameters to simulate a dissolved gasoline field experiment at the Canadian Forces Base (CFB) Borden. All input parameters were derived from independent laboratory and field measurements or taken from the literature a priori to the simulations. The simulated results match the experimental results reasonably well without model calibration. A sensitivity analysis on the most uncertain input parameters showed only a minor influence on the simulation results. Furthermore, it is shown that the flow field, the amount of electron acceptor (oxygen) available, and the Monod kinetic parameters have a significant influence on the simulated results. It is concluded that laboratory-derived Monod kinetic parameters can adequately describe field scale degradation, provided all controlling factors are incorporated in the field scale model. These factors include advective–dispersive transport of multiple contaminants and electron acceptors and large-scale spatial heterogeneities.

SIMULATING NEW ZEALAND FOREST DYNAMICS WITH A GENERALIZED TEMPERATE FOREST GAP MODEL

A generalized computer model of forest growth and nutrient dynamics (LINKAGES) was adapted for the temperate evergreen forests of New Zealand. Systematic differences in species characteristics between eastern North American species and their New Zealand counterparts prevented the initial version of the model from running acceptably with New Zealand species. Several equations were identified as responsible, and those modeling available light were extended to give more robust formulations.The resulting model (LINKNZ) was evaluated by comparing site simulations against independent field measurements of stand sequences and across temperature and moisture gradients. It successfully simulated gap dynamics and forest succession for a range of temperate forest ecosystems in New Zealand, while retaining its utility for the forests of eastern North America. These simulations provided insight into New Zealand conifer–hardwood and beech species forest succession.The adequacy of the ecological processes, such as soil moisture balance, decomposition rates, and nutrient cycling, embodied in a forest simulation model was tested by applying it to New Zealand forest ecosystems. This gave support to the model’s underlying hypothesis, derived from LINKAGES, that interactions among demographic, microbial, and geological processes can explain much of the observed variation in ecosystem carbon and nitrogen storage and cycling. The addition of a disturbance option to the model supported the hypothesis that large‐scale disturbance significantly affects New Zealand forest dynamics.

The magnetic field experiment onboard Equator-S and its scientific possibilities

Abstract. The special feature of the ringcore fluxgate magnetometer on Equator-S is the high time and field resolution. The scientific aim of the experiment is the investigation of waves in the 10–100 picotesla range with a time resolution up to 64 Hz. The instrument characteristics and the influence of the spacecraft on the magnetic field measurement will be discussed. The work shows that the applied pre- and inflight calibration techniques are sufficient to suppress spacecraft interferences. The offset in spin axis direction was determined for the first time with an independent field measurement by the Equator-S Electron Drift Instrument. The data presented gives an impression of the accuracy of the measurement.Key words. Magnetospheric physics (instruments and techniques) · Space plasma physics (instruments and techniques)

The magnetic field experiment onboard Equator-S and its scientific possibilities

The special feature of the ringcore fluxgate magnetometer on Equator-S is the high time and field resolution. The scientific aim of the experiment is the investigation of waves in the 10–100 picotesla range with a time resolution up to 64 Hz. The instrument characteristics and the influence of the spacecraft on the magnetic field measurement will be discussed. The work shows that the applied pre- and inflight calibration techniques are sufficient to suppress spacecraft interferences. The offset in spin axis direction was determined for the first time with an independent field measurement by the Equator-S Electron Drift Instrument. The data presented gives an impression of the accuracy of the measurement.Key words. Magnetospheric physics (instruments and techniques) · Space plasma physics (instruments and techniques)

ESTIMATING SOIL HYDRAULIC PROPERTIES USING TENSION INFILTROMETERS WITH VARYING DISK DIAMETERS

Tension infiltrometers have become a popular instrument for field determination of soil hydraulic properties, such as hydraulic conductivity (KS) of saturated soil and the parameter (α) used in exponential expressions of the hydraulic conductivity function. However, results different from other independent field or laboratory measurements are often obtained using the steady-state approximate solutions. This is likely caused by the variable sizes of the infiltrometer disk used for the infiltration measurement and/or the limitations of steady-state solutions for small disk dimensions. To determine the effect of disk sizes on parameter estimation, we measured infiltration in two soils (Arlington sandy loam and Sparta sand) with tension infiltrometers of several disk diameters (5.5-34.5 cm). For each disk size, the infiltration was repeated at multiple supply potentials and continued until steady-state so that replicated parameter estimates were obtained for each disk size. Results indicate that estimated values of KS and α appeared to vary with the size of the infiltrometer disk used. Variations in estimated KS and α values for different disk sizes or for different potential increments for the same disk were greater than the potential overestimation with the steady-state solution when compared with an improved solution for small disk sizes. Discrepancies between tension infiltrometer and other methods in practice are probably caused by variability within each method, such as soil heterogeneity or simplifying the hydraulic conductivity function to the exponential expression, rather than by limitations in the steady-state solution for small tension infiltrometer disk sizes.

Predicting Effects of Atmospheric CO 2 Partial Pressure on Forest Photosynthesis

A mechanistic (i.e. hierarchic or explanatory) model of forest canopy mass and energy exchange that has been previously tested with eddy-correlation measurements in the field albeit only at present ambient CO2 partial pressure was used to predict photosynthetic response of a deciduous Quercus-Acer forest in eastern North America to atmospheric CO2 partial pressure. Four partial pressures of CO2 were used in simulations: 28 (pre-industrial), 36 (present), 54 and 72 Pa. This is (one of) the first set(s) of predictions of forest photosynthetic response to CO2 partial pressure made by a mechanistic forest physiology model shown to accurately predict independent field measurements of whole-forest CO2 exchange at the hourly time scale. The model includes a biochemically based Farquhar-type model of leaf mesophyll CO2 assimilation, which is central to its ability to predict photosynthetic response to different CO2 partial pressures. Whole-forest photosynthesis was positively related to CO2 partial pressure, as expected. This was the case under both

Bernstein waves in the Io plasma torus: A novel kind of electron temperature sensor

During Ulysses passage through the Io plasma torus, along a basically north‐to‐south trajectory crossing the magnetic equator at R ∼ 7.8 RJ from Jupiter, the Unified Radio and Plasma Wave experiment observed weakly banded emissions with well‐defined minima at gyroharmonics. These noise bands are interpreted as stable electrostatic fluctuations in Bernstein modes. The finite size of the antenna is shown to produce an apparent polarization depending on the wavelength, so that measuring the spin modulation as a function of frequency yields the gyroradius and thus the local cold electron temperature. This determination is not affected by a very small concentration of suprathermal electrons, is independent of any gain calibration, and does not require an independent magnetic field measurement. We find that the temperature increases with latitude, from ∼1.3 × 105 K near the magnetic (or centrifugal) equator, to approximately twice this value at ±10° latitude (i.e., a distance of ∼1.3 RJ from the magnetic equatorial plane). As a by‐product, we also deduce the magnetic field strength with a few percent error.

Generation and dispersion of near-surface bubble densities

In four ocean experiments conducted in 1985, 1991, and 1992, extensive near-surface microbubble densities were obtained by both optical and acoustical techniques. The bubble radius (r) range for the former is 8–200 μm and the range for the latter is 34–1200 μm. Detailed analyses of the temporal variations (both on the order of 10 and 60 s) of these bubble densities reveal some common characteristics existing in all four sets of independent field measurements. Two maxima appear in the bubble density with the first one near r1=100–150 μm and the second one near r2=20 μm. The r1 peak occurs immediately when a fresh breaking wave plume is generated near the measuring sensor, while the r2 peak is more or less unchanged during the entire measurement period. These two peaks essentially divide the bubble density into two distinct radius ranges: the first range is 100<r<1200 μm with the power law of r−3, and the second range is 30<r<80 μm with a power law of r−4. It is speculated that the first and second range are, respectively, controlled by generation and subsequent dispersion of bubble plumes initially generated by wave breaking. [Work supported by ONR Acoustical Reverberation SRP.]

A comparison of vector magnetograms from the marshall space flight center and mees solar observatory

We compare completely independent vector magnetic field measurements from two very different polarimetric instruments. The Marshall Space Flight Center's imaging vector magnetograph is based on a birefringent filter, routinely measuring all four Stokes parameters integrated over the filter bandpass (1/8 A) which is tunable across the Fei λ5250 line in 10 mA steps. The Haleakala Stokes Polarimeter of the Mees Solar Observatory (MSO) is based on a spectrometer, routinely measuring all four Stokes parameters of the Fei λ6302.5 line simultaneously and then spatially scanning to build up a vector magnetogram. We obtained active region magnetic field data with both the Marshall Space Flight Center (MSFC) and MSO systems on five days during June 1985. After interpolating the MSFC vector fields onto the more coarse spatial grid of MSO we make a point-by-point comparison of the two vector fields for data obtained on two of these days (June 8 and 9). From this comparison we conclude: (1) the spatially-averaged line-of-sight components agree quite well; (2) although the MSO spatial grid is coarser, the quality of the MSO image is better than that of the MSFC data because of better seeing conditions; (3) the agreement between the transverse magnitudes is affected by the poor image quality of the MSFC data; and (4) if the effects of Faraday rotation caused by including line-center linear polarization in the method of analysis are taken into account, the azimuths show good agreement within the scatter in the data caused by the averaging process.

Real‐Time Estimation of Furrow Infiltration

An analysis to determine infiltration parameters from the early stages of furrow advance is reported. The analysis uses a kinematic‐wave simulation model in conjunction with a simplex search procedure to minimize the differences between measured and predicted advance rates. The technique uses a variable and expanding data set to forecast advance times and is therefore a useful prelude to real‐time feedback control systems for the future automation of furrow irrigation systems. Results from four furrow evaluations involving a sandy loam and a silty clay loam are given to demonstrate the capability of the analysis. The analysis was first verified by using the complete advance data sets for the evaluations. The analysis yielded the same values of the three Kostiakov‐Lewis infiltration parameters found in independent field measurement. In a second investigation, the basic intake‐rate parameter in the Kostiakov‐Lewis function was fixed and the analysis was repeated with advance data from the early stages of advance to determine if and how soon the analysis would estimate reasonably accurate values of the other two infiltration parameters. The results indicate that infiltration parameters can be estimated with sufficient accuracy from early advance data to allow an accurate forecast of efficiency and uniformity at the end of the irrigation.