Large Potential Errors Research Articles

Towards the validation of SeaWiFS in southern African waters: the effects of gelbstoff

The effective application of future ocean colour data for southern African waters requires an in-depth assessment of bio-optical algorithm performance, given the productive and highly variable nature of the oceanographic region. Phytoplankton degradation products, commonly known as gelbstoff, represent a large potential error source to future remotely sensed chlorophyll data, particularly in highly productive regions, where there can be a dramatic lack of covariance between phytoplankton and gelbstoff concentrations. Data relevant to gelbstoff character in the Agulhas Bank and southern Benguela systems are examined to assess the variability of gelbstoff chromophoric structure in these regions. These take the form of gelbstoff absorption spectra, and fluorescence excitation-emission matrices, performed on filtered seawater samples. Modelled bio-optical data, using conditions typical of high biomass marine environments, are then used to assess the performance of a proposed SeaWiFS combined algorithm with respect to variations in the magnitude and nature of gelbstoff absorption. It is shown that expected algorithm performance can be poor in bloom type scenarios, and that an appreciation of algorithm application is needed for effective interpretation of ocean colour imagery.

FM sonar characteristics for normal-incidence sediment classification

The prediction of sediment properties from normal-incidence acoustic measurements made with a broadband sub-bottom profiler requires that several acoustic parameters be estimated from the reflection data using independent numerical techniques to reduce the large potential errors of any one parameter. Examples of acoustic parameters that are combined empirically to estimate the vertical profiles of physical sediment properties are acoustic impedance, interlayer volume scattering, and attenuation. From the analysis of normal-incidence data collected in many depositional environments, the characteristics of a quantitative sub-bottom profiler for estimating impedance and volume scattering include: (1) the acoustic bandwidth of the transmitted pulse must be at least 2 oct for the reliable measurement of signal phase from interlayer reflections; (2) the two-way transmission/reception beam should be a cone with a width between 15° and 20° at the 3-dB down points to ensure that interlayer reflection amplitude to volume and surface scattering noise ratios are at least 6 dB (narrower beamwidths result in reflection amplitude measurement errors from sensor motion or seafloor slopes less than 5°); (3) the transmitted pulses contain energy from 500 Hz to 10 kHz to ensure most interlayer impedance gradients can be accurately measured.

Estimating carbon, nitrogen, protein, and chlorophyll a from volume in marine phytoplankton

The size of 30 small (2–60 µm) phytoplankton species was examined with a microscope and a Coulter Counter before and after fixation. Acid Lugol’s iodine caused cells to shrink immediately. The shrinkage effect was constant for concentrations of 1–10% Lugol’s iodine (in seawater). For optically measured cells fixed in 2% Lugol’s iodine, volume of live cells = 1.33 × (volume of fixed cells). Coulter Counter and optically measured volumes did not agree. For live cells, optical cell volume = 1.24–2.04 × (Coulter Counter determined volume); this difference is likely due to inaccurate volume measurements of non‐spherical cells by the Coulter Counter and by inaccurate microscopy resulting from optical distortions (errors of <0.5 µm in cell dimensions). Cell quota estimates were presented following the relation y = axb, where x = optically measured cell volume (µm3), y = any cell constituent (pg cell−1), and a and b are constants. The constants a and b were 0.109 and 0.991 for carbon, 0.0172 and 1.023 for nitrogen, 0.043 and 1.058 for protein, and 0.00428 and 0.917 for Chl a. Our relation of carbon to volume differs from other literature values, in which there is no consensus. Our data can be used to determine carbon, nitrogen, protein, and Chl a estimates from field material that has been fixed with Lugol’s iodine, observed live, optically measured, or Coulter Counter measured; however, the variability in published data suggests that any of these estimates will have a large potential error.

Effect of Soil Properties on Unsaturated Hydraulic Conductivity Pore‐Interaction Factors

AbstractThe variability of the pore‐interaction factor, ρ, for a microscopic model proposed for predicting unsaturated hydraulic conductivity, K(θ), from soil water‐retention, Ψ(θ), data was examined in relation to soil particle‐size data and indices, bulk density, organic C, parameters for the van Genuchten Ψ(θ) function, an index (W) of total energy of drainage and soil‐series, toposequential, and geographic groupings. The ρ exhibited no trend relationship to any of the soil properties tested. However, a change in the distribution of ρ was observed in relation to the geometric‐mean particle diameter (Gd) and other soil textural variables. For Gd < 0.08 mm, variability was much larger than for soils with Gd ≥ 0.08 mm. The latter consisted entirely of soils in the sand and loamy sand textural groups. A similar distributional relationship also occurred for ρ vs. W. Examination of ρ on three sites of the Hecla soil series and on related toposequential soils indicated that classification of ρ on the basis of soil series or of soil‐association groupings is a feasible strategy for ρ parameter estimation on some soils. The exponential factor η for a macroscopic model was also investigated. The η was found to be related to W as an exponential function over the full data range. However, η vs. W was nearly identical to a linear function for W data extending from 0 to 400 cm‐γ (where γ is the density of water). The η was also strongly related to Gd as a power function, and to other textural variables as exponential functions. The increasing slope of the power function for Gd < 0.08 mm indicated a large potential error of prediction for K(θ) on fine‐textured soils.

Heat and mass balance of an ablating ice jam

This paper reports on the heat and mass balance of an ablating ice jam at the confluence of the Liard and Mackenzie rivers in the spring of 1983. From May 6 to May 9, sufficient data were collected to determine the magnitude and relative importance of the major heat fluxes that caused thermal decay of the ice jam. The total atmospheric heat input of 1.25 × 108 MJ and the much larger contribution from hydrothermal sources of 9.80 × 109 MJ resulted in a total melt of 3.24 × 107 m3 of ice within the jam. These data in combination with previous results regarding jam dimensions permitted calculation of ice jam porosity, a variable frequently used in hydraulic studies of river ice jams although unverified by field data. The largest potential error in the calculations was related to the accuracy of water temperature measurement. Accounting for this resulted in an estimate of porosity of 0.38 ± 10% which lends credence to the commonly assumed value of 0.40. Details of the field conditions, instrumentation, theoretical methods, and results of the energy and mass balance analyses are described. Key words: floating ice, ice breakup, ice jams, ice melt, ice porisity.

Errors in the isotopic estimations of hepatic glycogen synthesis and glucose output

Ignoring the reality of glucose ⇄ glucose-6P cycling in the liver in vivo produces large potential errors in the isotopic quantitation of percentage contributions of direct and indirect pathways of glycogen synthesis and of hepatic glucose output.

A test of the potential accuracy of the water-budget approach to estimating evapotranspiration

The water-budget approach to estimating evapotranspiration was tested against energy-budget calculations to determine its potential accuracy. Measurements of soil moisture change were made using both neutron attenuation and gravimetric techniques for one-day and four-day periods. Hydraulic head gradients were measured routinely in the field and moisture characteristics and capillary conductivities were calculated from both field and laboratory data. The energy-budget calculation of evapotranspiration was derived from measurements of net radiation, soil heat flow and Bowen ratios. The measurements were made in a corn field and the experimental period spanned 25 days in July, 1969. An error analysis of the water-budget approach and the experimental results lead to the same conclusion. The water-budget cannot accurately estimate evapotranspiration on a daily basis due to unavoidable large potential errors. Accuracy arises with an increasing time span between soil moisture measurements. If actual evapotranspiration is high, there is no precipitation and six sites are averaged a reasonable accuracy can be achieved for four-day periods. If only one measurement site is used or if evapotranspiration is low or if precipitation occurs, the necessary time lapse between measurements increases substantially. Moisture characteristics and capillary conductivities varied greatly from site to site and from soil horizon to soil horizon. For the entire measurement period the water lost by deep seepage was less than 10% of that lost by evapotranspiration.