Abstracts

Evett, S., Laurent, J.P., et al. 2002, Neutron Scattering, Capacitance, and TDR Soil Water Content Measurements compared on Four Continents., 17th World Congress of Soil Science: Confronting New Realities in the 21st Century, Thailand. 14-21 August 2002 pp. Paper No. 1021.

Neutron scattering, capacitance, and time domain reflectometry (TDR) methods of soil water content measurement were compared in a wide variety of soils and environments. Comparisons aimed to establish the accuracy and precision of each method and particular device, the need for and amenability of the device to soil-specific calibration, the volume of measurement, and the conditions of successful use. Measurements were made in a soil in Australia, three soils in Austria, five soils in France, two soils in Tunisia and three soils in the United States. Except for conventional TDR, the devices were used in access tubes. Several experiments included gravimetric sampling. Devices tested included the soil moisture neutron probe (SMNP), Sentek EnviroSCAN and Diviner 2000 capacitance probes, the IMKO Trime tube probe, and conventional TDR systems from Soil Moisture Inc., Tektronix, Inc. and Dynamax, Inc. The Sentik, IMKO, Delta-T and SMNP devices all required soil-specific calibration. The conventional TDR systems were reasonably accurate without calibration. Due to their small measurement vloume, installation of access tubes without soil disturbance outside the tubes was critical for the success with the Sentek, IMKO and Delta-T devices, but not for the SMNP. Successful tube installation was difficult with the smaller diameter devices (<30mm), and was difficult in some soils for all devices except the SNMP. Access tubes available from the manufacturers for the Suntek, IMKO and Delta-T devices were expensive compared with access tubes for the SMNP. Preliminary calibration vs. gravimetric sampling resulted in coefficients of determination (r²) values of 0.42 and 0.53 in two Austrian soils for the Diviner. Also, the Trime system, using its standard factory calibration, did not compare well with SMNP, Diviner or EnviroSCAN measurements. Both the Diviner and EnviroSCAN devices were highly sensitive to the electrical conductivity of soil water, with a 5% change in water content caused by an EC increase of 5 dSm^-1. Tests in Australia showed the EnviroSCAN to overestimate water content near saturation and to underestimate near wilting point compared with the SMNP and laboratory determined water holding characteristics. Plant available water capacity measured by the EnviroSCAN system was twice that indicated by laboratory measures and the SMNP. In Tunisia, the Trime system was calibrated vs. the SMNP in a silty clay loam with r² of 0.60, but in a more clayey soil in France, calibration resulted in r² of 0.92. In Tunisia, comparison of the Trime device to gravimetric samples showed a generally linear relationship with data centred around the 1:1 line. However, for all soils in France and Tunisia, the Trime factory calibration was not suitable and calibration for different soil horizons was necessary. In the USA, devices were compared in soil columns. The EnviroSCAN and Diviner overestimated water content by 0.03 m³ m^-3. Conventional TDR measurements were within 0.01 m³ m^-3 of values determined by mass balance.

Request Information                              Back to Reference List

Evett, S.R., Howell, T.A., et al. 1993, Evapotranspiration by Soil Water Balance Using TDR and Neutron Scattering, National Conference on Irrigation and Drainage Systems, Park City, Utah, USA. 21-23 July

The soil water balance method of estimating evapotranspiration (ET) has been widely used since it is considerably cheaper than alternative methods such as the use of weighing lysimeters. Neutron scattering (NS) is commonly used to measure soil water content. However, for water balance studies, the method has been criticized as imprecise due to difficulties of measurement near the soil surface. Precision can be improved with destructive soil sampling near the surface but this is incompatible with many cropping and experimental systems. We examined time domain reflectometry (TDR) for measuring near surface soil water contents, combined with NS measurements at deeper depths to achieve a non-destructive estimate of ET. TDR probes were installed at depths of 2, 4, 6, 10, 15, 20 and 30 cm at two locations in a large weighing lysimeter and measured every half hour. Neutron scattering measurements were made at two access tube sites on the lysimeter at depth increments of 20 cm from 10 to 190 cm. For a 16 day period, daily change in soil profile water storage in the top 40 cm of soil, as measured by TDR, averaged 88% of total change in storage measured by the lysimeter. Estimates of ET from TDR based change in storage and precipitation data were inaccurate on many days due to water fluxes through the bottom of the layer measured by TDR. However, the soil water storage, computed by the combined TDR measurements for the surface to 40-cm layer and NS measurements below 40 cm, was within 0.7 mm of that measured by lysimeter whereas change in storage based only on NS was in error by 3.6 mm. A combination of daily NS measurements at depth with TDR measurements near the surface holds potential for accurate daily ET estimation.

Request Information                              Back to Reference List

Evett, S.R., Tolk, J.A., et al. 2003, 'A Depth Control Stand for Improved Accuracy with the Neutron Probe', Vadose Zone Journal, vol. 2, pp. 642-649.

The neutron thermalization method for soil water content measurement is well established as being accurate for deep soil profile measurements. However, the method has been criticized as inaccurate for shallow measurements (<30 cm). It is in this shallow zone that many plants have the largest root density and water uptake and where infiltration and evaporation typically cause the largest changes in water content. We show how neutron probe depth influences soil water readings in the top 30 cm of soil, and we describe a depth control stand that serves to control probe depth relative to the soil surface so that probes may be accurately calibrated and successfully used in the field for measurements at shallow depths. Using the stand, calibrations for the 10-cm depth may be obtained routinely with linear regression r² values >0.98 and RMSE values of calibration <0.01 m³ m^-3. The stand is also useful for elevating the gauge high enough above the surface so that standard counts are not influenced by the water content or nature of the surface, thus enhancing accuracy of both the calibration and subsequent water content readings, both of which depend on standard count values. Also, the stand serves to prevent repetitive strain injuries to backs and knees caused by bending and kneeling to place the gauge on top of access tubes, but without additional occupational exposure to radiation.

Request Information                              Back to Reference List

Li, J., Smith, D.W., et al. 2003, 'The Effect of a Gap between the Access Tube and the Soil during Neutron Probe Measurements', Australian Journal of Soil Research, vol. 41, pp. 151-164.

The neutron probe is a tool employed for the measurement of water content in a soil mass. The presence of a gap between the soil and the neutron probe access tube, filled with either air or water, inevitably introduces a systematic error in neutron probe readings. In this study, experimental investigations and numerical analyses were carried out to evaluate the effects of this gap on neutron probe calibration. The numerical model was developed based on the multigroup neutron diffusion equations and the finite element method. The experiments were conducted in a heavy clay soil. The results show that an air gap of 2.5-30 mm between the soil and a 50-mm-diameter aluminium tube could lead to an underestimation of soil water content by 5-45%, but significant underestimation was apparent for air gaps <10 mm. It is also found that the neutron count is significantly overestimated if the gap around the access tube is filled with water rather than air, but this effect is most significant for larger gaps. The results of this research clearly indicate that a gap between the neutron probe access tube and the soil profile should be avoided during field installation, and that if a gap between the access tube and soil develops during service, a systematic error will be introduced into measurements.

Request Information                              Back to Reference List

Li, J., Smith, D.W., et al. 2003, 'Numerical Analysis of Neutron Moisture Probe Measurements', International Journal of Geomechanics, vol. 3, no. 01/02, pp. 11-20.

The neutron probe has proven to be an effective means for monitoring long term in situ soil moisture variations. However, it is difficult to experimentally correlate neutron probe data (i.e., neutron counts) with accurate estimates of absolute soil moisture content, particularly for unsaturated clay soils. In this paper, a numerical model based on multigroup neutron diffusion theory is employed to predict the distribution of neutron flux in a neutron probe system. The model discretizes the neutron energy spectrum into seven intervals, with energy-dependent diffusion coefficients and parameters for each energy interval. The finite element method is employed to solve the coupled seven-group neutron diffusion equations. It is demonstrated that the numerical results compare very well with both laboratory experimental results and field measurements. The theoretical approach to neutron probe calibration described herein offers significant time and cost savings over traditional calibration methods, and potentially opens up new applications for neutron probe monitoring.

Request Information                              Back to Reference List

McKenzie, D.C. and Hucker, K.W. 1990, 'Field Calibration of a Neutron-gamma Probe in Three Agriculturally Important Soils of the Lower Macquarie Valley', Australian Journal of Experimental Agriculture, vol. 30, pp. 115-122.

A neutron-gamma probe was field calibrated in a grey clay, a grey-brown clay and a red-brown earth near Trangie, New South Wales. These are the main soils used for irrigated agriculture in this region. Accurate but simple calibrations and sampling procedures are required by commercial irrigators and researchers.

Request Information                              Back to Reference List

Mwale, S.S., Azam-Ali, S.N., et al. 2005, 'Can the PR1 Capacitance Probe Replace the Neutron Probe for Routine Soil-water Measurement?', Soil Use and Management, vol. 21, no. 3, pp. 340-347.

A study was conducted to compare the perfomance of the PR1 capacitance probe (Delta-T Devices) in measuring soil volumetric moisture content (Θ) with two established standard methods: the neutron probe (Didcot Instruments) and the gravimetric method. Over a two-year period, the three methods were used to measure Θ in glasshouse plots containing crops of either oilseed rape (Brassica napus L.) or bambara groundnut (Vigna subterranea L.). The PR1 probe gave variable performance depending on the depth and soil profile where the measurements were made. In most cases, the PR1 probe significantly overestimated Θ. In some cases, it significantly underestimated Θ or gave similar values to the other methods. Despite this frequent overestimation of Θ, the PR1 probe adequately registered the relative changes in Θ. Its measurements were linearly and positively related to those of the gravimetric and neutron-probe methods. However, the regression lines significantly deviated from the 1:1 line. This paper discusses the possible reasons for the unsatisfactory performance of the PR1 probe and the practical problems associated with the installation of its access tubes.

Request Information                              Back to Reference List

Snyder, K.A., Donovan, L.A., et al. 2004, 'Extensive Summer Water Pulses do not necessarily Lead to Canopy Growth of Great Basin and Northern Mojave Desert Shrubs.', Oecologia, vol. 141, pp. 325-334.

Plant species and functionally related species groups from arid and semi-arid habitats vary in their capacity to take up summer precipitation, acquire nitrogen quickly after summer precipitation and subsequently respond with ecophysiological changes (e. g. water and nitrogen relations, gas exchange). For species that respond ecophysiologically, the use of summer precipitation is generally assumed to affect long-term plant growth and thus alter competitive interactions that structire plant communmities and determine potential responses to climate change. We assessed ecophysiological and growth responses to large short-term irrigation pulses over one to three growing seasons for several widespread Great Basin and northern Mojave Desert shrub species: Chrysothamnus nauseosus, Sarcobatus vermiculatus, Atriplex confertifolia and A. parryi. We compared control and watered plants in nine case studies that encompassed adults of all four species, juveniles for three of the species, and two sites for two of the species. In every comparison, plants used summer water pulses to improve plant water status or increase rates of functioning as indicated by other ecophysiological characters. Species and life history stage responses of ecophysiological parameters (leaf N, δ¹⁵N, δ¹³C, gas exchange, sap flow) were consistent with several previous short-term studies. However, use of summer water pulses did not affect canopy growth in eight out of nine comparisons, despite the range of species, growth stages, and site conditions. Summer water pulses affected canopy growth only for C. nauseosus adults. The general lack of growth effects for these species might be due to close proximity of groundwater at these sites, colimitation by nutrients, or inability to respond due to phenological canalization. An understanding of the connections between short-term ecophysiological responses and growth, for different habitats and species, is critical for determining the significance of summer precipitation for desert community dynamics.

Request Information                              Back to Reference List

Tennakoon, S.B. and Milroy, S.P. 2003, 'Crop Water Use and Water Use Efficiency on Irrigated Cotton Farms in Australia', Agricultural Water Management, vol. 61, pp. 179-194.

Increasing public debate regarding the allocation of water resources within Australia has introduced a new level of significance to the issue of water use efficiency (WUE). Irrigated crops have been criticised for inefficient water use, but there is little information available to quantify performance. Production and water use data were obtained from 25 cotton farms and over 200 individual fields representing the six largest production areas, which produce over 80% of the annual crop. A daily water balance was calculated for each crop to estimate evapotranspiration (ET). Irrigation efficiency (IE) was calculated as the proportion of irrigation water input to the farm for cotton production that was used by the crop as ET over the growing season. Crop water use efficiency (CWUE) was calculated as lint production per unit of ET. The average on-farm total water input was 12 Ml/ha including 7 Ml/ha of irrigation water. The estimated mean seasonal ET was 735 mm. The observed average CWUE was 2.5 kg/mm/ha and the average farm level IE was 57%, but both efficiency measures showed large variability. These values compare well with published data on cotton industries in other countries. The figures can be used as a basis for future water management activities in the cotton industry. The wide variation in both CWUE and IE indicates that there is significant potential for some producers to increase their efficiency under the current reduction in water allocation.

Request Information                              Back to Reference List

Ulery, A., Stewart, S., et al. 2000, 'Vacuum Method for Field Installation of Pipes and Casings in Sandy Soils', Soil Science, vol. 165, no. 3, pp. 269-273.

Soil moisture-monitoring equipment is difficult to install in poorly consolidated sand or sediments using hand tools because the loose material tends to collapse. The technique described herein uses a 5.5-hp wet/dry vacuum cleaner, powered by a portable gasoline generator, to remove the soil while an operator pushes a conductor pipe or casing into the profile. After initiating the hole using a hand bucket auger, an open-ended metal pipe or polyvinyl chloride (PVC) casing is inserted vertically into the shallow hole. A smaller tube, or stinger, attached to a wet/dry vacuum is inserted into the pipe to extract loose material while downward pressure is applied on the pipe. Once the casing is installed, instrumentation such as lysimeters, gypsum blocks, or tensiometers can be placed at the desired depth and backfilled with native soil. The casing is then raised and the soil allowed to collapse around the equipment, or the pipe can be left in place for neutron probe access. Measurements of soil water content after an infiltration experiment demonstrated uniform downward movement with minimal preferential flow or soil disturbance as a result of the vacuum installation of gypsum blocks and a neutron access tube.

Request Information                              Back to Reference List