Neutron Access Tubes Research Articles

A lysimeters study of Chinese wheat and maize varieties: I. The lysimeters-rain shelter facility and the growth and water use of wheat

Excessive application of N fertilizer contributes substantially to high levels of nitrate (NO3−–N) in surface and groundwater on Northern China. A trial was set up to quantify the fate of N within intensive wheat maize rotation system with a view to improve N and water use efficiencies. This paper describes the construction and testing of a lysimeter facility used for this trial. A 44 lysimeter/rain shelter facility was constructed at Shandong Agricultural University (SDAU). Each lysimeter was equipped with a neutron access tube for soil water monitoring and ceramic solution samplers for soil solutions collection. In order to precisely quantify water input, two rain shelters were used to exclude rainwater. The water balance showed that water outputs and inputs agree within 10% for all lysimeters, and that the average water used being 5% less than the total irrigation water supplied was considered as an acceptable error for such large lysimeters. Wheat grown in these devices was consistently higher than those grown with similar fertilizer management in a field located in Ling Xian due to enhanced soil fertility and irrigation in the devices. The facility was shown to be suitable for investigating water and nutrient balances of the root zone.

Comparison of Methods for Estimating Evapotranspiration in a Small Rangeland Catchment

Evapotranspiration (ET) was quantified for two rangeland vegetation types, aspen and sagebrush‐grassland, over an 8‐yr study period by comparing the following approaches for estimating ET: eddy covariance systems (EC, available for only 6 yr); soil water storage loss measured by time domain reflectometry (TDR) and neutron probe; and model simulation. The research site, the Upper Sheep Creek catchment in the Reynolds Creek Experimental Watershed, is part of a study on the effects of prescribed fire and vegetation removal. Estimates of seasonal ET for the aspen using EC with the turbulent fluxes adjusted to force energy balance closure, a 180‐cm TDR soil profile, two 225‐cm neutron access tubes, and the Simultaneous Heat and Water (SHAW) model agreed well with each other. If the two neutron probes were averaged, the RMSD of all approaches over the 6 yr was within 8% of the average. For the sagebrush‐grassland, a 120‐cm TDR profile underestimated seasonal ET for all years except the year immediately following the prescribed fire, when rooting depth likely had not recovered. A 195‐cm neutron probe access tube located within 30 m of the stream underestimated ET for every year except when there was no streamflow, suggesting that lateral flow may have biased the results for this tube. A comparison of the other methods (EC flux adjusted to force energy balance closure, soil water loss measured from a 225‐cm neutron access tube, and SHAW model simulation) agreed within 3% during the 6 yr with EC measurements at that site.

Spatial characterization of scaled hydraulic conductivity functions in the internal drainage process leading to tropical semiarid soil management

Knowledge of water movement is critical to agricultural water use. An internal drainage experiment for assessing downward movement of soil water under the influence of gravity was conducted on a semiarid sandy soil at ICRISAT-Sadore in Niger. The objectives were to estimate unsaturated hydraulic conductivity functions K(θ) at multiple scales using soil water (θ) data determined over the drainage process, and to determine spatial characteristics of K(θ) functions for water management in the semiarid environment. The study site was a naturally rolling field and the soil was a sandy Labucheri soil series. The experimental design consisted of a nested grid with three scales, 1 × 1-m, 5 × 5-m and 20 × 20-m, using 182 neutron access tubes. Soil water movement in space was examined in the 0.15–1.50 m depth throughout the drainage course. Mean soil water downward movement varied between 0.029–1379 mm h−1, depending on water content and soil depth. Elevation features contributed to 16–40% of the variation in hydraulic conductivity functions (K(θ) = e·θf). The best fit for K(θ)-model coefficients was a normal distribution and K(θ) functions were correlated at the three scales (0.81 ≤ R2 ≤ 0.96, P < 0.01). The scaled K(θ) functions were auto-correlated in space within 25–60 m. Management zones based on geostatistical semivariograms and interpolated map patterns were practical for water management planning in the naturally rolling environments.

Saturated‐unsaturated flow to a well with storage in a compressible unconfined aquifer

Mishra and Neuman (2010) developed an analytical solution for flow to a partially penetrating well of zero radius in a compressible unconfined aquifer that allows inferring its saturated and unsaturated hydraulic properties from responses recorded in the saturated and/or unsaturated zones. Their solution accounts for horizontal as well as vertical flows in each zone. It represents unsaturated zone constitutive properties in a manner that is at once mathematically tractable and sufficiently flexible to provide much improved fits to standard constitutive models. In this paper we extend the solution of Mishra and Neuman [2010] to the case of a finite diameter pumping well with storage; investigate the effects of storage in the pumping well and delayed piezometer response on drawdowns in the saturated and unsaturated zones as functions of position and time; validate our solution against numerical simulations of drawdown in a synthetic aquifer having unsaturated properties described by the van Genuchten [1980]–Mualem [1976] model; use our solution to analyze 11 transducer‐measured drawdown records from a seven‐day pumping test conducted by University of Waterloo researchers at the Canadian Forces Base Borden in Ontario, Canada; validate our parameter estimates against manually‐measured drawdown records in 14 other piezometers at Borden; and compare (a) our estimates of aquifer parameters with those obtained on the basis of all these records by Moench [2008], (b) on the basis of 11 transducer‐measured drawdown records by Endres et al. [2007], (c) our estimates of van Genuchten–Mualem parameters with those obtained on the basis of laboratory drainage data from the site by Akindunni and Gillham [1992], and (d) our corresponding prediction of how effective saturation varies with elevation above the initial water table under static conditions with a profile based on water contents measured in a neutron access tube at a radial distance of about 5 m from the center of the pumping well. We also use our solution to analyze 11 transducer‐measured drawdown records from a 7 day pumping test conducted by University of Waterloo researchers at the Canadian Forces Base Borden in Ontario, Canada. We validate our parameter estimates against manually measured drawdown records in 14 other piezometers at Borden. We compare our estimates of aquifer parameters with those obtained on the basis of all these records by Moench (2008) and on the basis of 11 transducer‐measured drawdown records by Endres et al. (2007), and we compare our estimates of van Genuchten–Mualem parameters with those obtained on the basis of laboratory drainage data from the site by Akindunni and Gillham (1992); finally, we compare our corresponding prediction of how effective saturation varies with elevation above the initial water table under static conditions with a profile based on water contents measured in a neutron access tube at a radial distance of about 5 m from the center of the pumping well.

Time Domain Reflectometry Range of Accuracy for High Surface Area Soils

Time domain reflectometry (TDR) is commonly used to determine water content. Many laboratory studies have shown accurate water determination across a wide range of soils, often with a unified calibration equation. Recent data has shown a strong temperature influence on some TDR data. The purpose of this study was to calibrate TDR for water content under both laboratory and field conditions for Mollisols. TDR waveguides and thermocouples were installed horizontally into the sides of pits at four sites and 9 or 10 depth positions on a 5% slope. Neutron access tubes were installed within 3.3 m of the TDR sites. After the field study, soil was collected around the waveguides and repacked into columns for laboratory calibration. The columns were progressively wetted and then dried over a few months. At each water content and at two or three temperatures, the waveforms, bulk electrical conductivity, and square root of apparent dielectric εa1/2 were saved for the laboratory data, but memory limitations prevented saving waveforms from the field data. Much of the field calibration data were shifted to higher values for εa1/2 than for the laboratory data, apparently due to problems with internal waveform analysis. High bulk electrical conductivities (up to 0.13 S m−1) were apparent for some of the sites and depths even though these are not saline soils. For the laboratory data, adding a temperature term to the calibration equation reduced the root mean square error (378 data points) from 0.54 to 0.34 m3 m−3, and for the field data (939 points) reduced it from 0.78 to 0.54 m3 m−3

Monitoring soil moisture status in a Black Vertosol on the Liverpool Plains, NSW, using a combination of neutron scattering and electrical image methods

Electrical image data were used to estimate soil moisture content beneath smectite-dominated black Vertosol in the Liverpool Plains catchment in northern New South Wales. Measurements carried out over a range of soil moisture conditions included: water use by lucerne sown into a full profile, recharge of profiles after lucerne and long fallow after a cereal crop. Measurements were repeated approximately every 2 months between September 2000 and August 2001. Estimates of bulk electrical conductivity (EC), determined from inversion of apparent conductivity electrical images, were compared to soil moisture measurements made using neutron access tubes. A correlation of 0.77 (n = 228, P &lt; 0.0001) between the 2 datasets indicates that electrical imaging can be used to satisfactorily predict bulk moisture content in soils of this type. Interpretation of the bulk EC images indicates drying of the soil profile to 6 m depth beneath lucerne.

Interpreting INEEL Vadose Zone Water Movement on the Basis of Large‐Scale Field Tests and Long‐Term Vadose Zone Monitoring Results

A review of the results from two large‐scale field tests and vadose zone monitoring data at two facilities at the Idaho National Engineering and Environmental Laboratory (INEEL) has provided information to support development of a conceptual model of water movement in the highly heterogeneous vadose zone beneath two INEEL facilities. Both of the large‐scale field tests were ponded water infiltration tests that approached the size of the two facilities. An assembly of perched water wells, neutron access tubes, and tensiometers have been used to monitor water movement beneath these sites. Once water has percolated through the surficial sediment at these facilities, the presence of the surficial sediment–basalt interface results in some perching of the percolated water with limited lateral movement. Water movement in basalt formations is mainly gravity‐dominated in preferential fracture pathways and rubble zones and results in rapid vertical flow especially frequently under conditions of positive hydrostatic head. Detection of wetting front advance is typically not sequential with depth and has been shown to vary between episodic infiltration events. At the basalt–sediment interface of the interbeds, perched water is often detected and has resulted in more lateral spreading of recharge than at the surficial sediment–basalt interface. Gaps in the sedimentary interbeds will allow some of the recharge water to bypass the interbeds and rapidly continue toward the aquifer. This complex water movement in the vadose zone beneath the INEEL supports the need to control surface infiltration, identify gaps and the lateral continuity of the interbeds, and investigate the influence of nearby surface water sources on perched water movement beneath these two facilities.

Interpreting INEEL Vadose Zone Water Movement on the Basis of Large-Scale Field Tests and Long-Term Vadose Zone Monitoring Results

A review of the results from two large-scale field tests and vadose zone monitoring data at two facilities at the Idaho National Engineering and Environmental Laboratory (INEEL) has provided information to support development of a conceptual model of water movement in the highly heterogeneous vadose zone beneath two INEEL facilities. Both of the large-scale field tests were ponded water infiltration tests that approached the size of the two facilities. An assembly of perched water wells, neutron access tubes, and tensiometers have been used to monitor water movement beneath these sites. Once water has percolated through the surficial sediment at these facilities, the presence of the surficial sediment–basalt interface results in some perching of the percolated water with limited lateral movement. Water movement in basalt formations is mainly gravity-dominated in preferential fracture pathways and rubble zones and results in rapid vertical flow especially frequently under conditions of positive hydrostatic head. Detection of wetting front advance is typically not sequential with depth and has been shown to vary between episodic infiltration events. At the basalt–sediment interface of the interbeds, perched water is often detected and has resulted in more lateral spreading of recharge than at the surficial sediment–basalt interface. Gaps in the sedimentary interbeds will allow some of the recharge water to bypass the interbeds and rapidly continue toward the aquifer. This complex water movement in the vadose zone beneath the INEEL supports the need to control surface infiltration, identify gaps and the lateral continuity of the interbeds, and investigate the influence of nearby surface water sources on perched water movement beneath these two facilities.

The effect of improved pastures and grazing management on soil water storage on a basaltic plains site in south-west Victoria

Soil salinity of non-irrigated farmlands in Australia has been largely attributed to tree clearing and their replacement by annual pasture and crop species. This paper deals with the effects of sowing perennial ryegrass and greater inputs of fertiliser, and the effect of grazing management, on water use and the potential to improve recharge control on a gravelly soil derived from basalt.In 1991, neutron access tubes were inserted into plots on a project established in 1989 to examine the impact of upgrading the pasture on sheep productivity. These plots were subdivided in 1996 to examine the impact of grazing management (tactical v. set-stocking) and pasture type (pastures dominated by annual species v. upgraded pastures) on productivity. Neutron probe readings were taken periodically from tubes in each plot, at depth intervals of 25 cm (December 1991–March 1995) or 20 cm (August 1995–April 1999) to 170 cm. There was no effect of treatment on soil moisture. Data for 2 wet years (1995 and 1996) indicate that the effective soil-water storage capacity to 170 cm depth for these pastures was a mean of 125 mm of water. This represents the potential buffer before winter rainfall exceeds the water use by the pasture, fills the soil profile to capacity and then either runs off or allows deep drainage to occur.We did not achieve a significant reduction in soil-water storage, and therefore potential recharge of groundwater, by re-sowing the pasture with perennial ryegrass and applying more fertiliser, or by altering the grazing management to a form of rotational grazing. Compared with set-stocked annual pasture, the impact of such treatments was to reduce soil-water storage to a depth of 170 cm in autumn by less than 20 mm/year. There was no association between total herbage production and soil-water storage, however an increased percentage of perennial ryegrass in the pasture was associated with a small reduction in soil-water storage in 1 year. Greater use of soil-water may depend upon using deeper-rooted perennials or maintaining a higher proportion of perennial species in the sward (the perennial ryegrass in the re-sown pastures declined from 53% in October 1996 to 4% in October 1998).

Interpreting INEEL Vadose Zone Water Movement on the Basis of Large-Scale Field Tests and Long-Term Vadose Zone Monitoring Results

A review of the results from two large-scale field tests and vadose zone monitoring data at two facilities at the Idaho National Engineering and Environmental Laboratory (INEEL) has provided information to support development of a conceptual model of water movement in the highly heterogeneous vadose zone beneath two INEEL facilities. Both of the large-scale field tests were ponded water infiltration tests that approached the size of the two facilities. An assembly of perched water wells, neutron access tubes, and tensiometers have been used to monitor water movement beneath these sites. Once water has percolated through the surficial sediment at these facilities, the presence of the surficial sediment–basalt interface results in some perching of the percolated water with limited lateral movement. Water movement in basalt formations is mainly gravity-dominated in preferential fracture pathways and rubble zones and results in rapid vertical flow especially frequently under conditions of positive hydrostatic head. Detection of wetting front advance is typically not sequential with depth and has been shown to vary between episodic infiltration events. At the basalt–sediment interface of the interbeds, perched water is often detected and has resulted in more lateral spreading of recharge than at the surficial sediment–basalt interface. Gaps in the sedimentary interbeds will allow some of the recharge water to bypass the interbeds and rapidly continue toward the aquifer. This complex water movement in the vadose zone beneath the INEEL supports the need to control surface infiltration, identify gaps and the lateral continuity of the interbeds, and investigate the influence of nearby surface water sources on perched water movement beneath these two facilities.

Comparative water-use efficiency of Sporobolus arabicus and Leptochloa fusca and its relation with carbon-isotope discrimination under semi-arid conditions

Water-use efficiency (WUE) of Leptochloa fusca (L.) Kunth (Kallar grass) and Sporobolus arabicus Boiss. was determined under different soil moisture regimes. Plants grown in lysimeters were subjected to three soil moisture regimes, viz. well-watered (100%), medium-watered (75%), and low-watered (50%) of total available water (TAW). The soil moisture was restored on alternate days by adding the required volume of water on the basis of neutron moisture meter readings taken from neutron access tubes installed in each lysimeter. The grasses were harvested after suitable intervals (∼4 months) to obtain maximum biomass. Leaf samples collected at each harvest were analyzed for carbon-isotope discrimination (δ13C) with an isotope ratio (13C/12C) mass spectrometer. Results indicated significant differences in WUE of both grasses subjected to different water regimes. Sporobolus arabicus showed higher WUE than Kallar grass. However, Kallar grass showed better value of yield response factor (ky = 0.649) compared with Sporobolus (ky = 1.06) over the entire season. The data confirm that these grasses can be grown successfully in water-limited environments by selecting an optimum soil moisture level for maximum biomass production. The mean carbon-isotope discrimination (δ13C) of Kallar grass (−14.4‰) and Sporobolus (−12.8‰) confirm that both are C4 plants. The carbon-isotope discrimination (Δ) was significantly and negatively correlated with WUE of the two species studied. The results of the present study confirm that δ13C or Δ of leaves can be used as good predictor of WUE in some C4 plants.

Cotton lint yield variability in a heterogeneous soil at a landscape scale

Landscape variability associated with topographic features affects the spatial pattern of soil water and N redistribution, and thus N uptake and crop yield. A landscape-scale study was conducted in a center pivot irrigated field on the southern High Plains of Texas in 1999 to assess soil water, soil NO 3-N, cotton ( Gossypium hirsutum L.) lint yield, and N uptake variability in the landscape, and to determine the spatial correlation between these landscape variables using a state-space approach. The treatments were irrigation at 50 and 75% cotton potential evapotranspiration (ET). Neutron access tubes were placed at a 15-m interval along a 710 m (50% ET) and 820 m (75% ET) transect across the field. Soil NO 3-N in early spring was autocorrelated at a distance varying between 60 and 80 m. Measured soil volumetric water content (WC), total N uptake, and lint yield were generally higher on lower landscape positions. Cotton lint yield was significantly correlated to soil WC ( r=0.76), soil NO 3-N ( r=0.35), and site elevation ( r=−0.54). Differences of site elevation between local neighboring points explained the soil water, NO 3-N and lint yield variability at the micro-scale level in the landscape. Soil WC, cotton lint yield, N uptake, and clay content were crosscorrelated with site elevation across a lag distance of ±30–40 m. The state-space analysis showed that cotton lint yield was positively weighted on soil WC availability and negatively weighted on site elevation. Cotton lint yield state-space models give insights on the association of soil physical and chemical properties, lint yield, and landscape processes, and have the potential to improve water and N management at the landscape-scale.

VACUUM METHOD FOR FIELD INSTALLATION OF PIPES AND CASINGS IN SANDY SOILS

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.

Volatilisation and biodegradation during air sparging of dissolved BTEX-contaminated groundwater

The relative contributions of volatilisation and biodegradation are quantified for a field trial of air sparging for the remediation of groundwater contaminated with dissolved petroleum hydrocarbons. Groundwater in the unconfined sand aquifer at Kwinana in Western Australia was grossly contaminated by benzene, toluene, ethylbenzene, xylene (BTEX) and other dissolved organics from spills of gasoline. Multi-depth sampling bores, in situ oxygen sensors and neutron access tubes were used to determine changes in groundwater chemistry, oxygen utilisation and the fate of injected air in the aquifer. Oxygen utilisation was used to infer rates of biodegradation. A vadose zone soil vapour extraction system was used to quantify the volatile organic compounds (VOCs) that partitioned from the aqueous phase into the gas phase. Volatilisation was found to be the dominant mechanism for the removal of dissolved VOCs. This was indicated by the close correspondence between calculated masses and the timing of losses. The rate of removal was very rapid, with most organics removed within 3 days of the start of sparging. The rate of loss was also observed to follow the Henry's Law constant for the particular compounds. Estimating biodegradation of dissolved petroleum hydrocarbons was complicated by other sinks for dissolved O 2, the presence of residual entrapped air in the aquifer and bulk movement of groundwater. However, biodegradation rates were at least an order of magnitude less than volatilisation rates over the period of greatest losses. It was also notable that dissolved VOCs were reduced over a larger volume of the aquifer than directly contacted by injected air. This may have been due to groundwater movement enhanced by stopping and starting sparging during the trial.

Application efficiency of micro-sprinkler irrigation of almond trees

Micro-sprinklers are becoming a preferred irrigation method for water application in orchards. However, there is relatively little data available to support a particular irrigation scheduling method. The objective of this study is to quantify the components of the water balance of an almond tree under micro-sprinkler irrigation. For that purpose, an experimental plot around an almond tree with an area of 2.0 m X 2.0 m without vegetation, representing about one quarter of the wetted area of the micro-sprinkler was instrumented with neutron access tubes, tensiometers and catch cans. Twenty-five access tubes with catch cans were distributed in a square grid of 0.5 m × 0.5 m, to a depth of 0.9 m. Eight pairs of tensiometers were installed at depths of 0.825 and 0.975 m within the experimental plot. During a 7-day period in August, 1995 the plot was sprinkler-irrigated on three days, and water application rates and uniformity coefficients were calculated for each irrigation event. Neutron probe readings at 15 cm depth increments and tensiometer readings were taken 4 to 6 times daily. Results showed large evaporation losses during and immediately after the irrigations. Evaporation losses of the wetted area was estimated to be between 2 and 4 mm/irrigation event. Consequently, application efficiencies were only 73–79%, the wetting of the root zone was limited to the 0–30 cm depth interval only, the soil profile was depleted of soil water, and daily crop coefficient values at days between irrigation events were between 0.6 and 0.8. The study recommends irrigation in the evening and night hours, thereby largely eliminating the evaporation losses that occur during daytime irrigation hours.

Noninvasive Soil Water Content Measurement Using Electromagnetic Induction

The feasibility of soil water content measurement using electromagnetic induction was investigated in an arid region of southern New Mexico. Soil water measurements were taken monthly with a neutron probe at 65 equally spaced stations along a 1950‐m transect. At the same time, noninvasive electrical conductivity measurements of the soil were taken with a Geonics EM‐31 ground conductivity meter. Using 16 months of measurements, we found a linear relationship exists between bulk soil electrical conductivity and total soil water content in the top 1.5 m of the profile. A simple linear regression model was developed to describe the relationship between soil water content and bulk soil electrical conductivity. The spatial and temporal accuracy of the regression model is addressed as well as the total number of neutron access tubes needed to accurately calibrate the model. By comparison with the neutron scattering method the electromagnetic induction method is quite accurate for the prediction of water content changes over time. The speed and ease of use combined with the accuracy of the measurements make the ground conductivity meter a valuable tool for rapid, noninvasive soil water measurements.

Field-Scale Neutron Probe Calibration and Variance Analysis for Clay Soil

Although widely used for field measurements of soil moisture, problems continue with the use of the neutron probe in clay soils comprised of over 40% clay fractions, and in the determination of the intensity of field sampling necessary to characterize a site. We examine issues associated with field practice and calibration, data analysis and estimation of variance, and error in soil moisture measurements for silty clay soils in the Imperial Valley, California, using large calibration data sets from several different layouts of neutron access tubes. Our analysis indicates that for field calibrations: (1) bulk density corrections of the count ratio data do not necessarily improve the standard errors of the water content measurements; (2) soil sampling for calibration within the access tube hole at the time of tube installation results in smaller measurement errors; (3) calibrations improve as the average soil moisture of the profile at the time of the original sampling decreases; and (4) field-wide calibrat...

Subsidence and rock creep in a cross-valley fill

Cross-valley fills composed cf waste rock from mining operations pose environmental concerns related to their long-term stability. A large (2.06×107 m3) phosphate waste dump in southeastern Idaho was monitored from 1981 to 1988 for subsidence and horizontal displacement in conjunction with different surficial rock types and long-term precipitation patterns. Most of the 19 surveyed benchmarks on the dump face moved in the northeast direction during the 7-yr study period, roughly the same orientation as the aspect of the fill face. Average vertical subsidence rate decreased exponentially with time from a high of 0.15 m/yr in 1981–1982 to a fairly stable level of 0.008 m/yr by 1988. Net displacement similarly decreased from 0.19 m/yr shortly after dump construction to 0.023 m/yr by 1988. Both subsidence and net displacement were initially higher in the shale cap on the west side of the chert-filled dump; however, by 1986–1988 subsidence and displacement rates were actually lower in the shale material. Subsidence and net displacement appeared unaffected by temporal precipitation patterns; both subsidence and net displacement rates declined significantly during the very wet snowmelt seasons of 1983 and 1984. The temporal patterns of movement at the dump site suggest that the breakdown of the rock (especially shale) occurs primarily during the first 1.5 yr following dump construction. Periodic saturation of the upper 6 m of spoil occurred in nine of 60 neutron access tubes, normally during the spring snowmelt seasons. Saturation occurred at depths ≥1.8 m and is thought to be associated with uneven compaction or heterogeneities in the fill. Overall, the dump appeared to be internally well drained with rates of rock creep unaffected by seasonal water inputs.

Design and Evaluation of a Neutron Access Tube Extractor

AbstractThe labor and material costs associated with extracting neutron access tubes used for neutron probe soil water determinations can be substantial. Present commercially available access tube extractors are clumsy to use, and can ruin tubes that are being extracted from high clay content soils. To remedy these problems, a new access tube extractor was designed. The extractor slips over the tube to be removed and is secured in place by a threaded collar that provides the initial pressure to hold it in place until the extraction force is applied. As the extractor pulls on the access tube during removal, conically‐tapered jaws exert additional clamping pressure on the tube in greater proportion than the pulling force required to remove the tube. The device easily extracts access tubes against a resistance of up to 13.7 kN (3086 lb.) with no damage to the tubes. Either a mechanical jack or a hydraulic lift on a tractor may be used to extract access tubes.

Estimating the Number of Soil-Water Measurement Stations Required for Irrigation Decisions

Measurements of soil-water are an important and frequently frustrating consideration in many soil research and irrigation planning endeavors. Numbers of sensors required for irrigation scheduling with 95% confidence were investigated in five studies. Three methodsneutron probe, tensiometers and resistance blocks (Delmhorst and Larson)were evaluated. Two to four measurement stations, each consisting of sensors placed at several depths, were located in five field sites. Average residual error mean square values, obtained from an analysis of variance of repeated measures designs at each site for each technology formed the basis for estimating the required number of sensors needed to confidently make irrigation scheduling decisions under the conditions of each study. The results suggested that more than the customary four to six sensors per depth per field are needed with resistance blocks and tensiometers. Four to six neutron access tubes would have sufficed in uniform soil conditions under conditions of our studies. In obviously nonuniform soils, it is proposed that sensor numbers per station be increased. Stations could be located based on site specific soils and irrigation system considerations. The test rationale proposed herein can be used for evaluating new soil-water measurement technologies as they become available.