Neutron Probe Access Tubes Research Articles

Using infrared thermometry to improve irrigation scheduling on variable soils

An experiment was conducted to assess a modified Priestley and Taylor calculation (developed in a companion paper) that utilises surface temperature (Ts) to estimate evapotranspiration (ETMPT) and to assess the benefits of variable rate irrigation (VRI) based on ETMPT measurements. A ryegrass seed field with Low, Mid and High available water capacity soil zones was used. Four replicates of two irrigation treatments were established in each zone; uniform rate irrigation (URI) where the same amount of irrigation was applied to all three zones based on neutrone probe measurements; and VRI where ETMPT was measured and different amounts of irrigation applied to each zone to replace this. TS was measured continiously in each plot using stationary infrared radiometers, normalised difference vegetation index measured twice weekly with a hand-held instrument and meteorological variables were recorded in the field. Neutron probe access tubes were installed to 1.6m depth in all plots and monitored weekly to provide alternative evapotranspiration estimates (ETNP). The ETMPT decreased from 333mm in the high zone to 282 mm in the low zone. This in combination with faster canopy senescence and lower seed yields showed the low zone encountered water stress even though irrigated weekly. There was good agreement between ETMPT and ETNP in the VRI treatments and the High zone of the URI treatment. The higher values of ETNP in the Low and Med zones of the URI treatment were attributed to drainage. The URI treatment had 254.2 mm of irrigation applied to all zones during the season whereas VRI treatment had 183.4, 210.2 and 220.2 mm applied to the Low, Mid and High Zones, respectively. There, were no differences in ryegrass seed yield between the two irrigaiton treatments and less drainage from the VRI treatments. From this we conclude that VRI based on ETMPT is effective and field application of ETMPT is worth further development.

Transport of Atrazine and Dicamba through Silt and Loam Soils

The objectives of this research were to determine the role of preferential flow paths in the transport of atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine) and dicamba (3-6-dichloro-2-methoxybenzoic acid) through silt and loam soils overlying the High Plains aquifer in Nebraska. In a previous study, 3 of 6 study areas demonstrated high percentages of macropores; those three areas were used in this study for analysis of chemical transport. As a subsequent part of the study, 12 intact soil cores (30-cm diameter by 40-cm height), were excavated sequentially, two from each of the following depths: 0-40cm and 40-80cm. These cores were used to study preferential flow characteristics using dye staining and to determine hydraulic properties. Two undisturbed experimental field plots, each with a 3-m2 surface area, were installed in three study areas in Nebraska. Each was instrumented with suction lysimeters and tensiometers at depths of 10cm to 80cm in 10-cm increments. Additionally, each plot was planted with corn (Zea mays). A neutron probe access tube was installed in each plot to determine soil water content at 15-cm intervals. All plots were enclosed with a raised frame (of 8-cm height) to prevent surface runoff. All suction lysimeters were purged monthly for three months and were sampled immediately prior to pre-plant herbicide application to obtain background chemical concentrations. Atrazine and dicamba moved rapidly through the soil, but only after a heavy rainfall event, probably owing to the presence of preferential flow paths and lack of microbial degradation in these soil areas. Staining of laboratory cores showed a positive correlation between the percent area stained by depth and the subsequent breakthrough of Br- in the laboratory and leaching of field-applied herbicides owing to large rainfall events. Suction lysimeter samples in the field showed increases in concentrations of herbicides at depths where laboratory data indicated greater percentages of what appeared to be preferential flow paths. Concentrations of atrazine and dicamba exceeding 0.30 and 0.05µg m1-1 were observed at depths of 10-30cm and 50-70cm after two months following heavy rainfall events. It appears from the laboratory experiment that preferential flow paths were a significant factor in transport of atrazine and dicamba. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government or GNS.

Dynamic prescription maps for site-specific variable rate irrigation of cotton

A prescription map is a set of instructions that controls a variable rate irrigation (VRI) system. These maps, which may be based on prior yield, soil texture, topography, or soil electrical conductivity data, are often manually applied at the beginning of an irrigation season and remain static. The problem with static prescription maps is that they ignore spatiotemporal changes in crop water status. In a two-year study (2012 and 2013), a plant feedback system, including a wireless sensor network of infrared thermometers (IRTs), was used to develop dynamic prescription maps to accomplish adaptive irrigation scheduling for cotton (Gossypium hirsutum L.). One-half of a center pivot field was divided into manually and plant feedback-controlled irrigation treatment plots. Irrigation treatments were at three levels, 75, 50 and 25 percent of full as defined by either replenishment of crop water use to field capacity or by the equivalent threshold of the IRT sensed crop water stress. The system accepted user input to control irrigation for the manual treatment plots (I75M, I50M, and I25M), and calculated and compared a thermal stress index for each plant feedback-controlled treatment plot (I75C, I50C and I25C) with a pre-determined threshold for automated irrigation scheduling. The effectiveness of the plant feedback irrigation scheduling system was evaluated by comparing measured lint yield, crop water use (ETc), and water use efficiency (WUE) with the manually scheduled treatment plots. Results for both years indicated that average lint yields were similar between the manual and plant feedback-control plots at the I75 level (181 and 182gm−2, respectively, in 2012; 115 and 103gm−2, respectively, in 2013) and I50 level (146 and 164gm−2, respectively, in 2012; 95 and 117gm−2, respectively, in 2013). At the I25 level, average lint yield was significantly greater for the plant feedback-compared with the manual-control treatment plots (142gm−2 and 92gm−2, respectively), but the mean amount of irrigation was twice that of the manual-control plots. Mean water use efficiencies (WUE) within the same irrigation treatment levels were similar between methods. Importantly, the automatic plant feedback system did not require the time consuming and expensive manual reading of neutron probe access tubes that was required to schedule the manual treatments. These results demonstrate that the integration of a plant feedback system with a commercial VRI system could be used to control site-specific irrigation management for cotton at higher irrigation treatment levels, i.e., I75 percent and I50 percent of full. Such a system can facilitate the use of a VRI system by automating prescription map coding and providing dynamic irrigation control instructions to meet variable crop water needs throughout the irrigation season. As of yet, further research is required to maintain automatic deficit irrigation at a level equivalent to 25 percent replenishment of crop water use relative to field capacity.

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.

Effect of Aluminum Neutron Probe Access Tubes on the Apparent Electrical Conductivity Recorded by an Electromagnetic Soil Survey Sensor

Correlating soil moisture content to apparent electrical conductivity (σa), derived from above-ground, electromagnetic induction (EMI) dipole sensors, requires capacitance or neutron probe moisture meters. To this end, plastic or metallic access tubes (ca. 0.5-2 m long, 40-50 mm internal diameter, and 1-2 mm thickness walls) are inserted vertically into the soil to allow the probe to be lowered for moisture readings at a series of soil depths. The impact of these tubes on measurements derived from above-ground EMI sensors, when the sensor is in proximity or adjacent to these buried tubes, in unknown. We report on the impact of widely used aluminum (Al), as well as popular plastic alternatives of polyethylene (PE), polyvinylchloride (PVC) access tubes on the lateral σa profiles of an EM38 EMI meter as it is moved along survey transects that pass beside the access tubes. There was no significant difference observed between the EMI meter readings of the bare soil and the vertical holes created to house the access tubes nor when the plastic access tubes were in place. However, the Al tubes showed a considerable variation in readings once the EM38 meter was within 50 cm of the tube location. A theoretical model, based on a single dipole transmitter and receiver coil, and a thin, cylindrical conducting shell located beneath the earth' s surface confirmed the horizontal eddy currents, traveling around the tube shell to be responsible for the observed deviation in the sensor response when in proximity to the metallic tube.

Apparent electrical conductivity (ECa) as a surrogate for neutron probe counts to measure soil moisture content in heavy clay soils (Vertosols)

Site-specific measurements of the apparent electrical conductivity (ECa) of soil using the EM38 were correlated with near-simultaneous neutron probe readings over periods of moisture extraction by an irrigated cotton crop. Thirty sites were monitored from three ECa zones within a 96-ha field of grey Vertosol soil 30 km west of Moree, New South Wales, Australia. This study differs from previous approaches by reporting the effect on ECa of a wetting front (irrigation) reaching a single ECa measurement point in a field and by using polyethylene neutron probe access tubes so that the EM38 could be operated directly over the same site measured by a neutron probe. We report strong correlations (r = 0.94) between neutron probe counts (CRR) averaged to a depth of 40 or 60 cm and ECa from an EM38 held in the vertical mode 20 cm above the soil surface. All combinations of EM sensor height (0–1.2 m) to neutron probe measurement depth (0.2–1.4 m) returned correlations >0.85. The relationship between CCR and ECa was linear for the purposes of estimating water content over a range of background ECa levels. More critical modelling suggested a slight curve (logarithmic model) fitted best. The range of surface-surveyed ECa from the start of irrigation (refill point) to fully irrigated (full point) was ~27 mS m–1 for this Vertosol, where surface ECa readings typically ranged from 50 to 200 mS m–1. We suggest that the calibration of ECa to CRR might be effected by a two-point measurement of the soil, namely at both upper (field capacity) and lower (wilting point) ECa values, and a site-specific calibration template generated by extending these point measures to whole-field surveys.

Effects of a plastic cover on soil moisture change in a Mediterranean climatic regime

AbstractWater deficit is a serious problem for most agricultural crops, especially in arid–semiarid regions, and limits sustainable development. Production can be improved by reducing evapotranspiration and loss of infiltrated water by the use of a plastic cover. We monitored soil moisture fortnightly over 1 yr using a neutron probe near four trees in an olive grove (Olea europaea, var. Arbequina), two of which had a plastic cover buried in the topsoil around them. These trees were monitored using three neutron probe access tubes per tree to compare the behaviour of soil moisture over time with two other identically instrumented and nearby trees with no cover. Analysis was based on the resultant moisture profiles. The plastic cover retained moisture and increased soil water residence time. During the dry season, the amount of water retained in the plots was at most 15–20% greater in the mid‐season and at least 5–6% greater at the end of season than in the central part of the plots near the trunk. The plastic cover was effective to ca. 50 cm with maximum water content near the soil surface. During the wet season, the cover did not affect soil water. Soil moisture was greater near the tree trunk as a result of stemflow and throughfall.

Physico-chemical assessment of a fixated flue-gas desulfurization sludge cap emplaced along with other coal-combustion residues to abate acid mine drainage

Long term monitoring of the physical and chemical effects of using coal-combustion residues (CCRs), in particular fixated flue gas desulfurization (FGD) sludge, as a major component in the reclamation of a pyritic refuse deposit was undertaken to determine the beneficial and detrimental consequences of placing these controversial materials in an unrestricted environment. Monitoring wells, neutron probe access tubes, and weirs were installed before and after reclamation to observe hydrologic conditions and determine how the use of FGD sludge as a recharge barrier was affecting hydrochemical response to ambient weather conditions. Data were collected for six months prior to reclamation and then for an additional 13years (more intensively during the first 5years). Statistical analyses of water levels in the pyritic refuse deposit indicate a shift from precipitation- to barometric-controlled fluctuations. These findings, along with minimal variability in soil moisture within the CCR cap and transient perching of groundwater above the cap, are evidence that recharge of the refuse aquifer has been minimized. Statistically significant improvements in the quality of groundwater on-site and surface water leaving the site include long-term declines in acidity, As, and Fe concentrations within the refuse aquifer, attributed to a decrease in recharge of oxygenated water as supported by an analysis of calculated mineral saturation indices. Long-term declines in acidity and associated trace metals discharging from the site are attributed to the post-reclamation loss of sulfate salts brought to the surface by capillary forces. The results of this study indicate that strategic usage of CCRs in reclamation programs can produce beneficial effects, including acid drainage reductions, that are beyond those achieved using traditional reclamation approaches such as the utilization of mine spoil as capping and fill material.

Estimativa da capacidade de campo pela curva de retenção e pela densidade de fluxo da água

A capacidade de campo é um parâmetro de inegável relevância para o manejo adequado da irrigação. A partir da determinação confiável do conteúdo de água no solo na capacidade de campo, pode-se otimizar a produtividade das culturas agrícolas, maximizando a eficiência do uso da água pelas plantas e evitando a contaminação do lençol freático por lixiviação de fertilizantes e agroquímicos. Nesse sentido, o trabalho teve como objetivo avaliar a capacidade de campo (a) pelo cálculo da densidade de fluxo da água durante o processo de redistribuição no experimento clássico de determinação desse parâmetro e (b) a partir de uma dada tensão da água na curva de retenção, em um Latossolo Vermelho-Amarelo textura média, localizado em área experimental da Escola Superior de Agricultura Luiz de Queiroz, município de Piracicaba, Estado de São Paulo (Brasil). Para isso, instalaram-se no solo, até a profundidade de 1,9 m, 60 tubos de acesso a uma sonda de nêutrons, numa rede de malha quadrada de 5 m (6 x 10 pontos amostrais), e, adjacente a cada tubo, dois tensiômetros, um a 0,75 m e outro a 0,85 m de profundidade. Nos 60 pontos (locais) amostrais, desenvolveu-se um experimento similar ao experimento clássico de determinação da capacidade de campo, no qual o solo foi devidamente submetido a uma lâmina de infiltração visando à saturação do seu perfil; sua superfície, coberta com lona plástica para evitar fluxo de água através dela; e a redistribuição da água, monitorada até a profundidade de 0,8 m durante 20 dias. Observou-se que o conteúdo de água correspondente a uma densidade de fluxo de 1,0 mm dia-1 é a melhor estimativa da capacidade de campo para esse solo, uma vez que as densidades de fluxo de 0,1 e 0,01 mm dia-1, também recomendadas para estimar a capacidade de campo, são muito baixas, a ponto de não terem sido alcançadas neste estudo. Quanto aos resultados obtidos pelo método baseado na curva de retenção, utilizando-se o conteúdo de água correspondente à tensão de 10 kPa em curvas de retenção elaboradas no campo e no laboratório também para os 60 pontos, na profundidade de 0,8 m, observou-se que os valores obtidos a partir das curvas de retenção elaboradas no campo e no laboratório subestimaram e superestimaram, respectivamente, aqueles baseados na densidade de fluxo da água de 1,0 mm dia-1, e a medida de campo foi mais confiável.

Sealing Neutron Probe Access-Tubes in Rocky Soils using Expandable Polyurethane Foam

Installation of neutron probe access tubes for measuring soil water content requires the use of a water-tight seal between the tube and the soil to prevent preferential flow of water along the tube–soil interface. We tested expandable polyurethane foam as a sealant. The foam generating kit consisted of two liquid components that when mixed expanded up to 15 times the original volume and hardened within minutes. The foam density was 0.19 g cm−3 when hardened and was not wettable. Count ratios (CRs) in moist sand, with foam and air in the void space between the tube and the sand, differed by <1.5% for the small hole diameters (9.0 and 11.5 cm), but in a larger 20-cm diam. hole in saturated sand, they were 8% higher with foam in the void space than with air, evidence of some thermalization of the fast neutrons by the foam. Count ratios decreased as hole diameter increased, but the loss of sensitivity was due mainly to changes in the zone of influence of the void space, not the foam. In rocky soil, insertion of tubes in bore holes to a depth of 1.6 m and sealing with foam took <2 min per hole. We found no evidence of preferential flow between the tubes and the soil, confirming the foam created water-tight seals. The foam is an acceptable material for sealing neutron probe access tubes. It is not a significant source of the thermalized neutrons at hole diameters typical of field installations, is easy to use, and allows access tubes to be quickly inserted and sealed.

Soil Nitrogen Balance under Wastewater Management: Field Measurements and Simulation Results

The use of treated wastewater for irrigation of crops could result in high nitrate-nitrogen (NO(3)-N) concentrations in the vadose zone and ground water. The goal of this 2-yr field-monitoring study in the deep silty clay loam soils south of Dodge City, Kansas, was to assess how and under what circumstances N from the secondary-treated, wastewater-irrigated corn reached the deep (20-45 m) water table of the underlying High Plains aquifer and what could be done to minimize this problem. We collected 15.2-m-deep soil cores for characterization of physical and chemical properties; installed neutron probe access tubes to measure soil-water content and suction lysimeters to sample soil water periodically; sampled monitoring, irrigation, and domestic wells in the area; and obtained climatic, crop, irrigation, and N application rate records for two wastewater-irrigated study sites. These data and additional information were used to run the Root Zone Water Quality Model to identify key parameters and processes that influence N losses in the study area. We demonstrated that NO(3)-N transport processes result in significant accumulations of N in the vadose zone and that NO(3)-N in the underlying ground water is increasing with time. Root Zone Water Quality Model simulations for two wastewater-irrigated study sites indicated that reducing levels of corn N fertilization by more than half to 170 kg ha(-1) substantially increases N-use efficiency and achieves near-maximum crop yield. Combining such measures with a crop rotation that includes alfalfa should further reduce the accumulation and downward movement of NO(3)-N in the soil profile.

Estimating in situ soil–water retention and field water capacity in two contrasting soil textures

A priori knowledge of the in situ soil field water capacity (FWC) and the soil-water retention curve for soils is important for the effective irrigation management and scheduling of many crops. The primary objective of this study was to estimate the in situ FWC using the soil-water retention curve developed from volumetric water content (θ), and water potential (ψ) data collected in the field by means of soil moisture sensors in two contrasting-textured soils. The two study soils were Lihen sandy loam and Savage clay loam. Six metal frames 117 cm × 117 cm × 30 cm high were inserted into the soil to a depth of 5–10 cm at approximately 40 m intervals on a 200 m transect. Two Time Domain Reflectrometry (TDR) sensors were installed in the center of the frame and two Watermark (WM) sensors were installed in the SW corner at 15 and 30 cm depths to continuously monitor soil θ and ψ, respectively. A neutron probe (NP) access tube was installed in the NE corner of each frame to measure soil θ used for TDR calibration. The upper 50–60 cm of soil inside each frame was saturated with intermittent application of approximately 18–20 cm of water. Frames were then covered with plastic tarps. The Campbell and Gardner equations best fit the soil–water retention curves for sandy loam and clay loam soils, respectively. Based on the relationship between soil ψ and elapsed time following cessation of infiltration, we calculated that the field capacity time (t FWC) were reached at approximately 50 and 450 h, respectively, for sandy loam and clay loam soils. Soil-water retention curves showed that θ values at FWC (θ FWC) were approximately 0.228 and 0.344 m3 m−3, respectively, for sandy loam and clay loam soils. The estimated θ FWC values were within the range of the measured θ FWC values from the NP and gravimetric methods. The TDR and WM sensors provided accurate in situ soil–water retention data from simultaneous soil θ and ψ measurements that can be used in soil-water processes, irrigation scheduling, modeling and chemical transport.

Impact of Gully on Soil Moisture of Shrubland in Wind-Water Erosion Crisscross Region of the Loess Plateau

The most serious erosion on the Loess Plateau of China exists in the wind-water erosion crisscross region where the annual precipitation is about 400 mm, the ecological environment is very fragile, and water is the key limiting factor for improving the environment. In this study, changes of soil moisture content for Caragana korshinskii Kom. shrubland in the gully bank of the Loess Plateau were studied using the methods of soil sampling and neutron probe. A typical gully (75 m long, 28 m wide, and 10 m deep) was selected, and six neutron probe access tubes (6 m long) were installed at points 50, 100, 200, 300, 400, and 500 cm from the gully border for obtaining soil moisture data from July to October 2004 at approximately 10 d intervals. Soil samplings were simultaneously carried out for moisture determination at the six points. Results showed that the soil moisture of the shrubland in the gully bank significantly varied between 300 and 400 cm in the horizontal direction and up to 600 cm in vertical direction of the gully. Seasonal changes in soil moisture revealed a curve with a single peak that occurred at the end of August or early September. A linear regression equation was fit for soil water storage and the distance from the gully border, with coefficients depending on rainfall characteristics, sampling point, and time of measurement.

Quantifying reductions in consumptive water use under regulated deficit irrigation in pistachio ( Pistacia vera L.)

The reduction in agricultural water use in areas of scarce supplies can release significant amounts of water for other uses. As improvements in irrigation systems and management have been widely adopted by fruit tree growers already, there is a need to explore the potential for reducing irrigation requirements via deficit irrigation (DI). It is also important to quantify to what extent the reduction in applied water through DI is translated into net water savings via tree evapotranspiration (ET) reduction. An experiment was conducted in a commercial pistachio orchard in Madera, CA, where a regulated deficit irrigation (RDI) program was applied to a 32.3-ha block, while another block of the same size was fully irrigated (FI). Four trees were instrumented with six neutron probe access tubes each, in the two treatments and the soil water balance method was used to determine tree ET. Seasonal irrigation water in FI, applied through a full-coverage microsprinkler system, amounted to 842 mm, while only 669 mm were applied in RDI. Seasonal ET in FI was 1024 mm, of which 308 mm were computed as evaporation from soil ( E s). In RDI, seasonal ET was reduced to 784 mm with 288 mm as Es. The reduction in applied water during the deficit period amounted to 147 mm. The ET of RDI during the deficit period was also reduced relative to that of FI by 133 mm, which represented 33% of the ET of FI during the deficit irrigation period. There was an additional ET reduction in RDI of about 100 mm that occurred in the post-deficit period.

Ranking of the depth of water extraction by individual grass plants, using natural 18O isotope abundance

The depth of water extraction is a key trait in grassland species. For comparing individual plant forms in the field, different ecological studies have proven the efficiency of the isotopic signal of water ( δ 18O). To date, there has been no assessment of the aforementioned method within closely related grasslands species. For this study, Festuca arundinacea and Lolium multiflorum plants were grown in nutrient solution and the δ 18O of different plant compartments were compared to the δ 18O of the solution under both high and low transpiration conditions. In both species, the basal part of the tillers, excluding the eldest sheath, closely reflected the isotopic water composition of the nutrient solution, whereas other parts of the plant exhibited a significant enrichment in 18O as a consequence of transpiration. In a second experiment, using the genetic variability within plants obtained from crossing F. arundinacea with Lolium multiflorum, the study explored the variability of the depth of water extraction and δ 18O under conditions of soil water deficit. A plant nursery of hybrids was established in a greenhouse with plants sewn directly in the soil. Ten neutron probe access tubes were placed close to ten individual plants, which were sampled twice at 38 day intervals, following a period of 14 and 52 days of water deprivation. On these same dates, soil samples were extracted down to 1 meter in the vicinity of the 10 studied plants. Water was fully extracted from both plant and soil samples and the isotopic composition was measured. The predawn leaf water potential exhibited large variations in water status among the ten plants studied. The measured plant δ 18O were similar to the values computed from estimated soil water extraction profiles (using neutron probe measurements) and soil δ 18O of the soil water profile. On the second date of sampling, the δ 18O and leaf water potentials of three individual plants ( L. multiflorum) were observed throughout the day, revealing a gradual decrease in 18O content as the plant water potential decreased. The role of short term changes in the soil–root hydraulic conductance on depth of water extraction is discussed in this study.

Spatial and temporal distribution of root water uptake of an almond tree under microsprinkler irrigation

The spatial and temporal pattern of root water uptake in partially wetted soil was studied in the root zone of a 6-year-old microsprinkler-irrigated almond tree. The water balance of about one quarter of the root zone's wetted soil volume (2.0·2.0·0.9 m 3 ) was deter- mined by catch cans, neutron probe and tensiometer measurements. Twenty-five neutron probe access tubes with catch cans were distributed in a square grid of 50 cm spacing. Eight pairs of tensiometers were installed at depths of 82.5 and 97.5 cm in a regular pattern be- tween the access tubes. Neutron probe readings at 15 cm depth increments and tensiometer readings were taken at time intervals of 4-24 h. The rate of soil water depletion was calculated and used to estimate the spatial and temporal distributions of root water uptake. Soil water dynamics was studied in two stages: (1) during a week of conventional irrigation management with three irriga- tion events; and (2) during a period of 16 days without irrigation, after the monitored soil volume was thor- oughly moistened so that soil water was easily available everywhere, initially. The zones of maximum root water uptake were the same for both stages in periods of high local rates of water application. After water applica- tions, root water uptake occurred initially near the tree trunk and then progressed towards the root system periphery, thereby changing locations of maximum root water uptake and shifting to root zone regions with minimum soil water stress.

Subirrigation of land bordering small reservoirs in the semi-arid region in the Northeast of Brazil: monitoring and water balance

In Northeast Brazil, more than 70,000 small dams retain superficial runoff and underground water flow, creating small reservoirs. Recession agriculture consists of cropping the margins of the reservoirs, on slight slopes, while the water level progressively decreases, and it is a widespread cropping system in this area. The water balance for this particular system was quantified on two contiguous plots, one with “Marreca” grass ( Paspalum conjugatum Berg.), a forage crop, and the other one on bare soil, in the recession zone of the basin of Flocos dam, municipality of Tuparetama, PE (7°36′S and 37°18′W). The soil, classified as Fluvent, consists of a superficial clay textured layer overlying a sandy layer. Next to the dam, a meteorological station was installed, equipped with automatic sensors for recording rainfall, air temperature and wind speed, and pan evaporation. In the reservoir, the water level was recorded. The plots were instrumented with one neutron probe access tube, one piezometer and tensiometers at different soil depths. Water retention and hydraulic conductivity characteristics were determined through field and laboratory measurements. The water balance was determined for the cropped and the bare soil plots, during 67 days. Average values for the actual evaporation and evapotranspiration rate, were respectively 5.0 and 8.0 mm day −1, with an average reference evaporation rate of 8.7 mm day −1. The contribution of the aquifer recharge to evaporative demand was estimated and found to be very important. Field observation and the estimated aquifer recharge from the reservoir suggest the presence of preferential flow. The study of water dynamics in this system is fundamental for establishing a basis for the sustainable development of agriculture in this area.

Transport of bromide in the Bainsvlei soil: Field experiment and deterministic/stochastic model simulation. I. Continuous water application

A field-scale tracer study was carried out in a sandy loam Bainsvlei soil of South Africa. The objectives were to study the leaching of solutes in this soil and analyse the result using deterministic and stochastic solute transport models. A rainfall simulator was used to apply water and solution on an area 100 by 100 cm at a flux rate of 5.41 mm/h. A neutron probe access tube was drilled at the centre of the plot to a depth of 200 cm. Four tensiometers were installed at 30, 45, 90, and 120 cm depths. After steady-state condition was attained, a conservative tracer Br– was then applied as KBr at a rate of 13.5 g Br/m2. Soil samples were taken 10 times at 20-cm depth intervals to a depth of 160 cm and Br– concentration was analysed using ion chromatography. Transport parameters were determined using a convection–dispersion equation (CDE) and the stream tube model (STM). The average Br– mass recovery was almost 100%. The average pore-water velocity determined from soil water balance was 2.08 cm/h. Pore-water velocities determined with CDE and STM were 2.24 cm/h and 2.20 cm/h, respectively. Bromide velocities of peak concentration and solute centre of mass were 2.05 cm/h and 2.02 cm/h, respectively. Generally, these results indicate that the Br– moved almost as a piston flow in this soil. The deterministic CDE and the stochastic STM performed almost equally well in estimating the transport parameters.

The effect of a gap between the access tube and the soil during neutron probe measurements

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 &lt;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.

Spatial variability of the effective retardation factor in an unsaturated field soil

A precisely controlled field study was conducted to determine flow and transport of water and bromide through an unsaturated soil. A 50 m×50 m plot was instrumented with neutron probe access tubes, tensiometers, and solution samplers. Water containing bromide part of the time was applied at a steady flux of 1.85 cm d −1 for 24 days. The average degree of water saturation during water application was about 56%. The Hydrus 1-D model was used to optimize the saturated hydraulic conductivity parameter K s, and the transport parameters D (dispersion coefficient) and R (used here as a ‘bulk retardation coefficient’). The van Genuchten flow parameters θ r, θ s, α, and n were obtained from laboratory measurements on 11 cores taken 1.5 m below the soil surface along a transect through the plot. The estimated field K s value increased with depth due to higher sand and gravel contents at depth. The mean dispersion coefficient for 13 locations at the 3 m depth was found to be 5.35 cm 2 d −1, with a coefficient of variation of 52%. This resulted in a relatively small mean dispersivity value of 0.64 cm. The average R value was 0.63 with a range of 0.45–1.02 at 3 m (CV=28%). The low R value is indicative of anion exclusion, immobile water, or some other phenomenon difficult to identify from field data. These field data indicate that if a transport model with a bulk retardation factor is used for predicting bromide transport through unsaturated soil a range of retardation values may need to be used. For our soil, the highest R value needed to be at least twice its lowest value.