Measuring Soil Matric Potential Research Articles

A methodology to optimize site-specific field capacity and irrigation thresholds

The determination of field capacity (FC), irrigation thresholds, and irrigation amounts is characterized by site-specific soil hydraulic properties (SHPs). This study, conducted in two zones (zone 1 and zone 2) delineated based on soil, topography, and historical crop yield in Alabama (USA), focused on determining zone-specific FC using negligible drainage flux qfc criterion. The HYDRUS-1D model was used to optimize zone-specific SHPs using measured soil matric potential (h). The zone-specific FCs were determined using optimized and raw SHPs at 0.01 cm/day as qfc. The results showed that the optimized FC at qfc was at −39 kPa in zone 1 and raw FC was at −15 kPa. However, in zone 2, optimized FC was at −25 kPa and raw FC was at −59 kPa. To validate that optimized values are more accurate than raw values, a relationship between accumulated crop evapotranspiration (ETc) and required irrigation amount was determined using optimized parameters (SHPs and FC) and showed a stronger correlation in both zones than using raw parameters (SHPs and FC). At flux-based FC, the optimized irrigation thresholds and amounts in zone 1 were −88 kPa and 20 mm, and raw irrigation threshold and amount were −58 kPa and 33 mm, respectively. In zone 2, the optimized irrigation thresholds and amounts were −45 kPa and 18 mm, and raw irrigation threshold and amount were −116 kPa and 14 mm, respectively. Therefore, using raw and benchmark FC can result in inefficient irrigation strategies. The proposed novel method of optimizing zone-specific FC and irrigation thresholds can help with adopting timely best irrigation management schemes in respective zones.

Evaluating soil water and salt transport in response to varied rainfall events and hydrological years under brackish water irrigation in the North China Plain

• Responses of soil water and salt dynamics to varied rainfall events and hydrological years were evaluated. • Salt leaching and deep drainage occurred mainly due to summer-concentrated rainfall during wet year. • Salt decrease percentage in the CL was greater than that in the GL in all hydrological years. • One brackish water irrigation application is avail salt leaching both in cropland and grassland. Soil salination challenges sustainable agricultural production and management in the lowland area of the North China Plain (NCP). This study investigated the effect of varied rainfall events and hydrological years on deep drainage and salt leaching in Nanpi County in the NCP. Field experiments were conducted to measure soil matric potential (SMP) and soil water electrical conductivity (EC w ) in 300 cm profiles of an irrigated cropland (CL) and unirrigated grassland (GL). The HYDRUS-1D model was calibrated and validated to optimize soil water and salt transport parameters using the SMP and EC w data obtained in 2015 and 2016, respectively. The results showed that the HYDRUS-1D model was reliable to simulate soil water and salt migration in 300 cm profiles at both sites. The salt leaching depths and degrees were positively correlated with precipitation amount. The heavy rain (45 mm) and rainstorm (68 mm) favored salt leaching within 100 cm, and severe and extreme rainstorms resulted in salt leaching within 150–200 cm both in the CL and GL. The SDP in the CL was greater than that in the GL in all hydrological years owing to agricultural cultivation and irrigation. Salt leaching in the 300 cm profiles mainly occurred in the rainy seasons of wet year, with the salt decrease percentage (SDP) being 61% in the CL and 42% in the GL. Salt leaching due to precipitation was sufficient in the irrigated CL in all hydrological years, and precipitation in normal rainfall years could remove 70–80% of soil salts within 50–63 cm soil depth. Grass growth without irrigation led to water deficits and salt accumulation when the annual precipitation amount was less than that in wet years. The results demonstrated that precipitation, combined with irrigation using of brackish and freshwater, could be usefully for soil salt leaching. A recommended irrigation scheme for water saving and salt control in CL and GL is one brackish water irrigation application and three freshwater irrigation applications each year in the NCP.

Can streambank height indicate soil moisture regime of riparian zones? A case study in deep soils of a first-order watershed in Southeast Brazil

ABSTRACT A number of riparian ecosystem functions such as reducing chemicals are associated with high soil moisture conditions. Finding easy-to-measure riparian features that indicate soil moisture regime in riparian ecosystem may of use in environmental management. In this study, we answered the following question: can streambank height, on which these riparian zones are, indicate soil moisture regime? By measuring soil matric potential (0.15–0.9 m depth) and water table depth on a weekly basis at three forested riparian zones on different bank heights (low, moderate and high) and under a similar soil type, we demonstrate that riparian zones on high-to-moderate streambank height (h ≥ 1 m) generally have lower water table compared to riparian zones on low streambanks (h ≤ 0.3 m). This difference on water table depth led to significant differences in soil matric potential as water table depth lowers. However, in order to predict soil moisture regime more accurately, streambank height must be associated with a detailed field description of local factors such as soil type which can help in explaining deviations from the expected pattern of soil moisture as shown in moderate and high.

In-situ determination of soil water retention curves in heterogeneous soil profiles with a novel dielectric tube sensor for measuring soil matric potential and water content

Techniques of determining soil water retention curve (SWRC) have been developed for several decades, but very few have considered the heterogeneity of SWRC at different depths caused by texture, structure and other constituents. In this study, we designed a novel dielectric tube sensor (DTS) moving in a pair of tubes installed in-situ to measure soil water content (SWC) and soil matric potential (SMP) in heterogeneous soil profiles for determining wetting-path SWRCs. A series of experiments were conducted to test the optimal size of DTS, calibrate the DTS for SWC measurement and SMP measurement, test the water proof function of the impermeable sheet and equilibrium rate of SMP tube and determine SWRCs in a layered and stony soil tank with a relatively slow drip irrigation. Commercial sensors were also employed in comparison with the DTS during the drip irrigation experiment. The results showed that when the distance of the two electrodes of the DTS was 5 mm, the measurement range, volume of sensitivity and coefficient of variation were optimal. The calibration results verified that the DTS was suitable to measure volumetric SWC (R2 > 0.9914) and SMP (R2 = 0.9992). The SMP tube exhibits good water-proof property among different layers, and the average equilibrium rate was 2.1 kPa·s−1 in loamy soil and 7.3 kPa·s−1 in pure sand. The dynamics of SWC and SMP measured by the DTS approximately agreed well with the data measured by the commercial sensors. The relatively reduced agreement between the DTS and TEROS21 for SMP measurements may attribute to their different volume of sensitivity and installed position. Based on these, the wetting-path SWRCs were determined in-situ in a soil tank containing four kinds of soil samples. Furthermore, the determined SWRCs in four soil samples verified that stones decreased soil water holding capacity.

A Novel Frequency Domain Impedance Sensor with a Perforated Cylinder Coaxial Design for In-Situ Measuring Soil Matric Potential.

Soil matric potential is an important parameter for agricultural and environmental research and applications. In this study, we developed a novel sensor to determine fast and in-situ the soil matric potential. The probe of the soil matric potential sensor comprises a perforated coaxial stainless steel cylinder filled with a porous material (gypsum). With a pre-determined gypsum water retention curve, the probe can determine the gypsum matric potential through measuring its water content. The matric potential of soil surrounding the probe is inferred by the reading of the sensor after the soil reaches a hydraulic equilibrium with the gypsum. The sensor was calibrated by determining the gypsum water retention curve using a pressure plate method and tested in three soil samples with different textures. The results showed that the novel sensor can determine the water retention curves of the three soil samples from saturated to dry when combined with a soil water content sensor. The novel sensor can respond fast to the changes of the soil matric potential due to its small volume. Future research could explore the application for agriculture field crop irrigation.

Comparison of devices for measuring soil matric potential and effects on soil hydraulic functions and related parameters

The tensiometer represents an excellent instrument for measuring in situ soil water status. However, measuring soil matric potential requires a tensiometer reading system sensitive enough to accurately record the matric potential. Hence, an instantaneous profile-type experiment was conducted in the field, to measure the soil matric potentials within a moisture range from saturation to field capacity. After that, matric potential, soil moisture, total potential gradient, flow density and hydraulic conductivity were calculated and van Genuchten equation parameters were estimated through inverse modeling. This study aimed to test the Bourdon pressure gauge and digital tensiometer compared with the mercury manometer to measure the soil matric potential and to examine the differences related to the estimations of soil water content and to the associated variables. In addition, the study also aimed to evaluate soil hydraulic parameters by inverse modeling, based on the matric potentials from each reading system. Bourdon pressure gauge replaces the Hg manometer in the measurement of soil water matric potential within the moisture range from saturation to field capacity; The use of digital tensiometer and Bourdon pressure gauge reduced hydraulic conductivity by four and three times and flow density by approximately three and two times, respectively, at 6 kPa tension and, therefore, are not recommended for the estimation of these hydraulic parameters; Regardless of reading system used in the tensiometer, inverse modeling estimates well van Genuchten equation parameters and, consequently, soil water matric potential.

Do roots mind the gap?

Aims Roots need to be in good contact with the soil to take up water and nutrients. However, when the soil dries and roots shrink, air-filled gaps form at the rootsoil interface. Do gaps actually limit the root water uptake, or do they form after water flow in soil is already limiting? Methods Four white lupins were grown in cylinders of 20 cm height and 8 cm diameter. The dynamics of root and soil structure were recorded using X-ray CT at regular intervals during one drying/wetting cycle. Tensiometers were inserted at 5 and 18 cm depth to measure soil matric potential. Transpiration rate was monitored by continuously weighing the columns and gas exchange measurements. Results Transpiration started to decrease at soil matric potential = between �5 kPa and �10 kPa. Air-filled gaps appeared along tap roots between =0�10 kPa and =0�20 kPa. As = decreased below �40 kPa, roots further shrank and gaps expanded to 0.1 to 0.35 mm. Gaps around lateral roots were smaller, but a higher resolution is required to estimate their size. Conclusions Gaps formed after the transpiration rate decreased. We conclude that gaps are not the cause but a consequence of reduced water availability for lupins.

Patch vegetation and water redistribution above and below ground in south‐east Spain

AbstractTwo sites, one dominated by Anthyllis cytisoides and the other by Retama sphaerocarpa in south‐east Spain were instrumented to measure soil matric potential to a soil depth of 1 m and soil water content to 2·75 m depth within shrubs and grass areas. Soil cores and underground surveys were taken to estimate root length and soil texture, ground cover volume was measured and runoff traps were installed to estimate overland flow in grass and shrub areas. The outcome of the results was summarised as a conceptual model to describe the flow of water within patch vegetation, where the main dominating factor of patch vegetation was found to be the function of shrub canopy/root systems. The results illustrated that not only the dynamics of runoff/runon between grass and shrub areas were significant in redistributing rainfall, but also the morphology of shrub species caused significant differences to the accumulation and movement of water in shrub zones. A comparison of growth and flowering times of grass and shrub species showed that soil water availability for each species differs throughout the year. Such an understanding of water movement within patch vegetation of different species shows that sustainable management practices of semi‐arid areas must take into account not only natural vegetation patterns, but also the function of each plant type. Copyright © 2011 John Wiley & Sons, Ltd.

A Combined Frequency Domain and Tensiometer Sensor for Determining Soil Water Characteristic Curves

There is increasing interest in the development of a technique that can simultaneously measure soil matric potential and water content for rapidly determining a soil water characteristic curve. In this study, we developed a new combined soil water content and potential sensor. The new sensor adapted the stainless steel tube of a conventional water‐filled tensiometer into a single sensing electrode for measuring soil water content. This novel design has the advantage of utilizing the whole length of the tensiometer without adding additional components to the sensor. To verify its feasibility, the sensor was tested on three soils (sand, sandy loam, and clay loam) while they were drying under laboratory conditions to produce the soil water characteristic curves in the matric potential range of 0 to −80 kPa. Water characteristic curves of the three soils were also obtained using standard laboratory techniques (tension table and pressure plate). The results show that water characteristic curves of the three soils from the new sensor are in good agreement with those obtained using the standard methods.

Coiled Time Domain Reflectometry Matric Potential Sensor

Matric potential is a measure of the combined capillary and adsorptive forces of soil particles and is imperative for determining both the direction and magnitude of water flow in unsaturated soils. Measuring soil matric potential in semiarid and arid environments is a challenge. Current methods cannot measure the entire range of values, from 0 to −1.5 MPa. The objective of this research was to develop a new sensor capable of measuring the relationship between the dielectric constant of a ceramic core and the range of field soil matric potential values. This instrument uses two Cu TDR rods wrapped around a dual‐threaded Plexiglas core, which is inserted into a porous ceramic block salvaged from damaged Campbell Scientific 229 heat dissipation sensors. The relationship of the ceramic's measured dielectric constant to soil matric potential is determined through a laboratory calibration curve. Calibration of the probe involved the use of a tension table and pressure plate apparatus. The dielectric constant of the probe was measured at a range of points between 0 and −1.5 MPa, and a modified van Genuchten water retention model was used to fit the data (R2 = 0.84). The proposed instrument provided an effective method of measuring soil matric potentials from 0 to −1.5 MPa.

A New Resistance Sensor for Monitoring Soil Matric Potential

The electrical resistance sensor is an appropriate and inexpensive tool for measuring soil matric potential (ψ). These sensors are widely used in irrigation agriculture. One drawback of the resistance sensors, however, is that the measurements often are not accurate across a wide range of moisture conditions. The objective of this study was to design a resistance sensor that can measure accurately and precisely across a wide range of moisture conditions, thereby extending the measurement capabilities of currently available sensors. The sensor consists of two electrodes embedded in a sand–plaster matrix that is stabilized with polyacrylamide. The sensor was calibrated and tested to determine its sensitivity to soil texture, temperature, and electrical conductivity. Experimental results showed that the sensor was able to determine matric potentials in the range of −7.5 kPa to −10 MPa, and showed excellent precision, with a RMSE <5 kPa across the range of −5 to −80 kPa. A standard electrical resistance–water potential curve can be established in the laboratory on a soil, and can then be applied to different soil types.

Topographic position affects the water regime in a semideciduous tropical forest in Panama

The effects of topographic position on water regime in a semideciduous tropical forest on Barro Colorado Island in Panama were assessed by measuring soil matric potential using the filter paper technique and by using measured soil water release characteristics to convert a long-term (20 years) gravimetric water content data-set to matric potential. These were also compared against predictions from a simple water balance model. Soil matric potentials on slope sites were significantly higher than on plateau sites throughout the measurement interval and slopes experienced a shorter duration of drought during the annual dry-season. Measured values of matric potential agreed with those predicted from converting the gravimetric measurements using water release characteristics. Annual duration of drought predicted by the simple water balance model agreed with values determined from the converted long term water content data-set and was able to predict the annual duration of drought on plateau sites. On slope sites, the water balance systematically and significantly overestimated the duration of drought obtained from the water content data-set, suggesting that slope sites were supplied with water from upslope. Predictions of annual drought duration from sites with higher annual rainfall than Barro Colorado Island (BCI), suggest that while plateau sites on BCI experience a water regime consistent with annual rainfall, slopes experience a water regime more similar to that of forests with much higher rainfall. We conclude that such large variations in water regime over small spatial scales may play a role in maintaining high species richness through providing opportunities for niche specialisation and by buffering slopes against possible climate change.

Temporal Stability of Spatially Measured Soil Matric Potential Probability Density Function

Estimation of mean water status in a field is crucial to effective irrigation water management. Problems encountered with the estimation of mean field soil water status may be attributed to spatial variability of soil physical properties. Several investigators have shown temporal stability of spatial patterns of field measured soil water content, but temporal stability of field measured soil matric potential (ψm), a measure of soil water status more appropriate for irrigation scheduling, has not previously been reported to last for more than a few days within one irrigation cycle. This study investigated the temporal stability of spatial patterns of ψm both within and between sequential irrigation cycles. Sixty locations in a 1‐ha field were outfitted with a 1‐m neutron probe access tube and three tensiometers placed at 0.15‐, 0.3‐, and 0.5‐m depths. The observations obtained from 14 d of soil water content measurements and 46 d of ψm measurements within eight irrigation cycles were analyzed with Spearman's rank correlation coefficients and a relative differencing technique. The results showed temporally stable soil water content spatial patterns and also indicated temporally stable ψm spatial patterns if assumptions of full soil wetting at the beginning of the cycle and uniform evapotranspiration among locations were satisfied. Several locations in the field estimated the field mean ψm to within 10% within a given range of potentials, and a few estimated the field mean to within 20% across the entire range of potentials tested. Other locations estimated the lower and higher percentiles of ψm with similar accuracy.

A new soil metric potential sensor based on time domain reflectometry

We developed and tested a new sensor based on time domain reflectometry (TDR) to measure soil matric potential (h). The TDR‐matric (TM) sensor is constructed of porous disks having different known maximum pore sizes and stacked within a coaxial cage. The constant relationship between water content (θ) and h of the TM porous matrix is initially calibrated and subsequently used to infer matric potential of the surrounding soil, similar to existing porous heat dissipation and electrical resistance sensors. The θ of the sensor's porous matrix in hydraulic equilibrium with the surrounding soil is measured by TDR travel time analysis. Prototype sensors were constructed using porous ceramic and plastic disks having maximum pore diameters between 120 μm(2.5 kPa) and about 0.6 μm (0.5 MPa). Calibration tests to evaluate sensor θ‐h relationships were completed in a pressure chamber apparatus using four soils. These results and those from sensors installed in soil lysimeters in the presence of growing plants showed consistent θ‐h relationships and synchronized responses of soils and TM sensors to changing water status. Pairing standard TDR probes with the new TM sensors facilitates in situ determination of soil θ(h) relationships, using conventional TDR instrumentation. The sensor design accommodates construction of media‐ or application‐specific sensors using combinations of disks having different pore sizes. There is a trade‐off between the TM sensor's matric potential range and its sensitivity to changes in the surrounding soil. Additionally, a mismatch between the pore size distributions of the TM sensor and the soil (mostly relevant to coarse‐textured soils) can lead to hydraulic decoupling of these and other porous sensors.

IRRIGATION SCHEDULING METHODS FOR POPCORN IN THE NORTHERN GREAT PLAIN

Irrigation of popcorn requires knowledge of methods of applying and scheduling irrigations. A four-year field plot study of four application-scheduling methods for popcorn was undertaken on a sandy loam soil near Oakes, North Dakota (ND), using a randomized block design. The study was designed to assess the influence of the methods on grain yields, popped volumes, and total irrigation water amounts applied for “White Cloud” popcorn. The reference method used aboveground drip irrigation (AGDI) to apply water and a scheduling method based on estimates of when 40% depletion of root zone available water (40% D) would be reached. The other irrigation application-scheduling methods were: AGDI and scheduling based on plant temperature, i.e., crop water stress index (CWSI) of 0.4; subsurface drip irrigation (SDI) and scheduling based on measured soil matric potential (SMP) of 30 kPa using a feedback and control system to automate irrigation applications through an SDI system; and AGDI and CERES-Maize (CM) growth model estimates of water use. Due to difficulties in implementing the CWSI method (high relative humidity and intermittent cloudiness), irrigations were also scheduled based on tensiometer-measured SMP of 40 kPa at 0.3-m depth for the CWSI treatment. Compared to the 40% D method, all other methods achieved statistically significant (0.05 level) irrigation water savings for popcorn without significant reductions in yields or popped volumes. Four-year average irrigation water amounts were 235, 134, 142, and 156 mm for the 40% D, CWSI, SDI, and CM methods, respectively, with corresponding popcorn yields of 4.63, 4.33, 4.44, and 4.47 Mg ha–1 and popped volumes of 26.3, 28.0, 28.3, and 28.0 L kg–1.

Evaluation of a Line Heat Dissipation Sensor for Measuring Soil Matric Potential

AbstractThe accuracy and operational limits of a new heat dissipation sensor for measuring soil matric potential are evaluated. The measurement is based on the rate of temperature rise from a line heat source embedded in a cylindrical porous ceramic that is in equilibrium with soil water. Different heating currents or times can be accounted for by basing calibration on sensor thermal conductivity rather than temperature rise. Six sensors were calibrated against pressure plate and psychrometer measurements of matric potential. Inverse of sensor thermal conductivity was linearly related to the logarithm of matric potential between −10 and −1200 J kg−1. This range of measurable matric potentials is much larger than previous heat dissipation sensor designs. The heat dissipation sensor was accurate to within 20% of the independently measured matric potential. Normalizing sensor readings by sensor thermal conductivity after oven drying results in a calibration that accounts for between‐sensor variability. In a growth chamber experiment with fluctuating soil temperatures, heat dissipation sensor estimates of soil matric potential compared favorably with tensiometer and psychrometer measurements. Heat dissipation sensors based on this design appear useful when a wide range of matric potentials needs to be studied.

Field Comparison of Irrigation Scheduling Methods for Corn

Irrigation scheduling for corn requires knowledge of methods for timing irrigation applications. A three-year field plot study of irrigation scheduling methods for corn was undertaken on a sandy loam soil near Oakes, North Dakota, using a randomized block design. The study was designed to assess the influence of the methods on grain yields and total irrigation amounts applied. The reference irrigation timing method was based on allowable root zone available water depletion (40%). The other irrigation timing methods used were: partial evapotranspiration (ET) (0.5 ET) replacement, crop water stress index (CWSI of 0.2, 0.4, and 0.6), measured soil matric potential (30 and 50 kPa), and growth model (CERES-Maize) estimates of dry matter accumulation and water use. In terms of crop yield per unit ET, the best method was the 50-kPa treatment. The 50-kPa method resulted in the maximum average yield of 12 200 kg ha1 while achieving a statistically significant average reduction of 139 mm (40%) in irrigation application compared to the reference treatment. The nonreference methods, except for CWSI = 0.6, appear to offer the potential for significant irrigation water savings without significant yield reductions. The 0.6 CWSI treatment suffered a statistically significant yield reduction of 1 600 kg ha1 (13%).

Measurement of soil matric potential under 'Williams' Bon Cretien' pear comparing regulated deficit with normal irrigation

Soil matric potential (SMP) was measured under close planted 'William Bon Cretian' pear (Pyrus communis L.) after a period of withholding irrigation and during a period of regulated deficit irrigation (RDI). Trees were trickle irrigated with 20% replacement of pan evaporation over the planting square (Eps) after an initial period without irrigation, and compared with trees constantly irrigated at 80% Eps. Midway between 2 pairs of trees in each treatment (henceforth the 20% and 80% trees), gypsum blocks were installed in a grid pattern at depths of 10, 25, and 40 cm in the tree line and at 30 and 60 cm on the west and east sides of the tree line, to measure soil matric potential (SMP). The blocks were measured prior to, and 12 h, after irrigation. Consistent with previous results under RDI, shoot growth decreased and yields increased on the 20% relative to the 80% trees. Measured SMPs thus represent suitable levels for RDI. After withholding irrigation, SMPs at the 10 and 25 cm depths ranged between -1.25 and -2.61 MPa in the central position, but were less negative away from the tree line (-0.4 to -2.08) and at the 40 cm depth (-0.35 to -0.91). When averaged over all measurement dates during RDI, pre- and post-irrigation SMPs were more negative at all sites under the 20% compared with the 80% treatments. The pattern of wetting under the 20% treatment was restricted to the central tree line, with some skewing towards the east. Within this soil volume SMP fluctuated between irrigations, and prior to irrigation ranged between -0.3 and -0.7 MPa. As it was likely that only a small fraction of the available subsoil moisture was utilised, available water under the 20% treatment probably was confined to this small volume of soil.

Measuring Soil Matric Potential in situ by Sensing Heat Dissipation within a Porous Body: II. Experimental Results

AbstractA sensor described previously to measure the matric potential of soil water in situ was tested in soil‐plant systems. Experiments were performed in the laboratory in a controlled environment and in the field. In the field, temperatures obtained by the sensor were used to predict optimum time for measurement to avoid error caused by diurnal temperature drift in the soil. The error caused by temperature drift was eliminated completely by using two matched diode sensors and taking a temperature difference measurement.The accuracy of the matric potential sensor proved to be as good as or better than that of other techniques used to measure matric potential. Sensors with high sensitivity in the 0 to −2 bar matric potential range had an accuracy of ±0.2 bar. The accuracy decreased progressively to ±1 bar at a matric potential of −10 bars.

Measuring Soil Matric Potential in situ by Sensing Heat Dissipation within a Porous Body: I. Theory and Sensor Construction

A matric potential sensor is described that measures heat dissipation to sense the water content of a porous block in equilibrium with soil. It consists of a P-N junction diode that is surrounded by a heating coil and embedded in a porous medium. A theoretical model is developed that describes the sensor in terms of its limiting parameters. Geometry, size, rate of heat input, and duration of heating are chosen to obtain a certain degree of accuracy and to make the measurements independent of the surrounding soil. Construction, calibration, measurement, and temperature correction techniques are described. Since the electrical output of the sensor is linear with temperature, a single calibration curve is adequate at all temperatures within the accuracy reported. By varying the composition of the porous body, the range and sensitivity of the sensor can be altered.