Dual-probe Heat-pulse Sensors Research Articles

Use of the Dual‐Probe Heat‐Pulse Technique to Monitor Soil Water Content in the Vadose Zone

The dual‐probe heat‐pulse (DPHP) technique is emerging as a useful technique for measuring soil volumetric water content (θ). However, few published data are available regarding the performance of the DPHP technique under field conditions. The objective of this study is to evaluate the effectiveness of the DPHP technique for measuring θ under field conditions. We used 24 DPHP sensors to monitor θ in a soybean [Glycine max (L.) Merr.] field during the 2001 and 2002 growing seasons. The DPHP sensors demonstrated durability in field conditions and clear sensitivity to temporal and spatial variations of θ at the scale of measurement. The mean θ measured by the DPHP sensors (θDPHP) was on average 0.040 m3 m−3 larger than the mean θ measured by soil sampling (θSS). The response of the DPHP sensors was linear. Regressions of θDPHP vs. θSS yielded r2 values of 0.949 and 0.843 at depths of 7.5 and 37.5 cm. The DPHP technique showed good resolution with RMSE values for the regression of 0.009 and 0.011 m3 m−3 at the two measurement depths. The slopes of the regressions were 0.75 rather than 1.0. Errors in θSS are a likely cause of this low slope. We shifted all the θ values for each sensor up or down by a constant value to make the first θ measurement from each sensor equal θ determined from soil sampling near that sensor at the time of installation. This simple matching point procedure improved the accuracy of the DPHP technique, resulting in a −0.024 m3 m−3 average difference between θDPHP and θSS Also, the matching point procedure markedly reduced the variability between sensors, reducing the average SD from 0.063 to 0.026 m3 m−3 This procedure requires no additional soil sampling and is recommended for field applications of the DPHP technique.

Evaluation of Microlysimeters Used in Turfgrass Evapotranspiration Studies Using the Dual‐Probe Heat‐Pulse Technique

Microlysimeters (ML) are commonly used in turfgrass evapotranspiration (ET) studies. No standard exists for ML, which has resulted in multiple designs that may affect soil moisture. The effects of ML design on volumetric soil water content (θv) were investigated with the dual‐probe heat‐pulse (DPHP) technique. DPHP sensors were installed at 5, 15, and 25 cm in the ambient soil profile and in three designs of ML: (i) 15‐cm diam. by 30‐cm, mesh base, soil fill (MSL); (ii) 15‐cm diam. by 30‐cm, Plexiglass base (one drainage hole), soil fill (PSL); 3) 10‐cm diam. by 20‐cm, mesh base, soil (intact cores) (MSNL). Sleeves and a 5‐cm layer of gravel were placed in MSL and PSL. DPHP estimates of θv revealed that soils consistently dried faster in MSL and PSL than in the ambient profile, probably because of higher leaf area index (LAI) and biomass in MSL and PSL than in surrounding turf, limitations of roots to extract soil water only from mL, and evaporation through open bases. In MSNL, θv was similar to but may have been in hydraulic contact with ambient soils. The correlation was good between θv determined by DPHP and θv determined by gravimetric methods; DPHP sensors on average (all ML) measured θv to within 0.025 m3 m−3 of gravimetric estimates. ET estimates varied significantly among ML and were strongly correlated to LAI and aboveground biomass (r = 0.85). Results suggest that establishment–maintenance of similar LAI and biomass between ML and surrounding turf may be more important than ML design in providing accurate ET estimates, and bases should be sealed during ET measurements to prevent hydraulic contact with soil, drainage, or evaporation through bases.

Field Evaluation of the Dual‐Probe Heat‐Pulse Method for Measuring Soil Water Content

The dual‐probe heat‐pulse (DPHP) method is useful for measuring water content (θ) and change in water content (Δθ) near the soil surface. The method has been evaluated in laboratory and greenhouse experiments, but not in a field setting. Our objective was to test the DPHP method under field conditions and for a range of soil properties. Twenty‐five DPHP sensors and five monitoring stations were constructed and installed at five locations in northeastern Kansas to measure θ and Δθ at 3‐h intervals for 3 mo. In addition, θ was estimated by coupling Δθ measurements with independent measurements of θ obtained by soil sampling at sensor installation. Additional soil samples were collected from each location during the monitoring period to provide independent measurements of θ. Regression of DPHP and independent θ measurements revealed slight bias but substantial offset error (about 0.1 m3 m−3) in the DPHP method. The offset error could not be fully attributed to bias in any single input parameter, but could have been caused by a combination of biased parameters. Estimates of θ from Δθ measurements also revealed slight bias, but offset error was considerably smaller. Use of a published empirical calibration for DPHP sensors almost completely eliminated this bias and further reduced the offset error to approximately 0.01 m3 m−3 Thus, the Δθ approach combined with use of the empirical calibration appears to have practical utility.

Laboratory Evaluation of the Dual‐Probe Heat‐Pulse Method for Measuring Soil Water Content

The dual‐probe heat‐pulse (DPHP) method provides a means of estimating volumetric soil water content (θ) and change in volumetric water content (Δθ) from measurements of volumetric heat capacity. The purpose of this investigation was to characterize the accuracy and precision that can be achieved in measuring θ and Δθ with the DPHP method. Tempe pressure cells fitted with DPHP sensors were used to conduct desorption experiments in which DPHP‐based estimates of θ and Δθ were compared with values estimated by the gravimetric method. For water contents corresponding to soil water pressure potentials below −100 kPa, comparisons were made by packing the pressure cells with soil wetted to known water contents. The investigation was conducted with seven soil materials representing a wide range of physical properties for mineral soils. The DPHP sensors slightly overestimated θ at low water contents, but it was shown that the bias could be removed by using an empirical calibration equation, θ = 1.09 θDPHP − 0.045. This relationship appears to be general inasmuch as it was shown to be applicable for all seven soil materials and for water contents ranging from 0.02 to 0.59 m3 m−3 The general calibration equation was also shown to be effective in removing bias in Δθ estimates. Pooled regression analysis (all soil materials) showed that θ can be measured with a root mean square error (RMSE) of 0.022 m3 m−3 Greater precision can be achieved with Δθ measurements (RMSE = 0.012 m3 m−3); however, the results indicated a decrease in precision with increasing magnitude of Δθ.

Laboratory Evaluation of the Dual-Probe Heat-Pulse Method for Measuring Soil Water Content

The dual-probe heat-pulse (DPHP) method provides a means of estimating volumetric soil water content (θ) and change in volumetric water content (Δθ) from measurements of volumetric heat capacity. The purpose of this investigation was to characterize the accuracy and precision that can be achieved in measuring θ and Δθ with the DPHP method. Tempe pressure cells fitted with DPHP sensors were used to conduct desorption experiments in which DPHP-based estimates of θ and Δθ were compared with values estimated by the gravimetric method. For water contents corresponding to soil water pressure potentials below −100 kPa, comparisons were made by packing the pressure cells with soil wetted to known water contents. The investigation was conducted with seven soil materials representing a wide range of physical properties for mineral soils. The DPHP sensors slightly overestimated θ at low water contents, but it was shown that the bias could be removed by using an empirical calibration equation, θ = 1.09 θDPHP − 0.045. This relationship appears to be general inasmuch as it was shown to be applicable for all seven soil materials and for water contents ranging from 0.02 to 0.59 m3 m−3 The general calibration equation was also shown to be effective in removing bias in Δθ estimates. Pooled regression analysis (all soil materials) showed that θ can be measured with a root mean square error (RMSE) of 0.022 m3 m−3 Greater precision can be achieved with Δθ measurements (RMSE = 0.012 m3 m−3); however, the results indicated a decrease in precision with increasing magnitude of Δθ.

Field Evaluation of the Dual-Probe Heat-Pulse Method for Measuring Soil Water Content

The dual-probe heat-pulse (DPHP) method is useful for measuring water content (θ) and change in water content (Δθ) near the soil surface. The method has been evaluated in laboratory and greenhouse experiments, but not in a field setting. Our objective was to test the DPHP method under field conditions and for a range of soil properties. Twenty-five DPHP sensors and five monitoring stations were constructed and installed at five locations in northeastern Kansas to measure θ and Δθ at 3-h intervals for 3 mo. In addition, θ was estimated by coupling Δθ measurements with independent measurements of θ obtained by soil sampling at sensor installation. Additional soil samples were collected from each location during the monitoring period to provide independent measurements of θ. Regression of DPHP and independent θ measurements revealed slight bias but substantial offset error (about 0.1 m3 m−3) in the DPHP method. The offset error could not be fully attributed to bias in any single input parameter, but could have been caused by a combination of biased parameters. Estimates of θ from Δθ measurements also revealed slight bias, but offset error was considerably smaller. Use of a published empirical calibration for DPHP sensors almost completely eliminated this bias and further reduced the offset error to approximately 0.01 m3 m−3 Thus, the Δθ approach combined with use of the empirical calibration appears to have practical utility.

Laboratory Evaluation of the Dual-Probe Heat-Pulse Method for Measuring Soil Water Content

The dual-probe heat-pulse (DPHP) method provides a means of estimating volumetric soil water content (θ) and change in volumetric water content (Δθ) from measurements of volumetric heat capacity. The purpose of this investigation was to characterize the accuracy and precision that can be achieved in measuring θ and Δθ with the DPHP method. Tempe pressure cells fitted with DPHP sensors were used to conduct desorption experiments in which DPHP-based estimates of θ and Δθ were compared with values estimated by the gravimetric method. For water contents corresponding to soil water pressure potentials below −100 kPa, comparisons were made by packing the pressure cells with soil wetted to known water contents. The investigation was conducted with seven soil materials representing a wide range of physical properties for mineral soils. The DPHP sensors slightly overestimated θ at low water contents, but it was shown that the bias could be removed by using an empirical calibration equation, θ = 1.09 θDPHP − 0.045. This relationship appears to be general inasmuch as it was shown to be applicable for all seven soil materials and for water contents ranging from 0.02 to 0.59 m3 m−3 The general calibration equation was also shown to be effective in removing bias in Δθ estimates. Pooled regression analysis (all soil materials) showed that θ can be measured with a root mean square error (RMSE) of 0.022 m3 m−3 Greater precision can be achieved with Δθ measurements (RMSE = 0.012 m3 m−3); however, the results indicated a decrease in precision with increasing magnitude of Δθ.

Root-zone hydraulic lift evaluated with the dual-probe heat-pulse technique

Roots are movers of water in the soil. One method of movement is through hydraulic lift, which occurs when plants extract water from a moist subsoil and release it into a dry topsoil. Detection of hydraulic lift has been hampered by the lack of instruments sensitive enough to measure the small amount of water moved. Recently, the dual-probe heat-pulse (DPHP) technique has been used to monitor with fine spatial resolution the soil water content in root-zones. The objective of this research was to determine if water is released by hydraulic lift, using the DPHP technique. Sunflower (Helianthus annuus L.) was grown in a column (38 cm height; 25 cm diam.; bulk density = 1.45 Mg/m3) packed with a Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents; FAO-Eutric Fluvisols) with its roots divided between a top dry layer and a lower wet layer. Eight DPHP sensors installed in the soil column were used to monitor soil water content. During 24 measurement days, hydraulic lift was evident only when the plant was wilted. This occurred when the lower ‘wet’ layer had been allowed to dry and then it was re-watered. At this time, the roots in the upper dry layer released water, increasing the soil water content in the centre of the root mass by 0.019 m3/m3 (increase from 0.121 m3/m3 to 0.140 m3/m3). The soil-water increase was similar to other values reported in the literature and show it to be small.

Dual probe heat pulse technique for measuring soil water content and sunflower water uptake

Soil water content is the most sought-after soil physical parameter. Recent experiments have shown that dual probe heat pulse (DPHP) sensors can be used to determine volumetric water content of soil without roots. Little work has been done to document the performance of DPHP sensors in the presence of roots, and no work has been done with a taprooted plant. Thus, the objective of this experiment was to determine the accuracy of DPHP sensors in measuring volumetric water content ( θ v) and changes in volumetric soil water content (Δ θ v) in soil with a branched taproot system. Another objective was to determine plant water use. A sunflower plant ( Helianthus annuus L. `Hysun 354') was grown in a column (0.20 m in diameter and height) with Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents; FAO-Eutric Fluvisols) containing 11 DPHP sensors. Results from the sensors were compared with those from the gravimetric method. Discrepancies between measurements of soil volumetric water content and changes of soil volumetric water by the DPHP and gravimetric methods were small (within 0.018 and 0.01 m 3 m −3, respectively). The sunflower had a small amount of nocturnal transpiration, and roots took up water at a higher rate near the surface of the soil than at deeper depths. The results showed that the DPHP technique can monitor volumetric soil water content in the presence of a taproot.

Measuring Soil Water Content under Turfgrass Using the Dual-probe Heat-pulse Technique

Measurements of soil water content near the soil surface often are required for efficient turfgrass water management. Experiments were conducted in a greenhouse to determine if the dual-probe heat-pulse (DPHP) technique can be used to monitor changes in soil volumetric water content (θv) and turfgrass water use. `Kentucky 31' Tall fescue (Festuca arundinacea Schreb.) was planted in 20-cm-diameter containers packed with Haynie sandy loam (coarse-silty, mixed, calcareous, mesic Typic Udifluvents). Water content was measured with the DPHP sensors that were placed horizontally at different depths between 1.5 and 14.4 cm from the surface in the soil column. Water content also was monitored gravimetrically from changes in container mass. Measurements started when the soil surface was covered completely by tall fescue. Hence, changes in θv could be attributed entirely to water being taken up by roots of tall fescue. Daily measurements were taken over multiple 6- or 7-day drying cycles. Each drying cycle was preceded by an irrigation, and free drainage had ceased before measurements were initiated. Soil water content dropped from ≈0.35 to 0.10 m3·m-3 during each drying cycle. Correlation was excellent between θv and changes in water content determined by the DPHP and gravimetric methods. Comparisons with the gravimetric method showed that the DPHP sensors could measure average container θv within 0.03 m3·m-3 and changes in soil water content within 0.01 m3·m-3.