Heat-pulse Sensors Research Articles

POROUS MEDIA MATRIC POTENTIAL AND WATER CONTENT MEASUREMENTS DURING PARABOLIC FLIGHT

Control of water and air in the root zone of plants remains a challenge in the microgravity environment of space. Due to limited flight opportunities, research aimed at resolving microgravity porous media fluid dynamics must often be conducted on Earth. The NASA KC-135 reduced gravity flight program offers an opportunity for Earth-based researchers to study physical processes in a variable gravity environment. The objectives of this study were to obtain measurements of water content and matric potential during the parabolic profile flown by the KC-135 aircraft. The flight profile provided 20-25 s of microgravity at the top of the parabola, while pulling 1.8 g at the bottom. The soil moisture sensors (Temperature and Moisture Acquisition System: Orbital Technologies, Madison, WI) used a heat-pulse method to indirectly estimate water content from heat dissipation. Tensiometers were constructed using a stainless steel porous cup with a pressure transducer and were used to measure the matric potential of the medium. The two types of sensors were placed at different depths in a substrate compartment filled with 1-2 mm Turface (calcined clay). The ability of the heat-pulse sensors to monitor overall changes in water content in the substrate compartment decreased with water content. Differences in measured water content data recorded at 0, 1, and 1.8 g were not significant. Tensiometer readings tracked pressure differences due to the hydrostatic force changes with variable gravity. The readings may have been affected by changes in cabin air pressure that occurred during each parabola. Tensiometer porous membrane conductivity (function of pore size) and fluid volume both influence response time. Porous media sample height and water content influence time-to-equilibrium, where shorter samples and higher water content achieve faster equilibrium. Further testing is needed to develop these sensors for space flight applications.

Mathematical analysis of heat pulse signals for soil water flux determination

Soil water flux is an important parameter in studies of runoff, infiltration, groundwater recharge, and subsurface chemical transport. Heat pulse sensors have been proposed as promising tools for measuring soil water fluxes. To date, heat pulse methods have required cumbersome mathematical analyses to calculate soil water flux from the measured data. We present a new mathematical analysis showing that a simple linear relationship exists between soil water flux and the natural log of the ratio of the temperature increase downstream from the line heat source to the temperature increase upstream from the line heat source. The simplicity of this relationship makes heat pulse sensors an attractive option for measuring soil water fluxes. In theory, this method is valid for fluxes with magnitudes between 10−4 and 10−7 m s−1. The range of measurable fluxes is defined by temperature measurement resolution at the lower end and by the assumptions used in the analysis at the higher end.

The role of soil hydraulic conductivity on the spatial and temporal variation of root water uptake in drip-irrigated corn

A multiplexed TDR system and a heat-pulse system for stem sap flow measurements were used to determine the spatial and temporal pattern of root water uptake in field-grown corn. The TDR probes, 0.15 and 0.30 m in length, were buried vertically in the soil profile to a depth of 0.95 m below the soil surface and heat-pulse sensors were installed on the plant base. Nocturnal readings from TDR probes were used successfully to differentiate the two components of moisture change: root uptake and net drainage. The instantaneous rate of water extraction by the plant measured by the heat-pulse system agreed well with the integrated rate of root water uptake measured frequently (at half-hour or hourly intervals) by the TDR probes. This agreement enabled further exploration into the cause of the evolution of the spatial and temporal patterns of root water uptake during a drying cycle. The results indicated that right after irrigation in the well-watered soil profile, it is the spatial distribution of the roots that mainly determines the typical pattern of root extraction, in addition to the fact that the roots near the plant base are more effective than those farther away. The higher density and effectiveness of the roots near the plant base dry the soil rapidly so that soil hydraulic conductivity soon becomes a limiting factor for water uptake. Further analysis revealed that a decrease in root uptake occurs near the plant base under a given atmospheric demand when the relative bulk soil hydraulic conductivity decreases to 0.002Kr. This suggests that low conductivity (high resistance) in the soil near the plant base is the initial cause for downward and lateral shifting of the root uptake pattern. Note that this critical value of hydraulic conductivity is not universal since it depends on the soil type and atmospheric water demand during the period under observation. Therefore, prior to the application of moisture content or suction head as measures of water availability or to control irrigation scheduling, it is suggested that these parameters be calibrated by the soil K(θ) or K(ι) curves, respectively, for the expected atmospheric water demand for the specific crop and growing period.

‘Daleks’ and ‘Whirligig’ used to uncover the fate of intercepted light by an apple tree canopy

Canopy light interception and distribution has a major influence on CO2 fixation and the growth and productivity of fruit trees. Spatial and temporal variations in microclimate alter the gas exchange from the leaves and other organs (fruit, branch, etc.). The variable nature of the light environment within the tree crown, and other factors such as leaf age and various stresses (water, diseases, pests) can often impair our ability to extrapolate whole-canopy net carbon exchange (NCE) and transpiration from a small sample of single-leaf measurements. This paper compares two methods for estimating whole-canopy gas exchange. An open-top, through-flow whole-canopy cuvette (Wunsche & Palmer, 1997, HortScience, 32:653-658) was used to enclose the above-ground portion of a 4 year-old apple tree (Malus domestica Borkh. cv. `Braeburn? on M.9 rootstock). NCE and transpiration from the whole canopy was measured over a one week period in mid-season. Measurements of light interception were made on a neighbouring apple tree of the same age and of a similar growth habit using a `whole-plant? radiometer known as the Whirligig (McNaughton et al., 1992, Plant Cell Environ., 17:1061-1068). Data from the Whirligig were used to estimate the distribution of PAR radiation over the total leaf area of the tree. This PAR distribution was then combined with leaf stomatal conductance and leaf net photosynthesis response functions to estimate the rates of whole-canopy NCE and transpiration. The Whirligig calculations of transpiration were in good agreement with data from the whole-canopy cuvette, and with independent measurements of sap flow recorded using heat-pulse sensors located in the trunk of the cuvette tree. The Whirligig estimates of NCE were also comparable to values obtained from the whole-canopy curvette. We conclude that both instruments provide an integrated approach for whole-plant productivity studies

Determination of transpiration in irrigated grapevines: comparison of the heat-pulse technique with gravimetric and micrometeorological methods

The use of a heat-pulse technique to monitor sap flow from which transpiration can be deduced was evaluated in ungrafted grapevines (Vitis vinifera L., cv. Sultana) under glasshouse and field conditions. There was a significant degree of agreement between daily transpiration deduced from heat-pulse velocity (T hp) and that determined directly by gravimetry in the glasshouse and by calibration using the Penman-Monteith equation in the field. Comparison throughout the growing season of T hp to transpiration calculated with the full Penman-Monteith equation produced a high coefficient of determination (r 2=0.69). A similar comparison of T hp with transpiration calculated with only the aerodynamic component of the Penman-Monteith equation produced a non-linear relationship, due to the equation over-estimating transpiration relative to T hp at high vapour pressure deficits (i.e. above 2.5 kPa). Values for total seasonal transpiration measured with heat-pulse sensors or calculated with either the full Penman-Monteith equation or with only the modified aerodynamic component of the equation were within 10% of each other. Transpiration (T hp) in grapevines with an adequate supply of soil water was shown to be coupled to ambient air conditions.

295 Controlling Crop Growth Media Moisture with Sensors

Tight control of growth media moisture content is needed when plant growth systems employ shallow root zones or for cultivating fast-growing plants (i.e., crops). Poor control of moisture can affect both growth rate and plant quality by either excessive watering (waterlogging) or drought events. We evaluated the performance of two types of moisture sensors: tensiometers and heat-pulse moisture sensors. The output from each sensor type was evaluated as a function of volumetric moisture content in 1 to 2 mm Turface. The tensiometers were more sensitive between 30% and 60% volumetric moisture content, and their output was nonlinear because they measure water potential directly. In contrast, both the sensitivity and the output of the heat-pulse moisture sensors, as a function of volumetric moisture content, were linear. The heat-pulse moisture sensors were used to control moisture content in a shallow root zone, whereby water was added or removed from the media through a porous tube using peristaltic pumps. Moisture content in the media could be maintained within ±2% of setpoint for moisture contents ranging from 20% to 100% volumetric moisture content. The heat-pulse sensors were better suited for controlling media moisture because of their linear output and because of their constant sensitivity as a function of volumetric moisture content.

Observations of evapotranspiration in a break of slope plantation susceptible to periodic drought stress.

Break of slope (BOS) plantations are advocated as a means of water table control in areas where groundwater flows through colluvial deposits overlying low permeability bedrock. It is also believed that BOS plantations can supplement their water use requirements by exploiting shallow groundwater at the breaks in topographic slope. We compared measurements of BOS plantation and pasture evapotranspiration during spring, when the weather was warm and soils moist, and late summer when drought conditions prevailed. Microlysimeters and ventilated chambers were used to determine pasture and plantation floor evaporation, and heat pulse sensors were used to determine transpiration of the plantation. In spring, pasture evapotranspiration was 65% of that of the plantation, whereas, in summer, pasture evapotranspiration was equivalent to only 35% of that of the plantation. Rainfall interception by the canopy of the plantation was twice that of the pasture, reinforcing the notion that trees can help reduce groundwater recharge and alleviate dryland salinity and waterlogging. During the summer drought period, daily plantation transpiration was only 20% of that measured during spring, suggesting that the plantation was not utilizing groundwater supplies but was instead drawing from soil water supplies. This hypothesis was supported by the comparison of relative abundances of the isotopes of water ((2)H and (18)O) in soil and wood samples. We conclude that the BOS plantation is not behaving in the manner predicted, and our findings raise doubts about the predicted advantages of establishing plantations in break of slope positions.

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.

Partitioning of seasonal evapotranspiration from a commercial furrow-irrigated Sultana vineyard

Seasonal partitioning of evapotranspiration (ET) between transpiration by grapevines (Vitis vinifera) (T gp) and by cover crops of a ryegrass/clover mixture (T cc), and soil evaporation (E s) was performed for a furrow-irrigated vineyard during the 1994/1995 and 1995/1996 growing seasons in south-eastern Australia. ET, determined with a water balance approach, averaged 622 mm. The ET rate averaged over the two seasons increased from 2 mm day–1 in spring (September to November), when it was dominated by T cc, to peak rates of around 5 mm d–1 in summer (December to February) when it was dominated by E s. T gp, determined with either heat-pulse sensors or the Penman-Monteith equation, attained peak rates of 0.75 and 0.98 mm d–1, or 6.2 and 8.1 l vine–1 day–1 in the first and second seasons, respectively. Total seasonal T gp of 109.1 mm (900 l vine–1) in 1994/1995 and 118.8 mm (980 l vine–1) in 1995/1996 constituted just 18 – 19% of total ET. T cc totalled 214 mm (34% of ET) in the first season, when pasture cover was sparse and present for 5 months of the growing season (September to February), and 196 mm (30% of ET) in the second season when pasture cover was heavy but present for only 3 months (September to November). E s averaged 49% of total ET over both seasons. At least 30% of water used for ET was contributed by antecedent soil water in both seasons. The crop factor (K c) was largely constant throughout the season with an average value of 0.48. The depletion pattern of soil water indicated that the vine explored the soil profile well beyond 1.0 mm depth and almost evenly up to a distance of 1.5 m from the trunk. Water use efficiencies for fresh fruit yield (WUE), i. e., the ratio of fruit weight to total water use at harvest,were 13.3 and 40.5 kg ha–1 mm–1 when based on ET in 1994/1995 and 1995/1996, respectively, and 84.0 and 211.1 kg ha–1 mm–1, respectively, when based on T gp. The T gp data were used to verify three models of vine transpiration developed in an earlier study. Models based on the green area index or on fraction of incident radiation intercepted by the vine canopy produced good agreement with T gp. The model based on canopy resistance performed poorly, indicating the difficulty of extrapolating the stomatal response to environmental variables from one set of experimental conditions to another.

Seasonal Water Use of a Kiwifruit Vine: Measurements and a Model

We recorded canopy development and stomatal function of a kiwifruit vine for the purpose of calculating the seasonal water use by the crop. Canopy development was described using an empirical “S-shaped” curve fitted to weekly measurements of the vine's leaf area. Stomatal conductance was described using a semi-empirical model based on the incident radiation, and the ambient vapor pressure deficit of the air. These two descriptors, leaf area and stomatal conductance, were combined with meteorological data to calculate vine transpiration via the Penman–Monteith model. Transpiration rates calculated at 30-min intervals were in good agreement with the instantaneous rates of sap flow measured by heat-pulse sensors located in the vine stem. The measured and calculated transpiration remained in concert throughout the experiment, thereby confirming the Penman-Monteith model as a robust and suitable model to describe the seasonal water use by kiwifruit vines. The model validation enables confident predictions of crop water use and thus aids irrigation allocation for kiwifruit crops.