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a Department of Soil Science, University of Wisconsin, 1525 Observatory Drive, Madison, WI 53706
b Department of Crop, Soil, and Environmental Sciences, University of Arkansas, 115 Plant Sciences Building, Fayetteville, AR 72701
c USDA-ARS, Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108
* Corresponding author (kmasarik{at}uwsp.edu).
Received for publication May 26, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
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Abbreviations: AETL, automated equilibrium tension lysimeter • ETL, equilibrium tension lysimeter • HDS, heat-dissipation sensor
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
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Various technologies exist for measuring the drainage flux through the soil matrix by intercepting water and manually or automatically sampling the accumulated volume of water. Two types of sampling equipment commonly used for this purpose are zero-tension and fixed-tension lysimeters. Both have potential problems that can cause large errors in drainage flux measurements (Radulovich and Sollins, 1987; Jemison and Fox, 1992). Zero-tension lysimeters require that soil above the collection pan be saturated before water will drain into them. During unsaturated conditions water flow will bypass a zero-tension lysimeter and underestimate actual drainage amounts (Jemison and Fox, 1992). With fixed tension a constant tension is applied to the lysimeter to remove water from the soil matrix. Fixed-tension lysimeters are often used to obtain the chemical concentration of soil solution, but no useful relationship was found between the water collected and the amount of water drainage (van der Ploeg and Beese, 1977; Angle et al., 1991).
Technology exists to sample water flux in unsaturated soil. Passive capillary samplers (PCAPS) have been used to measure water drainage (Boll et al., 1992; Knutson and Selker, 1996) and chemical leaching from the soil, and have been shown to have greater collection efficiency than zero-tension pan lysimeters (Zhu et al., 2002). Passive capillary samplers rely on the capillary potential of a fiberglass wick to apply tension and sample water drainage in unsaturated soil. The tension applied by the PCAPS is a function of the wick material, as well as the wick length and diameter, which must be carefully selected to match soil characteristics at installation sites (Boll et al., 1992). Although many studies show the effects of capillary wicks on the chemistry of the solution collected to be negligible (Holder et al., 1991; Boll et al., 1992; Knutson and Selker, 1996), issues have been raised regarding the resistance of the wick material to weathering and the suitability of PCAPS for geochemical studies of dilute soil solutions (Goyne et al., 2000). Recently, Gee et al. (2002) developed a vadose zone water fluxmeter using a PCAPS and a tipping bucket to continuously measure collected water volumes. Unfortunately, this device has a relatively small surface-sampling area (346 cm2) and installation requires the backfilling of soil above the collection device, which is a serious flaw in many studies that require drainage from undisturbed soil profiles.
Equilibrium tension lysimeters (ETLs) were developed by Brye et al. (1999) to address the problems associated with zero-tension lysimeters and fixed-tension lysimeters and can be installed below an intact soil profile. By adjusting lysimeter suction to match soil-water matric potential, ETLs maintain equilibrium between lysimeters and the bulk soil. In their design, heat-dissipation sensors (HDS) were used to measure the soil-water matric potential adjacent to the lysimeter. A human operator visited each ETL periodically, typically two or more times per week, and manually adjusted ETL suction to match that of the surrounding soil. Essentially this represents a frequently adjusted, fixed-tension lysimeter. Equilibrium tension lysimeters have maintained their integrity for over 8 yr at the original installation sites, demonstrating the suitability of ETLs for long-term studies. They have been used successfully to measure drainage fluxes (Brye et al., 2000) as well as nitrogen and carbon (Brye et al., 2001) and soluble phosphorus leaching (Brye et al., 2002) through the soil of prairie and corn (Zea mays L.) agroecosystems. Despite the improved performance over previous vadose zone samplers, ETLs are still prone to error due to fluctuations in soil-water matric potential that occur in between adjustments.
A refinement suggested by Brye et al. (1999) was to automate the vacuum controls to maintain a near-constant equilibrium with the soil. Recently, Lentz and Kincaid (2003) designed an automated system for ceramic-cup samplers to maintain equilibrium with the bulk soil. Ceramic-cup samplers were placed in the bottom of a stainless steel beaker filled with 15 to 20 cm of tamped soil and soil slurry before being pushed up into a soil-cavity ceiling to sample below an undisturbed soil column. Although laboratory experiments obtained an extraction–soil tension ratio to optimize the sampler for field soils, no attempt was made, in the field, to verify that equilibrium was maintained between the undisturbed soil and the ceramic cup in the bottom of the stainless steel beaker. Thus, even though the lysimeter tracked soil potential changes over time, there was no way to determine whether the soil potential at the top of the stainless steel beaker was near equilibrium with the bulk soil. Not only could the system chronically over- or underestimate the soil drainage, but large lags could occur between the time a drainage pulse reaches the top of the lysimeter and the lysimeter suction cup responds, leading to transient divergence of flow around the lysimeter. The system used a single vacuum source to maintain suction for 36 samplers, requiring a large network of tubing to connect individual sites to the vacuum source and a large power supply to operate.
The objective of our work was to develop and field-test a low-power, automated equilibrium tension lysimeter (AETL) to improve the response time of ETLs to soil-water matric potential fluctuations in the bulk soil and expand their use to more remote research locations. In addition, we also realized that an ETL, if it is properly controlled in true equilibrium mode, could be used as a water-flux meter if the water level inside of the lysimeter were continuously monitored. To keep up with the growing need for new technology to improve measurement and monitoring of drainage flux, we developed and tested, in the laboratory, a method for automated measurement of water level in an AETL. The level measurement device was not installed in the existing lysimeters because of the need to dig up the field site and remove lysimeters for sensor insertion, which would have disrupted the ongoing experiment.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
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The system was tested on four lysimeters in two replicate plots for chisel plow and no-tillage treatments in an agroecosystem at the UW Agricultural Experiment Station in Arlington, WI. All plots were planted continuous corn rotation and were fertilized at optimum application rates.
Automated Control System
Each system was designed to control equilibrium for two pan lysimeters and a schematic of the electrical and pneumatic components is shown in Fig. 1
. The AETLs were controlled by a datalogger (Model CR10X, Campbell Scientific, Logan, UT) and powered for 2-wk periods by a 12-V, 7.2 amp-hour rechargeable battery. Heat-dissipation sensors (HDS; 229-L, Campbell Scientific) connected to an excitation module (CE8; Campbell Scientific) were used to measure soil-water matric potential fluctuations in the bulk soil adjacent to each lysimeter at a depth of 1.4 m and in the soil directly above each lysimeter. The suction applied to the bottom of the porous plate was measured with a differential pressure transducer (PX170-014GV; Omega Engineering, Stamford, CT). Suction in the lysimeter was increased with a battery-operated vacuum pump (TD-2N; Brailsford and Company, Rye, NY) or decreased by bleeding lysimeter vacuum to the atmosphere, until matching was achieved between the HDS in the soil and the lysimeter. Four, 3-way, normally open, standard-mount solenoid valves fitted with 3.2-mm (1/8-in.) barb fittings (ETO-3-12 and 11752-3; Clippard Instrument Laboratory, Cincinnati, OH) were connected to the vacuum pump and pressure transducer with 3.2-mm (1/8-in.)-i.d. clear flexible PVC tubing (14-169-7A; Fisher Scientific, Pittsburgh, PA). A 12-V relay driver (A6REL-12; Campbell Scientific) in connection with the CR10X logger controlled the operation of the vacuum pump and 3-way valves. All equipment was enclosed in a 41- x 46-cm (16- x 18-in.) fiberglass enclosure (ENC 16/18; Campbell Scientific). Copper tubing (6.4-mm-o.d.) connected the vacuum line of each lysimeter to the enclosure and clear flexible PVC tubing connected the copper tubing to a 3-way valve. The total cost of this control system for two AETLs, excluding the datalogger and heat-dissipation sensors, is approximately $900.
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Every 10 min the program recorded soil-water matric potentials using the HDSs for the two lysimeters (A, B) in the bulk soil and in the soil directly above the lysimeters. The soil-water matric potential measurement of sensors in the bulk soil was used to control the suction inside the lysimeter, while the HDSs above the lysimeter were used to validate the equilibrium between the lysimeters and the soil. Because HDSs operate near the voltage resolution of the dataloggers, the signals were averaged over a 10-interval period (approximately 100 min). With the recorded soil-water matric potential, the program assigns 5 min to achieve equilibrium for each lysimeter, and continues this process every 10 min until equilibrium is achieved between the lysimeter and the bulk soil. The suction inside the lysimeter is set 2 kPa greater than the soil-water matric potential of the bulk soil to overcome the resistance of the porous plate.
The control program also contained an algorithm to prevent continuous operation of the pump if cavitation of the lysimeter occurred. The arbitrarily chosen indicator for this is a continuous duty cycle for 12 consecutive 10-min execution intervals; when this occurs, the program stops controlling the lysimeter and requires human attention for manual reset. Hourly averages of the following data were stored: soil-water matric potentials for all four HDSs, Lysimeter A and B suctions, vacuum pump time for Lysimeter A and B, and battery voltage.
Water drainage was collected throughout the year since the lysimeters were located below the frost layer in the soil. Lysimeters were sampled approximately once every 14 d for the majority of the year and once every 30 d during periods when the soil surface was frozen. During sampling the program operation was suspended while the leachate was collected from the pan lysimeter. The first liter of leachate was collected for chemical analysis, while any remaining volume was measured and discarded (Brye et al., 1999).
Heat-Dissipation Sensors
The ability of the AETL to maintain equilibrium with the soil is dependent on accurate soil-water matric potential measurements. Therefore, an accurate calibration of the HDSs is critical to maintaining equilibrium of the AETLs. Each HDS's individual response curve was determined to ensure accurate measurements when placed in the field. The air-dry value was obtained by measuring the response of the sensor when it was dry and suspended in air, while the saturated value was obtained after soaking the sensors in water overnight. It is important to note that vacuum saturation was not used in calibrating these sensors because sensors did not return to vacuum saturation values after a drying cycle. Because water drainage is minimal when soils are below field capacity and tension was not applied when the soil-water matric potential measured less than –35 kPa, the most critical part of the HDS calibration was near saturation. A pressure plate was used to force water out of sensors placed in a silty-clay-loam slurry (same soil as field installation) at pressures of 5, 10, 20, 30, 40, 50, and 100 kPa. Sensors were allowed to equilibrate at each pressure and measurements were obtained using a Campbell Scientific datalogger (Reece, 1996; J. Bilskie, personal communication, 2001). Heat-dissipation sensors were excited with electrical current for a period of 10 s and the difference in temperature of the thermocouple inside of the ceramic cylinder before and after heating was recorded. A linear calibration equation was determined by plotting the natural logarithm of the calibration tensions (kPa) against the measured temperature (°C) difference (Campbell Scientific, 1998). The logarithmic relation between tension and temperature differential held for all sensors to a tension of at least 100 kPa. However, for some sensors the linear relation did not fit the 10-MPa point for the air-dry temperature difference as suggested by Reece (1996); as a result, the calibration may lose accuracy when measuring tensions above 100 kPa and can only be considered as semiquantitative measurement at tensions greater than 100 kPa. On each calibration curve sensors at saturation appeared to have a temperature difference consistent with a 5-kPa tension, which may be the air-entry value of the ceramic. One remarkable observation related to the calibration stability of the HDSs was the agreement within a few kPa of the HDS above the lysimeter and the lysimeter suction, suggesting that the calibrations on the HDSs have been stable during 8 yr of continuous use.
Water Level Detection
A wide range of options were available for measuring liquid level. We chose to test a sensor that probably would already be in use at locations where AETLs might be deployed. A new and inexpensive soil moisture gauge was chosen that measures in the frequency domain (ECH2O probe; Decagon Devices, Pullman, WA). It requires little power and can be read directly using a CR10X datalogger. It was installed in an open-top lysimeter, diagonally from the upper front of the lysimeter internal cavity, just below the level at which the porous surface would be attached, to the lower rear of the cavity across the width of the lysimeter. Diagonal rather than vertical orientation was used to maximize the depth resolution of the measurement. For testing purposes the top was left off of the lysimeter. Water was added incrementally, and the water depth was measured with a measuring tape while the datalogger collected data from the ECH2O probe.
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Results and Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
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Hourly averages of the soil-water matric potentials and the lysimeter suction for a 9-d period are shown in Fig. 2 . The graph shows the response of soil-water matric potential above the lysimeter to the lysimeter suction, which was applied to match that of the bulk soil. The soil-water matric potential above the lysimeter decreased as the soil-water matric potential of the bulk soil decreased, verifying the operation of the lysimeter control system. The soil-water matric potential above the lysimeter measured slightly less than the soil-water matric potential in the bulk soil due to the 2 kPa of excess suction applied to the lysimeter.
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The soil-water matric potential of the bulk soil and daily precipitation amounts, along with cumulative drainage collected from the lysimeters and cumulative precipitation, are shown in Fig. 3 . The soil-water matric potential measurements show the response of the bulk soil to the conditions experienced within the corn agroecosystem. During the summer little drainage occurred because of the high evapotranspiration rates of the corn crop, which usually exceed the amount of precipitation during the growing season. It is generally during this time when the soil-water matric potential decreased below –35 kPa and the program stops the operation of the vacuum pump; however, data collection continued as normal. The program automatically reactivated the pump operation anytime the soil-water matric potential increased above –35 kPa. As the corn matured and evapotranspiration rates increased, the amount of water within the soil matrix decreased. The corresponding decrease in soil-water matric potential is evident during both growing seasons, in particular during 2002 from 15 August to 10 October when it reached a low of –230 kPa. The total precipitation from May to October was 503 mm in 2001 compared with only 356 mm in 2002, which may explain the nearly 200-kPa-lower minimum soil water matric potentials for the 2002 growing season. Since precipitation in 2002 was less than in 2001, the water demand of the crop was met by removing more of the water stored within the soil.
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A comparison was performed using data collected from the original ETLs designed by Brye et al. (1999) and the automated design. Results in Table 1 compare the variability of sampling methods using data collected during years with similar precipitation patterns. Similar precipitation patterns result in similar patterns of collected drainage volumes and drainage variability from the mean for the two periods should also be similar if the manual adjustment and automated control systems are equivalent. Two periods were chosen in which precipitation events of varying degrees of magnitude occurred with similar frequency. Precipitation events during the growing season were not considered when choosing periods, because very little drainage occurs during this period due to evapotranspiration. Results indicated that the variability from the mean of the new system to maintain equilibrium and measure leachate volumes was comparable with that of the original ETL system.
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Conclusions |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
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