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Abstract. Strategies for characterizing subsurface hydraulic properties often lack observations on actual water movement in soil. The goals of this study were to investigate the variability of wetting front velocities during a field-scale infiltration experiment at the Maricopa Environmental Monitoring site, Maricopa, Arizona (surface area: 50 by 50 m). Wetting front velocities and soil textural components were analyzed as local and effective parameters. Local representation was done by measuring the wetting front travel time through specific depth increments (normally 0.5 m) to a depth of 3 m. Effective representation was done by measuring travel time from ground surface to the depth of interest in 0.5-m increments from the 1-m to 3-m depths. Textural components were represented similarly. One-way analysis of variance (ANOVA) tests were run on wetting front velocities and soil textural components, with spatial location of neutron probe access tubes and soil sampling points classified as replicates and depth intervals classified as independent treatments. The results showed that mean local velocities varied significantly with depth (range = 20-44 cm d-l). Effective front velocities varied much less (range = 22-25 cm d -1) and were found to be statistically the same, even though differences in soil texture and strong layering were observed. As the total wetting depth increased, variance in effective wetting front velocity decreased, with a coefficient of variation of wetting front arrival of only 9% at the 3-m depth. The decline in the variance of the front velocity is consistent with previous findings that the variance of effective parameters reduces as the measurement scale increases. IntroductionUnderstanding the spatial variability of soil hydraulic properties is needed to more accurately predict field-scale behavior of water movement and solute transport in unsaturated soils. The properties can be determined using two basic approaches. Core samples can be collected in the field and analyzed in the laboratory. These data are representative only at the local scale. Moreover, data obtained using this approach could be affected by (1) the disturbance of soil samples during collection and transport and (2) errors and uncertainties in the laboratory methods used to characterize the physical, hydraulic, and transport properties. Correctly integrating the core data into representative averages that maintain the statistical structure of the point-scale samples, so that the information can be used in predictive models, is another problem.The approach, employed in this paper, uses observed spatial variability of field-scale water flow or solute transport to make inferences about spatial variability of soil textural, hydraulic, or
Abstract. Strategies for characterizing subsurface hydraulic properties often lack observations on actual water movement in soil. The goals of this study were to investigate the variability of wetting front velocities during a field-scale infiltration experiment at the Maricopa Environmental Monitoring site, Maricopa, Arizona (surface area: 50 by 50 m). Wetting front velocities and soil textural components were analyzed as local and effective parameters. Local representation was done by measuring the wetting front travel time through specific depth increments (normally 0.5 m) to a depth of 3 m. Effective representation was done by measuring travel time from ground surface to the depth of interest in 0.5-m increments from the 1-m to 3-m depths. Textural components were represented similarly. One-way analysis of variance (ANOVA) tests were run on wetting front velocities and soil textural components, with spatial location of neutron probe access tubes and soil sampling points classified as replicates and depth intervals classified as independent treatments. The results showed that mean local velocities varied significantly with depth (range = 20-44 cm d-l). Effective front velocities varied much less (range = 22-25 cm d -1) and were found to be statistically the same, even though differences in soil texture and strong layering were observed. As the total wetting depth increased, variance in effective wetting front velocity decreased, with a coefficient of variation of wetting front arrival of only 9% at the 3-m depth. The decline in the variance of the front velocity is consistent with previous findings that the variance of effective parameters reduces as the measurement scale increases. IntroductionUnderstanding the spatial variability of soil hydraulic properties is needed to more accurately predict field-scale behavior of water movement and solute transport in unsaturated soils. The properties can be determined using two basic approaches. Core samples can be collected in the field and analyzed in the laboratory. These data are representative only at the local scale. Moreover, data obtained using this approach could be affected by (1) the disturbance of soil samples during collection and transport and (2) errors and uncertainties in the laboratory methods used to characterize the physical, hydraulic, and transport properties. Correctly integrating the core data into representative averages that maintain the statistical structure of the point-scale samples, so that the information can be used in predictive models, is another problem.The approach, employed in this paper, uses observed spatial variability of field-scale water flow or solute transport to make inferences about spatial variability of soil textural, hydraulic, or
Abstract. Pressure fluctuations in tensiometers in response to temperature changes are examined. Mechanisms considered include temperature variation within an air gap at the top of the tensiometer, the air gap size, saturated water vapor pressure, and hydraulic conductivity of the soil. Pressures measured in a tensiometer generally fall between two simplified, limiting cases. The first limiting case assumes that the tensiometer cup is impermeable for water. This leads to very high fluctuations as air and soil temperatures change. For a cyclical temperature of 35 ___ 15øC, variations in water pressure inside the cup can be ___70 cm water head. For the second limiting case the water moves freely between the tensiometer and the soil, which leads to more stable readings, within _+ 1 cm for the above 15øC fluctuation. While cup impedance was found to be a negligible factor for all cases considered, the analysis presented here suggests that conductivity of the soil immediately around the cup is the main factor governing temperature-induced pressure fluctuations inside the cup. BackgroundTensiometers are widely used to evaluate soil water potentials in the wet range. Documentation and criteria for successful operation are extensive and well known [cf. Cassel and Klute, 1986]. An assumption for using tensiometers is that the soil water is in equilibrium with water inside of the device where the pressure is measured, either within standing water inside of the cup or in a gas phase existing at the upper end of the column. For the older style tensiometers the gas phase is minimized by servicing the device to remove gases which enter by leaking through the cup or coming out of solution in the water.The newer "puncture-type" tensiometer [Martbaler et al., 1983] is designed to include an air gap of ---2 cm at the top of the device to allow insertion of a needle to measure the pressure with a portable transducer. The air gap allows for a more gradual (seconds instead of milliseconds) change in pressure at the transducer membrane, and thus it eliminates membrane rupture. It also prevents the needle from filling with water and leads to a device which requires less servicing. An increase in air pressure due to insertion of the needle can be taken into account by a "double-puncture technique" [Greenwood and Daniel, 1996]. An intentional air gap can also be used for monitoring in the deep vadose zone by measuring the soil water pressure using a pressure transducer beiow the standing water level. In this case, a considerable air gap can be used, but the soil water pressure at the cup follows as long as the pressure transducer is below the standing water level.
We study the probabilities that monitoring systems will be capable of detecting subsurface contaminant plumes. The analysis is an extension of previous research which focused on detection of ore bodies and contaminant releases to the subsurface using probabilistic models; the previous research assumed that releases to the subsurface are elliptical in shape, and that detections are absolute when a monitoring point intercepts a release. New features are introduced into the analytical framework that include irregular, rather than regular, sampling arrays, as well as nonuniform probabilities of a release occurring at a particular location of the site. These features allow the user to optimize the location of sampling devices based on site knowledge, by concentrating monitoring locations where a release is believed more likely. The stratified Monte Carlo approach used in this paper is tested on a number of cases with uniform and nonuniform sampler distribution and release probabilities. The results provide statistical probabilities that one is capable of finding releases of different sizes with a system of monitoring points.
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