Michael H. Young

Two- and three-parameter calibrations of time domain reflectometry for soil moisture measurement

Time domain reflectometry (TDR) is widely used to measure and monitor soil water. The commonly used calibration curve is the third-degree “universal polynomial” of Topp et al. [1980]. The most common refinement is calibration to a specific soil but still using four parameters (coefficients) from fitting a third-degree polynomial. Here we demonstrate that a three-parameter expression, θυ = aKaα +  )where the three parameters a, b, and α are determined by fitting water content θυ to the dielectric coefficient Ka). This form is consistent with the well-known mixing model. For an isotropic soil with homogeneous water distribution this expression is further simplified to two parameters by taking α=0.5. When α is 0.5, its calibration is equivalent to the linear calibration between θυ and the travel time along the waveguide. In addition, the simple three-parameter expression can be easily inverted without losing accuracy with regard to the original calibration. The TDR calibration expressed in a three-parameter form not only achieves a good fit but also conveys a physical connotation.

Flexible Time Domain Reflectometry Probe for Deep Vadose Zone Monitoring

Accurate determination of water content is an important aspect of most vadose zone monitoring programs. Real-time, continuous, in situ measurements of water content in relatively undisturbed conditions are usually limited to shallow soil horizons. We present a new methodology using time domain reflectometry (TDR) for water content monitoring in deep vadose zone horizons. The method uses flat, flexible, waveguides pressed against the wall of a borehole. The flexible TDR waveguides are attached to the outer side of a flexible sleeve filled with a liquid resin. The resin (e.g., a two-component urethane) generates hydrostatic pressure that forces the flexible waveguides against the borehole wall, ensuring a close fit to the irregular shape of the borehole walls. The probe can be used with either a standard TDR technique, which uses a cable tester (e.g., Tektronix 1520, Tektronix, Beaverton, OR) for collecting waveforms, or a water content reflectometer (e.g., model CS505, Campbell Scientific, Inc., Logan, UT), which provides a direct electrical output, which may be sampled using a data logger. Laboratory calibration experiments and a full-scale field experiment showed that the method is reliable and capable of providing accurate water content measurements in deep vadose zone horizons.

Quantifying the effects of phenology on ecosystem evapotranspiration in planted grassland mesocosms using EcoCELL technology

Use of plant phenological variables in models predicting evapotranspiration (ET) has largely relied on relatively simple (e.g., linear) relationships which may not be sufficiently accurate to predict small—yet ecologically significant—changes in plant phenology that are expected to occur in response to global climate change. A dearth of experimental data reflects the difficulties in quantifying these relationships against the background of large environmental variability that occurs in the field. Our main objective was to quantify how plant phenology (leaf area index [LAI] and root length density [RLD]) affect ET and its components during an entire vegetation cycle in large-scale model grassland (Bromus tectorum) ecosystems using the Ecologically Controlled Enclosed Lysimeter Laboratory (EcoCELL)—a unique open flow and mass balance laboratory. We also aimed to compare the three methods employed by the EcoCELL laboratory to measure ecosystem ET (whole-ecosystem gas exchange, weighing lysimetry, and weighing lysimetry combined with time domain reflectometry [TDR]) in order to independently confirm the performance of the unique gas exchange technology. Cumulative ET during the 190 days of the experiment measured with the three different methods compared very well with each other (mean errors <1%). We found that ET reached maximum levels at relatively low LAI (2–3), but as LAI increased beyond this value, small increase in transpiration were more than offset by decreases in soil evaporation, thereby causing declines in ET. A combined rectangular hyperbola (effects on transpiration) and linear (effects on soil evaporation) function between LAI and ET accounted for almost 90% of all variability in measured daily ET. RLD showed relationships to ET similar to those observed for LAI due to high covariance between RLD and LAI, but root length densities did not explain any additional variability in daily ET beyond that explained by LAI under the well-watered conditions of the experiment. Taken together, our results show that: (i) the EcoCELL mesocosm laboratory can precisely and accurately quantify hydrologic processes of large soil–plant monoliths under controlled environmental conditions; (ii) plant canopy phenological changes affect ecosystem ET, and the contribution of transpiration, in non-linear ways; (iii) these non-linear responses must be accounted for when assessing the consequences of changes in plant phenology—e.g., due to global environmental change—on ecosystem hydrology.

Diurnal fluctuations of tensiometric readings due to surface temperature changes

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 ± 15°C, 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 15°C 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.

A New Technique for Characterizing the Efficacy of Fugitive Dust Suppressants

Abstract The Portable In-Situ Wind Erosion Laboratory (PI-SWERL) instrument was evaluated for testing the effectiveness of dust suppressants for a range of native and constructed soils. The PM10 (particles with diameter ≤10 µm) emissions from dust suppressant-treated and untreated soil surfaces were measured periodically over 14 months. No statistically significant differences were found among soil surfaces treated with three dilution mixtures of the dust suppressant. The temporal variation of PM10 emissions from treated and untreated plots for native and constructed soil textures indicated that: (1) reductions of PM10 emissions by the dust suppressant were significant within 2–3 months after the application and diminished substantially thereafter, (2) decomposition of the protective treated layer resulted in high PM10 emissions for longer environmental exposure times, and (3) emissions from untreated soil surfaces declined over time because of the formation of a natural crust. These results demonstrated that the PI-SWERL can provide qualitative and quantitative information on PM10 emissions for a range of soil textures and can be used to estimate the effectiveness of dust suppressants exposed to actual environmental (i.e., weather and solar radiation) conditions over long periods of time.

Controls on Water Use for Thermoelectric Generation: Case Study Texas, U.S.

Large-scale U.S. dependence on thermoelectric (steam electric) generation requiring water for cooling underscores the need to understand controls on this water use. The study objective was to quantify water consumption and withdrawal for thermoelectric generation, identifying controls, using Texas as a case study. Water consumption for thermoelectricity in Texas in 2010 totaled ∼0.43 million acre feet (maf; 0.53 km3), accounting for ∼4% of total state water consumption. High water withdrawals (26.2 maf, 32.3 km3) mostly reflect circulation between ponds and power plants, with only two-thirds of this water required for cooling. Controls on water consumption include (1) generator technology/thermal efficiency and (2) cooling system, resulting in statewide consumption intensity for natural gas combined cycle generators with mostly cooling towers (0.19 gal/kWh) being 63% lower than that of traditional coal, nuclear, or natural gas steam turbine generators with mostly cooling ponds (0.52 gal/kWh). The primary control on water withdrawals is cooling system, with ∼2 orders of magnitude lower withdrawals for cooling towers relative to once-through ponds statewide. Increases in natural gas combined cycle plants with cooling towers in response to high production of low-cost natural gas has greatly reduced water demand for thermoelectric cooling since 2000.

A gas‐phase partitioning tracer method for the in situ measurement of soil‐water content

The purpose of this paper is to describe a gas-phase partitioning tracer method for the in situ measurement of soil-water content and to illustrate the application and performance of the tracer method in well-defined systems. The method is based on the use of a tracer test with nonpartitioning and partitioning tracers introduced into the system in the gas phase. Partitioning tracers dissolve into the water, which retards their gas-phase transport relative to that of the nonpartitioning tracers. Retardation of the partitioning tracers is a function of the amount of water present. The method provides an integrated field-scale value that complements smaller-scale methods, such as neutron moderation, and regional-scale methods based on remote sensing. Experiments were conducted in the laboratory and in a large weighing lysimeter to test the performance of the gas-phase partitioning tracer method. Soil-water contents estimated from the tracer tests were reasonably close to values obtained using gravimetric and time domain reflectometry measurements, indicating the tracer method has the potential to provide accurate measurements of soil-water content at the field scale.

Monitoring Vegetation Phenological Cycles in Two Different Semi-Arid Environmental Settings Using a Ground-Based NDVI System: A Potential Approach to Improve Satellite Data Interpretation

In semi-arid environmental settings with sparse canopy covers, obtaining remotely sensed information on soil and vegetative growth characteristics at finer spatial and temporal scales than most satellite platforms is crucial for validating and interpreting satellite data sets. In this study, we used a ground-based NDVI system to provide continuous time series analysis of individual shrub species and soil surface characteristics in two different semi-arid environmental settings located in the Great Basin (NV, USA). The NDVI system was a dual channel SKR-1800 radiometer that simultaneously measured incident solar radiation and upward reflectance in two broadband red and near-infrared channels comparable to Landsat-5 TM band 3 and band 4, respectively. The two study sites identified as Spring Valley 1 site (SV1) and Snake Valley 1 site (SNK1) were chosen for having different species composition, soil texture and percent canopy cover. NDVI time-series of greasewood (Sarcobatus vermiculatus) from the SV1 site allowed for clear distinction between the main phenological stages of the entire growing season during the period from January to November, 2007. NDVI time series values were significantly different between sagebrush (Artemisia tridentata) and rabbitbrush (Chrysothamnus viscidiflorus) at SV1 as well as between the two bare soil types at the two sites. Greasewood NDVI from the SNK1 site produced significant correlations with chlorophyll index (r = 0.97), leaf area index (r = 0.98) and leaf xylem water potential (r = 0.93). Whereas greasewood NDVI from the SV1 site produced lower correlations (r = 0.89, r = 0.73), or non significant correlations (r = 0.32) with the same parameters, respectively. Total percent cover was estimated at 17.5% for SV1 and at 63% for SNK1. Results from this study indicated the potential capabilities of using this ground-based NDVI system to extract spatial and temporal details of soil and vegetation optical properties not possible with satellite derived NDVI.

Spatiotemporal patterns in nutrient loads, nutrient concentrations, and algal biomass in Lake Taihu, China

Abstract Lake Taihu, Chinas third largest freshwater lake, exemplifies the severity of eutrophication problems in rapidly developing regions. We used long term land use, water quality, and hydrologic data from 26 in-lake and 32 tributary locations to describe the spatiotemporal patterns in nutrient loads, nutrient concentration, algal biomass, measured as chlorophyll a (Chl-a), in Lake Taihu. Point and nonpoint sources, as determined by chemical oxygen demand, contributed approximately 75 and 25% of the total nutrient loads to the lake, respectively. Spatial patterns in total phosphorus (TP) and total nitrogen (TN) concentrations in Lake Taihu strongly corresponded with observed loads from adjoining rivers with high concentrations proximate to densely populated areas. Chl-a concentrations exhibited spatial patterns similar to TP and TN concentrations. Generally, nutrient and Chl-a concentrations were highest in the northwestern region of the lake and lowest in the southeastern region of the lake. Seasonally, the largest nutrient loads occurred during summer. The annual net retention rate of TP and TN in Lake Taihu was approximately 30% of the total load. This study identifies regions of the lake and the watershed that are producing more nutrients to develop targeted management strategies. Reducing external P and N input from both point and nonpoint sources is obviously critical to address water quality issues in the lake. In addition, atmospheric deposition and resuspension of existing lake sediments also likely play a role in eutrophication processes and harmful algal blooms occurrence.

VARIABILITY OF WETTING FRONT VELOCITIES DURING A FIELD-SCALE INFILTRATION EXPERIMENT

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−1). 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.