|
| Soil Hydraulic Property Measurement Setup at the U.S. Salinity Laboratory |
| Data | Soil Properties Page |
- Saturated Hydraulic Conductivity
- Soil Water Retention
- Soil Water Retention (Hysteresis)
- Soil Water Retention (Temperature Effect)
- Saturated Hydraulic Conductivity (Temperature Effect)
Scientists have long recognized the critical role of soil properties in hydro-climate modeling. To better understand the complex and interdependent geophysical processes in the near-surface, we conducted an extensive soil property measurement campaign during the Southern Great Plains 1997 (SGP97) Hydrology Experiment,. Major objectives of this effort included: (1) measurement of soil physical, hydraulic, and thermal properties across the study region, and (2) use of our measurements as a basis for pedo-topo-vegetation transfer function modeling for extrapolating point estimates of soil properties to the larger scales. The second effort should be viewed within the context of regional-scale hydrology and soil-atmosphere-vegetation transfer (SVAT) schemes in general circulation modeling.
We collected soil cores from different depths of representative (soil, topography, and vegetation) sites based on a-priori information (Geographic Information System coverages and overlay maps) and concurrent site inspection. A total of 157 soil cores were collected from 46 quarter sections covering the more intensively monitored Little Washita (LW), El Reno (ER), and Central Facility (CF) areas (Figure 1). In addition to the surface cores, 4 or 5 subsurface soil cores were collected (based on soil morphologic characteristics) at depths of up to one meter at selected sites within the LW, ER, and CF areas. Soil cores collected from different sites and depths were analyzed in the laboratory for soil water retention, saturated and unsaturated hydraulic conductivities, bulk density, thermal conductivity, heat diffusivity, heat capacity, organic carbon, and soil texture. Selected soil cores were also used to measure soil water hysteresis (drying vs. wetting) and the effects of temperature on the hydraulic properties.
Site Description and Field Procedures
General landscape features in across the SGP97 region are shown in Figure 2. Typical vegetation in the study region are shown in Figure 3 and typical soil profiles in Figure 4. Soil cores (in brass cylinders, 5.3 cm diameter and 5.9 cm long, (Figure 5-B) at different depths were collected from representative (soil, slope, and vegetation) sites using thematic polygons of GIS overlay. A pick-up truck-mounted Giddings drilling rig (Figure 5-D) and a hand core or bulk density sampler (Figure 5-C) were used to extract soil cores. The Giddings rig was used for extracting profile core samples that were on the order of 100 cm or slightly longer. These core samples were used to tentatively classify the soil profile and determine the location of each soil horizon by using the texture (field method), and soil color. We took soil surface (0-9 cm depth) samples from 46 out of a total of 48 quarter-sections in the focus regions of LW, ER, and CF, Oklahoma. Using a-priori information and field site confirmation, one or more representative soil cores were collected from each quarter-section, with additional subsurface soil cores retrieved (based on soil stratigraphy) from depths up to 1-m at selected sites, leading to a total of 157 cores. Other features that we looked for in terms of stratifying the soil horizons were worm holes or animal burrows, root channels, and macroporous anomalies. We observed and noted the extent and density of the root systems. Once the soil had been classified and the depths of the different soil horizons defined, we decided to sample the depth increments for subsequent hydraulic property determination. For this purpose we excavated the soil above a particular depth with a large soil probe (4-6 in. o.d.) attached to the Giddings rig, and then took an "undisturbed" soil core using the bulk density sampler. The soil sample was encapsulated in 5.3 cm i.d. by 5.9 cm long brass ring (within the sampler). The brass rings were customized to fit inside the available Tempe cell apparatus (to be discussed below). At all but 12 locations we collected duplicate soil samples (approx. 300-500 g per depth) in order to measure field soil water content, soil organic carbon, and particle size distribution in the laboratory.
Antecedent soil moisture (water content at the time of sampling) and soil temperature measurements were made corresponding to most soil core sampling locations. The profile soil moisture content was measured gravimetrically using duplicate Giddings extracted soil cores that were sectioned into appropriate depth increments, placed in heavy-duty paper bags, and weighed immediately after being taken from the profile. At locations where only surface soil cores were taken, duplicate bulk density cores were used for measuring the (antecedent) field soil moisture content. At the end of the day all soil moisture samples were placed in an oven at 105 °C for 24-48 hours, then re-weighed. The soil temperature was measured in the field at different depths using a thermocouple thermometer (Baranant 100, Model 600-2820, Industrial Instruments and Supply, Southampton, Pennsylvania, (Figure 5-A) with a readout unit. Other pedological information such as soil color and structure were determined during the soil core extraction process. Correspondingly, the local (30-m diameter) topography, including the east-west and north-south slopes, were measured with a hand level (Abney Level, PECO). Positive or negative slope with respect to east and north were reported. For example, "-3 Degree E" indicates that east is the down gradient direction. General landscape, relative field position, microtopography, vegetation type, canopy height, canopy density, rooting depth, current weather, and other site specific observations were measured or approximated by visual inspection. Each sampling location was identified with a Differential Global Positioning System (DGPS, Figure 6) having a precision of ±10-m. Because of technical difficulties, some field data could not be measured/recorded on several occasions.
A complete particle size distribution function for each soil was measured. The basic hydrometer method outlined in Gee and Bauder (1986) (Figure 7) was used to determine the silt and clay fractions from approximately 50 microns to 1.4 microns. The same sample used for determining the particle size by the hydrometer method was used for determining the sand fractions between 2000 microns and 50 microns using a wet sieve method. We quantitatively transferred the sediment and suspension from our 1-L sedimentation cylinders through a nest of 11 sieves (2000, 1400, 1000, 700, 500, 355, 250, 180, 147, 105, and 90 microns). The sediment was washed through each sieve with a combination of a wash bottle and a gentle stream of deionized water from the tap. The sand fraction defined by each sieve was placed on a tarred evaporation dish, put in an oven at 105 °C for 24-48 hours, and subsequently weighed.
Bulk Density: The core method (Blake and Hartage, 1986) was used to measure the bulk density. We measured the soil bulk density of the surface soil horizons using a commercial sampler with known volume (Figure 5-C). For deeper depths we also used the Giddings drilling rig samples. The sample mass was measured in the field using a small portable battery-operated balance. These samples were weighed, dried, and reweighed to determine the field moisture content and the dry bulk density. In most cases we obtained at least two different estimates of the bulk density for each site and depth. Average of the two estimates was reported.
Soil Organic Carbon: Soil organic carbon content was measured directly by furnace combustion at 375 °C using a UIC Full Carbon System150 with a CO2 coulometer 5011 (UIC Inc., Joliet, IL).
Soil Structure and Color: Soil color, structure, and other pertinent profile data were gathered during soil coring with the truck-mounted Giddings probe. A Mansel soil chart was used to identify soil color under wet and dry conditions. Soil structure was defined by manual, hand and visual inspection.
Hydraulic Properties: The individual undisturbed soil cores were capped using air-tight plastic caps and weighed using a portable battery-operated scale after extraction from the soil profile. The samples were re-weighed upon delivery to the U.S. Salinity Laboratory, and the weight loss was recorded (weight loss was always less than 0.1g). The samples were stored at 4 °C until they could be processed.
Saturated Hydraulic Conductivity: All refrigerated samples were first allowed to temperature equilibrate for 24 hours at 25 °C. We used a constant head permeameter (Figure 8) to measure the saturated hydraulic conductivity as described in Klute and Dirksen (1986). Using a Mariott bottle, a dilute calcium chloride solution was allowed to infiltrate for 24 hours at a constant positive head of 2~3 cm. Then the effluent was subsequently collected in tarred bottles at regular time intervals. After reaching steady-state flow conditions we measured the flow rate over three timed intervals. The effluent was weighed after each interval and the saturated hydraulic conductivity calculated and averaged.Soil Water Retention: The soil water retention curve was measured using a combination of measurement techniques. We used tempe cells (Figure 9; Reginato and van Bavel 1962; Klute, 1986; Eching et al., 1994) for retention measurements between 5 cm and 500 cm of pressure head, and used the pressure plate apparatus (Klute 1986) for measurements at 333, 500, 1000, 3000, 8000, and 15000 cm of pressure head. The method is detailed in Klute (1986). A semi-automatic tempe cell apparatus was used to control and record the measurement of 22 tempe cells at one time (Figure 10; Jobes et al., 1999). The apparatus produced a series of outflow data as a function of time. We allowed the outflow to cease completely before changing to a higher pressure. With this approach we measured the dynamic multi-step outflow as well as an equilibrium retention point. The dynamic outflow data can be analyzed using inverse procedures to calculate the soil hydraulic parameters. The more conventional static measurements can be used to help refine the inverse problem as auxiliary data, thereby lessening non-uniqueness problems and improving convergence.
The soil cores were trimmed flat (as needed) and placed in a Tempe cell apparatus. With the soil core surface exposed to atmospheric pressure, the cores were saturated slowly (usually over 36-48 hours) from the bottom of the core. We used a deaired, calcium-rich infiltrating fluid as suggested by Klute (1986) to saturate the core. The water pressure head at the bottom of the core was never greater than 5 cm. This was done in an attempt to reduce air entrapment inside the core. During the final several hours of the saturation process a small vacuum was applied to the top of the core to remove any entrapped air.
After the soil cores were totally saturated, a 5-cm negative pressure head was applied for 24-48 hours to the cores until they reached equilibrium. Pressures were next sequentially (10, 20, 50, 100, 333, 500 cm of water) applied to the top of the soil cores until they reached equilibrium using an automated control system, with each pressure step approximately 48 hours. The cumulative outflow was recorded every 6 minutes using a data acquisition system and pressure transducers calibrated to measure water volumes (dynamic outflow). The mass of water present at each equilibrium pressure step was calculated using a mass balance approach. When 500 cm of pressure was reached, the cores were allowed to imbibe water for 24-48 hours at a pressure of 333 cm. The cores were subsequently removed from the Tempe cell, and the plastic caps replaced on the two ends and cores weighed. The cores were allowed to stay at room temperature (22 °C) for 24 hours, after which the thermal properties were measured. After measuring also the hysteresis and temperature effect on soil water retention (on selected cores as described below), and the thermal properties (on all cores as described below), the soil cores were dissected into five equal parts. The five sub-samples were placed on pressure plates and equilibrated at 333, 1000, 3000, 8000, and 15000 cm of pressure. The equilibration time varied between 10 days and 60 days depending upon the texture of the soil, and the pressure head. After equilibration, the soils were removed from their respective pressure plates and weighed, dried at 105 °C for 48 hours, and reweighed.
Static retention measurements are available for all 157 soil cores while dynamic outflow data are available only for approximately 127 soil cores. Dynamic outflow for the other soil cores were not obtained because of variety of reasons, including technical difficulties during the laboratory measurements.
Soil Water Retention (Hysteresis):Forty-four samples were selected for measurement of soil water retention hysteresis. We measured hysteresis using Tempe cells; each core was equilibrated at 500 cm pressure head following the drainage sequence, after which the pressure was decreased sequentially for imbibition to occur until the core was again saturated. The dynamic inflow rate was measured in exactly the same manner as the outflow, while a mass balance equation was used to calculate the water content.
Soil Water Retention (Temperature Effect): The same forty-four soil cores were also used to measure the temperature dependence of the soil water retention curve. This consisted of measuring the retention at 20 °C, 25 °C, and 30 °C by changing the room temperature before the cores were allowed to saturate. The equilibration time was 24 hours. The dynamic outflow was measured and the mass balance was again used to calculate the water content of the cores.
Saturated Hydraulic Conductivity (Temperature Effect): Besides soil water retention, we also measured the saturated hydraulic conductivity for selected soil cores at 20°C, 25°C, and 30°C room temperatures.
Soil Thermal Properties: We used the dual-probe heat-pulse technique of Bristow et al. (1994) (Figure 11) to measure the soil thermal properties, including thermal diffusivity, heat capacity and thermal conductivity. The basic dual-probe heat-pulse unit used in this study consisted of a heater and a sensor needle probe mounted in parallel. The probe was designed and built by Klutenberg et al 1998 (personal communication) and was made from thin stainless steel tubing. The spacing between the sensor and the heater probes was approximately 6 mm. Additional details about the probe construction can be found in Campbell et al. (1991) and Bristow et al. (1993, 1994).
The heat pulse was generated by applying a voltage from a direct current source to the heater for a fixed period of time (in our case 8 sec). We used a CR7 (Campbell Scientific Inc., Logan, UT) data logger to control the heat pulse, monitor the current through the heater, and measure the temperature of the sensor as a function of time. In our procedure we allowed the soil core samples to come to thermal equilibrium with the room temperature for periods exceeding 24 hours. At those times one end cap of the soil core was removed and the dual-probe inserted. A heat pulse of 8 sec duration was initiated and the temperature data collected using the data logger. The same procedure was used on the other end of the cores. Thus we obtained two measurements per core. The dynamic temperature data collected by the data logger was used next to estimate the soil thermal properties using a numerical inversion procedure developed by Welch et al. (1996).
Particle Size Distribution and Soil Texture:Our 18-20-point particle size data were grouped into sand (<2000- .05 µm), silt (50- 2 µm), clay (<2 µm) according to the USDA standard. The grouped data (% sand, % silt, and % clay) were used to derive the textural class of each soil sample according to the USDA textural triangle (Gee and Bauder, 1986).
Bulk Density and Porosity: Total soil porosity (Danielson and Sutherland, 1986) for each sample was calculated using the measured bulk density of the sample and a particle density of 2.65 g/cm3.
Soil Hydraulic Properties: The dynamic outflow and static retention data were used in the HYDRUS-1D inverse modeling procedure (Simunek et al., 1998) to estimate the van Genuchten-Mualem hydraulic parameters (van Genuchten, 1980) for each core. The inverse optimization procedure was based upon minimization of an objective function which expresses the discrepancy between the observed values and the predicted system response in terms of cumulative outflow and/or soil water retention. Initial estimates of the optimized hydraulic parameters were iteratively improved during the minimization process until a desired degree of precision is obtained. We performed the inverse optimization procedure twice with (i) equal weights for both dynamic flow and static retention data, and (ii) double weights for the static retention data as compared to the dynamic outflow data. For the soil cores with no dynamic outflow data, we used the RETC estimation procedure (van Genuchten et al., 1991) for estimating the soil hydraulic parameters from the retention data only (no value of Ks could be estimated in this manner). For completeness, the van Genuchten-Mualem hydraulic functions are defined as
Soil Thermal Properties: Similar
to the hydraulic properties, an HPC inverse optimization procedure (Welch et al., 1996) was used to estimate the soil
thermal properties based on the dual-probe heat-pulse data. The
computer model allows for simultaneous measurement of the volumetric
heat capacity (
c), thermal diffusivity (
), and the thermal
conductivity (
=c). The parameter estimation scheme
requires extraction of the temperature maximum Tm
and the time needed to reach the maximum temperature,
tm, from a temperature-time record. The constants
Tm and tm are subsequently used in
a heat conduction model for an infinite line heat source in order to
determine c,
and . The
approach involves a "single-point" parameter estimation scheme since
the model is fitted to the data at a single point, the maximum
temperature. Except for few exceptions, all heat pulse data produced
good fits with the model predictions, which suggests that reasonable
estimates of the soil thermal properties were generated.
Summaries of the soil physical, hydraulic, and thermal properties, and associated topographic, vegetation, and other ancillary information are available. Raw dynamic outflow, soil water retention (including hysteresis and temperatue effects), and particle-size distribution data are available only upon request to the PI. A more comprehensive report that contains all raw data will be made available to interested SGP participants from the U.S. Salinity Laboratory at a later date. Data are organized by ascending sample ID (1...157). Corresponding quarter section ID (LW1..23, ER1..16, CF1..9), allocated site number (1, 2,...), and/or their geographic location (latitude and longitude) are also tagged. In few occasions certain measurements were made twice which were identified as "DO OVER." Blank cells indicate "missing data." Any unreported sample for optimized hydraulic parameters indicates that simulation was "not successful." Updated results will be provided at a future date.
| ||||||||||||||||||||||||||||
The Excel soil property files for SGP97 is in the following GES DISC ftp site:
Excel soil property files for SGP97
Note: Users of the data should acknowledge and refer to this page and the PI
The Principal Investigator for the SGP97 Soil Property Measurement data set is
Binayak P. MohantyFor more information using SGP97 data from the GES DISC, contact:
U. S. Salinity Laboratory
450 W. Big Springs Road
Riverside, CA 92507
E-mail:bmohanty@ussl.ars.usda.gov
Voice: 909-369-4852 Fax: 909-342-4964
Hydrology Data Support Team
Goddard Earth Sciences Data and Information Services Center (GES DISC)
Code 610.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
E-mail: hydrology-disc@listserv.gsfc.nasa.gov
Voice: 301-614-5165 Fax: 301-614-5268
 |
Last update:Mon Oct 18 13:30:51 EDT 1999 Page Author: Hydrology Data Support Team -- hydrology-disc@listserv.gsfc.nasa.gov Web Curator: -- Stephen W Berrick NASA official: Steve Kempler, DAAC Manager -- Steven.J.Kempler@nasa.gov |
