Southern Great Plains 1997 (SGP97) Hydrology Experiment Plan
Section 4 - Soil Physical and Hydraulic Properties

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Table of Contents and Executive Summary            
1. Overview
2. Soil Moisture and Temperature
3. Vegetation and Land Cover
4. Soil Physical and Hydraulic Properties
5. Planetary Boundary Layer Studies
6. Satellite Data Acquisition
7. DOE ARM CART Program
8. Oklahoma Mesonet Program
9. Operations
10. Data Management and Availability
11. Science Investigations
12. Sampling Protocols
13. Local Information
14. References
15. List of Participants


4.1. Introduction

This section of the experiment plan for SGP97 describes supporting work in the area of soil physical and hydraulic property characterization and soil and landscape information resources that will be used by SGP97 scientists in both pre-mission preparation and post-mission analysis. The nature and properties of soil are a controlling element in the distribution of soil moisture and, ultimately, in land surface-atmosphere interaction processes. Knowledge of the physical and hydraulic properties of the soil in the SGP97 study area will facilitate correlation of ground and remotely sensed observations of soil moisture and support extrapolation into unmonitored areas. Ultimately, the labor and expense in collecting ground observations of soil moisture in remote sensing mission support will require heavier reliance on existing soil survey and characterization information. This will be of particular importance as we use satellite platforms with global coverage

4.2. Soils of the Region

The SGP '97 area lies predominantly within the Central Rolling Red Prairies and Central Rolling Red Plains land resource areas of Oklahoma (Gray, 1976). A small part of the total experimental area is contained in the Cross Timbers and Bluestem Hills land resource areas. The Central Rolling Red Prairies and Plains areas are smooth to rolling land which are underlain by dominantly red sedimentary strata. Stream gradients are gentle and relief averages only 30 m in smoother portions, with some local relief being greater, particularly in the more rugged southwestern portion of the area. Annual rainfall ranges from 35 inches in the east (Red Prairies) to about 28 inches in the west (Red Plains). Soils in the region are dominantly Mollisols (grassland soils) which reflect slow leaching (low precipitation - subhumid to semiarid climate) with relatively large annual additions of organic matter.

4.3. Soil Survey Resources

4.3.1. SSURGO

The NRCS has not yet created certified digital SSURGO (Soil Geographic Database) products for counties in the SGP97 study area. The detailed county-level soil survey maps for the three counties (Grady, Caddo, and Commanche) that contain the Little Washita River watershed have been digitized independently by the USDA-ARS. These data will be available from the USDA-ARS Grazinglands Research Center in El Reno, OK

4.3.2. STATSGO

A multi-layer soil characteristics data set for the conterminous United States (CONUS-SOIL) that specifically addresses the need for soil physical and hydraulic property information over large areas has been developed at Penn State's Earth System Science Center (ESSC). The State Soil Geographic Database (STATSGO) developed by the United States Department of Agriculture - Natural Resources Conservation Service (USDA-NRCS) served as the starting point for CONUS-SOIL. Geographic information system (GIS) and Perl computer programming language tools were used to create map coverages of soil properties including: soil texture and rock fragment classes, depth-to-bedrock, bulk density, porosity, rock fragment volume, particle-size (sand, silt, and clay) fractions, available water capacity, and hydrologic soil group. Complete documentation of the elements of the data set, as well as the original STATSGO data, and the procedures used to generate each of the elements of CONUS-SOIL are described on a WWW server at A subset of the full 48-state CONUS-SOIL is located at

4.3.3. MIADS

The Map Information Assembly and Display System (MIADS) was developed by the Oklahoma state office of the NRCS in 1981. MIADS is a 200 m (4 ha.) digital raster data set in a UTM map projection. It is based on previously published detailed county-level soil survey maps and was developed with adherence to NRCS standards. The data were originally captured as a series of county map files and are available from the Oklahoma NRCS office in ASCII file format. The Biosystems and Agricultural Engineering Department at Oklahoma State University developed a statewide data set by merging the 77 county map files. For this map coverage, the soil attributes database and the corresponding spatial data base were joined in ARC/INFO. The resulting data format and structure is quite similar to the STATSGO and CONUS-SOIL products mentioned previously. Further information on this statewide coverage is available from Gabriel Senay, Postdoctoral Fellow at Oklahoma State University, Stillwater, OK (e-mail:

4.4. Soil Characterization Data

A concerted effort including literature review, and contacting scientists at the universities and federal and state laboratories will be made to locate water retention, hydraulic conductivity and associated physical soil property data on Oklahoma soils. The data will be catalogued and reformatted into a consistent format and made available to SGP97 collaborators.

4.4.1. NRCS

The NRCS National Soil Survey Center (NSSC), through its Soil Survey Laboratory (SSL) maintains analytical data for more than 20,000 pedons of U.S. soils. Standard morphological pedon descriptions are available for about 15,000 of these pedons. This information includes physical, chemical, and mineralogical data on samples taken in support of the soil survey activities of the NRCS. Although soil hydraulic properties are rarely found within this database, the information that is available on other aspects of soil physical properties may be of relevance for soil moisture modeling research. The relevant pedons for the SGP97 study area will be extracted from this database and the tabular data made available to SGP97 researchers via the WWW.

4.4.2. Oklahoma State Mesonet

Eleven Oklahoma Mesonet sites are within the SGP97 study area or very close to its boundaries. Six of these sites (Acme, Apache, El Reno, Kingfisher, Marshall, and Blackwell) are equipped with soil moisture sensors -- automated heat-dissipation sensors at four depths, and a single TDR probe for periodic monitoring of five soil layers. At the time of sensor installation, soil samples were collected from each of the four depths (5, 25, 60, and 75 cm). A particle size analysis has been conducted for each sample according to ASTM D422-63, and the soil textural classification has been determined.

4.4.3. Sampling of Soil Physical and Hydraulic Properties

Soil moisture content in the shallow subsurface can most often be described as a variably-saturated phenomenon governed by mass and energy balance. Driving forces for this include (transient) precipitation, antecedent soil moisture content (soil factor), overland runoff (soil and landscape factors), downward infiltration (soil factor), upward exfiltration or evapotranspiration (vegetation factor), soil water retention, and hydraulic conductivity properties. In most soils the constitutive relationships between soil moisture content and soil hydraulic or soil matric potential are nonlinear in nature and thus complicate the transient flow problems. This warrants site-specific measurement of soil hydraulic properties to correlate and extrapolate the transient soil moisture information to deeper depths. This information will be valuable for testing soil moisture data from other sources as well as for modeling variably-saturated flow from shallow surface horizons to deeper soil profile and groundwater aquifers for global water balance. Sites

Site selection will be based primarily on whether there are profile soil moisture measurements at a site, ease of access, and representativeness of the site. Potential sites have been indicated in Table 5. Sampling Core Extraction

Soil cores at different depths will be collected from representative (soil, slope, and vegetation) sites using thematic polygons generated via GIS overlay. Nested grids encompassing two or more adjacent quarter sections of higher variability will be used for this purpose. At least two samples (replicates) will be collected for different combinations of soil, topography, and vegetation. Moreover, sampling will be repeated at nearby locations at least two times (e.g., before and after harvest of crop) during the SGP-97 experiment for estimating any temporal variability of soil hydraulic properties. These soil cores will be used for soil water retention, hydraulic conductivity, and texture analysis in the laboratory. Surface Characterization

Alabama A & M University will also analyze soil samples for their hydraulic characteristics at the sites of the HSCaRS soil profile stations (6 sites at 5 depths). Soil profiles will be described and sampled for texture, hydraulic conductivity, bulk density and water retention characteristics. A representative grass and winter wheat field in the Little Washita basin will also be sampled (up to 50 samples per field) for surface hydraulic properties. They will use a 3 inch diameter coring tool and aluminum rings. Additional sampling is possible if support is provided for rings and laboratory labor. Laboratory Analysis

The U.S. Salinity Lab will determine the soil water retention and hydraulic conductivity functions for up to 100 soil cores. Approximately 5 different depths based on soil stratigraphic information at 20 different sites will be selected at Little Washita, El Reno, and Lamont. Soil cores (e.g., using brass cylinders of 2 1/4" O.D. and soil core sampler cat. no. 200, Soil Moisture Equipment, Santa Barbara, CA) collected from different selected sites and depths will be preserved and transported to the U.S. Salinity Lab, Riverside, CA. Soil water characteristics (draining curve) of these cores at several soil water suctions between 0 - 15 bar will be measured using pressure cells and pressure plat extractors following a multi-step outflow experiment. All these experiments will be conducted in constant temperature chambers to minimize any temperature effect on soil hydraulic properties. Subsequently these data will be used to determine unsaturated hydraulic conductivity functions adopting predicative approaches (Mualem 1986) by means of the RETC computer code (van Genuchten et al. 1991).

4.5. Topographic Data

4.5.1. USGS 1 km and 3-arc second

Digital elevation information for the SGP97 study area is available in three grid resolutions: 1-km, 3-arc second (~100 m), and 30 m. The 1-km and 3-arc second data were obtained from the USGS and are available at the Penn State SGP WWW site:

4.5.2. ARS Little Washita 30 m

The 30 m (7.5 minute ) USGS DEM's for the Little Washita River Basin are available from the USDA-ARS Grazinglands Research Center in El Reno, OK. The other intensive study sites in the SGP97 study area (El Reno and the ARM/CART sites) do not have 30 m DEM data available.

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