1. OVERVIEW

Soil moisture is fundamental in several disciplines of the Earth sciences. The need for a suitable approach to global measurement of soil moisture has been emphasized, in particular, by the World Climate Research Program (National Research Council, 1992) wherein the Global Energy and Water Cycle Experiment (GEWEX) was created to study the "fast" component of the climate system. One of the objectives of the GEWEX Continental-scale International Project (GCIP) is to improve the predictive capability of coupled hydrologic-meteorological models; an improved capability in modeling the large-scale soil moisture dynamics and its verification is essential.

As in the case of many geophysical variables, global measurement and interpretation of soil moisture might be best accomplished by a combination of spaceborne and ground-based techniques. The SGP97 Hydrology Experiment builds upon the success of the Little Washita 1992 experiment in demonstrating the viability of L-band radiometry for remotely sensing surface moisture. The insight gained from the Little Washita 1992 experiment (e.g., Rodriguez-Iturbe et al., 1996, Mattikali et al., 1996; ) and the emerging research needs from GCIP form the basis of the scientific objectives of SGP97.

1.1. Scientific Objectives

The difficulty with the measurement of soil moisture and the understanding of its dynamics has often been attributed to the heterogeneity of soil properties and land surface attributes. However, at the large scale, the complicating factor in soil moisture dynamics also lies with the complex control of the land surface energy and water balance by the atmosphere and the soil; this control is further complicated by plant activities in the root zone.

SGP97 is set in a subhumid environment during early summer. Within this setting, the objectives of SGP97 are

1. to establish that the retrieval algorithms for surface soil moisture developed at higher spatial resolution using truck- and aircraft-based sensors can be extended to the coarser resolutions expected from satellite platforms;

2. to verify spatial-temporal estimators of soil moisture and to examine the utility of pedotransfer function in hydrologic modeling;

3. to examine the feasibility of inferring soil moisture and temperature profiles using surface observations in conjunction with in situ measurements, and

4. to examine the effect of soil moisture on the evolution of the atmospheric boundary layer and clouds over the southern great plains during the warm season.

1.2. Approach

SGP97 was originally conceived as an airborne experiment for daily mapping of surface soil moisture. In expanding its scope to meet interdisciplinary interests, the main considerations in the experimental design have been (1) maintaining as much spatial airborne coverage as possible on a daily basis; (2) nesting when- and where-ever possible to allow observations at a hierarchy of scales; and (3) making maximum use of existing facilities in the area.

The core of this project is the large scale aircraft soil moisture mapping. Within logistic and fiscal constraints, this experiment will attempt to map surface soil moisture over an area of ~10,000 km² (order of magnitude larger than previously observed) at a spatial resolution compatible with known data interpretation algorithms (~1 km). The resulting data base would allow the scaling up to projected satellite sized footprints (~10 km) and cover an area large enough to provide over 100 pixels of this size. These data would allow the examination of the information content of coarse resolution data as well as the analysis of the spatial/temporal scales generally utilized in hydrological and hydrometerological models. We will attempt temporal coverage on a daily basis over a period of one month. Logistical issues (rain events and FAA regulations) will result in a some missing days.

Data will be collected using an L band passive microwave mapping instrument called ESTAR which will be flown on a P-3 aircraft. In addition to the L band system we are attempting to include a thermal infrared sensor and a simulator of the AMSR satellite passive microwave radiometer that will be part of the Japanese ADEOS-II platform and possibly the EOS PM.

The temporal analysis will be enhanced by making continuous 24-hour observations using a truck based microwave radiometer system to complement the once-a-day aircraft measurements. This system consists of L, S, and C band single polarization instruments as well as thermal infrared. It would be located at the DOE ARM CART Central Facility which will provide the most comprehensive temporal observations.

This region selected for investigation is the best instrumented site for surface soil moisture, hydrology and meteorology in the world. Figure 1 shows the location and general features. Reasons for selecting this area included established ties with ARS programs and the possibility of integrating this project with other ongoing programs (DOE ARM and Oklahoma Mesonet).

In selecting the region, three key facilities play a critical role. They are the ARS facilities in the Little Washita watershed southwest of Chickasha, the ARS facility at El Reno, and the ARM CART Central Facility (CF) near Lamont (see Figure 1). One of the first changes made in the plan was the expansion to the north of the aircraft mapping so that it included the CF. Figure 2 is a Landsat Thematic Mapper (TM) bands 2, 3, and 4 false color composite of the region. This image is from July 9, 1991 and illustrates the typical conditions that might be encountered during SGP97. The red areas are mostly grasses. Whites and blues are areas of harvested winter wheat. Within the study area, there is a transition from mostly grass in the south to winter wheat in the north. There is also a demarcation between the area of interest and the obviously redder area to the east.

The Little Washita Watershed is the most critical study area in the project. It has been the focus of extensive hydrologic research for over 35 years. There is an ongoing data collection of unique and relevant data by the Agricultural Research Service, and the experience the local personnel have had in similar studies such as Washita'92 and the Shuttle Imaging Radar experiments in 1994. The watershed is located in southwest Oklahoma in the Great Plains region of the United States and covers an area of 603 sq. km. ( Figure 1). Landsat TM data (described above) were used to generate Figure 3 which shows many features and details. The climate is classified as sub­humid with an average annual rainfall of 75 cm. Within the watershed there are a total of 42 continuous recording rain gages distributed at a 5 km spacing over the watershed that are called the ARS Micronet system. The rain gage network of the Little Washita Watershed is fully described in Allen and Naney (1991). The topography of the region is moderately rolling with a maximum relief less than 200 m. Soils include a wide range of textures with large regions of both coarse and fine textures. Land use is dominated by rangeland and pasture (63%) with significant areas of winter wheat and other crops concentrated in the floodplain and western portions of the watershed area. Additional background information on the watershed can be found in Allen and Naney (1991) and Jackson and Schiebe (1993). Figure 4 is a collection of photos showing typical field conditions.

Recently the ARS Micronet system has been integrated in a remote data collection system capable of providing nearly real time hourly observations that include the following:

Rainfall

Air Temperature

Humidity

Soil Temperature (5, 10, 15, and 30 cm)

Equipment has been installed at 12 ( Figure 3) sites to monitor soil moisture at several depths (5, 10, 15, 20, 30 and 60 cm). Soil heat flux at three depths and soil temperature at eight depths will also be measured. Also scheduled are measurements of specific heat capacity, thermal diffusity, and thermal conductivity at four depths.

ARS also operates a grasslands research center at El Reno, OK. This consists of 6000 acres of federally operated grasslands ranging from winter wheat to natural prairie. Figure 5 is a schematic map of the area and Figure 6 is a TM image. In addition to the ready site access and variety of conditions, this site also can provide logistic support and is located approximately half way between Chickasha and Lamont. Figure 7 is a collection of photos showing typical field conditions.

The third facility that will be used is the ARM CF. This area consists of a grassland and a winter wheat field side by side. This facility is extensively instrumented and a great deal of descriptive information can be found on the home page at URL www.arm.gov/docs/sites/sgp/sgp.html. Scaling results to larger regions will be possible using the ARM Extended Facilities (EF). Figure 8 is a TM image of the CF area and Figure 9 is an aerial photo of the CF site available from the ARM web site.

1.3. Summary of key measurements and data products