2. SOIL MOISTURE AND TEMPERATURE

2.1. Introduction

Up to now there have been few soil moisture data sets that could in any way represent the types of observations that a satellite observing system might provide. Data were either good quality over short durations and small areas or sparse point samples over large regions. Therefore, hydrologic and climate studies have relied almost exclusively on simulated data sets, which of course are limited by our ability to describe the physical features and processes through a model representation. Since the science of modelling these large systems is still evolving, these models cannot be expected to fully reflect the exact nature of these processes at this stage. The feedback of actual observations, both spatial and temporal, would be a significant contribution to the development of these interdisciplinary studies.

The critical issues we are proposing to address here involve the scales of temporal and spatial observation of surface soil moisture. Specific objectives are: 1) to establish that higher resolution soil moisture-brightness temperature algorithms developed using truck and aircraft sensors can be extended to the coarser resolutions expected from satellite platforms, 2) to examine the spatial and temporal dynamics of surface soil moisture at an order of magnitude greater than previous investigations, and 3) to develop a data base for soil hydrology and land atmosphere interaction investigations.

2.2. Electronically Scanned Thinned Array Radiometer (ESTAR)

L band passive microwave radiometers are capable of providing surface soil moisture maps. Recent experiments such as Washita'92 (Jackson et al., 1995) have demonstrated the capabilities of this approach. Further information on the approach can be found at URL hydrolab.arsusda.gov/RSatBARC/soilmoisture.html

The electronically scanned thinned array radiometer (ESTAR) is a synthetic aperture, passive microwave radiometer operating at a center frequency of 1.413 GHz and a bandwidth of 20 MHZ. As installed it is horizontally polarized. This instrument is the most efficient mapping device currently available.

Aperture synthesis is an interferometric technique in which the product (complex correlation) of the output voltage from pairs of antennas is measured at many different baselines. Each baseline produces a sample point in the Fourier transform of the scene, and a map of the scene is obtained after all measurements have been made by inverting the transform. ESTAR is a hybrid real and synthetic aperture radiometer which uses real antennas (stick antennas) to obtain resolution along-track and aperture synthesis (between pairs of sticks) to obtain resolution across-track (Le Vine et al., 1994). This hybrid configuration could be implemented on a spaceborne platform.

The effective swath created in the ESTAR image reconstruction (essentially an inverse Fourier transformation) is about +/- 45^o wide at the half power points. The field of view is restricted to +/- 45^o to avoid distortion of the beam but could be extended to wider angles if necessary. The image reconstruction algorithm in effect scans this beam across the field of view in 2^o steps. The beam width of each step varies depending on look angle from 8 to 10^o, therefore, the individual original data are not independent, since each data point overlaps its neighbors. Contiguous beam positions can be achieved by averaging the response of several of these data points. This results in approximately nine independent beam positions. Another approach to using the data, especially in a mapping mode, is to interpret each of the original nonindependent observations as a sample point and then use a grid overlay to average the data. The final product of the ESTAR is a time referenced series of data consisting of the set of beam position brightness temperatures at 0.25 second intervals.

Calibration of the ESTAR is achieved by viewing two scenes of known brightness temperature. By plotting the measured response against the theoretical response, a linear regression is developed that corrects for gain and bias. Scenes used for calibration include black body, sky, and water. During aircraft missions, a black body is measured before and after the flight and a water target during the flight. Water temperature is determined using a thermal infrared sensor. The match in level and pattern is quite good and in general the ESTAR calibration should be considered accurate and reliable. For interpretation purposes it should be noted that the sensitivity of soil moisture to brightness temperature is 1% for 3^oK.

The ESTAR instrument will be flown on a P-3 aircraft operated by the NASA Wallops Flight Facility. Additional details on the aircraft can be found on the URL http://www.wff.nasa.gov/. Current assignments show that the aircraft will be available for flights in Oklahoma from June 18 and July 18, 1997. Instrument installation and check flights will be conducted at Wallops between June 9 and 16. In addition to the ESTAR, a two channel single beam thermal infrared radiometer will be flown. ESTAR will be installed in the bomb bay portion of the aircraft.

Flights will be conducted at an altitude of ~ 7 km and, therefore, the aircraft will be pressurized. It should be noted that radiometer calibration is based on its operating environment. At a particular aircraft altitude this is quite stable, however, operating at drastically different altitudes (and associated thermal environments) requires a separate calibration. All P-3 flights will be conducted at a single altitude to avoid this problem. Figure 10 shows the current flightline plan and Table 1 presents the details.

The same flights will be conducted daily (conditions permitting). Certain antecedent conditions may cause a change in the flight schedule. Nominal over target time will be 9:30 to 11:30 am local time. The decision to fly will be based on the following sequence of conditions; safety regulations (Aircraft Manager), aircraft operation (Aircraft Manager), ESTAR operations (Le Vine), weather conditions affecting flights (Le Vine and Aircraft Manager), experiment objectives (Jackson). In the past, the aircraft has been stationed at Will Rodgers Airport in Oklahoma City. The expected resources required for the aircraft area listed in the Table 2.

2.3 C Band Dual Polarization Observations

Two C band radiometers (wavelength of 6 cm) are being leased from Geoinformatics. These will be incorporated into the P3 and an appropriate data collection system by the University of Massachusetts. One antenna will be oriented for H polarization and the other for V polarization. Both will look behind the aircraft at an angle of 50 degrees to simulate future satellite systems. Data is time integrated for a single swath with a width of 5 km at the proposed altitude of 7 km. Data will be collected on all ESTAR P3 lines. The flightlines have been arranged to attempt to fly directly over the critical sampling sites.

2.4 Scanning Low Frequency Microwave Radiometer (SLFMR)

The scanning low frequency microwave radiometer (SLFMR) was designed and built for NOAA to measure ocean surface salinity from a small-engine aircraft by Quadrant Engineering, Inc. This is a 1.4 GHz L Band microwave radiometer. with its own GPS receiver. This will be flown on a Piper Navajo Chieftain aircraft operated by the Provincial Remote Sensing Office (PSRO) in Canada. There is only a limited time frame that this aircraft can be on site (operating out of Oklahoma City), probably the last week of June. The number of flight hours is also very limited. CASI (described in a later section) will also be flown on this aircraft.

The SLFMR has an electronically steered antenna beam and is capable of viewing any of six footprints across the flight track. Footprint size is nominally 0.3 of the altitude. The total swath covered is approximately 2 times the altitude. Since this instrument was designed for salinity mapping the sensitivity and thermal resolution are high. Components of the system which must be placed outside the aircraft are housed in a thermally controlled and aerodynamically shaped enclosure measuring 0.2 m high by 1 m wide by 1.5 m long and weighing 52 kg (115 lbs.). Components of the system which are placed inside the aircraft include a power supply, an IBM compatible computer which is used for control and acquisition of the microwave, infrared and GPS data. The computer, GPS and power supply are mounted in a rack measuring 0.7 m high by 0.5 m wide by 0.5 m long and weighing 32 kg. The infrared radiometer views the surface through a 0.15 m diameter hole in the underside of the aircraft. The infrared radiometer measures 0.2 m high by 0.15 m wide by 0.08 m long and weighing 2.3 kg. The system operates from standard 115 VAC power and requires a maximum of 320 W during normal operation of which 200 W is allocated to the computer and another 70 W is allocated to thermal control of the SLFMR electronics. The system can be placed in a fast warmup mode during which it would require a maximum of 1600 W.

The primary objective of the SLFMR is to provide multiscale L band observations. With the limited hours, it is necessary to focus on a single area (El Reno because it is closest to the airport and has the most concentrated variety of conditions and sampling). The proposed flightlines are described in Table 3. These would be flown twice over the course of one week. Any increase in operating hours could be used to either have more flights at this area or adding a second area.

2.5. Thermal Infrared Multispectral Scanner (TIMS)

TIMS is a simulator for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), an imaging instrument that will fly on EOS AM­1, a satellite planned for launch in 1998 as part of NASA's Earth Observing System (EOS). ASTER will be used to obtain detailed maps of surface temperature. Such information can then be used in studies of the surface energy balance, plant evaporation and evapotranspiration, vegetation and soil characteristics, and the hydrologic cycle.

TIMS is a six channel NASA aircraft scanner operating in the thermal infrared (8 to 12 /m) region of the electromagnetic spectrum. The channels and bandwidths (in microns) are; 1(8.2 ­ 8.6), 2 (8.6 ­ 9.0), 3 (9.0 ­ 9.4), 4 (9.4 ­ 10.2), 5 (10.2 ­ 11.2), and 6 (11.2 ­ 12.2). The instrument has a 2.5 mrad IFOV, 77° FOV spread over 638 pixels. The scan rate can be varied from 7.3 to 25 scans/second in four steps. Typical swath width and resolutions are

For calibration, the system is equipped with cold and warm reference sources or black bodies, approximately covering the temperature range of interest. All pixels are assigned a digital count value between 0 and 256 (DN). Reference source 1 is scanned at the beginning, and the second at the end of a line.

The TIMS instrument is flown on a DOE Cessna Citation aircraft. Eight flight hours are being provided by the EOS project. It is anticipated that the mission will consist of two days of coverage over the course of one week with differing antecedent conditions. Data collection will focus on collecting data over areas with intensive flux station measurements. It is anticipated that the aircraft will base out of Oklahoma City and be on site for one week, either the last week of June or the first week of July. Ideally, these flights should be integrated with the higher resolution L band flights of the SLFMR on the PSRO aircraft.

There are two planned flight lines (Table 4) for the Cessna Citation with TIMS during this summer's Great Plains Experiment in Oklahoma. One line will cover the El Reno test area, just west of OK city, and the other will be over the CART-ARM central facility. The altitude for these lines will be 16,000 feet (5000 m) above ground, yielding a spatial resolution of about 12 m and a useable swath of 5.6 km (+/- 30 deg), resolution of 12 m. The El Reno flight line will cover winter wheat fields south of the ARS rangeland. The CART-ARM line will go directly over the central site and will provide coverage to 3 km on both sides. There would be 3 flights per day, the first 1 hr after sunrise or about 6:00 AM LST; the second at about 10:30 LST, the time of ASTER overpass; and third at the times of the AVHRR overpass, or about 2 to 3 PM (LST) in the afternoon if resources permit.

For the water target coverage at will be obtained at several altitudes (5000', 10000' and 16000' AGL) as the aircraft takes off out of Oklahoma City. This coverage will be obtained at least once a day.

Table 4. DOE Cessna Citation Flightlines
Line	Start		Stop		Area
	Latitude	Longitude	Latitude	Longitude
1	35.4666	-98.0475	35.6333	-98.0475	El Reno
2	36.5000	-97.4842	36.7500	-97.4842	Central Facility
3	35.1500	-98.4417	35.2500	-98.5400	Lake Cobb

2.6. Split Window Thermal Infrared Radiometer (SWTIR)

The Split Window Thermal Infrared Radiometer is a two channel instrument that collects data along a single swath. It will be installed on the P3 and integrated into the ESTAR data collection system.

2.7. Soil Moisture Sampling

2.7.1 Surface Soil Moisture Sampling

2.7.1.1. Site Selection

Soil moisture observations in SGP97 will be used to address the following objectives of various investigators;

1. Validating and calibrating hydrologic and GCM land processes models

2. Atmospheric boundary layer interactions

3. Verification of the ESTAR microwave radiometer soil moisture algorithm

4. Geostatistical and scaling studies

5. Development of C band microwave radiometer-soil moisture relationships

6. Enhanced calibration of the existing insitu profile systems

7. Correlation of the insitu near surface observation and gravimetric sampling

8. Surface to profile extrapolation

9. Evaluation of alternative soil moisture measurement devices

Items 6 to 8 build on the extensive networks that already exist as part of the DOE ARM, USDA ARS, and Oklahoma Mesonet programs. These efforts will provide a vital link for larger scale and longer term satellite and modeling studies. Analyses will be conducted cooperatively with scientists from these organizations.

The sampling strategy is influenced by some important logistic issues which include the existing and proposed locations of instrumentation (i.e. the insitu profile soil moisture networks), facility support (ARS Little Washita, ARS El Reno, and the ARM Central Facility), and site access. This set of potential sites can be increased to a limited degree to address specific issues related to the items listed above. This is when other factors such as time (all surface samples must be collected within a window of about 3 hours) and manpower resources must be considered.

Data collection and sample coding will be related first to an area; Little Washita (LW), El Reno (ER), and Central Facility (CF). For each area there will be a two digit site code, i.e. LW01. The following table is the current set of sampling sites.

Network: S-ARS SHAWMS, M-Mesonet, A-ARM EF

Sampling Type: F-Full, P-Profile Only

Cover: R-Range, W-Wheat

2.7.1.2. Sampling Plan

2.7.1.2.1. Gravimetric Surface Sampling

For the most part, sampling will be performed on sites approximately a quarter section (0.8 km by 0.8 km) in size. Attempts will be made to sample several adjacent sites that can be clustered. In addition, some sites are being sampled solely for surface-profile correlations and consist of the area immediately surrounding a profile location.

Sites with "Full" sampling will involve two transects separated by 300 m with a sample every 100 m resulting in 14 samples per site. Profile only sites will consist of 9 samples collected over a 30 m by 30 m grid near the profile location. A standardized tool will be used to extract a sample of the 0-5 cm soil layer. Sample location is not critical in this approach. The grid is used only as an aid in stratifying the distribution of samples.

2.7.1.2.2. TDR Surface Sampling

The primary objective of the Southern Great Plains 1997 (SGP97) Experiment is to map soil moisture (0-5 cm surface soil layer) using an airborne passive microwave radiometer. These daily, 1 km² resolution, measurements are not detailed enough to capture the high degree of variability exhibited by soil moisture in both space and time. This variability must be better understood to enable full utilization of the larger-scale remotely sensed averages. Therefore, to assess these variations over large areas the remotely sensed observations must be combined with high resolution ground based monitoring. The SGP97 experiment offers a unique opportunity to characterize soil moisture variability at high spatial resolution and determine how well that variability is represented in 1-km (approximately) remotely sensed soil moisture maps. Selected fields will be more intensively sampled using a fixed grid and a time domain reflectometry (TDR) device. The choice of technique is under investigation.

Variability sites will be collocated with gravimetric sampling sites. To the degree possible (allowing for logistics, access to private lands and collocation with other equipment), selection of these quarter sections should reflect the range of variability in surface conditions (e.g. in topography, soils, vegetation, precipitation) encountered within the region, while at the same time providing adequate spatial coverage across the experimental domain. Quarter section sites will lie within three focus areas (Little Washita El Reno, ARM-CF). Studies of horizontal variability will be concentrated within the Little Washita basin. The number of sites will depend upon several factors that should be resolved shortly (number of instruments, time required for measurement, and personnel available).

A detailed study of horizontal variability is critically dependent upon a fast, portable sampling technique. Three TDR-based sensors are currently under consideration, including models offered by Campbell Scientific, Delta-T and Mesa. The feasibility of using of each of these in portable mode is under investigation at Alabama A & M University in conjunction with the Global Hydrology and Climate Center.

Basic gravimetric sampling of quarter sections will consist of 12 samples in the 0-5 cm soil layer (2 parallel rows of 6 samples/row). Supplementary sampling in support of this

variability investigation will utilize a grid-based sampling scheme. Forty-nine samples will be collected on a 7 x 7 square sampling grid (approximately 100 m between sampling points) centered within the quarter section. Sampling locations will be marked in the field with spray paint and accurately located using GPS. Additional samples can be taken in more variable regions within quarter sections. This basic sampling plan will be repeated in each of the four sections at full section sites. The same plan could also be repeated for the remaining quarter sections within a subwatershed, or could be modified if the subwatershed is too large. The final sampling plan for the subwatershed is TBD until it is selected. The time frequency of the horizontal variability sampling is TBD and depends upon the method of sampling chosen and the rate at which that sampling can be conducted.

Equipment required for each observing package will include; surface portable TDR units (including TDR sensor, sensor reader, data recorder), GPS unit, utility belt and pouch (manufacturers TBD), Recorder-PC interface cables and extra TDR sensors.

2.7.1.2.3. Bulk Density and Surface Roughness

Bulk density is used to convert the gravimetric samples to volumetric. A standard volume extraction technique will be used. Sampling will be performed by a single team and include 4 samples per site. Surface roughness will be recorded using a photograph of a grid board that will later be digitized. One bulk density sample will be retained per site for possible laboratory soil texture characterization.

2.7.2. Profile Soil Moisture and Temperature Sampling

As noted in Schneider and Fisher (1997), the SGP region is rich in observations, including three research networks: the Department of Energy's Atmospheric Radiation Measurement Program's Southern Great Plains Cloud and Radiation Testbed (ARM/CART SGP Site; Stokes and Schwartz, 1994); the Oklahoma Mesonet (jointly operated by the Universities of Oklahoma and Oklahoma State; Brock, et al, 1995); and the USDA/ARS Micronet in the Little Washita watershed (Elliott, et al, 1993). It was generally agreed that the data from these networks would be more valuable to scientists if the networks were augmented with continuous, automated measurements of volumetric soil water through and below the rooting zone. Each network has made significant progress toward this (Schneider and Fisher, 1997).

All of these networks employ the same type of soil water sensor, the Campbell Scientific Inc. heat dissipation sensor (Model 229-L). Analysis indicated that the CSI 229-L sensor produces reasonable measurements of matric potential over a wide range of wetness, and responds quickly and accurately to changing soil wetness conditions. These evaluations have since been corroborated by Reece (1996). The 229-L also measures soil temperature before each soil wetness measurement cycle. And it is a simple device, with an expected unattended field lifetime greater than 5 years.

The CSI 229-L sensor is designed to produce a point measurement of soil matric potential (the tension with which water is held onto the soil particles) by measuring the temperature change after a heat pulse is introduced (hence "heat dissipation"). This is a distinctly different measurement from the layer average of volumetric water produced by gravimetric measurement, neutron probes, or time domain reflectometry [TDR] systems. Matric potential can be related to volumetric water, given a soil water retention curve (unique for each soil). Thus, computation of volumetric water from the 229-L measured temperature change requires: a) laboratory calibration of each sensor to relate observed temperature changes to water matric potential; and b) determination of the soil water retention curve for the soil surrounding each sensor.

Alternatively, the raw data (temperature changes) could be calibrated against collocated direct measurements of volumetric water. This second route would require a longer calibration period, and would need to be repeated whenever a sensor is replaced in the field.

2.7.2.1. DOE ARM CART

DOE ARM CART refers to the soil moisture systems as Soil Water and Temperature System, or SWATS. To create a minimal redundancy, as well as an opportunity to examine local variability, they deployed the sensors in two profiles, separated horizontally by 1 m. The SWATS takes observations once every hour, with data transmitted automatically via phone line every 8 hours. Data is also stored locally, and manually downloaded during biweekly maintenance checks.

The final design consists of electronics in a surface-mounted enclosure (data logger, multiplexor, constant-current source, power supply, storage module, and telecommunications equipment) supporting 16 CSI 229-L sensors, deployed in two profiles of 8 sensors each. Sensors are located at depths of 5, 15, 25, 35, 60, 85, 125, and 175 cm, rock permitting. The installation procedure was designed to minimize the disturbance of the soil, and maximize the contact between the sensor and the surrounding soil, while satisfying DOE Safety requirements.

The Department of Energy's ARM/CART SGP Site is centered near Lamont, OK, and covers an area roughly 325 by 275 km, extending from the Little Washita watershed in Oklahoma north into central Kansas. The data produced by the SGP Site is part of the DOE contribution to GCIP. The SGP Extended Facilities are of particular interest to GCIP: 22 installation providing observations of air temperature, wind speed and direction, humidity, rainfall, and snow depth; several measures of up welling and downwelling visible and near-infrared radiation; and estimates of sensible and latent heat fluxes in the atmospheric surface layer. SWATS have been added to each of these Extended Facilities.

These instruments are still undergoing calibration. Data for all ARM sites will be available, however, there are four ARM Extended Facilities that will receive more attention for the current study; the two at the Central Facility, EF24, EF26, and the planned installation at El Reno. Locations of these stations are shown in Figure 1.

2.7.2.2. USDA ARS SHAWMS

SHAWMS stands for Soil Heat and Water Measurement System. These sensor packages are managed by Pat Starks and installed within the Little Washita as shown in Figure 3 (12 sites) and at the El Reno facility (4 locations collocated with flux stations, exact locations TBD). Each system includes 3 sensors at 5 cm, then single sensors at 10, 15, 20, 25, and 60 cm. Readings are acquired every hour, and are calibrated against the capacitance probe measurements. These data are downloaded once a week. Data for May through August will be made available to the SGP97 data base. Any additional data must be independently negotiated with Pat Starks (USDA ARS El Reno)

2.7.2.3. Oklahoma Mesonet

Two types of soil water sensors have been added to 60 of the 114 stations comprising the Oklahoma Mesonet. The CSI 229-L has been installed at depths of 5, 25, 60, and 75 cm. Particle size analyses have been conducted for each of these sites. The 229-L sensors are read by a data logger every 30 minutes, and the data are reported in real-time as part of the Mesonet data stream. The second type of sensor is a time domain reflectometry (TDR) system, the Environmental Sensors MoisturePoint probe, installed to a depth of 90 cm. A Model MP-917 instrument is carried to the site, connected to the probe, and then used to make readings of volumetric water content in 5 soil layers (0-15, 15-30, 30-45, 45-60, and 60-90 cm). Because of the manual nature of the measurement, the TDR observations are made fairly infrequently (whenever a Mesonet technician or interested researcher visits the site). The TDR measurements will be used to perform site-specific, in situ calibration of the 229-L sensors.

Data from this network will be available as part of the data set for the project. The following sites fall within the aircraft mapping area; ACME, APAC, ELRE, KING, and MARS.

2.7.2.4. Cross Calibration with TDR Probes

As noted in Schneider and Fisher (1997), the method most frequently used to calibrate heat dissipation sensors involves the use of high-pressure vessels. Unfortunately, this method requires expensive, specialized equipment and facilities which are not commonly available. Therefore, they are testing several methods in order to develop an alternative method employing readily available equipment, with the goal of providing an efficient and accurate means of calibrating the 229-L sensors before field deployment. The methods differ in the way water potentials are generated, measured, and imposed on the sensors. All sensors deployed in the ARM/CART SGP network have been calibrated using the vapor pressure method, with a number of sensors cross-calibrated to support comparison of the methods. This calibration study is being conducted in collaboration with scientists at the Oklahoma Mesonet.

Schneider and Fisher (1997) reported that data quality analysis is just beginning. Current indications are promising: there is clearly a signal in the raw (temperature change) data associated with rain events and drying, with the expected trends and differences between depths. Scientists at OSU are making pairs of gravimetric measurements at each SGP SWATS Site, one during a relatively wet period, the other drier. Those results will provide a preliminary indication of the accuracy of the 229-L estimates of volumetric water.

There are also longer term plans for the collocation of instruments.

With the variety of installations and the potential problems in calibration the heat dissipation sensors, scientists in Oklahoma had initiated a program utilizing insitu TDR probes that are read on site. The technique used involves Moisture Point probes. A very extensive description of this technique can be found at the following web site (URL www.esica.com). The use of these probes will provide both individual site calibration and some cross calibration. As part of the current project, the number of sites will be increased and observations will be made every day. In addition, at selected sites (see Table 5 for possible locations, however, this is still TBD) additional probes will be installed to examine the spatial aspects of these point probe observations.

2.7.2.5. Dielectric Profiling Stations

HSCaRS will install up to 6 supplemental soil profile stations. Two of these stations will be installed at the Central Facility, one on grass and the other on winter wheat. The remainder will be installed at sites TBD within the Little Washita area. Installation involves digging a pit (about 1 m x 1 m x 1 m) for instrument installation. Soil moisture and temperature measurements will be made at several depths down to about 75 cm in each pit. Soil moisture will be measured using Water Content Reflectometers (Campbell Scientific, Inc), a device based on time domain reflectometry, and using Soil Moisture Probes (Radiation and Energy Balance Systems), a device based on electrical resistance. Soil temperature will be measured in each pit using soil thermistors. Ground heat flux will be determined using a heat flux plate installed at 5 cm depth plus the heat storage in the upper 5 cm layer calculated from the time rate of change of temperature, which is measured using 4-sensor averaging thermocouple probes installed at 1, 2, 3, and 4 cm depths. Techniques to derive the soil dielectric constant from Water Content Reflectometers (or similar sensors) are currently under investigation. If these prove feasible, dielectric constant profiles will be provided at one or more of the profile stations. Stations operate from battery power. These stations will have to be installed approximately one month prior to the experiment.

2.8. Truck Based Microwave Radiometer System

The S and L Microwave Radiometer (SLMR) is a dual frequency passive sensor system operating at S band (2.65 GHz or 11.3 cm) and L band (1.413 GHz or 21.2 cm) managed by the ARS Hydrology Lab and maintained in cooperation with the University of Massachusetts. The staging platform used is a 1990 Navstar hydraulic boom truck belonging to the Hydrological Sciences Branch at NASA's Goddard Space Flight Center. This vehicle is equipped with a hydraulic boom which permits deployment of sensor packages up to a height of approximately 19 m above the ground. The instrument platform at the end of the boom can be moved to vary incidence angle from 0^o (nadir) to 180^o (sky), while the boom itself can be rotated 360^o in azimuth. The antennas are mounted to observe horizontal polarization. At the nominal operating height of 7 m with the specified field of view of the radiometers (20^o), the footprint size is on the order of 2.5 m at a viewing angle of 10^o off nadir. Incidence angle is provided by internal inclinometers.

Recently, a 6 channel stepped-frequency C band radiometer has been added to this system. This operates between 4 and 8 GHz and has a nominal field of view of 18^o. Like the other instruments, this is a single polarization radiometer.

In addition to the microwave radiometers, several other supporting instruments are also mounted on the truck platform. A small portable thermal infrared radiometer by Everest Interscience (Model 110) is used to estimate the surface temperature by measuring thermal emission in the 8-14 m> wavelength range. Target location for the microwave radiometers is achieved with a color video camera installed on the platform between the two antennas. A portable generator on the truck provides electrical power at remote sites. Figure 11 shows the truck with the SLMR installed.

System operation and control is maintained by a personal computer. The software monitors the thermal status of the radiometers and attempts to maintain thermal equilibrium of the defined goal temperature through the distributed heater network. Data collection can be either operator controlled or automatic. The former is used in circumstances where the boom is moved from one target to another or the effect of specific changes are to be observed. In the automatic mode, the system can be set to make observations at specified intervals for extended periods. Due to the low data rates, high temporal frequency is possible.

The main purpose of the truck radiometers is to provide continuous 24-hr brightness temperature measurements to complement the once-a-day aircraft microwave data. The radiometers would be deployed at a representative site. Flux station and other insitu measurements would be made simultaneously providing a high temporal resolution data set for energy and water balance modelling.

The acquisition of SLMR data on a continuous basis during the one month field experiment will provide a context for interpreting any potential temporal variations occurring due to the duration of the day's aircraft mapping flight or ground sampling activities, and will also produce a continuous record for filling in data gaps due to aircraft down time (i.e. weather). In addition, the temporal nature of the SLMR data will permit diurnal effects in the microwave/soil moisture relationship to be calibrated. The resulting data base which combines coverage (aircraft mapping) with high temporal resolution (ground based radiometers) along with supporting meteorological and other insitu observations will be unique, and should have significant impact on the study of surface hydrology and land/atmosphere interactions at different scales.

Deployment of the truck will involve the consideration of several scientific and logistic factors:

1. Side by side grass and winter wheat fields

2. Representativeness of conditions

3. Ancillary observations

4. AC power availability

5. Security

6. Access roads and stability of deployment site

7. Impact of truck operations on pre-existing site operations

At the present time, the truck will be deployed at the Central Facility. A possible backup (in case there are RFI issues) is the ARS lab at Chickasha, OK. If time permits, the truck might be sent to several key sites for a short series of observations.