Airborne soil moisture measurement is based on the difference between natural terrestrial gamma radiation flux measured for comparatively wet and dry soils. The presence of moisture in the soil causes an effective increase in the soil density resulting in an increased attenuation of the gamma flux for relatively wet soil and a correspondingly lower flux at the ground surface. As part of the FIFE experiment, natural terrestrial gamma radiation data over a network of 24 flight lines were collected. The data acquisition procedure was designed to accumulate and store spectral radiation data along a flight line from which estimates of soil moisture could be computed. Ground-based soil moisture measurements were used to make a one-time calibration of the natural terrestrial radioisotope signal over the flight line network. A time-series of airborne soil moisture measurements (to a depth of 20 cm) was compared to an extensive, independent data set of ground-based soil moisture measurements. Estimates for flight line segments were found to have an average RMS error of approximately 2.5 % soil moisture (Peck et al., 1990).

Gravimetric soil moisture data collected in conjunction w/ gamma ray soil study

Airborne gamma surveys were conducted over each of the three Cold Land Processes Field Experiment (CLPX) Meso-cell Study Areas (MSAs) in northern Colorado, USA, during September 2001 and 2002, and during the three Intensive Observation Periods (IOPs) in February 2002 (IOP1), February 2003 (IOP3) and March 2003 (IOP4). Data collected in September 2001 and 2002 provided background gamma radiation measurements necessary to calculate measurements of snow water equivalent (SWE) and soil moisture during subsequent winters.

Soil moisture collected at 25mm, 75mm, and 150mm

,The Southern Great Plains 1997 (SGP97) Hydrology Experiment originated from an interdisciplinary investigation, "Soil Moisture Mapping at Satellite Temporal and Spatial Scales" (PI: Thomas J. Jackson, USDA Agricultural Research Service, Beltsville, MD) selected under the NASA Research Announcement 95-MTPE-03. The GEWEX Continental-Scale International Project (GCIP) Near-Surface Observation Data Set (NESOB) 1997 ARM 5 Minute Surface Composite is composed of data from the Department Of Energy (DOE) Atmospheric Radiation Measurement Surface (ARMSFC) Network within the NESOB 1997 domain. This Surface Composite contains data for the NESOB 1997 time period (01 April 1997 through 31 March 1998) and for the NESOB 1997 area which is approximately 34N to 39N latitude and 94.5W to 100.5W longitude. The DOE ARM Surface (ARMSFC) 5-minute values were derived from ARMSFC 1-minute data. The ARMSFC instrument readouts were every second for all variables except 1 minute for barometric pressure. The detailed descriptions of the algorithms used to produce ARMSFC one minute data are not currently available. When not present in the raw data, the dewpoint is computed using the formula from Bolton (1980). Calculated Sea Level pressure is computed from station pressure, temperature, dewpoint, and station elevation using the formula of Wallace and Hobbs (1977). Specific Humidity values were computed from dew point and station pressure using formulas from Wexler and Wildhack (1963). The NESOB 1997 ARM 5-Minute Surface Composite contains ten metadata parameters and 41 data parameters and flags. The metadata parameters describe the station location and time at which the data were collected. The time of observation is reported both in Universal Time Coordinated (UTC) Nominal and UTC actual time. Days begin at UTC hour 0000 and end at UTC hour 2355. The data parameters are valid for the reported times. Missing values are reported as 9's in the data field.,

This data set contains percent soil moisture ground measurements. These data were collected on the ground along the various flight lines flown in the Southern and Northern Study Areas (SSA and NSA) during 1994 by the gamma ray instrument. This data set contains information on the locations of field in-site measurements of soil moisture, depth of moss/humus layer, and water content of the moss/humus layer and contains information on soil conditions and vegetative cover around the sites.

,Gamma-ray spectroscopy (GRS) enables continuous estimation of soil water content (SWC) at the subfield scale with a noninvasive sensor. Hydrological applications, including hyper-resolution land surface models and precision agricultural decision making, could benefit greatly from such SWC information, but a gap exists between established theory and accurate estimation of SWC from GRS in the field. In response, we conducted a robust three-year field validation study at a well instrumented agricultural site in Nebraska, United States. The study involved 27 gravimetric water content sampling campaigns in maize and soybean and 40K specific activity (Bq kg−1) measurements from a stationary GRS sensor. Our analysis showed that the current method for biomass water content correction is appropriate for our maize and soybean field but that the ratio of soil mass attenuation to water mass attenuation used in the theoretical equation must be adjusted to satisfactorily describe the field data. We propose a calibration equation with two free parameters: the theoretical 40K intensity in dry soil and a, which creates an “effective” mass attenuation ratio. Based on statistical analyses of our data set, we recommend calibrating the GRS sensor for SWC estimation using 10 profiles within the footprint and 5 calibration sampling campaigns to achieve a cross-validation root mean square error below 0.035 g g−1.,

,The Southern Great Plains 1997 (SGP97) Hydrology Experiment originated from an interdisciplinary investigation, "Soil Moisture Mapping at Satellite Temporal and Spatial Scales" (PI: Thomas J. Jackson, USDA Agricultural Research Service, Beltsville, MD) selected under the NASA Research Announcement 95-MTPE-03. The Global Energy and Water Cycle Experiment (GEWEX) Continental-scale International Project (GCIP) Enhanced Observing Period (EOP) takes place in the Mississippi River basin during 1995-2000. The Mississippi River basin provides a number of watershed areas that are potentially useful for hydrologic focused studies. During this period, there will be a number of Enhanced Seasonal Observing Periods (ESOPs) in various smaller scale areas. The temporal coverage for this dataset is as follows: Begin date: 1995-10-01 00:00:00, End date: 2000-10-01 23:59:59. This dataset contains the National Centers for Environmental Prediction (NCEP) Environmental Modeling Center (EMC) 4 KM GRIB gage-only analysis using 24h accumulated ("RFC") dataset. A prototype, real-time, hourly, multi-sensor National Preciptation Analysis (NPA) has been developed at NCEP in cooperation with the Office of Hydrology (OH). This analysis merges two data sources that are currently being collected in real-time by OH and NCEP. Hourly digital precipitation (HDP) radar estimates are created by the WSR-88D Radar Product Generator on a 131 X 131 4-km grid centered over each radar site. Data analysis routines, including a bias correction of the radar estimates using rain gage data, have been adapted by NCEP on a national 4-km grid from algorithms developed by OH and executed regionally at NWS River Forecast Centers (RFC). The format of the files is GRIB. The files are compressed using the UNIX "compress" command and "uncompress" must be used before decoding.,

Spectral reflectance of soils, Atlas of Soil Reflectance Properties (Stoner '80)

This ancillary climatology soil porosity and wetlands coverage information were derived from the daily GEWEX fusion of satellite active and passive microwave measurements. These quantities are re-mapped to a 1.0 degree equal-area grid from the original 0.25 degree equal-angle map grid.