Michael Cosh, William White, Andreas Colliander, Thomas Jackson, John Prueger, Brian Hornbuckle, E. Raymond Hunt, Heather McNairn, Jarrett Powers, Victoria Walker, Paul Bullock
Vegetation water content (VWC) is an important land surface parameter that is used in retrieving surface soil moisture from microwave satellite platforms. Operational approaches utilize relationships between VWC and satellite vegetation indices for broad categories of vegetation, i.e., “agricultural crops,” based on climatological databases. Determining crop type–specific equations for water content could lead to improvements in the soil moisture retrievals. Data to address this issue are lacking, and as a part of the calibration and validation program for NASA’s Soil Moisture Active Passive (SMAP) Mission, field experiments are conducted in northern central Iowa and southern Manitoba to investigate the performance of the SMAP soil moisture products for these intensive agricultural regions. Both sites are monitored for soil moisture, and the calibration and validation assessments had indicated performance issues in both domains. One possible source could be the characterization of the vegetation. In this investigation, Landsat 8 data are used to compute a normalized difference water index for the entire summer of 2016 that is then integrated with extensive VWC sampling to determine how to best characterize daily estimates of VWC for improved algorithm implementation. In Iowa, regression equations for corn and soybean are developed that provided VWC with root mean square error (RMSE) values of 1.37 and 1.10 kg / m2, respectively. In Manitoba, corn and soybean equations are developed with RMSE values of 0.55 and 0.25 kg / m2. Additional crop-specific equations are developed for winter wheat (RMSE of 0.07 kg / m2), canola (RMSE of 0.90 kg / m2), oats (RMSE of 0.74 kg / m2), and black beans (RMSE of 0.31 kg / m2). Overall, the conditions are judged to be typical with the exception of soybeans, which had an exceptionally high biomass as a result of significant rainfall as compared to previous studies in this region. Future implementation of these equations into algorithm development for satellite and airborne radiative transfer modeling will improve the overall performance in agricultural domains.
Andreas Colliander, Simon Yueh, Seth Chazanoff, Steven Dinardo, Ian O'Dwyer, Thomas Jackson, Heather McNairn, Paul Bullock, Grant Wiseman, Aaron Berg, Ramata Magagi, Eni Njoku
NASA’s (National Aeronautics and Space Administration) Soil Moisture Active Passive (SMAP) Mission is scheduled
for launch in late 2014. The objective of the mission is global mapping of soil moisture and freeze/thaw state. Merging of
active and passive L-band observations of the mission will enable unprecedented combination of accuracy, resolution,
coverage and revisit-time for soil moisture and freeze/thaw state retrieval. For pre-launch algorithm development and
validation the SMAP project and NASA coordinated a field campaign named as SMAPVEX12 (Soil Moisture Active
Passive Validation Experiment 2012) together with Agriculture and Agri-Food Canada, and other Canadian and US
institutions in the vicinity of Winnipeg, Canada in June-July, 2012. The main objective of SMAPVEX12 was acquisition
of a data record that features long time-series with varying soil moisture and vegetation conditions over an aerial domain
of multiple parallel flight lines. The coincident active and passive L-band data was acquired with the PALS (Passive
Active L-band System) instrument. The measurements were conducted over the experiment domain every 2-3 days on
average, over a period of 43 days. The preliminary calibration of the brightness temperatures obtained in the campaign
has been performed. Daily lake calibrations were used to adjust the radiometer calibration parameters, and the obtained
measurements were compared against the raw in situ soil moisture measurements. The evaluation shows that this
preliminary calibration of the data produces already a consistent brightness temperature record over the campaign
duration, and only secondary adjustments and cleaning of the data is need before the data can be applied to the
development and validation of SMAP algorithms.
Microwave remote sensing can provide reliable measurements of surface soil moisture. However, some land surface conditions can have a perturbing influence on soil moisture retrievals. In the soil moisture experiments in 2005 (SMEX05), we attempted to contribute to the understanding of the effect of dew using concurrent ground and aircraft observations. Early morning flights were conducted with an airborne microwave radiometer from June 19 to July 2, 2005, in Iowa, USA over an agricultural domain. Results of the experiment indicated that dew had a small but measurable effect on the observed 10.7-GHz brightness temperatures. The results indicate that the H-pol emissivity increased 0.015 to 0.04 for the corn sites, 0.014 to 0.02 for soybean, and 0.01 for forest sites as dew evaporated. These results suggest that the presence of dew decreases X-band land surface emissivity slightly and the effect of dew varies with vegetation types. Our findings are consistent with other works in the literature that has found that the effect of dew depends on both the type of vegetation and the wavelength of observation, but further studies should be conducted to verify this hypothesis.
Passive microwave soil moisture algorithms must account for vegetation attenuation of the signal in
the retrieval process. One approach to accounting for vegetation is to use vegetation indices such as
the Normalized Difference Vegetation Index (NDVI) to estimate the vegetation optical depth. The
passive microwave sensor platforms typically do not include sensors for providing this information
and the data must be acquired independently. This presents challenges to data processing and
integration and concerns about data availability. As an alternative to routine updating of the NDVI, it
is possible to use a global vegetation index climatology. This climatology is based on the long term
set of observations from the MODIS instrument (10 years). A technique was developed to process
the NASA NDVI and Enhanced Vegetation Index (EVI) data base to produce a 10-day annual cycle
(climatology) for each 1 km pixel covering the Earth's land surface. Since our focus was on soil
moisture, the classification rules and flags took this into consideration. Techniques developed for
processing the indices, development of flags, and expected utilization in soil moisture retrieval
algorithms are described.
Previous investigations have established the basis for a new type of vegetation index, Microwave Vegetation Indices
(MVIs), based on passive microwave satellite observations. In this technique, the quantitative basis of the MVIs can be
derived from the zeroth-order radiative transfer solution. However, the zeroth-order solution is only applicable when the
scattering contributions within the vegetation are negligible. As a result, the first-order radiative transfer solution is
superior to the zeroth-order solution due the fact that it considers volume scattering. In this paper, we evaluated the
applicability of the zeroth-order solution at different microwave frequencies and for vegetation with different densities.
Next, a parameterized vegetation microwave emission model for the first-order solution was developed that was used to
improve the MVIs. The superiority of MVIs derived from the parameterized model was demonstrated by comparison to
the original approach. The refinement of MVIs presented in this study will be helpful in improving their accuracies and
expanding their applications, and will contribute to improved information on vegetation coverage, biomass, and water
content.
Mapping land cover and vegetation characteristics on a regional scale is critical to soil moisture retrieval using microwave remote sensing. In aircraft-based experiments such as the National Airborne Field Experiment 2006 (NAFE'06), it is challenging to provide accurate high resolution vegetation information, especially on a daily basis. A technique proposed in previous studies was adapted here to the heterogenous conditions encountered in NAFE'06, which included a hydrologically complex landscape consisting of both irrigated and dryland agriculture. Using field vegetation sampling and ground-based reflectance measurements, the knowledge base for relating the Normalized Difference Water Index (NDWI) and the vegetation water content was extended to a greater diversity of agricultural crops, which included dryland and irrigated wheat, alfalfa, and canola. Critical to the generation of vegetation water content maps, the land cover for this region was determined from satellite visible/infrared imagery and ground surveys with an accuracy of 95.5% and a kappa coefficient of 0.95. The vegetation water content was estimated with a root mean square error of 0.33 kg/m2. The results of this investigation contribute to a more robust database of global vegetation water content observations and demonstrate that the approach can be applied with high accuracy.
A recent study established the theoretical basis for a new type of index based on passive microwave vegetation indices
(MVIs). The approach was then calibrated for use with data from the Advanced Microwave Scanning Radiometer
(AMSR-E) on the Aqua satellite under the assumption that there is no significant polarization dependence of the
vegetation emission and attenuation properties. To demonstrate the potential of the new microwave vegetation indices,
these were compared with the Normalized Difference of Vegetation Index (NDVI) derived using MODIS at continental
and global scales. These results verified that the microwave vegetation indices can provide new and complementary
information on vegetation to NDVI for the global monitoring of vegetation and ecosystem properties from space. The
next phase of analysis has focused on quantifiable vegetation parameters, specifically vegetation water content that is a
valuable parameter in soil moisture retrievals using microwave data. Data sets collected in several recent large scale field
campaigns included vegetation water content over domains in addition to conventional indices. Comparisons to date
indicate that the MVI does provide vegetation water content information, however, further analysis of vegetation type
effects are needed.
Soil Moisture Active/Passive (SMAP) Mission is one of the first satellites being developed by NASA in response to the
National Research Council's Decadal Survey. SMAP will make global measurements of the moisture present at Earth's
land surface and will distinguish frozen from thawed land surfaces. Direct observations of soil moisture and freeze/thaw
state from space will allow better estimates of water and energy transfers between Earth's surface and atmosphere, which
are primary driving factors for weather and climate. Soil moisture measurements are also of great importance in
assessing flooding potential and as input to flood prediction models. Conversely, observations of widespread low soil
moisture levels can provide early warning of drought conditions, reduced water supply and crop loss. SMAP
observations can help mitigate these natural hazards, resulting in potentially great economic and social benefits. SMAP
freeze/thaw timing observations will also reduce a major uncertainty in quantifying the global carbon balance and will
help resolve the problem of the missing carbon sink. The SMAP mission concept would utilize L-band radar and
radiometry. These instruments will share a rotating 6-meter mesh antenna to provide high-resolution and high-accuracy
global maps of soil moisture and freeze/thaw state every two to three days.
Soil moisture has long been recognized as one of the critical land surface initial conditions for numerical weather, climate hydrological predictions, particularly for transition zones between dry and humid climates. However, none of the currently existing soil moisture products has been used operationally in these models because of their consistency and reliability issues. A consistent and qualitatively reliable global soil moisture product is thus in desire to make good use of observations from different microwave sensors, such as AMSR-E, WindSat and TMI. This study explores the potential of WindSat data for producing such a product using the single channel algorithm (SCA) for soil moisture retrieval in conjunction with field observations for calibrating the algorithm and for validation. The preliminary results show good agreement between the results from WindSat and NASA AMSR-E product both in terms of spatial pattern
and magnitude. The validation results show that the differences between the retrieved soil moisture from WindSat data and the ground measurements are below 0.05 (vol/vol) in most cases, meaning a great potential of WindSat data for producing a blended product. Further cross calibration between the brightness temperatures from different sensors might be needed for producing such a blended product.
Vegetation indices are valuable in many fields of geosciences. Conventional, visible-near infrared, indices are often
limited by the effects of atmosphere, background soil conditions, and saturation at high levels of vegetation. In this
study, the theoretical basis for a new type of passive microwave vegetation indices (MVIs) based on data from the
Advanced Microwave Scanning Radiometer (AMSR-E) on the Aqua satellite is developed. Numerical simulation results
were used to establish relationships of bare soil surface emissivities at different frequencies. Using a radiative transfer
model, a linear relationship between the brightness temperatures observed at two adjacent radiometer frequencies can be
derived. The intercept and slope of this linear function depend only on the vegetation properties and can be used as
vegetation indices. These can be derived from the dual-frequency and dual-polarization satellite measurements under the
assumption that there is no significant impact of the polarization dependence on the vegetation signals. To demonstrate
the potential of the new microwave vegetation indices, we compared them with the Normalized Difference of Vegetation
Index (NDVI) derived using MODIS at continental and global scales. The results indicate that the MVIs provide a
complementary dataset for monitoring global short vegetation and seasonal phenology from space.
WindSat is a spaceborne multi-frequency polarimetric microwave radiometer and has the potential of contributing to the
retrieval of land variables and complementing efforts directed at the Aqua AMSR-E. In this study, a previously
established algorithm was applied to WindSat data to estimate global soil moisture. Comprehensive validation was
performed by comparing the retrievals with in situ soil moisture observations from networks located at four soil moisture
validation sites. The overall standard error of estimate for surface soil moisture was 0.038 m3/m3. This analysis shows
that the WindSat soil moisture retrievals are reasonable and fall within the generally accepted error bounds of 0.04
m3/m3. Larger scale qualitative assessments were performed by analysis of the spatial distribution of soil moisture,
which were found to be consistent with the known global climatology. There are other soil moisture algorithms under
investigation, however, these result show the potential of the WindSat sensor for soil moisture as well as future
operational satellite instruments.
WindSat is the first spaceborne fully polarimetric radiometer. It measures all four Stokes components; Tv
(vertically polarized), Th (horizontally), U (difference between polarizations at +45° and -45°) and V (difference
right hand minus left hand circular polarized) and is primarily developed to retrieve wind speed and direction over
ocean. Here we investigate the WindSat observations over Dome C, Antarctica, consisting of nearly flat terrain at
about 3200 m above sea level. The seasonal cycles of Tv and Th reflect the surface temperature cycle and the
penetration depth decreasing with increasing frequency (6 to 37 GHz), while the difference Tv - Th is nearly
constant. The U and V signals are most pronounced in Austral winter (July-August). The differences between
ascending and descending overpasses, corresponding to different azimuth observing directions, take values up to
1.2 K (U) and 3 K (V). Fitting the data of for and aft swath to a second order harmonic function of the azimuth
angle reveals a consistent orientation of the structures at the used frequencies 10 and 37 GHz and Stokes
components U and V in the direction of about 153° with respect to north, consistent with the small overall slope
direction of the terrain (145°) and ERS scatterometer observations.
Validation is an important but particularly challenging task for passive microwave remote sensing of soil moisture from Earth orbit. The key issue is spatial scale; conventional measurements of soil moisture are made at a point whereas satellite sensors provide an integrated area/volume value for a much larger spatial extent. For microwave remote sensing from space it is necessary to consider the kilometer to 40 km scale, which presents new challenges. This issue of spatial scale is common to both current and future satellite missions. Regardless of the degree of difficulty, ground based sampling must remain a core component of validation. An integrated approach using in situ networks, field campaigns and comparison to other satellite products is described.
This paper reports an attempt in improving surface soil moisture radar algorithm for Hydrosphere State Mission (Hydros). We used a Radiative Transfer Model to simulate a wide range surface dielectric, roughness, vegetation with random orientated disks database for our algorithm development under HYDROS radar sensor (L-band multi-polarizations and 40º incidence) configuration. Through analyses of the model simulated database, we developed a technique to estimate surface soil moisture. This technique includes two steps. First, it decomposes the total backscattering signals into two components - the surface scattering components (the bare surface backscattering signals attenuated by the overlaying vegetation layer) and the sum of the direct volume scattering components and surface-volume interaction components at different polarizations. From the model simulated data-base, our decomposition technique works quit well in estimation of the surface scattering components with RMSEs of 0.12, 0.25, and 0.55 dB for VV, HH, and VH polarizations, respectively. Then, we use the decomposed surface backscattering signals to estimate the soil moisture and the combined surface roughness and vegetation attenuation correction factors with all three polarizations. Test of this algorithm using all simulated data showed that an accuracy for the volumetric soil moisture estimation in terms of Root Mean Square Error (RMSE) of 4.6 % could be achievable.
We evaluate the response of a passive microwave soil moisture retrieval algorithm to errors in the estimation of input variables and parameters. The model is run varying one parameter at a time within a specified range to quantify the effects individual parameters have on soil moisture retrieval. Although errors in the estimation of most parameters yield total variations in soil moisture of less than about 4% volumetric water content (vwc), variations in the estimates of vegetation water content, vegetation b parameter, percent clay, and surface roughness yield the greatest total variations in calculated soil moisture. The effects of these parameter variations on calculated soil moisture are greater for wetter soils (above 25% vwc) and can result in total variations in soil moisture retrieval up to 24% vwc. These same parameters have a compound effect on calculated soil moisture when they vary collectively; variations in soil moisture retrieval with errors in vegetation water content and surface roughness may be as high as 38% vwc (-12%, +26%). Even over more common conditions between 10% and 25% vwc, errors in vegetation water content, percent clay, and surface roughness result in total soil moisture variations of 9% to 15% (plus or minus 4.5% to plus or minus 7.5%), which are unacceptably high for many applications. When random errors are imposed on these three parameters of the Southern Great Plains 1997 (SGP97) Hydrology Experiment data set, the macrostructure of the soil moisture distribution remains intact compared to the original calculations, but the moisture field is significantly more heterogeneous. It is demonstrated that the distribution (plus or minus 2(sigma) ) of soil moisture for given values of brightness temperature ranges between plus or minus 5% vwc from random errors imposed on the same three parameters. Improvements in parameter estimation in SGP97 contributed to a decrease in the soil moisture uncertainty ((alpha) equals 0.05) by about 67% to plus or minus 3% vwc.
An Impedance Probe (IP) was used during the Southern Great Plain 1997 field experiment to support the remote sensing soil moisture mapping effort in Oklahoma, USA. Soil moisture sampling techniques used as ground-truth for remote sensing are most often not adequate due to the time and cost associated with them. The need for fast, accurate, less expensive, and less destructive measurement techniques within the remote sensing discipline is increasingly growing. The IP offers an alternative for measuring soil moisture in remotely- sensed regions where the logistics of gravimetric sampling can restrict the number of samples obtained. The gravimetric technique has been the traditional and most accurate technique used to validate remotely measured soil moisture. This technique is very destructive and time consuming, thereby limiting our ability to apply it over large areas expected by coarse resolution microwave remote sensing products. Recent advances in the Time Domain Reflectometery (TDR) techniques have resulted in sensors with less noise and require minimum human intervention. Field sampling and laboratory experiments were conducted to evaluate and calibrate soil moisture data measured using an IP. The objectives were: to examine the best calibration technique that can be used to estimate the spatial distribution of soil moisture and/or a representative mean moisture level for a given location, and to evaluate the correlation of soil moisture values using the probe and gravimetric measurement techniques. Results indicated that a site specific calibration curve developed by using a pressure plate apparatus provided a representative mean value was equivalent to the gravimetric technique. Both, the simulated and manufacturer calibration techniques provide soil moisture values that are spatially correlated with the gravimetric measurement technique. Our results indicated that, the instrument can provide reproducible results comparable to the gravimetric technique. This instrument can be utilized to meet the requirement of measuring the soil moisture content in the upper 6-cm of the soil depth.
William Belisle, Narayan Rajbhandari, Teferi Tsegaye, Ahmed Fahsi, Andrew Manu, Yanming Liu, G. Robertson, Tommy Coleman, T. Jackson, Peggy O'Neill, M. Collins
One of the present efforts in the field of microwave remote sensing is to estimate soil water status throughout the soil profile using radiometers. This study describes a simple, ideal algorithm relating microwave brightness temperature (TB) and soil water status in the upper (10 cm) and lower (greater than 10 cm) soil depths. The algorithm description considers a simple, homogeneous, and sandy soil system with negligible amounts of capillary-stored water, organic matter content, and surface roughness. The microwave TB was estimated from the dielectric constant, k, as a function of the amount of water remaining in the soil during drainage. The effect of downward fluxing, free water on the value of TB was greater and occurred over a shorter period than that of the slower draining water near field capacity. The predictability of the algorithm decreased as the moisture content decreased to that of approximately field capacity.
This paper demonstrates that the soil physical properties can be estimated using temporal variation of surface soil moisture derived from remote sensing. Passive microwave remote sensing was employed to collect daily soil moisture data across the Little Washita watershed, Oklahoma, for the period between June 10-18, 1992. The ESTAR instrument operating at L band was flown on a NASA C-130 aircraft. Brightness temperature data collected at a ground resolution of 200 m were used to derive the spatial distribution of surface soil moisture. Analysis of temporal soil moisture information and soils data reveals a direct relationship between changes in soil moisture and soil texture. Areas identified by loam/silt loam soils are characterized by higher changes of total soil moisture and those of sand/sandy loam by remarkably lower amounts. Analysis suggests that two-day initial drainage of soil, measured from remote sensing, is related to saturated hydraulic conductivity (Ksat). A methodology has been developed to employ remotely sensed data for estimation of profile Ksat using a hydrologic model and a GIS. Model simulations have yielded good correlations between soil moisture change and Ksat. The results have potential applications to obtain quick estimates of spatial distributions of soil properties over large area for input to mesoscale hydrologic and global circulation models.
A major application for a 21 cm radiometer is the remote sensing of soil moisture which is possible because of the large contrast between the dielectric constant of dry soil (approximately equals 3.5) and that of liquid water ( approximately equals 80). One of the major problems with the utilization of long wavelength radiometers from satellite platforms has been the large antenna size required with its substantial mass. For example, at satellite altitudes an antenna size of at least 10 m is required to obtain resolutions in the 10-20 km range. The size requirement is fundamental but the mass can be reduced by using unfilled arrays or as will be described here a thinned array antenna. Such a system operating at L-Band ((lambda) equals 21 cm or 1.42 GHz) has been developed and tested from an aircraft platform. It is called ESTAR (Electronically Scanned Thinned Array Radiometer) and it uses linear (stick) antennas in the along-track direction and aperture synthesis between pairs of sticks separated by odd multiples on half wavelengths in the cross track direction. The approximate dimensions of the antenna are 1 meter by 1 meter. Results from an evaluation series of flights over a study watershed in Oklahoma indicate that such a system can provide useful soil moisture information.
Methods for estimating profile soil moisture using remotely sensed surface moisture data are reviewed. Primary attention is given to four basic approaches: regression, knowledge-based, inversion, and combinations of remotely sensed data and water balance models. Results of ground-based and aircraft experiments using microwave radiometers for soil moisture measurements are discussed to illustrate the methods.
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