Brian K. Hornbuckle

Vegetation canopy anisotropy at 1.4 GHz

We investigate anisotropy in 1.4-GHz brightness induced by a field corn vegetation canopy. We find that both polarizations of brightness are isotropic in azimuth during most of the growing season. When the canopy is senescent, the brightness is a strong function of row direction. On the other hand, the 1.4-GHz brightness is anisotropic in elevation: an isotropic zero-order radiative transfer model could not reproduce the observed change in brightness with incidence angle. Significant scatter darkening was found. The consequence of unanticipated scatter darkening would be a wet bias in soil moisture retrievals through a combination of underestimation of soil brightness (at H-pol) and underestimation of vegetation biomass (at V-pol). A new zero-order parameterization was formulated by allowing the volume scattering coefficient to be a function of incidence angle and polarization. The small magnitude of the scattering coefficients allows the zero-order model to retain its limited physical significance.

Initial Validation of SMOS Vegetation Optical Thickness in Iowa

The European Space Agencys Soil Moisture and Ocean Salinity (SMOS) satellite mission provides microwave L-band measurements of vegetation optical thickness over the Earth. Optical thickness is related to water held in vegetation. The water content of crops varies over the growing season from a minimum during planting to a maximum during reproduction and back to a minimum during senescence. We found that in Iowa in 2010 the change in SMOS optical thickness over the growing season can be related to crop yields. However, there are inconsistencies in the optical thickness data, particularly high-frequency variation and unexpected changes outside of the growing season. We hypothesize that the unexpected changes during the dormant periods are due to changes in soil surface roughness caused by land management activities and show a relationship between changes in roughness and changes in optical thickness, which may be confusing the SMOS retrieval algorithm.

Radiometric sensitivity to soil moisture at 1.4 GHz through a corn crop at maximum biomass

[1] Radiometric sensitivity to soil moisture at 1.4 GHz through a corn canopy at a maximum biomass of 8.0 kg m � 2 (water column density of 6.3 kg m � 2 ) was much higher than expected. The magnitude of the measured sensitivity of horizontally polarized brightness temperature to the 0–3 cm volumetric soil water content was at least 1.5 K per 0.01 m 3 m � 3 and could have been as high as 2.5 K per 0.01 m 3 m � 3 . Vertically polarized brightness temperature was 0.5 K per 0.01 m 3 m � 3 less sensitive than horizontally polarized brightness temperature. A widely used radiative transfer model that assumes a uniform distribution of vegetation in the canopy underestimated this soil moisture sensitivity at horizontal polarization by over 1 K per 0.01 m 3 m � 3 . Given an appropriate emission model that correctly accounts for the differences in transparency between heterogeneous canopies (as compared to the wavelength) such as corn and relatively homogeneous canopies such as grass, it appears that there will be practical sensitivity to soil moisture through corn (and most, if not all, row crops) throughout the growing season. INDEX TERMS: 1866 Hydrology: Soil moisture; 6969 Radio Science: Remote sensing; 1894 Hydrology: Instruments and techniques; 6994 Radio Science: Instruments and techniques; KEYWORDS: field experiment, hydrology, microwave radiometry, radiative transfer, remote sensing, soil moisture

Diurnal variation of vertical temperature gradients within a field of maize: implications for Satellite microwave radiometry

We present the diurnal variation of vertical temperature differences measured within and beneath a maize canopy over the course of a growing season, and we analyze the implied temperature gradients in the context of microwave radiometry and soil moisture retrieval in particular. We find that the temperature differences can be as large as 9 K in magnitude within the vegetation canopy and as large as 10 K between the soil surface and a depth of 4.5 cm. Satellite overpass times at 1:30 A.M. and 1:30 P.M. occur close to when the magnitude of the temperature differences are largest. For 6 A.M. and 6 P.M. overpass times, temperature differences were smaller in magnitude at 6 P.M. This contradicts the widely held assumption that surface temperature gradients are more uniform at 6 A.M. than at 6 P.M.

Reconsidering Leaf Wetness Duration Determination for Plant Disease Management

Relationships between leaf wetness and plant diseases have been studied for centuries. The progress and risk of many bacterial, fungal, and oomycete diseases on a variety of crops have been linked to the presence of free water on foliage and fruit under temperatures favorable to infection. Whereas the rate parameters for infection or epidemic models have frequently been linked with temperature during the wet periods, leaf wetness periods of specific time duration are necessary for the propagule germination of most phytopathogenic fungi and for their penetration of plant tissues. Using these types of relationships, disease-warning systems were developed and are now being used by grower communities for a variety of crops. As a component of Integrated Pest Management, disease-warning systems provide growers with information regarding the optimum timing for chemical or biological management practices based on weather variables most suitable for pathogen dispersal or host infection. Although these systems are robust enough to permit some errors in the estimates or measurements of leaf wetness duration, the need for highly accurate leaf wetness duration data remains a priority to achieve the most efficient disease management.

Comparisons of Evening and Morning SMOS Passes Over the Midwest United States

This study investigates differences in the soil moisture product and brightness temperatures between 6 p.m. and 6 a.m. local solar time Soil Moisture Ocean Salinity (SMOS) passes for a region in the Midwest United States. This region has uniform land cover, consisting largely of maize and soybean row crops. The comparison was restricted to periods with no rainfall. There were 19 days available for analysis of the soil moisture product. It was found that there was a significant difference in the soil moisture product for all 19 days, with lower soil moisture for most mornings. The difference between the soil moisture products on some days exceeded the allowable error of 0.04 m3 m-3. In-situ and model results indicate that there should be virtually no change in soil moisture between the evening and morning. In order to investigate this discrepancy, measured brightness temperature was converted to a polarization index (PI), and evening and morning values were compared. Investigation of the measured brightness temperature was limited to five days where a large range in incidence angles was available. Large differences between evening and morning passes were found for incidence angles less than 40° that could not be explained with radiative transfer theory but may be attributed to technical issues. There was also a difference in the PI values between the evening and morning passes for incidence angles greater than 40°. This can be caused by a decrease in soil moisture from evening to morning or could be attributed to an increase in the volumetric water content of the vegetation.

The potential of the COSMOS network to be a source of new soil moisture information for SMOS and SMAP

The COSMOS network will eventually consist of several hundred sensors throughout the United States that report kilometer-scale soil water content via measurement of the intensity of neutrons immediately above Earths surface. We show that COSMOS sensors must be corrected for the effects of growing vegetation. Once this phenomenon is completely understood the COSMOS network could be a useful source of information for the validation of both soil moisture and vegetation products obtained from current and future microwave remote sensing satellites.

The Effect of Intercepted Precipitation on the Microwave Emission of Maize at 1.4 GHz

Intercepted precipitation was found to increase the 1.4 GHz brightness temperature of maize. This effect is opposite that of dew, another form of free water in a vegetation canopy. If the effects of intercepted precipitation and dew are to be modeled correctly, emission models must be used in conjunction with a land surface process model in order to separate the two effects

Effect of dew on aircraft-based passive microwave observations over an agricultural domain

Abstract. 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.

A non-linear relationship between terrestrial microwave emission at 1.4 GHz and soil moisture caused by ponding of water

We present an example of a non-linear relationship between terrestrial microwave emission at 1.4 GHz and soil water content. The brightness temperature record of a bare agricultural field exhibited strong variations during a rain event despite a steady increase in soil water content. We show that these variations can be explained by water ponding on the soil surface since ponded water can add coherency to the radiation emitted by a water–soil system.