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Abstract
Synthetic aperture radar (SAR) imagery was collected over a brown locust Locustana pardalina outbreak area to estimate soil moisture relevant to egg development. ERS-2/RadarSat overpasses and field studies enabled parameterization of surface roughness, volumetric soil moisture, soil texture, and vegetation cover. Data were analyzed both when the target area was assessed as nonvegetated and when treated as vegetated. For the former, using the integral equation model (IEM) and soil surface data combined with the sensitivity of the IEM to changes in surface roughness introduced an error of ∼±0.06 cm 3 cm −3 in volumetric soil moisture. Comparison of the IEM modeling results with backscatter responses from the ERS-2/RadarSat imagery revealed errors as high as ±0.14 cm 3 cm −3 , mostly due to IEM calibration problems and the impact of vegetation. Two modified versions of the water cloud model (WCM) were parameterized, one based on measurements of vegetation moisture and the other on vegetation biomass. A sensitivity analysis of the resulting model revealed a positive relationship between increases in both vegetation biomass and vegetation moisture and the backscatter responses from the ERS-2 and RadarSat sensors. The WCM was able to explain up to 80% of the variability found when the IEM was used alone.
Highlights
Locusts and grasshoppers (Orthoptera: Acrididae) are infamous pests responsible for destroying crops and pastures.[1,2,3] All locust species require moist soil for oviposition and varying degrees of moisture at different times to allow their eggs to develop and hatch
The focus of this paper is an assessment of the potential of synthetic aperture radar (SAR) to provide an accurate quantitative assessment of soil moisture, more applicable than rainfall amounts for use in brown locust life cycle modeling based on practical model inputs that can be clearly defined and measured in the field using field study results as inputs; we present comparisons of results using an integral equation model (IEM) and those from SAR imagery to estimate soil moisture below a bare surface and details of investigations of a vegetated surface
The relatively smooth conditions of the measured soil surfaces in the Karoo meant that slight variations of root mean square (RMS) over time and location can have dominant impacts on the backscatter response
Summary
Locusts and grasshoppers (Orthoptera: Acrididae) are infamous pests responsible for destroying crops and pastures.[1,2,3] All locust species require moist soil for oviposition and varying degrees of moisture at different times to allow their eggs to develop and hatch. Desert locusts Schistocerca gregaria need at least 25 mm of rain to have fallen after their eggs have been laid to allow successful breeding,[4] whereas other species, such as the Senegalese grasshopper Oedaleus senegalensis, can enter quiescence or diapause and their eggs may hatch as long as five years after being laid.[5,6] This is true of the brown locust Locustana pardalina that occurs in southern Africa. We present results of a study addressing this issue using synthetic aperture radar (SAR) images of soil in habitats where the brown locust breeds in the Republic of South Africa
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