What range of errors will be introduced by soil roughness and vegetative cover variations in microwave radiometry of soil moisture?
Surface roughness has influence on air dynamics by influencing its velocity due to friction. In high level of roughness it may also divert direction of wind flow through gullies or narrow troughs. As is known that wind speed, with humidity level and sunshine bear heavily in the rate of evaporation greater the wind velocity, the larger amount of the local moisture in the air, due to vapour pressure, will move away and increasing the process of evaporation of soil moisture. Surface roughness mostly retard the wind velocity therefore decreases the intensity of evaporation of soil moisture. However, in some situations roughness may act conversely. For example, if the wind is directed into a narrow valley, its velocity shall increase so may increase evaporation of soil moisture on valley floor.
Please consult FAO's Modified Evapotranspiration Model 65. I do think implication is transpiration from plants plus evaporation from soil. Even in the case of bare soil whatever thin film of water is on surface and vapour in the local 9supposed) stationary air, it moves to other place depending on the speed of wind. If air that replaces previous air is more moist, there will be less soil moisture loss otherwise it will increase. Wind speed is one of the variables required in above model and should be if not scientifically then intuitively Perhaps, there are very few evaporation and reference evapotranspiration models that do not take into consideration directly or indirectly wind speed..
Mohammad and Richard, Thanks for the information. I may get back to you with questions later on.
Vic K.
Victor, I am not an expert on the subject, but remembering stuff from some of the basics on MW remote sensing, here are some comments that I hope you will find useful.
I assume here that you mean *passive* microwave remote sensing of soil moisture. In that case, the dependance of surface microwave emissivity on surface roughness is an important parameter. Apart from dependencies on incidence angle and frequency (see Prigent, C., J. P. Wigneron, W. B. Rossow, and J. R. Pardo-Carrion, 2000: Frequency and angular variations of land surface microwave emissivities: Can we estimate SSM/T and AMSU emissivities from SSM/I emissivities?. IEEE Trans. Geosc. Remote Sensing, 38, 2373-2386),
for bare soil surfaces the emissivity may increase by several percentage points for larger roughness values, with increased sensitivity for soils with higher moisture levels. An important reference on this:
Choudhury, B.J., T. J. Schmugge, A. Chang, and R. W. Newton, 1979: Effect of surface roughness on the microwave emission from soils,. J. Geophys. Res., vol. 84, pp. 5699-5706.
Things get even more complicated with vegetation, with surface microwave emissivity depending on the surface area of the canopy and leaf wetness.
On this subject, see:
Jackson, T. J. and T. J. Schmugge, 1991: Vegetation effects on the microwave emission of soils. Rem. Sens. Environ., 36(3), 203-212.
That indeed looks interesting Ruxandra,. However, SAR is an *active* sensor. Its sensitivity to surface roughness is related to its impact on backscattering rather than emissivity. So the dependency of the signal and soil moisture retrievals to roughness will be different.
Soil moisture estimation using passive microwave radiometry depends on the emissivity of the surface. Roughness increases the surface area for emission. So more the roughness, more will be the brightness temperature at sensor.
Vegetation cover hinders the surface emission and we observe less brightness temperature at the sensor which in turn affects the soil moisture estimation.
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