Research

The depth through which energy is emitted and sensed by microwave radiometers has been the subject of research and discussion for many years. The emitting layer has been variously referred to as the penetration depth, sampling depth, effective depth, contributing depth, and emitting depth although these are not necessarily equivalent terms. Despite what it is called, the emitting layer is more easily regarded as a concept rather than a specific quantity because the thickness of the layer is defined somewhat arbitrarily, differs with regards to the wavelength of the emitted energy, and changes as a function of moisture and temperature conditions. Theoretical and empirical evidence suggest that the optimal frequency for remote sensing of soil moisture is L band (~1.4 GHz) (cf. Jackson, 1993) because at longer wavelengths the emitting layer is thicker due to less scattering and attenuation. Jackson and Schmugge (1989) and more recently Jackson et al. (1997) summarized early modeling and experimental results pertaining to emitting depth. A vast amount of literature has been generated on the subject and there is overwhelming agreement that most of the emitted energy is derived from a soil layer no thicker than about 10% to 25% of the wavelength.

Figure 1: Plots showing the cumulative contribution to emitted brightness temperature as a function of depth for a range of soil moisture conditions. Results are given for L and C band.

We performed simulations with a radiative transfer model in which the integral that defines brightness temperature was evaluated from the surface to various depths. We parameterize the model to reproduce direct observations of brightness temperature. In our first case, we used data from the ground-based S and L Microwave Radiometer, which is a dual-frequency single polarization passive sensor system that operates at S-band (2.65-GHz or 11.3 cm) and L-band (1.43-GHz or 21.2 cm). In addition, this system was supplemented with a C-band radiometer at a frequency of 6.34-GHz. The radiometers were mounted to observe horizontal polarization from a hydraulic boom truck during the Huntsville 1998 field experiment. Figure 1 shows emitted energy as a function of depth at two different microwave frequencies. The energy emitted from a particular depth is expressed in terms of the cumulative percentage of total brightness temperature. The differences in emitting depth among bands are a direct function of the proportional difference among frequencies. Thus, C band emitting depth is 25% of the L band emitting depth or 45% of the S band emitting depth. If one arbitrarily defines emitting depth as 1/e (~37% of total emission) as Ulaby (1986) has done, then the L and C band emitting depth is about 2.8 cm and 0.5 cm, respectively. If, on the other hand, we define emitting depth as the depth contributing say 50% of the total brightness temperature, then the L band and C band emitting depths are between 3.5-5.0 and 0.8-1.4 cm, respectively. These variations are from our wet to dry case.

Figure 2: Plots showing the cumulative contribution to emitted brightness temperature as a function of depth for corn and soybean cover over a range of soil moisture conditions. Results are given for L and C band. The clustering of lines in each plot is due to moisture differences.

For our second case, we evaluated emitting depth associated with the Soil Moisture Experiments in 2002. In this investigation, we simulated brightness temperatures from the Passive and Active L and S band (PALS) microwave instrument system. PALS is an aircraft-based system that operates at L band (1.41-GHz radiometer and 1.26-GHz radar) and S band (2.69-GHz radiometer and 3.15-GHz radar) with dual polarization (fully polarimetric radar). The ‘total’ TB was determined by integrating to a depth of approximately 67 cm for L band and 14 cm for C band. Figure 2 shows the cumulative contribution of the surface layers to total emitted TB for each of our study sites. A curve is plotted for each observation at the six locations at each study site. Under dry conditions, at least 75% of the total L band TB is derived from the upper 5-6 cm layer. Under wet conditions, 85-95% of the total L band TB is derived from the upper 5-6 cm layer. The L band emitting depth under corn is slightly less than that of soybean. For both crops, more than 90% of the total C band TB is derived from the 0-6 cm layer. These data define the significant contribution of the 0-6 cm soil layer with regards to total TB. They also indicate, especially for C band, that the uppermost 2-3 cm is far more important than the lower profile in contributing to TB.

Soil moisture values that are retrieved by inversion of passive microwave brightness temperatures are effective values dependent on the vertical distribution of moisture (and temperature) in the soil. Thus, there exists a discrepancy between the observed mean moisture and retrieved soil moisture. The error associated with correlating average moisture for a fixed layer with moisture retrieved from effective emissivity is lumped into overall error associated with numerous assumptions and generalizations used in the retrieval algorithm. The emitting depth function can be used to isolate the bias between observed near-surface mean soil moisture and retrieved effective moisture. If the observed near-surface moisture profile can be weighted by the emitting depth function, at least a portion of this bias could be removed. See “Converting Remotely Sensed Soil Moisture to Effective Moisture.”

Acknowledgements:
This research was supported by NASA through grant no. 291-07-75-90 to Universities Space Research Association and grant no. NCCW-0084 to Alabama A&M University , Center for Hydrology, Soil Climatology and Remote Sensing. This work was conducted in collaboration with Dr. Frank Archer of Alabama A&M University .

References Cited:
Jackson,T.J. 1993. Measuring surface soil moisture using passive microwave remote sensing. Hydrol. Proc., 7:139-152.

Jackson, T.J., and Schmugge, T.J. 1989. Passive microwave remote sensing system for soil moisture: some supporting research. IEEE Trans. Geosci. Rem. Sens. 27:225-235.

Jackson, T.J., O’Neill, P.E., and Swift, C.T. 1997. Passive microwave observation of diurnal surface soil moisture. IEEE Trans. Geosci. Remote Sensing, 35:1210-1222.

Laymon, C.A., Crosson, W.L., Jackson, T.J., Manu, A., and Tsegaye, T.D. 2001. Ground-based passive microwave remote sensing observations of soil moisture at S-band and L-band with insight into measurement accuracy. IEEE Trans. Geosci. Remote Sensing, 39:1844-1858.