Vegetation water content in source region of the Yellow River
This study uses the brightness temperature at the given microwave frequency (18.7 GHz) from the Microwave Radiation Imager (MWRI) on board Fengyun-3B (FY-3B) satellite to improve the τ-ω model by considering the radiative contribution from water body in the pixels over the wetland of the Yellow River source region, China. In order to retrieve vegetation optical depth (VOD), a dual polarization slope parameter is defined to express the surface emissivity in the τ-ω model as the sum of soil emissivity and water body emissivity. In the regions with no water body, the original τ-ω model without considering water body impacts is used to derive VOD. With use of the field observed vegetation water content (VWC) in the source region of the Yellow River during the summer of 2012, a regression relationship between VOD and VWC is established and then the vegetation parameter b is estimated. The relationship is employed to derive the spatial VWC during the entire vegetation growing period. The VOD retrieved is invalid and failed in some part of the study area by using the previous τ-ω model, while the results from improved τ-ω model indicate that the VOD is in the range of 0.20 to 1.20 and the VWC is in the range of 0.20kg/m2 to 1.40kg/m2 in the entire source region of the Yellow River in 2012. Both VOD and VWC exhibit a pattern of low values in the west part and high values in the east part. The largest regional variations appear along the Yellow River. The comparison between the remote-sensing-estimated VWC and the ground-measured VWC gives the root-mean-square error of 0.12kg/m2. These assessments reveal that with considering the fractional seasonal wetlands in the source region of the Yellow River, the microwave remote sensing measurements from the FY-3B MWRI can be successfully used to retrieve the VWC in the source region of the Yellow River.
In this study, the τ-ω model was improved by using the dual-polarized brightness temperature observations at the K-band obtained from the MWRI microwave radiometer on board the FY-3B meteorological satellite and removing the influence of water bodies. By constructing a microwave VOD model that is suitable for the source region of the Yellow River, one can be able to quantitatively retrieve the canopy VWC.
The VOD scheme considering the influence of water bodies demonstrated superior to the one directly using the τ-ω model for the inversion. The VOD with considering the influence of water bodies showed a large-value region occurring primarily in the area along the Yellow River and in the southeastern source region, where seasonal variation was dramatic. In particular, in the region from the west of Maqing to the east of Ngoring Lake, the VOD exhibited strong monthly variation. In the Maqu area, the maximum VOD of approximately 0.60 occurred in early-August. Due to low vegetation density in the source region of the Yellow River, the maximum VOD was less than 0.40.
The VWC in the entire source region was in the range of 0.20-1.40kg/m2, and the large-value area was mainly located along the Yellow River in the eastern source region. Large variation in the source region in the range of 0.20-0.50kg/m2 occurred in the Maqu area to the east, whereas in the western source region it did not show seasonal variation obviously, especially in the east of Ngoring Lake.
Although the VWC was derived by using the field observations, the number of samples is still insufficient. An analytic experiment about vegetation canopy samples with an even broader coverage range is necessary if these data are employed to derive the parameter b that is suitable for variation in vegetation coverage in the source region of the Yellow River to retrieve canopy VWC.
The error analysis of VWC indicated that the τ-ω model considering the influence of water body coverage was reasonable for derivation of vegetation canopy characteristics in the source region of the Yellow River. This study has shown that the brightness temperature data from the MWRI sensor on board the FY-3B could be used for land surface parameters retrieval in the source region of the Yellow River.