In: Wagner W., Székely, B. (eds.): ISPRS ТС VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
267 
Where T 37 is the threshold derived from the linear relationship 
between T B . 37V and near-surface soil temperature. It is set as 
258.2 К and 260.1 К for morning and evening satellite passes 
based on our previous results (Han, et al., 2010a). 
And the pixels from water-covered areas and regions of high 
elevation (>3000m) were filtered by considering their 
influences on brightness temperature. 
4.3. Soil moisture model 
A simplified water balance based model, as shown in Equation 
6, is used in this investigation (Yamaguchi and Shinoda, 2002). 
The top 30 mm soil moisture content at the end of 2003 and the 
snow water equivalent after consecutive maximum temperature 
higher than 0°C at the beginning of 2004 were considered in 
estimating the soil moisture conditions. 
dW{t)/dt = P(t)-E(t)-R{t) 
and 
R(t) = 
fW(t)-W /c ,W(t) > w fc 
I 0, W(t) < w fc 
(6) 
Where W, the soil moisture, is expressed as the equivalent depth 
of liquid water that exists from the surface to a 30-cm depth, 
and t is the time in days; p, E, R represent the precipitation, 
évapotranspiration and runoff, respectively; and \y^ represents 
the field capacity. 
5. RESULTS AND DISCUSSION 
5.1. Validations of the results 
Validations have done for both of the results from active and 
passive microwave remote sensing. For result from active 
microwave remote sensing, a reliability of R = 0.8, P < 0.05 was 
obtained between estimated primary thaw date and the date 
when consecutive days’ average air temperature keeping 
positive values; and R = 0.85, P < 0.05 was obtained in result 
from passive microwave remote sensing with comparison 
between estimated onset/offset of the thaw and field measured 
soil thaw event. And the soil moisture model was validated by 
comparison with the ground truth with root mean square error 
(RMSE) of 4.10±1.69 mm. Such reliabilities were strongly 
enhanced our confidence in making the further analysis. 
5.2. Result from active microwave remote sensing 
Primary thaw date and geographical boundary of the thaw was 
detected as shown in Figure 2. In the north: the thaw event 
boundary runs south of the Three River Plain, Mt. 
Xiaohingganling, and Mt. Dahingganling to the north of the 
Desert Basin, then to the south of Mt. Henteyn, Mt. Hangayn, 
and Mt. Altayn. The other area consists of regions of high 
elevation, such as Mt. Tianshan, Mt. Changbai, and the Tibetan 
Plateau. No thaw events occurred in the remaining (gray) areas, 
like the deserts regions and south warmer areas, due to the drier 
surface conditions or temperatures too high to freeze the surface 
layer in winter. The primary thaw date follows clear spatial and 
temporal patterns. In high elevation and high northern areas, 
thaw events happen later than in lower elevation and southern 
areas. On the Tibetan Plateau, at Mt. Hangayn, and at Mt. 
Altayn, the thaw event occurred at the end of the first 180 days. 
Those three areas are almost entirely covered by permafrost, so 
only the active layer thaws in the short summer. 
I30WJC ' N 
:mГ 351 - 60 ИИ III- 120 
mrm”E 
120 WE 
I3BWE 
Figure 2. Primary thaw date and geographical boundary of the thaw derived from QuikSCAT data (Julian day). (Gray-colored areas 
represent regions without water driven soil thaw occurrence.) 
5.3. Result from passive microwave remote sensing 
Onset and offset of springtime soil thaw event was obtained as 
shown in figure 3. The onset and offset of near-surface soil 
thawed in spring begins in the south and southeast of the study 
area and then progresses northwest and north, but these trends 
are somewhat disrupted in areas of complex topography. 
The main progression of the onset is from Mt. Qinling through 
the Guanzhong Basin, Loess Plateau, Ordos Plateau, and on 
toward Mt. Yinshan, the Mongolian Plateau, and finally to the 
Desert Basin and Hulunboir Plateau. This progression is 
disrupted by high elevations at Mt. Yinshan and Mt. Henteyn. 
Another extension of the onset branches northwest from the 
main northern trend at the Loess Plateau, and extends 
approximately northwest through the Hexi Corridor and Badain 
Jaran Desert to Mt. Altayn, where the latest onset of near 
surface soil thawed is evident. The third branch of the onset 
progresses approximately northeast through Haihe River Plain 
to the Horqin Sandy Land, Northeast Plain, and Songnen Plain,