The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
132
Then, these obtained maps together with DEM, the land use
map and soil type map participate in the model to get a rougher
soil moisture distribution map. This map is resampled into the
same size as the coherence Thus, a preliminary comparison and
analysis between coherence image and soil moisture map can be
done. In this section, the detail methods of acquiring soil
moisture and InSAR coherence are introduced.
3.1 Soil moisture
The distributed hydrologic model which is based on raster data
model in GIS and popular at presmt is used to build the
distributed model of soil moisture. There are four models: soil
évapotranspiration model, raster water flow model, rainfall
runoff model and soil moisture model. In the model, three main
factors that affect soil moisture are considered. That is rainfall,
runoff and soil évapotranspiration. The point data on the rainfall
provided is interpolated to get rainfall distribution maps of the
whole area on which the physical processes of rainfall runoff is
simulated. Other meteorological data are processed in the same
way of interpolation to calculate daily soil évapotranspiration of
the area by the function of raster calculator in ArcGIS
according to the évapotranspiration model.
Based on the soil water balance principle, the soil moisture
model uses the other three models to get the final soil moisture.
When no rainfall happens, soil évapotranspiration model is used.
In the area where there is rainfall, raster water flow model and
rainfall runoff model are needed.
Actual soil évapotranspiration is calculated according to
Penman equation.
Raster water flow model is based on DEM. Compare slope
drops between the processing cell with the nearest eight cells.
The direction of the line that join the processing cell center with
the one which has the largest gradient among the eight is
defined as the water flow direction of the processing cell, and
the water flow direction of a cell is represented by a number. 1,
2, 4, 8, 16, 32, 64 and 128 correspond to east, southeast, south,
southwest, west, northwest, north and northeast respectively.
The gradient equation is as follows
Z -Z
MD = —— - x 100 (i)
J{X k - Xi ) 2 HY k -Y t ) 2
0 (P-0.2S) 2
^ (P + O.&S)
Q = o
P>0.2S
(2)
P < 0.2S
Where, Q is daily runoff (mm); P is daily rainfall (mm); S is
retention parameter which is different with area and time and is
related with the soil, slope, land use, management and prior soil
moisture.
The following equation links S with curve number CN:
5' = 254(—-1)
CN
(3)
The range of CN is 0~100(does not equal 0). When CN is 100,
S is 0 and Q equals P.
3.2 Coherence
The coherence is the key product of SAR interferometric
process, which is the magnitude of the complex correlation of
both amplitude and phase information from two interferometric
signals. It is an important parameter to measure InSAR data
quality because the area with high coherence could usually
obtain qualified interferometric measurements. The coherence
of two co-registered complex SAR images Ij and I 2 can be
defined as:
Where, y stands for the coherence. The brackets () is the
estimated ensemble average and * denotes the complex
conjugate. The value of y is in the range [0.0, 1.0].
The degree of coherence that is estimated from a complex SAR
image pair can be considered as the product of different de-
correlation factors as long as the sources of decorrelation are
statistically independent.
Where, MD is the gradient between two cells; (X k , Y k ) and (X i;
Y ; ) represent the coordinates of the calculating cell and the
nearest ones respectively; Z k and Z\ represent their elevation
values.
Y ~ y System SNR N Processor y Baseline v R egistration ZTemporal
Curve number method created by Soil Conservation Board in
United States Department of Agriculture is widely used by all
countries so far and is accepted to calculate rainfall runoff on
our study.. In this method, daily rainfall data easily acquired is
used and runoff of land surface is connected with soil type, land
use, management, etc. This method links runoff with daily
rainfall directly and the relationship between runoff and rainfall
is:
Where, SNR is the radar system signal-to-noise-ratio. yp r0 cessor is
de-correlation due to the SAR processor, which is the
processing stages from the SAR raw data to the single-look-
complex (SLC) SAR image product. Baseline stands for the
across-track distance between the two satellite passes, and
Registration is referring to the accuracy a SLC SAR image pair
can achieve in the coregistration process.
The four first terms on the right-hand side of the equation are
factors that one will desire to minimize (i.e., obtain values close
to 1.0) so that the estimated coherence in an area is
corresponding more or less to the amount of temporal