International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
the soil moisture content in the first meter of the soil layer,
ranging between 0 and 100. If soil hydrologic soil properties are
known (wilting level, field capacity and total water holding
capacity) the SWI can be related to the volumetric soil
moisture content. ;
An advanced change detection approach fully exploits the
unique sensor design of the ERS scatterometers with three
antennas simultaneously observing the Earth surface at different
look directions and the availability of 10 years of high quality
backscatter data. The dependency of the reflected signal to the
incidence angle depends on the amount of vegetation on the
surface. For correcting vegetation effects, we use the fact that
there exist an incidence angle, which varies with the moisture
conditions, were the effect of vegetation is minimized.
M Forest ®
L] Frost/snow
Bl azimuthal artefart
Figure 12. Global SWI derived from ERS scatterometer
measurements in January (1992-2000 average).
Global soil moisture fields are also derived from passive
microwave measurements (Aqua / AMSR). The parameter is
here defined as the content of water within the uppermost meter
of soil. The starting point of the analysis is to consider ground
measurements from the former Soviet Union. The data set
comprises soil moisture measurements of the upper 1 meter soil
layer at 50 stations. Measurements are taken every 10 days
during the period 1952 to 1985.
Figure 13. Multi-
year mean soil
moisture as
derived from
SMMR (1978-
1987)
Longtime mean retrieved soil mobsiure / mm
A two step methodology is then applied: first, the time averaged
soil moisture at each grid point is calculated as a linear
regression function of the long-time mean precipitation from
GPCP (Global Precipitation Climatology Project), the
vegetation density cover from the UMD-1km land cover map,
and soil texture and terrain slope from FAO. The coefficients of
the regression are calculated using the ground measurements.
In a second step, the remaining temporal variance of soil
moisture can be inferred at each grid point using three
temporally varying parameters. These are the precipitation
(again from GPCP), the air temperature, and the horizontally
polarized 10 Ghz brightness temperature from AMSR. A further
linear regression yields the temporal varying part of soil
moisture.
4.8 Precipitation
The goal is to develop a global scale daily precipitation product
based on existing multi-satellite products and bias-corrected
precipitation gauge analyses. The objective is to improve the
GPCP-1DD global multi-satellite product with a better
calibration making use of bias-corrected rain gauge analyses
based on about 6000 synoptic stations.
01.07.2000 [EEE I I I I I EX Precipitation [mm/d)
no dela 025 us 1 2 4 8 16 31 4
Sauren: GPCP-1D0 Slabsics means 271,51: = 6.88, «Md 2 8 45, ruin a 1.80, maso 22 0t
Figure 14. GPCP-1DD multi-satellite estimates of precipitation
(mm) for July 01, 2000.
The GPCP-IDD multi-satellite product is derived from geo-
stationary sensors data in the 40° north-south belt, and TOVS
data from the polar orbiting NOAA 12/14. It is calibrated with
monthly precipitation data being near-real-time available based
upon reports from GPCP. Currently, there does not exist any
operational global daily precipitation product, which is based
on bias-corrected gauge analyses. The idea here is to collect
global daily rain gauge data for the period 1997-2003, and to
correct them for systematic measurement errors. The bias
correction of the ground-based precipitation measurements is
needed, because of the under-catch of operational rain gauges,
which is on the order of 5-30 percent on average. Then, the rain
gauge data will be interpolated to a global regular 1°
longitude/latitude grid. GPCP satellite product and rain-gauge
measurements will be co-kriged to estimate the final CSP
precipitation product.
4.9 Evapotranspiration
The actual evapotranspiration in energy units represents the
latent heat flux exchanged between the land surface and the
atmospheric boundary layer.
The actual evapotranspiration is calculated as the difference
between the daily net radiation and the daily sensible heat flux,
under the assumption that the soil heat flux can be neglected.
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