ERS Wind Scattermeter Data For Vegetation Monitoring In The Thar Desert
Result
Radar backscatter variation with Rainfall
Figs. 2a, 2b and 2c show time series plots of radar backscatter and rain three point in the study area during January 1992 and May 1996. Rainfall data for the years 1992 and 1993 is not available and station in desert also lacks rainfall data for 1995. The rainy season in the area starts in the month of July and may go until end of September. It can be noticed that even slight rainfall affects the backscatter because it depends upon the dielectric constant which change with moisture conditions.
Figure 2a: Time Series of daily rainfall and monthly average radar backscatter of fore-beam
and far range incidence angles for desert
Figure 2b: Time Series of daily rainfall and monthly average radar backscatter of fore-beam
and far range incidence angles for semi desert
Figure 2c: Time Series of daily rainfall and monthly average radar backscatter of fore-beam
and far range incidence angles for vegetated desert
Radar backscatter in the desert zone is as low as -19.5 dB due to the uniform land-form with very low vegetation and dry state of the soil, which causes also increases penetration and absorption of signal and increases specular reflection of microwave radiation. During the period from July to September rain an increase of -16 dB in radar backscatter.
The radar backscatter in the semi desert zone during the dry parts of the year retains its value around- 16 db during rainy season shows 4.5 dB increase mainly due to increased moisture and also due to increased vegetation density. The effect of rains reduces towards vegetated areas where it does not cause much variation in radar backscatter. The rise in rainy season for this area is about 1.5 dB.
Radar backscatter behavior of different zones
Fig.3 shows a map of spatial radar backscatter data for the July 1995 and the highest NRCS values are found in the mountains range. The radar backscatter in mountains ranges from -8 to -5 dB shown with red color and reduces towards the desert. The desert area can be seen as a dark purple colored patch and here the radar backscatter values lie between -22 and -17 dB. The low radar backscatter area is surrounded by a higher radar backscatter boundary of -16 to -13 dB which is actually an area with little vegetation cover.
Figure 3: Map showing backscatter in different zones of the study area
The vegetation patch around the river can be seen in green color and has radar backscatter ranging between -13 and -10 dB. A small low radar backscatter patch in the north of Thar Desert beyond Indus river is part of Thal desert. It can be noticed that various landforms behave differently to Scattermeter, and indication for land studies on regional scale.
Radar backscatter time series and Howoeller diagram
Fig.4 shows the time series of the fore been and far range incidence angles radar backscatter images from March 1992 to October 1995. The cyclic radar backscatter behavior though different years can be well noticed. It is worth noting that the area with low radar backscatter which is actually the desert shrinks until September and again starts expanding to a maximum in May.
Figure 4: Time series of spatial radar backscatter for fore-beam and far range incidence angles.
The mountainous areas show expansion of high radar backscatter area twice a year i.e. in May- June and November-December, while minimum area of this radar backscatter (-8 to -5 dB) occurs in January and February. The patch around river shows the increased radar backscatter during August and September changing in color from green to yellow. During August-September the areas of low radar backscatter decrease while the areas of high radar backscatter increase throughout the time series. This increase in the radar backscatter corresponds to the rains occurring in the areas. Another worth noting phenomenon is the cyclic behavior of area throughout the study period. If same months of four years are compared, they show that beside inter- annual cycle the low radar backscatter area in desert is reducing from 1992 towards 1995. This could potentially be a part of a long duration cyclic behavior of the desert which may be investigated with sufficiently long term data of Scattermeter when available.
Fig.5 shows the Hovmoeller diagram for a slice through desert area from 220 N to 320 N latitude and 710 E longitude where inter-annual cyclic behavior and probable long term cycle can be seen. The southern parts are more susceptible to increase in radar backscatter in rainy season than the north. Vegetation in the north is mainly fed by river Indus which supplies water throughout the year while the south is rain area and shows greater variations in rainy season. Another noticeable feature is the time lag between different movements. In the south during 1992 the high radar backscatter area (blue colored) has a peak in August while for the still higher radar
backscatter area (green colored), it is September with a lag of one month. Similar trends can be seen in other years except for 1993.
Figure 5: Rader backscatter hovmoveller diagram for a slice through desert area from 22 N to 32 N latitude and 71 E longitude
In each year if the shapes of the peaks are observed, it shows that th erise is drastic which is from July to September followed by a gradual fall until July of the next years. September 1994 slice shows maximum increase in backscatter which has been found to be the month of maximum rainfall in the study period.
