Agricultural Drought Indices - US Department of Agriculture

Monitoring Drought by Remote Sensing Method
NRSC (Department of Space, Government of India) has been assessing and monitoring
agricultural drought since 1989 under the National Agricultural Drought Assessment and
Monitoring Systems (NADAMS). Under NADAMS, agricultural conditions are monitored at the
district level using daily observed coarse resolution (1.1 km) NOAA-AVHRR data for the entire
country and at the subdistrict level using better spatial resolution Indian Remote Sensing Satellite
(IRS) AWiFS/WiFS data .
IRS series (IRS 1C, IRS 1D, and IRS P3) have WiFS (Wide Field Sensor) payload, which collects
data in two spectral bands: 0.62-0.68 µm (red) and 0.77-0.86 µm (near infrared) with spatial
resolution of 188 m and ground swath of 810 km with a revisit period of 5 days. The IRS P6
(Resource Sat) has advanced WiFS (AWiFS) sensors that provide data with spectra, radiometric,
and spatial (56 m) resolutions for better monitoring of agriculture. The combination of
AWiFS/WiFS would help increase the frequency of images with almost one coverage in two days
time, which is useful to minimize cloud contamination.
The crop/vegetation reflects high energy in the near infrared band because of its canopy geometry
and health of the standing crops/vegetation and absorbs high in the red band due to its biomass
and photosynthesis. Using these contrasting characteristics of vegetation in near infrared and red
bands, which indicate both the health and condition of the crops/vegetation, the Normalized
Difference Vegetation Index (NDVI) is derived by the difference of these measurements and
divided by their sums. The vegetation index is generated from each of the available satellite data
irrespective of the cloud cover present. To minimize the cloud, monthly time composite vegetation
index is generated.
The monthly vegetation index maps for the states with district boundaries overlaid are given in
specific colors for the vegetation index ranges. Yellow through green to violet indicate increasing
green leaf area and biomass of different vegetation types. Cloud and water are represented in
black and blue colors, respectively. The bare soil, fallow, and other non-vegetation categories are
represented in brown.
The composite NDVI images are generated for each month of the monsoon separately for the total
geographic area and for the agricultural area of the state. The seasonal progression of NDVI
compared to that of normal and complementary ground data on rainfall and crop sowing progress
are utilized in the assessment of agricultural drought. Figures 5 and 6 depict monitoring of
agricultural drought by remote sensing during the monsoon season of 2009 (NRSC 2009).
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