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Manipulation of Normalized Difference Vegetation Index (NDVI) for Delineating Drought Vulnerable Areas
Water availability is one of the limiting factors for vegetation growth. If the moisture requirements of vegetation (cultivated and/or natural) are being fulfilled, then this healthy and dense vegetation will be evident from higher reflectance in near infra red region. However, decrease in water availability may limit healthy vegetation and result in decreased reflection in the near infrared region of EM spectrum. This can be supported by the two findings of present study. Firstly, denser vegetation is found mainly in the districts along Indus River. These districts have an irrigation network to support the crop cultivation. A decrease in denser vegetation is prominent towards lower Sindh, eastern Sindh and Balochistan where the irrigation is less or impossible. Secondly, in the rain fed areas the temporal change in vegetation cover occurs due to the change in rainfall pattern. The wet years are characterized by healthy vegetation where as in drought years vegetation growth gets limited and chances of occurrence of sparse vegetation also become less (Fig 4.1).
In order to study temporal change in the vegetation cover at district level two districts (Umerkot and Khuzdar) were selected. Umerkot district lies between 24°-54' to 25°- 47' north latitudes and 69°- 11' to 70°- 18' east longitudes. Topographically the district has two distinct portions the irrigated area in the west and north and the desert area in the east and south. The climatic conditions in both portions differ considerably. In the irrigated portion the climate is temperate, being neither exceedingly hot in the summer not very cold in winter as compared to the eastern desert area. The mean annual total precipitation from 1961 – 1990 is above 200mm Most of the rain falls in the monsoon months between June and September. The winter rains are insignificant. The mean December/January total precipitation from 1961- 1990 is just 1.5 mm. The distribution of denser, dense and sparse vegetation in the district as obtained from the categorization of NDVI images of February 1997 clearly distinguishes between the irrigated and desert area. The irrigated area has both the forest area as well as the crop cultivation. The exact delineation of the portion covered by forest and cultivated land cannot be done by NOAA satellite imagery only. A satellite image of higher resolution is required to support this result. A decrease in the vegetation cover of both the irrigated area and the desert area in Umerkot as evident from Figure 4.2 can be due to decline in water availability. In irrigated area, shortage of water occurs as the district is situated at the tails of Nara canal and Mithrao canal. The data regarding the surface water level for the irrigated area was not available so the estimation of hydrological drought leading to agricultural drought was limited. However, the rainfall data available for the meteorological station in Chhor was evaluated for the identification of meteorological drought conditions. Below normal rainfall in December/January of years 1997-98 and 1999-00 resulted in emergence of severe drought conditions. The vegetation cover in this part became environmentally stressed and faced a severe decrease in area in February 2000. 
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