Five disadvantages observed fromtraditional NDVI/VCI methods are: 1. Theamplitudes and phases of NDVI vary by crop type, but the traditional NDVIcalculation ignores the difference of per-pixel crop type across years (VCI hasremedied the second part) 2. Thereis always a lagging period between NDVI and precipitation, and the NDVI failsto pick up the vegetation moisture difference after saturation; 3. Soiltype and wetness largely influence the NDVI values, so the NDVI is in fact acombined information of vegetation condition and soil situations4. NDVI/VCIis not directly related to the occurrence or the severity of drought.
Droughtcan be due to moisture-stress or thermal-stress. A lower NDVI/VCI could meanthat the crop land is suffering from vegetation stress due to many reasons –drought, flood, extreme temperature, wild fire, pesticides or lack offertilizers5. Operationaldrought definitions were developed by fitting an appropriate distributionfunction to a drought index. Five drought thresholds (ranging fromabnormally dry to exceptional drought) were defined based on the percentilesused by the USDM. Using an objective approach for determining droughtdefinitions ensures that droughts are accurately and correctly identified atthe local level. It is inappropriate to use a single set of drought definitionsfor an entire state, Overall, no single index can represent all aspects ofmeteorological or hydrological/water supply drought so it is best to use amulti-index approach for operational drought monitoring.
Due to these reasons,choosing the correct drought indicator and severity classification scheme, basedon the geospatial and environmental characteristics, is the key to successfullymonitoring and forecasting drought. Adopting a good drought indicator willenable us (1) to detect and monitor drought conditions, (2) to determine thetiming and level of drought responses, (3) to characterize and compare droughtevents, and (4) to tie together levels of drought severity with droughtresponses thereby forming an operationally workable drought management plan.With a universally usable drought indicator, the next step is to establish athreshold for the drought indicator, which is also important because it aims toassess the change thereby to determine drought risk and vulnerability of anarea. Normally, the thresholds should be region specific and are subject tomodifications in order to reflect market, climate, environment, public healthand socio-economic changes (Nagarajan, 2009).