Desert Vegetation during droughts: response and sensitivity

Desert ecosystems are characterised by large spatial and temporal variability mainly due to scarcity of moisture owing to insignificant precipitation. Desert vegetation is represented mostly by natural vegetation and least by agricultural crops. Desert plants are naturally adapted to hyper-arid climate where, rainfall is sparse and day-time temperature is very high. However, desert phenology is very sensitive to climatic parameters, particularly rainfall and temperature. Insufficient and erratic rainfall causes moisture-stress, whereas high heatflow imparts thermal-stress to vegetation. Due to higher sensitivity, slight decrease in rainfall and/or increase in temperature impart stresses on desert plants. Therefore, desert-plants suffer severely during intense droughts. Thar, the Great Indian Desert covers 446,000 km2 area in the north-western part of India and eastern part of Pakistan. The Indian portion of the Thar Desert covers 208,110 km2 area of which 61% lies in Rajasthan state. The frequent occurrence of drought in this region is due to poor and untimely monsoon, abnormally high temperature especially in the summer and various other unfavourable physico-climatic events such as dust storm. In the present work, multi-sensor satellite data have been used to derive various vegetation parameters together with actual ground data (rainfall and temperature) for detailed drought analysis of the part of Thar Desert falling in the Rajasthan state, for the years 1984–2003. Vegetative drought indices have been calculated using Normalised Difference Vegetation Index (NDVI) values obtained from Global Vegetation Index (GVI) of NOAA AVHRR data. Spatial and temporal variations in rainfall, temperature, and vegetation indices in the Thar Desert have been analysed and correlated for monsoon and non-monsoon seasons. The mean seasonal NDVI, the index representing greenness of vegetation is found to be strongly correlated with seasonal rainfall. The time series of Vegetation Condition Index (VCI) and Temperature Condition Index (TCI) show that in certain years they correspond each other, while in other years one counter the other and their resultant determined the occurrence and severity of drought, which is reflected in the Vegetation Health Index (VHI). The results show that seasonal average of the VCI is directly correlated with cumulative seasonal rainfall, and seasonal average of the TCI shows strong correlation with average seasonal above-ground temperature. The time series of VHI shows that drought developed and affected the desert vegetation mostly when both moisture- and thermal-stress were generated (years 1985, 1986, 1987, 1989, 1990, 1991, 2000, and 2002) and sometimes due to thermal-stress alone (years 1995). In certain years (1993, 1995, 1996, 1997, 1998, 1999), inspite of high thermal-stress the desert vegetation could avoid drought or reduced drought severity due to adequate moisture supply. Opposite were the cases in the years 1988 and 1994, when excellent thermal condition countered moisture-stress and drought was bypassed. The results point out that both moisture-stress and thermal-stress play role in drought development, whereas drought severity is governed by moisture supply, which is directly related to and dependent on rainfall and dew. Therefore, desert phenology during drought is dependent more on moisture than temperature. Since rainfall reduces both moisture-stress and thermal-stress, it is a saviour of vegetation under drought.