Rapidly assessing the 1997 drought in Papua New Guinea using composite AVHRR imagery
During 1997 Papua New Guinea (PNG) experienced an intense drought. Emergency famine relief operations provided many subsistence agricultural communities with food, water and health provisions during the height of the drought. The locations of relief operations were based on a rapid and spatially explicit extensive field survey conducted at the height of the drought for all PNG. We have tested the utility of composite Advanced Very High Resolution Radiometer (AVHRR) data to assist in a rapid assessment of drought conditions in PNG. Composited data were used to provide a means to overcome the frequent cloudy conditions that exist in PNG. To assess the drought we divided land surface temperatures (Ts) by the Normalized Difference Vegetation Index (NDVI). The ratio (Ts/NDVI) increases during times of drought. This is due to the increase in Ts associated with more net radiation being partitioned into the sensible heat flux and the decrease in NDVI associated with decreasing amounts of plant cover. A time series of Ts/NDVI is a rapid indicator of the drought at the country and province level. We calculated the integral under the Ts/NDVI curve for 1997, denoted ∫JanDec Ts/NDVI, which allowed us to validate the assessment in two ways. Firstly, it was compared with field-based assessments of food supply (undertaken at the height of the drought) which classified areas into five grades. Plotting ∫JanDec Ts/NDVI against the percentage area of each of the 14 mainland provinces experiencing food supply problems (so lives were at risk) showed a strong positively correlated relationship (r2=0.818). Secondly, for seven meteorological stations plotting ∫JanDecTs/NDVI against cumulative 1997 rainfall showed an inverse relationship (r2=0.809). Both forms of validation show that the composite AVHRR Ts/NDVI ratio provides a rapid means to assess drought conditions in a cloudy environment such as PNG.