Growing period and drought early warning in Africa using satellite data

Vegetation growing periods for 1983-84 were determined for 28 sites in Ethiopia using data from the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA series of meteorological satellites. Results offer promise for drought early warning from space. A strong correlation (r=0·99) was found between rates of change of the normalized difference vegetation index (NDVI) derived from the AVHRR data and threshold values of a soil moisture index at the beginning and ends of growing periods. The moisture index (P + S)/ET_p relates precipitation P, stored soil moisture S and potential evapotranspiration ET_p in a simple moisture balance (LPG) model that requires inputs of standard monthly meteorological data. A moisture threshold of (P + S)/ET_p = 0·5 was used to identify the beginning and end of the growing periods and to calibrate the time series of NDVI responses. Trends also detected in values of the NDVI during vegetation growth cycles suggest useful minima exist at the beginning and end of growing periods. Below respective minima of 0·10 and 0·22, growing periods are unlikely to have been initiated or to continue during a declining growth stage. Correlation analysis indicated a relation between moisture index and NDVI, with NDVI lagging in time, in most cases, by 5 or less weeks during the initial growth stage and 6 or more weeks during declining growth