Monitoring the length of the growing season with NOAA

A vegetation index (NDVI) time series of 1984 to 1993 was used to calibrate a pixel-based assessment of the start, end and length of the growing season in the sahelian country of Burkina Faso. Annual curves of rainfall/ETP, NDVI crop calendars were prepared for meteorological stations representative for three climatic zones. Agrometereological criteria for assessment of the start and the end were extracted from literature. NDVI parameters were assessed at the dekad of the agrometeorological onset and end of the growing season, and used for the definition of decision rules. Decision rules were implemented in a GIS to the NDVI image time series. The start of the growing season assessed with the NDVI dynamics was slightly better correlated with given farmer's sowing dates than with original agro-meteorological criteria. The normal start of the growing season, calculated from the NDVI image time series, was in good agreement with the recommended sowing dates. A delay of the onset of the growing season with regard to normal was monitored under near real time conditions in Burkina Faso in 1995. It was found that the delay of up to five dekads could lead to a partial crop failure and to changes in decisions of farmers. The model for the start and the end of the growing season was validated with independent data from different years. From the maps of the end and the start, maps of the length of the growing season were calculated by subtraction. The length of the growing season was found to be in agreement with agrometeorological literature, although in northern sahelian areas, the real growing season measured by the vegetation index tended to be shorter by about one dekad. In addition, the climatic map was improved by a per pixel classification of a normal (1984-1995 median) image time profile. The 36 normal NDVI images were classified using a hyperspectral image-processing tool.