Drought Monitoring in Zambia using Meteosat and Noaa Avhrr Data
System Outputs : From an input consisting of sets of dekadal raingauge data, daily and dekadal images of storm cloud duration (CCD), plus NOAA-AVHRR day time and night time imagery, the system is able to produce a set of outputs such as:
Ten day rainfall estimates: These are derived from a weighted average of gauge and satellite data (Grimes et al, 1999): A set of gauge rainfall values is interpolated by block kriging thus deriving a map of rainfall and a map of the associated uncertainty (error variance). A regression model provides an estimate of rainfall and associated error from an image of CCD output by a Meteosat. A combined estimate is derived as a weighted average of the two above estimates. The weights are a function of the uncertainty in each component - where the gauge network is sparse, the gauge derived estimate has higher error and hence smaller weight than the satellite estimate. The combined estimate therefore is weighted more heavily by whatever component is locally more accurate.
Figure 1 Rainfall Estimate for dekad 1, December 1999
Number of rain days in dekad: Although the amount of rainfall from Satellite Estimates is less reliable on a daily time step as compared to decadal, it can provide useful information on the probability of rainfall having occurred.
A logistic model was derived relating probability of rainfall above 0 to amount of daily CCD, calibrated on years of historical data. This model derives an image of probability of rainfall from an image of daily CCD. From the images of probability of rainfall we derive categorical images of rainfall occurrence by thresholding at a given probability level, i.e. set pixels to 1 if above this threshold, set to 0 if below. The threshold is derived according to criteria of optimal discrimination of rain/no-rain.
From these daily rainfall occurrence images we can trivially derive images of number of rain days per dekad or month. The quality of estimates of consecutive dry periods is under investigation.
Figure 2 Number of Rain Days in dekad 1, December 1999
Sowing rains occurrence: These are derived from the above as areas where it rains more than 25mm from 4 or more rain days in a dekad (ZMD internal definition). These are used to monitor the onset of the growing season and produced up to end of December
Figure 3 No. of Dekads with Sowing Rains, dekad 1 December 1999
Cumulative rainfall: Derived from the ten day rainfall images.
Figure 4 Cumulative Rainfall from 1st October 1999 to April 10, 2000
