Drought Monitoring in Zambia using Meteosat and NOAA AVHRR Data
Kawana Nawa
Remote Sensing Unit
Zambia Meteorological Department,
P.O. Box 30200, Lusaka, Zambia
Fax/Phone: (260-1) 252728/251889,
E-mail: knawa@yahoo.com
Keywords : Satellite rainfall estimates, Rainfall deficits, Meteosat, NOAA.
Abstract
Rainfall is of crucial importance for all life in Zambia not only because it is a resource for human and livestock but also because it is the major limiting factor to plant growth. Between 80 to 90% of agriculture in Zambia is rain-fed and as a result, any falls in the required amounts need to be identified as quickly as possible to enable decision makers take necessary measures to avoid catastrophe in human, plant and animal life. Rainfall in Zambia is highly irregular in space and time which makes cultivation difficult since small differences in the amounts and timing of rain received at a site may determine the success or failure of critical stages in Vegetation development and hence crop production. Determination of areas of rainfall deficit can be greatly improved by using satellite rainfall estimates.
Rainfall measurements have traditionally come from a network of raingauges which in the past years have dwindled in number. Since raingauge measurements are only valid for a small area in the immediate vicinity of the instrument, use of Geo-stationary satellites such as the Meteosat and polar orbiting satellites such as the NOAA series, provide data at a resolution sufficient for monitoring of rainfall events at a much wider scale. Currently different rainfall information is produced and disseminated to various users ranging from farmers, researchers to decision makers.
Introduction
Throughout much of Africa, rainfall is the most important meteorological factor in determining agricultural production and hence economic well being on a local or national scale. Accurate, real-time information on rainfall should therefore be valuable in providing an overview of the development of the current rainy season, and highlighting areas where drought or crop shortfalls are likely. Unfortunately, the raingauge network in Zambia is too sparse to provide this overview and is still reducing further as government funding on public services continues to be small.
The use of rainfall estimates based on satellite data should be of great benefit. In particular, Meteosat thermal infrared imagery can be used in detecting CCD's (an indicator of convective cloud activity) at different thresholds. The CCD images are used in a number of algorithms to give useful quantitative rainfall estimates. These estimates can easily be produced in near real-time and give full area coverage irrespective of the remoteness of a region given the temperal and spatial characteristics of the Meteosat satellite. Complementary information on vegetation and seasonal large scale flooding can be derived from NOAA-AVHRR satellite.
The problem remains of how to communicate this information to local institutions that need to use it. We argue in favour of local reception facilities - they provide more timely information, ensure local staff are familiar with remote sensing technology, improves their scientific background, promotes technical independence of local institutions and can provide information not usually available from international dissemination centres. Locally produced information has a better chance of reaching a wider local user base and is more flexible to varying user demands.
Aiming at improving this situation a joint project took place between the TAMSAT group, Department of Meteorology at the University of Reading, UK and the Zambian Meteorological Department (ZMD). Zambia was suitable for simple rainfall estimation algorithms based on Meteosat Thermal Infra-Red (TIR) imagery and ZMD had the basic technical capacity (Meteosat and NOAA-AVHRR receivers) already in place.
The Aims of the Project were:
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To refine rainfall related information from station and satellite data.
- To implement an operational system of dissemination of agro-meteorological information to a user community along the principles of sustainability and independence. Such a system was designated ZAMIS (Zambian AgroMeteorological
The Zambian Agrometeorological Information System (ZAMIS)
ZAMIS Design : The design of the ZAMIS obeyed the following requirements:
- that it should be adaptable to take account of changing user requirements;
- that the hardware and software used should be inexpensive and easily maintained
- that the staff involved should understand the limitations and benefits of the methodologies
- that the staff involved should be capable of developing new output products as appropriate
Software Development : From the operational sources of raw data, there was a need to produce a very specific set of information outputs using particular methodologies and algorithms. Given the requirements for flexibility, ease of maintenance and low cost, instead of developing a software package to deal with this need in a specific way, the system was built by breaking down the processing task into a sequence of small modular units, each coded in a third party (low cost or free) GIS/IP software macro language and some programs provided by TAMSAT. A scripting language (AWK) is used as a "software glue" to define, control and automate the sequence of processing modules. The AWK scripts have embedded all the required coefficients - e.g. calibration coefficients for rainfall occurrence and estimates, geographic region definitions, settings for image format conversions, etc,. Since all scripts are ASCII, updating and modification of settings and coefficients is a trivial task. IDRISI is the GIS/IP used at ZMD, but any with an ASCII macro language will do (e.g. IDA/WinDisp).
