SOWING RAINS ISOHYETAL METHOD OF PROJECTING PROBABLE 
MAXIMUM PLANTED AREA FOR RAINFED CROPS IN SOUTHERN AFRICA 
Francis A. Banda 
A. R. Sandoval 
Zambia National Early Warning System 
Department of Meteorology 
P.O.Box 30200 
Lusaka, Zambia 
ISPRS Commission VII / Working Group 1 
ABSTRACT 
In rainfed farming, commencement of the adequate rain is essential in the initial sowing of the principal 
crops. This is more so in Southem Africa (Zambia in particular) where its annual rainfall is received during 
the months on October through April when the country depends most for its agricultural operations (Das, 
1979). Das reported a statistical study on the commencement of sowing rains in any place in Zambia as: 
"Starting with any rainy day of at least 1 mm of rain, 7 consecutive days are counted and the total rain in this 
should be at least 20 mm". In addition there should be at least 4 rainy days of 1 mm or more in this period of 
7 days. 
Notice from his criterion (Das, 1979) that the sowing rain of 20 mm was the total amount for a period which 
has an interval of 7 days or one week. The Zambia National Early Warning System (ZNEWS), however, 
consider a period with a 10-day interval or a dekad. Based on these two reports, ZNEWS adopted this value 
of 30 mm of rainfall as the sowing rains. 
The commencement of the sowing rains once established during a growing season could be of interest to 
government planners and policy decisions makers. At planting, if the area that had been planted to a given 
crop could be projected, preliminary conservative food balance estimates for the country could be made much 
earlier since there is a lag between planting and the compilation of the survey of the planted areas during the 
growing season. The purpose of the present study is to describe a method of projecting the probable maxima 
area that could be planted to a given crop using the sowing rains isohyetal method. In turn you can make a 
forecast on the crop production for the on going season. 
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