Development of a Raster Database of Annual, Monthly and daily Rainfall for Southern Africa
Report No 1156/1/04
Rainfall measurements were recorded as early as 400 BC and, since its inception, both the principles and the purpose of rainfall measurement have remained unchanged (Biswas, 1967; Ward, 1975). The rainfall of an area helps to structure society in a geographical sense. Water is an essential element for life, thus the more water that exists in an environment, the more potential that environment has for sustenance of life. Early settlers settled in higher rainfall areas, and towns were built up in and around these areas, shaping settlement patterns as we experience them today (Antevs, 1938). In order to obtain rainfall data the rainfall needs to be measured. Rainfall measurements can be undertaken by numerous different methods. The most common method is the use of a standard non-recording rain gauge, but nowadays estimation by radar and satellite is practised as well. While radar and satellite imaging for rainfall estimates are able to give real time, areal estimates of rainfall data, the primary source of rainfall data is still provided by the daily non-recording rain gauge (Seed and Austin, 1990). This is mainly so because rain gauges are cheap and generally reliable. Rain gauge data are also available for longer time periods, which is advantageous in many respects. It is anticipated that over time, the rain gauge network density will increase, so as to provide better areal estimates of rainfall from the use of point measurements.
The measurement of rainfall is a simple procedure provided that accuracy is not essential, as an exact measurement of rainfall is impossible to obtain owing to the random and systematic errors occur in measuring rainfall (Schultz, 1985). As no `true rainfall amount' can be achieved, one can only attempt to improve the estimation of rainfall amounts by minimising the known errors, which are the systematic errors that are associated with a rain gauge used to measure rainfall amounts. Empirical equations have been derived which can be used to account for the systematic errors in point rainfall measurements (Schultz, 1985). Added to these errors, rainfall amounts are extrapolated to give an areal average of rainfall. Boughton (1981) stated that deficiencies of 10-20% could be expected in point measurements of rainfall and that a further 10-20% error is likely in extrapolating data from a point measurement to an areal average. To aid in decreasing this error, a sufficiently dense and suitably spaced rain gauge network needs to be used (Schultz, 1985).
Mean annual precipitation (MAP) is a widely used variable in hydrological design, water resources planning and agrohydrology. The basic sources for MAP in South Africa up to the end of the 1980s were the 1:250 000 average rainfall map series compiled and drawn in the early 1960s by the erstwhile Hydrological Research Division of what was then called the Department of Water Affairs. After that, with rainfall records having lengthened by more than 20 years and techniques of analysis and computerised mapping having increased dramatically, the next series of maps using data up to the mid1980s, and the first set of digital raster rainfall information, became available towards the end of the 1980s (Dent, Lynch and Schulze, 1989). Since then a further 15 years of data have been collected.
Project K5/1156 is titled "Research on the Development of a Revised Spatial Database of Annual, Monthly and Daily Rainfall and Other Hydroclimatic Variables for South Africa". This report covers the rainfall component of the project; a separate report (Schulze, 2003) deals with the other hydroclimatic variables. In addition to the development of revised spatial and temporal databases of annual, monthly and daily rainfall values and related rainfall statistics, this research was also to provide for: