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Rainfall Variability Analysis of rainfall variability and drought in the 1961-2002 period
The National rainfall index (NRI) in the 1961-2002 period

Global analysis  Annual rainfall | National rainfall index | Global rainfall pattern
Case studies  Burkina Faso | Cambodia | Nepal | Tanzania
Downloads  Images (IDA) | Images (GeoTiff) | Movies (AVI) | Windisp | Quantum GIS

1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 movie time series

NRI
A national rainfall index (NRI) is defined for areas where water is limiting for agricultural production. The definition of drought is no trivial assignment. Low values of total precipitation do not lead necessarily to drought and drought is not necessarily associated with low precipitation totals. The impact on food production depends on a great number of weighting factors such as the quality of land or the level of technological development of agriculture. Additionaly, these country specific impacts show variations under relatively stable climatological conditions, for example as a result of increasing population.

The NRI can be used as an indicator of the quality of the agricultural season. The definition as a national annual value allows a comparison of countries and years and allows a comparison with other country wide statistical indicators. The NRI is defined as the national average of the total annual precipitation weighted by its long-term average. In this study, the average rainfall covering the years 1986 to 2000 is used as reference rainfall. The calculation of the NRI is different in the northern and the southern hemisphere. In the northern hemisphere the indices were calculated based on the January-December rainfall; the rainfall indices coincide with the calendar year. But in the southern hemisphere, crops are planted at the end of a year to be harvested in the first half of the following calendar year. Consequently, the index of a special year is calculated on July of the previous year to June data of the year of interest for a crop harvested in this year.

The long-term average as weighting factor gives rainfall in wetter areas a higher relative weight than in dry areas. The result is a good correlation with crop yields. Especially peaks and low values coincide. But the relation is not so good in quantitative terms, because irrigation, technology trends, pests and diseases affect yields in addition to rainfall variability. Other reasons are the poor quality of early agricultural statistics, the unconsidered distribution of rainfall or the nonlinear relation between rainfall and yield except under dry climates.

The menu bar above gives the possibility to roam between the global maps of the years 1961 and 2002 of the NRI or to see the whole movie. In the following some interesting examples are given, worth to have a closer look at:
  • In Africa the observation period include very variable conditions. The years from 1960 to 1969 were among the wettest of the period, followed by years of lower precipitation totals in the seventies and eigthies. A comparison of the annual rainfall maps with the maps of the NRI show, that the impact of the downward trend on food production is restricted to low rainfall areas.
  • The sahel is one of the driest and most variable areas in Africa (see also the global rainfall pattern by classes). Worst drought years were recorded 1983 and 1884, but severe droughts also occured 1972, 1973 and 1977. It is also possible to see the time series of the NRI in the observation period of the countries concerned, for example Mali or Niger.
  • The ENSO impact produces below average rainfall during an El-Nino event in the northern part of Brasilia, the south-east of Africa and the south-east of Asia. Simultaneously, above average rainfall occures in California, Peru, Bolivia, Equador, Paraguay and South Brasilia. The reverse happens during La Nina. Intense El-Nino seasons are for example 1970/1971, 1982/1983, 1986/1987 or 1997/1998.

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