|The National rainfall index (NRI) in the 1961-2002 period
National rainfall index |
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
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
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.