Rainfall seasonality

Multimedia

Development area

Rainfall seasonality

Rainfall seasonality is related to the temporal distribution of rainfall on a monthly basis. Rainfall seasonality can be estimated by the Walsh and Lawler (1981) index.

Aim of the method/technique

The method aims to characterize the distribution of precipitation throughout the year and to classify the climate of an area. For example the climate of an area can be characterized as rather seasonal with a short dry season or marked seasonal with a long dry season, depending on the distribution of rainfall during the year.

Scale – spatial and temporal

The calculation of rainfall seasonality requires climatic data. Therefore, the spatial scale of the index requires regionalization according to the method proposed for the raw climatic data. A period of at least 30 years data is necessary.

Brief description

The Seasonality Index (SI) (Walsh and Lawler, 1981) can be estimated using the following equation:

Where Ri is the total annual precipitation for the particular year under study and Xin is the actual monthly precipitation for month n. The various categories of climate based on rainfall seasonality can be assessed by using the following values of the SI index.

SI	Precipitation Regime
<0.19	Precipitation spread throughout the year
0.20-0.39	Precipitation spread throughout the year, but with a definite wetter season
0.40-0.59	Rather seasonal with a short dry season
0.60-0.79	Seasonal
0.80-0.99	Marked seasonal with a long dry season
1.00-1.19	Most precipitation in <3 months

Data requirements

Data required for the calculation of the rainfall seasonality index are: (a) average monthly rainfall, and (b) average annual rainfall.

Main applications in cropland and forests & shrubland regions

Rainfall seasonality index is a critical environmental factor affecting the evolution of natural vegetation. The seasonality index classifies the type of climate in relation to water availability. The higher the seasonality index of a region the greater the water resources variability and scarcity in time, the more vulnerable the area to desertification.

Strengths and weaknesses

The proposed method is very simple with few data required for calculation. However, since meteorological data are point observations the seasonality index has to be regionalized based on the Thiessen method. A period of at least 30 years of rainfall data are required to describe the prevailing rainfall seasonality index of an area.