b. Description
The length of the "growing season" or "growing period" (LGS or LGP),
as defined by the Agro-Ecological Zones project (FAO, 1978. Report on
the agro-ecological zones project. Vol 1: Results for Africa. World
Soil Resources Report 48/1. FAO, Rome, 158pp. and 8 tables.), is the
period (in days) during a year when precipitation exceeds half the
potential evapotranspiration. A period required to evapotranspire an
assumed 100mm of water from excess precipitation stored in the soil
profile is sometimes added. No provision is made for stored soil moisture.
LGS is useful in determining crop cycle lengths and calendars under
average conditions. Actual years may sometimes depart significantly
from the average. The calculation of the growing period is based on a
simple water balance model, comparing water availability with crop
water demand (precipitation with PET), using monthly values. PET (or ETP)
is the Potential Evapotranspiration, i.e. a measure of evaporative
power of the atmosphere. A "normal" growing period (also called a Type
3 season) is characterised by a dry period, a moist period (also called
intermediate period) and a wet (or humid period)
Figure 1
A normal season is shown in fig. 1, has the following characteristics:
A Beginning Period
The beginning of the growing period occurs when precipitation (PPTN)
equals half PET and marks the start to the normal rainy season, shown
as a in fig. 1. A value of one half PET has been chosen as germinating
crops do not evapotranspire at the full rate of PET and false starts to
the rainy season are eliminated. The beginning marks the transition from
the dry period to the "intermediate" period when PET/2 < R < PET.
A wet (humid) Period
This is the period during which precipitation exceeds PET. The beginning
and ending dates (shown as b and c in fig. 1, respectively) are the two
points where the precipitation and PET curves cross.
An End to the Growing Period
The end of the growing period occurs at the point where the precipitation
curve crosses the one half PET curve (labeled as d in fig. 1).
Figure 2: six types of growing periods
In addition to a normal growing period (marked as graph number 3 in fig. 2),
five other types can be defined. Beside each season type described below
is the corresponding graph number in fig. 2. The additional season types
are as follows:
An All Year Round Dry Period (Type 1)
The average monthly precipitation for every month of the year is lower
than half the average monthly PET. Areas with all year round dry periods
have been inventoried separately as areas with a growing period of 0 days.
An Intermediate-Dry Growing Period (Type 2)
Throughout the year, the average monthly precipitation does not exceed
the full rate of the average monthly PET, but it does exceed half the
PET. The beginning and the end of such an intermediate growing period
are defined as the points where the precipitation curve crosses the
one half PET curve and no humid period exists.
An All Year Round Intermediate Growing Period (Type 4)
During the entire year, the rainfall stays permanently between PET and
one half PET. This is a very rare type of season with no beginning or
end.
An All Year Round Humid Growing Period (Type 5)
In this kind of season, the average monthly precipitation during every
month of the year exceeds the full rate of the average monthly PET.
Thus, there is no true start to the growing period or to the humid
period. Areas with all year round humid growing periods are inventoried
as areas with a normal growing period of 365 days.
An Intermediate-Humid Growing Period (Type 6)
This type of season has both an intermediate and a humid period but
no dry period (i.e., a period in which the precipitation curve drops
below one half PET).
