The growing season in Tanzania
1. The Franquin method and the different types of growing season
Impact of climate on crops can be expressed in terms of a certain loss of water.
In case of cropped surfaces, the continous loss of water in form of vapour
is called evapotranspiration, which is the combined evaporation from the
soil and transpiration from the plants. The potential evapotranspiration
(PET) is derived under the assumption of unlimited water availability and
depends on the available radiation amount, the humidity of the air and
from the wind speed. It represents the water demand of the atmosphere and
is referred to the water requirement of a conventional "reference crop".
This "reference crop" is cool season grass which is 4 inches tall. In order
to estimate real crop water requirements, PET is corrected through the use
of a crop coefficient (Kc). The crop coefficient depend on the type of crop
and the stage of crop. Values of Kc greater than 1 describe well developed
crops and values of Kc lower than 1 correspond to sparse crop. The crop
water requirements (and the crop coefficient) grow slowly at the beginning
of the crop cycle, exceed PET at mid-cycle and decrease again when the crop
matures.
The length of the "growing period" or "growing season" (LGP or LGS), 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, 15pp. and 8 tables.), is the period
(in days) during a year when precipitation exceeds half the potential
evapotransipiation (Franquin method). Consequently, the estimation of
the LGP is based on the comparison of the water availability with crop
water demand. The LGP is a useful quantity for determing crop cycle
lengths under average conditions. But single years may depart significantly
from that value.
A "normal" growing period, which is also called a growing season Type 3,
includes a dry period, a moist period (also called intermediate period)
and a wet period (or humid period). Figure 1 illustrates the definition
of the different periods.
Figure 1 The definition of the dry, intermediate and wet period.
The beginning of the growing period marks the transition from the dry period
to the intermediate period. It occurs when precipitation equals half PET.
The period during which precipitation exceeds PET is called the wet (humid)
period. In addition to the growig period Type 3, five other types are defined
(see Figure 2):
Figure 2 Six different types of growing periods.
The growing period Type 1 is an all year round dry period. In that case,
the average monthly precipitation is in every month of the year lower
than half the average monthly PET.
The growing period Type 2 is an intermediate-dry growing period. The
average monthly precipitation does not exceed the full rate of the average
monthly PET, but it exceeds half the PET. No humid period exist.
The growing period Type 4 is an all year round intermediate growing period.
The average monthly precipitation varies during the entire year between
PET/2 and PET. This type reveals no beginning or end of the growing season.
The growing period Type 5 is an all year round humid growing period. During
the entire year, the average monthly precipitation exceeds the full rate
of the average monthly PET. This type of season has no end or beginning of
the growing period or the humid period.
The growing period Type 6 is an intermediate-humid growing period. This
type has an intermediate and a humid period, but the average monthly
precipitation always exceeds one half PET. No dry period occures.
2. Type, beginning and length of growing season
In Tanzania for most grid nodes the total number of growing seasons is 1.
Grid nodes with 2 growing seasons are as could be expected because of the
monsoon located in the N-NE part of Tanzania. A small number of grid nodes
also reveal 3 growing seasons. The most frequent growing season type is 3.
The growing season types 6 and 2 are represented to a lesser extent.
Figure 1 The length of the first growing season (LGS).
Figure 2 Beginning of the first growing season.
Considering the region of Tanzania with a second growing season, we found that
the growing season type 2 is more frequent in the north-eastern part of Tanzania
and the growing season type 6 is observed at the north (see Figure 3).
Figure 3 The growing season type of the second growing season.
Figure 4 The length of the second growing season.
Figure 4 Beginning of the second growing season.
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