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Crop monitoring and forecasting Length of the growing season

| Prepared by: FAO/NRCB, Agrometeorology Group |

a. Maps
Legend LGS (2K)
AFRICA W. AFRICA N. AFRICA
 
  S. AFRICA E. AFRICA
legend ==>

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)

cropfor_01 (16K)
Figure 1

A normal season is shown in fig. 1, has the following characteristics:
  1. 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.
  2. 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.
  3. 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).

cropfor_02
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).

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