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Chapter 4
Growing period analysis

The analysis of growing period is required in the assessment in order to quantify the time period in each year when moisture supply from rainfall is considered adequate, from a climatic viewpoint, to permit crop1 growth. More specifically, quantification is needed in terms of:

  1. lengths of growing periods and number of such periods per year;

  2. the quality of moisture conditions during the various parts of the growing period;

  3. the year-to-year variation of each length of growing period and its moisture conditions.

Length of growing period from climatic viewpoint alone, and independent of a specific crop, soil and landform, can only be defined in a ‘reference manner’. During the matching exercise in the land suitability assessments, for field crops (Technical Annexes 3 and 4), pasture and livestock (Technical Annex 5) and fuelwood (Technical Annex 6), the reference length of growing period (with its associated features of moisture quality, long-term variability, temperature and agro-climatic constraints) is then interpreted and evaluated for suitability of specific land utilization types of field crops, pasture and livestock and fuelwood, taking into account soil factors and landform.

1 The word crop is used here in generic form and refers to annual and perennial field crops, including pasture and tree crops.

4.1 Computer Printout and Symbols

The full growing period analysis as used in the assessment is explained in the following sections using the computed results for one selected station (Lamu Meteorological Station, No 9240001, 2°16'S, 40°54'E) with 42 years of rainfall records as an example. The results are presented in Appendix 4.1 and are arranged in two parts, namely:

  1. analysis of moisture periods;
  2. statistical analysis.

In the computer printout the following terms, abbreviations and symbols are used:

ALTITUDEElevation above sea level (m).
TEMPERATUREAverage annual daily temperature (°C).
THERMAL ZONE
(MAJOR CLIMATE)
Represents the temperature regime class of the station (see explanation in Section 6).
LGPLength of growing period (days).
PoSoil moisture supply without contribution from soil moisture storage (S=0 mm).
P100Soil moisture supply with 100 mm contribution moisture storage (S=100 mm). S represents a reference soil moisture supply (mm).
BEGBeginning of growing period (date).
BHBeginning of humid period (date).
(BEG-BH)1st intermediate period (days).
EHEnd of humid period (date).
(BH-EH)Humid period (days).
(EH-BH)Inter-humid intermediate period (days).
*Artificial end/beginning (date).
ENDEnd of growing period (date).
(EH-END)2nd intermediate period (days).
(BEG-END)Growing period (days).
TYPType of growing period.
NNormal type of growing period.
IIntermediate type of growing period.
(END-BEG)Dry period (days).
DMoisture supply deficit (P100< ET), numerically negative (mm).
E or ExcessExcess moisture supply (P100> ET), numerically positive (mm).
D/E or DEF/EXCMoisture supply deficit or excess (mm).
P/E or P/ETRatio of moisture supply (P100) over potential evapotranspiration.
TOTAL MOIST. PER.Total moisture period, equal to the total length of growing period or periods (when more then one per year).
TOTAL DRY PER.Total dry period, equal to the total length of dry period or periods (when more than one per year).
AVRAverage, representing LGP (and the associated parameters) computed using average rainfall as an input.
SDStandard deviation.
CVCoefficient of variation (%).
DatesA month is regarded as having 30 days with 10 parts of 3 days. Calender months January, February, March, etc. are coded 1, 2, 3, etc. respectively. A date 12 July is shown as 7.4, 27 October as 10.9.

4.2 Growing Period Model

The quantification of the reference length of growing period is based on a water balance model comparing moisture supply from rainfall (P) and storage (S) with potential evapotranspiration (FAO 1978; Kassam, Higgins and Christoforides 1981; Kassam, van Velthuizen, Higgins, Christoforides, Voortman and Spiers 1982; Brammer, Antoine, Kassam and van Velthuizen 1988).

The following definitions and methodologies are employed in the model.

4.2.1 Beginning of the growing period (BEG)

The beginning of the growing period is taken as the time (in days) when moisture supply from rainfall and soil storage is equal to half potential evapotranspiration. This premise takes into account the fact that the amount of moisture required to sustain crop germination and emergence is much below full potential evapotranspiration and during crop emergence and establishment in the field it approximates to about 0.S ET (Kowal and Kassam 1978; Doorenbos and Kassam 1979). Therefore, the amount of moisture supply that is equal to (or greater than) 0.5 ET has been considered as being sufficient to meet the water requirements of establishing field crops. Consequently, in the model, the time when moisture supply is 0.5 ET is taken as the reference beginning of the growing period.

For the Lamu station, the computer printout (Appendix 4.1) shows that in the year 1931 the annual moisture supply without storage (i.e. Po, equal to rainfall) is 1231.1 mm.The reference beginning of the growing period is on 3rd April (date: 4.1).

4.2.2 Humid period (BH-EH)

A ‘normal’ type of growing period is defined as one with a period when there is a surplus of rainfall over potential evapotranspiration, i.e. a humid period. During such a period not only full or close to full evapotranspiration demands of crops (with complete canopy cover) are generally met, but also the soil moisture in the soil profile is replenished.

For Lamu, the humid period in the year 1931 is 60 days, from 15th April to 15thJune. There is an excess of moisture supply over potential evapotranspiration of 701 mm, and the ratio of moisture supply over potential evapotranspiration is 3.55.

4.2.3 End of growing period (END) and moisture storage (S)

During the post-humid period, the moisture supply is again less than potential evapotranspiration, and crops have to rely on water stored in the soil to meet full water requirements. Subsequently, the frequency and amount of rainfall diminishes and soil moisture deficit increases. This results in a marked alteration of the environment and triggers pronounced changes in the eco-physiological responses of crops. Under such conditions, and in the absence of soil moisture reserves, crops are forced to mature when rainfall is less than 0.5 ET.

The time when Pn = 0.5 ET in the post-humid period is taken as the reference end of rains and rainy season.

However, the growing period of crops may continue beyond the rainy season when there is stored moisture in the soil, and in reality, to a greater or lesser extent, crops rely on stored moisture between rainstorms and often mature after the end of the rains on moisture reserves in the soil profile. Such moisture storage is therefore considered in defining the reference length of growing period. However, the amount of soil moisture stored in the soil profile and available to a crop varies with depth of the profile, the soil physical characteristics, the rooting pattern and other crop factors (Kowal and Kassam 1978; Doorenbos and Kassam 1979).

In the model1 a general figure of up to 100 mm stored moisture has been assumed as being available to the crops. The choice of 100 mm is based on extensive evidence which indicates that annual crops grown during the rainy season (i.e. rainfed annual crops as opposed to annual crops wholly or largely produced from stored moisture in the dry season) can utilize stored moisture in the range of 75 – 125 mm, by the time of maturity (Kowal and Kassam 1978). Accordingly, the extra time taken to evapotranspire this 100 mm of available stored moisture has been added to the duration of the rainy season to set the end of the reference growing period. The actual status of the reference soil moisture is evaluated in the matching exercise for specific crop and soil situations. The 100 mm of available stored moisture originates during the humid period when there is a surplus of rainfall over potential evapotranspiration. When there is less than 100 mm of surplus precipitation, the storage term is also proportionately less.

For Lamu (Appendix 4.1), the reference end of the growing period in the year 1931 is on 6th August (date: 8.2).

1 For the model to cater for a range in moisture contribution from soil storage, the computer program is set up to process the soil storage term in the range 0–250 mm, and the computer printout automatically provides the computed results of length of growing period for S equal 0, 50, 100, 150, 200 and 250 mm (Kassam et al. 1981; Brammer et al. 1988).

4.2.4 Growing period (BEG-END)

According to the above definitions, the reference growing period, from a moisture viewpoint, is therefore defined as a continuous period where moisture supply is greater than half potential evapotranspiration. It includes the time required to exhaust up to 100 mm stored soil moisture.

For Lamu (Appendix 4.1), the length of growing period in the year 1931 is 123 days.

Also, a reference growing period is type coded as ‘normal’ (N) when it has a humid period as is in the case of Lamu in the year 1931: when it has no humid period, the growing period is type coded ‘intermediate’ (I). During an intermediate type of growing period, therefore, rainfall does not exceed the full rate of potential evapotranspiration. This situation occurred in Lamu in the year 1949 when the first of the two growing periods of 67 days had no humid period. The two types of growing period are schematically shown in Figure 4.1.

The distinction between ‘normal’ and ‘intermediate’ type growing periods is useful because in the latter it is unlikely that full crop water requirements can be met during the rainy season without moisture conservation.

A normal growing period is made up of three moisture periods. For example, for Lamu for the year 1931, the three moisture periods during the normal growing period (BEG-END) of 123 days are 13 days from the beginning of the growing period to the beginning of the humid period (BEG-BH), the first intermediate moisture period, 60 days from the beginning of the humid period to the end of the humid period (BH-EH), the humid period, and SO days from the end of the humid period to the end of the growing period (EH-END), the second intermediate moisture period.

An intermediate growing period is made up of one moisture period; and for Lamu examples of intermediate growing periods (BEG-END) of 15 days, 24 days and 82 days occur in the years 1936, 1945 and 1958 respectively.

Two more growing period types have also been defined (Figure 4.1). These are: (a) all year-round humid with rainfall exceeding full potential evapotranspiration throughout the year, and (b) all year-round dry with rainfall not exceeding half potential evapotranspiration throughout the year. These situations have not occurred in Lamu.

4.2.5 Dry period (END-BEG)

A reference dry period is defined as the time when moisture supply from rainfall and storage is less than half potential evapotranspiration.

For Lamu, the reference dry period in the year 1931 is 237 days, from 6th August to 3rd April.

4.3 Quality of Moisture Supply

In addition to the lengths of the various moisture periods and the total lengths of growing periods, it is important to assess the quality of the moisture supply during each of the moisture periods. This has been achieved by quantifying the extent of moisture supply deficit (P100 less than ET) or excess moisture supply (P100 more than ET), and the moisture supply as a ratio of potential evapotranspiration, i.e. P100/ET (but shown as P/E in the computer printout).

For Lamu (Appendix 3.1), in the year 1931, there is a moisture supply deficit (D) of 26 mm during the period BEG-BH with a P/E ratio of 0.58. During the period BH-EH, there is an excess moisture supply (E) of 701 mm and the P/E ratio 3.55. During the period EH-END, there is a deficit of 32 mm and the P/E ratio is 0.86.

4.4 Moisture Periods

4.4.1 One growing period in a year

A year with one normal growing period has four moisture periods of which one is a dry period and the rest make up the growing period, namely:

  1. first intermediate period (BEG-BH): from the beginning of the growing period to the beginning of the humid period; the moisture supply during the period is greater than 0.5 ET but less than full ET, and there is a moisture supply deficit with a P100/ET ratio of less than 1.0;

  2. humid period (BH-EH): the moisture supply during the period is greater than ET and there is an excess of moisture supply over ET, and the ratio P100/ET is greater than 1.0;

  3. second intermediate period (EH-END): from the end of the humid period to the end of the growing period; the moisture supply during the period is greater than 0.5ET but less than full ET, and there is a moisture supply deficit and the ratio P100/ET is less than 1.0;

  4. dry period (END-BEG): from the end of the growing period to the beginning of the growing period; the moisture supply during the period is less than 0.5ET and there is a moisture supply deficit and the ratio P100/ET is less than O.S.

A year with an intermediate length of growing period has two moisture periods, namely:

  1. intermediate period (BEG-END): from the beginning of the growing period to the end of the growing period; the moisture supply during the period is greater than 0.5ET but less than full ET, and the ratio P100/ET is less than 1.0;

  2. dry period (END-BEG): from the end of the growing period to the beginning of the growing period; the moisture supply during the period is less than 0.5ET and there is a moisture supply deficit and the ratio P100/ET is less than O.S.

4.4.2 More than one separate growing period in a year

In a year with more than one growing period, the pattern described above is repeated. In a year with two separate normal growing periods, there are a total of eight moisture periods of which four are intermediate, two are humid and two dry.

In a year with two growing periods of which one is normal and the other intermediate, there are a total of six moisture periods of which three are intermediate, one humid and two dry.

In a year with two separate intermediate growing periods, there are four moisture periods of which two are intermediate and two dry.

In a year with three normal growing periods, there are twelve moisture periods in all. If one of the growing periods is intermediate, then there are a total of ten moisture periods.

4.4.3 Inter-humid intermediate period

An additional moisture period has been defined to cater for situations when the end of a humid period (EH) is not followed by an end of growing period (END) but instead by a start of a second humid period (BH). This period is defined as an inter-humid intermediate period (EH-BH).

This period can occur in areas with a bimodal rainfall pattern when moisture supply between the two rainy seasons is greater than 0.5ET. The period can also occur where there is a ‘drought’ during a humid period.

When the inter-humid intermediate period occurs, its length and moisture characteristics are given under (EH-BH). For Lamu (Appendix 4.1), the year 1967 has an inter-humid intermediate period of 39 days with a moisture supply deficit of 58 mm and the ratio P100/ET of 0.68.

In the analysis of growing period, an inter-humid intermediate period has been treated as having two lengths of growing periods with no separation between the two periods at one end. To achieve this, the middle of the (EH-BH) period is taken to represent an artificial end (END) for the first growing period and an artificial beginning (BEG) of the next growing period. Consequently, there is no dry period between the end of the first growing period and the beginning of the second growing period. In the computer printout this is indicated by an asterisk.

4.5 Variability in Moisture Conditions

Variability in moisture conditions from year-to-year is quantified after the analysis of growing periods in each year is completed. This is done in three stages as described below.

4.5.1 Historical profile

Firstly, a historical profile is compiled showing groups of years each with a different number of growing periods per year, from zero to six. The proportional representation of each group in the total series of years is computed. Further, for each group, the number and percentage of years with normal and intermediate growing periods respectively are calculated.

In the case of all-year humid and all-year dry, the information on the number and percentage of years is provided separately.

For Lamu, the historical profile (Appendix 4.1) shows that of the 42 years, 28 years (67%) have one length of growing period and 14 years (33%) have two lengths of growing periods.

Of the 28 years with one length, 27 years have normal growing periods and 1 year has intermediate growing period. In the group of 14 years with two lengths, the first and the second lengths are normal in 13 years (93%) and 6 years (43%) respectively and are intermediate in 1 year (7%) and 8 years (57%) respectively.

4.5.2 Mean, standard deviation and coefficient of variation

Secondly, for each group of years with different number of growing periods, mean, standard deviation and coefficient of variation are calculated for the lengths of growing periods, length of individual moisture periods, deficit or excess and the ratio of moisture supply to potential evapotranspiration.

For Lamu, the mean length of growing period for the years with one growing period is 125.6 days, SD is 28.1 days and CV is 22%.

4.5.3 Frequency distribution tables

Thirdly, for each group of years with different number of growing periods, frequency distribution of each of the mean lengths of growing periods is computed at 10-day intervals with the corresponding ratio of moisture supply to potential evapotranspiration (P100/ET) and excess moisture supply. These are presented in the form of frequency distribution tables, one for each individual mean length of growing period. The group of years with one length thus has one frequency distribution table whereas the group of years with two growing periods per year has two frequency distribution tables.

For Lamu, the mean length of 125.6 days, for the group of years with one length per year represents a variation from 60 to 186 days. The mean length of 132.4 days for length 1 in the group of years with two lengths per year, represents a variation of 67 to 195 days. Similarly, the mean length of 44 days, for length 2 in the same group of years, represents a variation of 9 to 118 days.

FIGURE 4.1
Schematic presentation of types of growing periods

FIGURE 4.1

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