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5.4 Calculating the Length of the Growing Period (LGP)
The length of the growing period (LGP), as defined by the Agro-Ecological Zones project carried out at FAO during the last two decades [refs], is the period (in days) during a year when precipitation exceeds half the potential evapotranspiration, plus a period required to evapotranspire an assumed 100 mm of water from excess precipitation stored in the soil profile.
The LGP is a useful concept for calculating agricultural potential, and can be used as a criterion for classifying areas and in roughly determining crop cycle lengths. The calculation of the growing period is based on a simple water balance model, comparing precipitation with PET, using monthly values.
A "normal" growing period, pictured in fig.5.4a, has the following characteristics:
(1) A Beginning
The beginning coincides with the start of the normal rainy season and is taken as the date when precipitation equals half PET, denoted as a in fig.5.4a. A value of 1/2 PET has been chosed because the water requirements of germinating crops are much below the full rate of PET, reflected clearly in the magnitude of the crop coefficients, and false starts to the rainy season are eliminated.
(2) A Humid Period
This is the period during which precipitation exceeds PET. The beginning and ending dates (shown as b and c in fig.5.4a, respectively) are the two points where the precipitation and PET curves cross.
During this period, crops are able to meet their full water requirements and the soil moisture deficit is replenished. The ending date of the humid period coincides with the end of the rainy season and crops mature largely from water stored in the soil.
(3) An End to the Growing Period
This occurs at the point where the precipitation curve crosses the 1/2 PET curve (labeled as d in fig. 5.4a) and takes into consideration that most crops continue to grow beyond the end of the rainy season.
The soil water-holding capacity (defined as....) is assumed to be 100 mm and the time taken to deplete the remaining soil reserves at the end of the season is added on to the LGP. In a future version, there will be an option to change this value.
In addition to a normal period, three other types of growing periods can be defined:
(1) Intermediate Growing Period
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 0.5 PET curve and there is no humid period.
(2) All Year Round Humid Growing Period
The average monthly pptn, for 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 have been included and inventoried as areas with a normal growing period of 365 days.
(3) All Year Round Dry Period
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.
Figure 5.4a
Examine the sample data file, AL-LGP.DAT, shown in fig. 5.4b. The file contains station names in Africa, geographical coordinates between 2 North and 2 South, and 12 monthly values of rainfall and PET for each station. Notice that the input file requires the rainfall and PET to appear on the same line.
"Rainfall and PET from 67 stations between 2 and 2 degs. latitude" "Lon","Lat","Alt","R1","R2","R3","R4","R5","R6","R7","R8","R9","R10","R11","R12","E1","E2","E3","E4","E5","E6","E7","E8","E9","E10","E11","E12"
"GOMA ZAI",29.14, 1.41,1552,115,86,98,126,129,45,28,66,110,149,136,117,110,101,113,102,105,111,116,115,120,117,113,103
"GABIRO RWA",30.24, 1.32,1472,65,78,95,155,78,18,19,19,68,82,72,75,121,113,126,117,118,122,130,133,139,124,120,116
"MACHAKOS KEN",37.17, 1.31,1680,49,53,124,210,76,12,5,6,9,53,189,122,149,140,145,118,110,100,93,92,126,136,127,122
"GALOLE KEN",40.02, 1.3,100,35,19,34,56,30,20,21,13,49,38,94,61,152,151,164,151,150,132,136,145,166,152,146,137
"KITUI KEN",38.01, 1.22,1090,41,24,118,244,56,5,3,5,6,82,304,143,151,154,165,127,124,117,114,130,150,161,134,126
"RUMANGAB ZAI",29.22, 1.21,1620,114,115,170,216,156,112,83,140,193,203,200,154,104,95,108,98,98,96,103,97,106,100,99,95
"NAIROBI KEN",36.55, 1.19,1624,63,34,81,107,130,24,7,11,21,62,188,116,147,151,156,122,107,97,92,90,110,125,121,122
"NAIROBI KEN",36.45, 1.18,1798,55,43,87,190,175,41,15,24,20,46,136,83,133,131,141,114,95,86,79,85,109,126,116,116
"KABETE KEN",36.45, 1.16,1820,47,51,100,210,171,44,19,26,29,60,127,93,144,140,142,112,97,81,80,85,115,126,120,121
"KABETE KEN",36.46, 1.15,1820,32,49,102,222,164,46,15,26,30,55,103,87,139,138,142,112,96,85,82,87,114,122,113,116
"MASARA KEN",34.15, 1.01,1200,51,79,120,235,198,94,36,62,93,114,151,117,145,137,147,123,123,115,121,128,152,144,134,126
"THIKA KEN",37.06, 1.01,1463,33,30,96,213,130,25,15,18,17,60,145,76,149,157,157,125,110,98,92,93,123,135,130,124
"ORICHING UGA",30.47, 0.52,1280,56,95,179,240,116,32,25,147,169,56,148,128,103,99,110,93,96,93,102,101,108,100,97,91
Figure 5.4b : AL-LGP.DAT file
Open the sample file and select Lgp from the Agromet menu. A window appears and you will immediately be prompted to accept or change the default value of 1/2 PET to define the beginning and the end of the growing season as given in the original AEZ definition. The value can be changed by simply typing a new value into the box. But for this example, accept the value and click on the Compute button to perform the calculation.
The output is written to the file AL_AL-L1.DAT, partially shown in fig.5.4c.
"Length of growing season with F= .5"
,"La","Lo","Alt","SeasTot","Nr","Type","LGS","BegSd.m","BegSJul","EndSd.m","BegHd.m","EndHd.m"
"GOMA ZAI", 29.14 , 1.41 , 1552 , 1 , 1 , 3 , 231 , 9.08 , 221 , 27.03 , 23.09 , 23.01
"GABIRO RWA", 30.24 , 1.32 , 1472 , 1 , 1 , 3 , 250 , 18.09 , 261 , 25.05 , 29.03 , 1.05
"MACHAKOS KEN", 37.17 , 1.31 , 1680 , 2 , 1 , 3 , 93 , 23.02 , 54 , 26.05 , 21.03 , 7.05
"MACHAKOS KEN", 37.17 , 1.31 , 1680 , 2 , 2 , 3 , 81 , 19.1 , 292 , 7.01 , 3.11 , 16.12
"GALOLE KEN", 40.02 , 1.3 , 100 , 1 , 1 , 2 , 34 , 5.11 , 309 , 8.12 , 999 , 999
"KITUI KEN", 38.01 , 1.22 , 1090 , 2 , 1 , 3 , 84 , 15.1 , 288 , 6.01 , 25.1 , 20.12
"KITUI KEN", 38.01 , 1.22 , 1090 , 2 , 2 , 3 , 74 , 3.03 , 62 , 15.05 , 24.03 , 4.05
"RUMANGAB ZAI", 29.22 , 1.21 , 1620 , 1 , 1 , 6 , 340 , 999 , 206 , 999 , 25.07 , 29.06
"NAIROBI KEN", 36.55 , 1.19 , 1624 , 2 , 1 , 3 , 88 , 13.03 , 72 , 8.06 , 27.04 , 23.05
"NAIROBI KEN", 36.55 , 1.19 , 1624 , 2 , 2 , 3 , 88 , 16.1 , 289 , 11.01 , 1.11 , 13.12
"NAIROBI KEN", 36.45 , 1.18 , 1798 , 2 , 1 , 3 , 105 , 3.03 , 62 , 15.06 , 28.03 , 5.06
"NAIROBI KEN", 36.45 , 1.18 , 1798 , 2 , 2 , 3 , 78 , 21.1 , 294 , 6.01 , 9.11 , 27.11
"KABETE KEN", 36.45 , 1.16 , 1820 , 2 , 1 , 3 , 114 , 27.02 , 58 , 20.06 , 25.03 , 5.06
"KABETE KEN", 36.45 , 1.16 , 1820 , 2 , 2 , 3 , 78 , 17.1 , 290 , 2.01 , 13.11 , 21.11
"KABETE KEN", 36.46 , 1.15 , 1820 , 2 , 1 , 3 , 113 , 27.02 , 58 , 19.06 , 23.03 , 5.06
"KABETE KEN", 36.46 , 1.15 , 1820 , 2 , 2 , 2 , 72 , 19.1 , 292 , 29.12 , 999 , 999
Figure 5.4c : AL_AL-L1.DAT file
The first line of the file lists the limit you choose for determining the beginning and end of the growing season, i.e., 1/2 PET. This is followed by columns with the station name, latitude, longitude, altitude and several growing season characteristics. Table 5.4a describes the parameters in the output file and the range of possible answers:
Table 5.4a - Growing Season Characteristics
Column Heading Meaning of the column variable Range of Answers Seastot Seasonal total rainfall (mm) N.A. Nr Number of seasons 1 - single; > 1 - multiple Type Type of season 1 - dry; 2 - intermediate; 3 - normal; 6 - normal with no dry period LGS Length of the Growing Season (days) 0 to 365 BegSd.m Beginning of the season (day.month) BegSJul Beginning of the season (Julian days) 0 to 365 EndSd.m End of the season (day.month) BegHd.m Beginning of the humid period (day.month) -999 if no humid period EndHd.m End of the humid period (day.month) -999 if no humid period
Note that for a station with a year round dry period, indicated as 1 under the column "Type", the LGS will be listed as having 365 days instead of 0 days. This bug will be fixed in future versions.