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Validation of rainfall forecast given by AAU Monsoon Research Almanac-2010

Last modified November 01, 2010 14:06

In year 2010, the daily rainfall predictions were made for 26 districts of Gujarat state. The validation of this forecast on Yes/No basis for season as a whole indicated that accuracy varied between 64 to 75% for different zones (Table 1). Comparatively, the accuracy of forecast was good in the month of July to September except June. The average accuracy of daily rainfall forecast for state as a whole was 55.5% in June, 78.9% in July, 79.3% in August and 71.2% in September. The skill scores averaged for four months (June to September) indicated that, it was 75%, 63.5%, 73.7% and 72.8% for four regions viz. Middle Gujarat, North Gujarat, South Gujarat and Saurashtra respectively (Table 1). Gujarat state as a whole has recorded 71.3% skill score.

Prepared by V.B. Vaidya, Kedar Damle and Vyas Pandey Department of Agricultural Meteorology, BACA, Anand Agricultural University, Anand vidyadhar vaidya

 

            In year 2010, the daily rainfall predictions were made for 26 districts of Gujarat state. The validation of this forecast on Yes/No basis for season as a whole indicated that accuracy varied between 64 to 75% for different zones (Table 1). Comparatively, the accuracy of forecast was good in the month of July to September except June. The average accuracy of daily rainfall forecast for state as a whole was 55.5% in June, 78.9% in July, 79.3% in August and 71.2% in September. The skill scores averaged for four months (June to September) indicated that, it was 75%, 63.5%, 73.7% and 72.8% for four regions viz. Middle Gujarat, North Gujarat, South Gujarat and Saurashtra respectively (Table 1). Gujarat state as a whole has recorded 71.3% skill score.

 

Table 1. Yes/No skill score (%) of rainfall prediction given in AAU’s Monsoon

               Research Almanac-2010 during June to September 2010.

 

Sr. No.

Name of region

June

July

August

September

Seasonal

1

Middle Gujarat

65.8

78.0

80.0

76.2

75.0

2

North Gujarat

40.3

65.6

77.8

70.2

63.5

3

South Gujarat

56.1

84.8

85.7

67.9

73.7

4

Saurashtra

59.8

87.1

73.8

70.4

72.8

5.

Gujarat State

55.5

78.9

79.3

71.2

71.3

 

 

Validation of Rainfall Projection:

Rainfall intensity was predicted in AAU Monsoon Research Almanac-2010. The rainfall projection on monthly basis for each district of Gujarat state was given in Calendar. The validation was done with actual rainfall for each district and zone wise results are given in Table 2.

During 2010 monsoon season the AAU’s Monsoon Research Almanac has predicted above normal rainfall (i.e. 31.2%) for June to September months, whereas, actual rainfall occurred (June to Sept. month) is 29.8 % above normal for Gujarat state as a whole which was matching perfectly with state’s actual rainfall. Amongst four regions, AAU’s Monsoon Research Almanac has predicted above normal rainfall ranging between 14.6% for middle Gujarat to 46.7% for south Gujarat. The trend was positively predicted for all the regions while rainfall occurred above normal in all four regions. The above normal rainfall was recorded in Kutch district in North Gujarat.

Out of 26 districts of Gujarat state, 22 districts have recorded above normal rainfall. AAU’s Monsoon Research Almanac -2010 gave forecast of above normal rainfall projection in Gujarat which came true in most parts of Gujarat.

 

Table 2. Validation of rainfall projection for four regions of Gujarat during 2010.

 

Sr. No.

Name of Region

Rainfall projection (June-Sept.)

(mm)

Actual rainfall

(mm)

Normal Rainfall

(mm)

Rainfall Projection

(% departure from Normal)

Actual Rainfall (% departure from normal)

Rainfall projection (% departure from actual)

1

Middle Gujarat

893.7

812.3

779.8

14.6

4.2

10.0

2

North Gujarat

670.3

833.3

567.0

18.2

47.0

-19.6

3

South Gujarat

2050.3

1483.0

1402.5

46.2

5.7

38.3

4

Saurashtra

731.1

1169.4

562.7

29.9

107.8

-37.5

 

State

1086.4

1074.5

828.0

31.2

29.8

1.1

           

Validation of rainfall forecast given in Monsoon research Almanac 2005-2010:

 

Validation of rainfall forecast, for the year 2005, was done on Yes/No skill score (%) basis, for different zones of Gujarat. The validation of this forecast on Yes/No basis indicated that accuracy ranged between 52 to 73% for various zones (Table 3). During the year 2006, Yes/No skill score (%) for different zones of Gujarat ranged between 49 to 53%.  In August and September rainfall was well distributed spatially over Gujarat, hence, range of accuracy was less. During 2007 accuracy varied between 54 to 69% for various zones. The validation of rainfall for the year 2009, on Yes/No skill score basis for June, July, August and September month indicate that average accuracy was 46%, 56%, 49% and 52% for four regions viz. Middle Gujarat, South Gujarat, North Gujarat and Saurashtra respectively. On an average 51% accuracy was observed for 2009 for Gujarat state. During 2010,on an average 71.3% accuracy was observed which was highest amongst last six years (2005-2010).

The validation of rainfall forecast on Yes/No basis indicated that average accuracy varied between 37% (2008) to 71.3% (2010) for state as a whole. Overall state average skill score was 56%. Among the four regions average accuracy was highest in South Gujarat (59.1) and lowest in North Gujarat (52%) for the period of six years from 2005-2010.

 

 

Table 3: Validation of Yes/No Skill Scores of occurrence of Rainfall

 

Sr. No.

Name of Region / SAU

2005

2006

2007

2008

2009

2010

Average

1

Middle Gujarat

58

52

63

36

46

75

55.0

2

North Gujarat

52

53

59

35

49

64

52.0

3

South Gujarat

64

52

69

41

56

74

59.3

4

Saurashtra

73

49

54

35

52

73

56.0

 

State Average

62

52

61

37

51

72

55.8

 

           

 Validation of Rainfall Projection:

 Rainfall intensity was predicted for the first time in AAU Monsoon Research Almanac-2007 and has been predicted till 2010. The rainfall projection on monthly basis for each district of Gujarat state was given in Calendars. The validation was done with actual rainfall for each district. The percent deviation from actual rainfall for each zone and state as a whole was studied and presented in Table 4. The data reveals that in years 2007, 2008 and 2010, we had correctly predicted the above normal rainfall with more accuracy. The error was below 10%. For the year 2009 we have successfully predicted below normal trend. From the year 2007 to 2010 the average error was -4.3% for state as a whole. The most accurate prediction was done for 2010 monsoon with only 1.1% error.

 

Table 4. Validation of rainfall projection for four regions of Gujarat.

Sr. No.

Name of Region

Rainfall Projection (% deviation from actual)                                

Average

 

 

2007

2008

2009

2010

1

Middle Gujarat

6.1

16.2

-25

10

1.8

2

North Gujarat

37.4

6.6

-43

-19.6

-4.7

3

South Gujarat

-37.6

11.0

-15

38.3

-0.8

4

Saurashtra

49.6

-19.4

-45

-37.5

-13.1

 

State

4.6

6.3

-29

1.1

-4.9

 

Average error was lowest in south Gujarat (-0.8%), while it was highest in Saurashtra (-13.1%). In the year 2010, we were successful in predicting above normal monsoon rainfall which came true when compared with actual. The validation showed that during 2010 monsoon season the AAU’s Monsoon Research Almanac has predicted above normal rainfall (i.e. 31.2%) for June to September months, whereas, actual rainfall occurred (June to Sept. months) was 29.8 % above normal for Gujarat state as a whole which was quite accurate. Amongst four regions, AAU’s Monsoon Research Almanac has predicted above normal rainfall ranging between 14.6% for middle Gujarat to 46.2% for south Gujarat. The trend was positively predicted for all the regions while rainfall occurred above normal in all four regions of Gujarat. The rainfall was comparably less in Middle Gujarat (4.2% above normal) followed by south Gujarat (5.7%) where rainfall was just above normal. Only Saurashtra has recorded very heavy rainfall (107.8%) due to very heavy rainfall in the months of June, July and September. The above normal rainfall was also recorded in Kutch district in North Gujarat. Out of 26 districts of Gujarat state, 22 districts have recorded above normal rainfall. Amongst the months we have predicted less rainfall in June which came true. AAU’s Monsoon Research Almanac-2010 gave forecast of above normal rainfall projection with good distribution in Gujarat which came true in most districts of Gujarat.

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