Weekly rainfall probability over dry farming tract of Gujarat

Transcription

1 Annals of Arid Zone - 21 (3), , 1982 Weekly rainfall probability over dry farming tract of Gujarat B. C. BISWAS AND S. S. BASARKAR, Agricultural Meteorology Division, Meteorological Office, Pune ABSTRACT By fitting Incomplete Gamma Distribution week by week probability of rainfall amount at different levels (10 %- 90 %) has been computed for 81 stations in the dry farming tract of Gujarat. The analysis has brought out four distinct rainfajl patterns and homogeneous rainfall zones over Gujarat. The specific amount of assured rainfall at different probability levels in various weeks are computed for different stations. No break condition is noticed in rainfajl distribution in any part of the tract during the monsoon period at 50% probability level. High assured rainfall pockets are found around Junagadh, Rajkot and Palitana whereas an area of low assured rainfall comprising of Ranpur, Chuda. Dhandhuka and Dho1era is identified. Crop growing period on the basis of rainfall alone has been indicated for each zone. INTRODUCTION The philosophy of dryland agriculture revolves round the principle that water is the limiting factor in these areas and one has to maximise the use of rain water for agricultural production. The strategy for this agriculture is to narrow down the inter-annual variation, stabilise crop production at a higher level even in unfavourable years and maximise the outturns in favourable years to build up buffer stocks. Research, therefore, should be directed to evolve means to face variety of conditions, arising out of abnormal weather. As short period weather cannot be forecast sufficiently in advance, the emphasis should be given on proper advance planning of crop strategies and mid-season corrections, appropriate to the climate and soils of the place. Extensive studies of short period rainfall show that there is no trend, cycle or periodicity in any rainfall series. For agricultural purposes rainfall situations over short period and small areas are very important and have to be considered. As rainfall is highly variable over short period both in time and space, weekly rainfall probability may br;ng out certain features which may be useful for mapping out homogeneous rainfall zone and land use planning. Fourteen out of nineteen districts of Gujarat are covered by Dry Farming Tract where the annual rainfall varies from 400 to 1000 mm. The dry farming area is characterised by generally low production and higher instability. Hardly 11% of this area enjoys benefit of irrigation and in Saurashtra this is less than 8%. Dry land agriculture produces the bulk of crops like Iowar, Bajra and Groundnut and this will continue to be so in near future.

2 188: B. C. BISWAS AND S. S. BASARKAR Anantakrishnan et at. (1970) studied the normal pentad rainfall pattern over India including a few stations of Gujarat. National Commission on Agriculture ( 1976) tried to divide the state into different parts on the basis of monthly rainfall and average temperature. Gujarat State Government demarcated the state into different agroclimatic zones on the basis of monthly rainfall, and average maximum and minimum temperature. Khambete and Biswas (1978) studied run of wet and dry spells and computed weekly probabilistic rainfall of 15 stations taking one station in each district. An attempt has been made in this paper to compute week by week assured rainfall at different probability levels and demarcate the dry farming tract into homogeneous rainfall pattern zones, so that this information may be available to make use for the agricultural pla'nning and management. Data MA TERIAL AND METH'ODS Weekly rainfall data of any station available for more than 50 years could be used for probability analysis by fitting incomplete gamma distribution, so that the parameters in the distribution are stable. Eighty one stations of 15 districts hae been used in the present study (FIg. 1). Although district Kutch is having 23 % area of the State, it is presented by five stations only. Methods Many workers (Thorn, 1966; Mooley, 1973; Hargreaves, 1974) have used incomplete gamma distribution to compute seasonal or monthly probability at different levels. Sarker et at. (1977) have used this method to compute weekly rainfall at different probability levels for one station in each of 87 districts under dry farming tract of India. Biswas and Khambete (1979) have used the same' method to compute weekly probability taking one station in each taluk of dry farming area of Maharashtra. Considering each standard week as an' independent unit, gamma distribution is fitted to the weekly rainfall totals. The lowest amount' of niinrall that can be expected has been computed for each decile, i. e'.; 10%, 20%... 90% probability level. This has been designated in this paper here onwards as Assured Rainfall (AR.). The goodness of fit of gamma distribution probabi'iity m<xlel to weekly total' of rainfall is tested by means of variance ratio test (Cochran, 1959). Data are well fitted in 95% of the cases. To identfy rainfall, pattern assured rainfall (AR.) at,30;..., 80 per cent probability,level for each station are plotted on graph. The rainfall peak and shape of the curve which gives temporal variation have been used to classify the pattern. The spatial distribution of these patterns demarcate the dry farming tract into three zones, 1. e., A, Band C. Area D gives the AR. pattern of arid zone (Fig. 1). Conclusion of this paper has been drawn on the basis of weekly probabilistic rainfall amount at 30% to 80% levels and the isohyetal maps for all the weeks and all the levels. All these could not be reproduced here for want of space.

3 RAINFALL OVER DRY ZONE OF GUJARAT 189 :\;., <' "'r" r.... "- 'f- < '" w S z a: ' l?,..j lc % <r t: it a 0, cr ffj '0 C(.. D 9i «g. 3.. '" u..

4 190: B. C. BISWAS AND S. S. BASARKAR DISCUSSION Spatial variation of annual rainfall in the dry farming tract is quite prominent. It varies from 984 mm. at Rajpipla (Lat ', Long ') to 295 mm at Naliya (Lat 23 16', Long. 68 5/1'). Number of rainy days ranges from 47 to 15 days. Southwest monsoon normally sets in over the southern part of the State on 11th June and covers the whole State within a week. It starts withdrawing from the northwest part of the State around 18th September and leaves the boundary of the State within 8 days. Distribution of rainfall is not even. An examination of the weekly probability values brings out four distinct rainfall patterns. Areas of these patterns have been shown in Fig. 1 as A, B, C and D. Fig. 2 for Amod (Lat '. Long ') depicts the rainfall pattern of area A which covers the eastern part of Gujarat including the districts of Panchmahal, Sabarkantha, Kaira, parts of Ahmedabad, Mehsana, Banaskantha, major portions of Baroda and Broach (annual rainfall varies from 697 to 1000 mm). This area gets more assured rainfall (A.R.) in the dry farming tract. The main rainfall period is observed from 26th (25th June - 1 July) to 36th (3-9 September) week with a prominent peak at 30th (23-29 July) week and two minor peaks in 27th (2-8 July) and 36th (3-9 September) weeks. In the 24th week (11-17 June) the assured rainfall varies from 37 mm at Rajpipla to 13 mm, at Palanpur at 30% level and it reduces to 14 and zero mm at 50% level respectively. In the 30th week which is the peak rainfall week of the area A, AR. is 129 mm, at Anand and 70 mm at Dahej at 30% level. As sured rainfall of those two stations at 50% level is 69 and 32 mm respectively and at 70% probability it further reduces to 29 and 9 mm. After 30th (23-29 July) week AR. gradually comes down upto 33rd (13-19 August) week and then slightly increases in 35th (27 Aug. - 2 Sept.) week. In the 36th (3-9 Sept) week AR. at 50% level varies from 31 mm at Jumbughoda to 7 mm at Palanpur but the corresponding values are 20 mm and 1 mm in 37th (10-16 Sept.) week. Three high rainfall zones compared to surrounding areas are observed around Anand, Kapadvanj and!dar. This is clearly noticed in 30% probability charts. The pattern and distribution of week by week assured rainfall indicates that the prospect of kharif crops in this area is good and the duration of crop as revealed from rainfall alone may be from 25th to 38th week. As amount of A.R. is of the order of mm from 29th to 31st week, at 50% level, a good amount of moisture may be accumulated in those places where soil depth is deep. A double crop or mixed crop may be expected to be raised from this area. Needless to say, a better suggestion can be offered by taking into account the soil type and atmospheric demand in addition to rainfall. Area B comprises of Jamnagar, J unagadh, Amreli, Rajkot and part of Bhavnagar district. Fig. 3 gives the rainfall of Gondal ( ' N, 70 48' E) which represents the pattern of rainfall of area B. The annual rainfall of this area ranges from 400 to 650 mm. The

5 RAINFALL OVER DRY ZONE OF GUJARAT : 191 LOWEST ASSURED RAINFAl.L AMOO ( B' I E (La' zt"o 1.""9 7z"5Z'( E 14 z IZO 20 M III E E K S.. FIG.2 10"10 RANPUP,AhmedobOd) (Lot 22"2"11I 1.0<19 r1043"ei...1 « Z < cr \,,. : \20'\ GONDAl r\l\i/,'(.,. ' ".::, J:.. ",. J/\_ (Ilololl Ileo' Zo"!l8' N. Lon, rcfta'e1 I,/ H: \:,\". \. I' \ j ':':.,"-. \.\ '-/-- '..,'\ \ j ' ':; I:.\:< \,.l /' -'-\ \.,\. \, / I 'I -'.. y... '._"':'".:-./. \ WEEKS FIG. 3 8 H U J 1 K lch I 11.0' Zi'IS' H.Long6lt'48'EI.... < 6 :it weeks FIG. 4

6 192: B. C. BISW AS AND S. S. BASARKAR main rainfall period is from 27th (2-8 July) week to 30th (23-29 July) week with a slight depression in 29th (16-22 July) week. After 30th (23-29 July) week rainfall abruptly comes down and the minimum is observed in 32nd (6-12 Aug) week in mid-monsoon. A second peak is found in 35th (27 Aug - 2 Sept) week. The rainfall then gradually slides down with slight increase in AR. in 38th (17-23 Sept) week. AR. in 26th (25 June - I July) week varies from 24 mm for Mahuwa and J unagadh, to 5 mm, for Jamnagar at 50 % level. In the 28th (9-15 July) week AR. at 30% level is of the order of 70 mm around Mahuwa and Jamnagar which reduces to the order of 35 and 20 mm respectively, at 50% level. During the period 33rd (13-19 August) to 35th (27 Aug - 2 Sept) week AR. is more or less steady at 30 mm in this area at 30% level. The high AR. zone is found around Junagadh and low AR. area observed around Amreli. Rainfall practically ceases after 38th week and amount of week by week rainfall from 32nd to 38th week is not high. Rabi crop normally does not suit in this area. Prospect of kharif crop is good and its duration may be from 26th to 38th (25 June - 23 Sept) week. Area C includes northern part of Bhavnagar district, Western Parts of Ahmedabad, Surendranagar, Mehsana and major portion of Banaskantha districts. Fig. 4 shows the rainfall pattern of Ranpur, which represents the rainfall distribution of area C. In this zone peak AR. is found in the 28th (9-15 July) week. During 32nd (6-12 Aug) to 36th (3-9 Sept.) week AR. is more or less steady and slightly increases in the 38th week, in three out of ten years. In the 26th week AR. at 30% level is about 43 and 14 mm at Chuda and Tharad respectively. Assured rainfall in the 30th week is of the order of 60 mm at both the stations. At 50% level AR. is of the order of 25 mm in most of the area C, except Ranpur and Chuda where it is of the order of 15 mm. Though AR. in the area C is more or less uniform, a low rainfall area could be identified around Ranpur, Chuda and Dhandhuka. The prospect of short duration karif crop except in low rainfall area is quite good at least once in two years. Assured rainfall pattern and amount suggest that the duration of crop may be from 27th to 37th (2 July - 16 Sept) week. The whole of the Kutch is under area D where annual rainfall is less than 400 mm. The duration of rainfall period is short and weekly amounts of A.R. are small. Fig. 5 of Bhuj depicts the rainfall pattern of this area. The main rainfall period is from 27th to 31st week. At 3C'% level AR. is more than 10 mm in most of the stations during 26th to 36th week. Although dry land agriculture without irrigation is speculative in this area, the rainfall pattern reveals that short duration crops may be successfully raised once in five years. CONCLUSION (a) Rainfall normally increases from southwest to northeast from 26th (25 June - 1 July) to 34th (20-25 August) week. Assured rainfall is more in 35th week than that of 34th week at

7 RAINFALL OVER DRY Z8E OF GUJARAT : 193 south Saurashtra and in 35th to 36th weeks at southeast Saurashtra. (b) Duration of monsoon is about 15 weeks in Gujarat. There is hardly any break in monsoon specially at 50% level of probabiiity. (c) Four district areas could be demarcated from the rainfall patterns ovd the State. In area A, the highest peak is observed in 30th week. Main rainfall period is spread from 26th to 36th week \vith a lull period in 32nd week. A Kharit crop of weeks duration or a mixed crop may be raised from this area. In area Band C, A.R. attains itpeak in 28th week. In area B major niidfall period is found during 26th to 35th week and in C it is limited from 26th to 33i:-d week. A shorter duration crop 12 to 14,yeeks from area Band 10 to 12 weeks duration crop from area C. may be ex: pected to be harvested one it).two years, Area D comprises of Kutch where annual raiiifall is less than 400 mm and. monsoon activities are mainly confind from 27th to 31st week. Rainfed cultivation does not appear to be feasible in this area even at 30% of the years. (d) There are pockets of high A.R. around J unagadh, Rajkot and Palitana. A large area of low A.R. comprising of Ranpur, Chuda, Dhandhuka and Dholera and also small pockets of A.R. around each of Amreli and Sanand are observed. (e) The crop life period on the basis of rainfall has been suggested at different areas. It is required to superimpose on the assured rainfall characteristics, soil and evaporation features so as to enable one to suggest specific cropping patterns. Studies on the above lines are in progress. ACKNOWLEDGEMENTS The authors are thankful to the Director General of Meteorology for permission for the publication of the paper and to Dr. R. P. Sarker, Dy. Director General of Meteorology. (Climatology & Geophysics) for his guidance and encour.agemet).t..tey.wish to express their thanks to Smt. N.. N. Khambete, Shri. S. Venkataraman, Shri. P. E. Moray and Shri.. S. K. Saha for their constructive criticisms. Thariks are also due to the staff o.pry,farming Research Unit and Drawing Branch. REFERENCES Biswas, B. C. and Khambete, N. N. i979. Short Period Rainfall Analysis over Dry Farming Tract of Maharashtra. J. Maharashtra Agricultural Universities 4 (2). Cochran, W. G Some methods for strengthening the common Chi-square tests. Biometrics 10 (4): Hargreaves, C. V Climatic zoning for agricultural production in Northeast Brazil. Khambete, N. N. and Biswas, B. C Characteristics of short period rainfall in Gujarat. Indian J. Met. Hydrol. Geophys. 23 (3). Mooley, D. A Gamma Distribution Probability Model for Asian summer monsoon. Monthly Weather Review 101 (2).

8 194: B. C. BISWAS AND S. S. BASARKAR Patel, G. A., Kasad, G. R. and Shah, R. L Crop zones of Gujarat. Govt. of Gujarat Publication, Technical Bulletin No. 36. Sarker, R. P., Biswas, B. C. and Khambete, N. N Probability Analysis of Short Period Rainfall in Dry Farming Tract of India. Sci. Report 78/9. Thorn, H. C. S Some methods of climatological amlysis. WMO Technical Note. No. 82, pp National Commission on Agriculture. (1976) Vol. IV.