Dec 2, 2004 - seasons in Kerala are south-west monsoon (June-. September) ... rainfall is the actual source of water for crops. ... Here the soil is of red loam in.
Indian Journ al of Radi o & Space Ph ysics Vo l. 34, August 2005. pp. 269-273
Estimation of sub-soil temperature using air temperature P Tessy Chacko & G Renuka Department of Ph ys ics, University o f Kerala, Kariavattom 695 58 1. India
Received 2 December 2004. accepled 3 Jllll e 2005 Ex treme vari ability in the temperature o f soil and air affects plant life. An atte mpt has been made to develop reg ression mode ls to estim ate the soil te mperature at 5 and 10 cm depth using air temperature. Soil and air te mperature data for th e peri od 1998-2002 were collected from Vell anikara and Vellayani stati ons in central and south o f Kerala state. respecti vely . A c lose resembl ance in the wee kl y observed and co mputed sub-soil te mperatures has been es tabl ished. Keywords: S ub-soil tempe rature, Air temperature, Regression analys is PACS No.: 92.40. Lg ; 94.40.Je IPC Code: G 0 I S l7/00
1 Introduction The measurement of soil and air te mperature a re very important for many purposes like ag ri c ultura l me teorology, climato logy a nd the study o f heat storage vari ations. Extreme leve ls o f soil and air te mperature affect the plant life. Flow of heat in soil s is of co nsiderabl e impo rtance in plant cultura l practices and it affects plant root acti vity with respec t to the uptake of nutri ents, water, etc. The ecolog ical sig nifi cance of soil te mperature is obv io us fro m th e fact that an unfavo urable va lue of thi s para me te r may retard o r ruin the cro ps. Th e so il heat flu x at s urface and sub- surface layers can be estimated us ing so il temperatu res whi ch, in turn , ca n give a good indi cati o n of evapo ratio n. M any workers l -, have estim ated so il te mperatures at di ffe rent depths w ith fairl y good accuracy by harmo ni c ana lys is using a 4 seri es of so il te mperatures. Th e study by Math a n has indi cated that the so il temperature at di ffe re nt depths during cro p growth and fl o we rin g stages influ enced th e yie ld of g ra in and straw of ragi s ig nifi ca ntl y. So il te mpe rature measurement at large numbe r o f pl aces in a reg io n is diffic ult a nd impracti cab le. Thi s can be overcome by estimating so il te mperat ures fro m easil y avail abl e meteorol og ical data. The obj ect ive o f thi s study is to estimate th e so il te mperatures at two di ffe re nt depths usin g the a ir te mpe ratu re. T here are several mode ls to predi c t da il y a ir te mperaturess,r,. In th ese mode ls so il te mpe rature is no t conside red as a para meter. T he c limati c vari a ti o ns in d iffe re nt mo nth s affec t the te mperatu re and mo isture conten t of the so il. Vagaries of south-west mo nsoo n is a well know n feat ure in Ind ia. T he cl imate o f Kera la is tropi ca l
maritime and mon soona l in character. Th e main rain y seasons in Kerala are south-west monsoo n (JuneSepte mber) and no rth-east monsoon (O cto be r and Nove mber). The pre-monsoon mo nths (M arch-M ay) acco unt fo r major thundersto rm acti vity in th e state and winter mo nths (December-Fe bruary) are no ticed by minimum clo udin g and rainfall 7 . Tem pe ratures a nd humidities are hi g h throug ho ut the year and the rainfall is the actu a l so urce of water fo r crops. Solar e nergy is absorbed less in the form o f latent heat and mo re in the fo rm of sensible heat in the s ubso il stratum (0-20 cm). The a nnua l so il tem pe ratu re reg ime fo ll ows a wave-li ke patte rn wh ich is enti re ly anal ogous to that of the diurn a l o ne. Th e a mplitude of the te mperature wave decreases as th e depth of th e 8 so il increases . Heat sto red ii' the soil deCl'eases expo ne nti a ll y w ith depth . Beyond 20 c m, th e soil does not seem to be much affec ted by th e d iurn a l te mperatu re vari ati o ns. The layer up to 15 c m de pth ca n, as s uch, be call ed th e "s urface layer of th e so il " ana logous to s urface layer of the atmosphere'). In th is study, the regressio n models fo r es timatin g so il te mperatures at depths o f 5 c m and 10 c m are deve lo ped and tested . A si mi lar mode l of so iI lo te mpe rature has been de velo ped by Pate l et a l. in terms of weath er paramete rs and fo und that th e mean so il te mpe rature is the most sig nifi cant para mete r to estimate so il te mperature 111 plgeol/ epea and groundnllf cropping sy:;tem .
2 Data and method T he da il y so il a nd air temperature data were co ll ected for the period 1998-2002 from two d i ffere nt stat ions, name ly Ve ll ani ka ra (10°32' , 76° 16'E and
270
INDI AN J RADIO & SPACE PHYS , AUGUST 2005
22.5 m as l) in Trich'.lr Di strict and Vel\ayani (8°26'N, 76°59'E and 8 m asl ) in Thiruvananthapuram Di strict of Kerala state. Vellanikara station is located at th e College of Ho rticulture, Kerala Agricultural University, Vellanikara, where th e so il is sandy loa m in tex ture. Vellayani station is located at th e College of Agriculture, Kerala Agricultural University Campus, Ve ll ayani . Here the so il is o f red loam in tex ture. The main criteria for the se lection of th e station s was th e availability of complete sub- so il and air temperature data for the peri od under study and also they be long to different types o f so il. Dail y va lues of air temperature and so il te mperatures at 5 c m and 10 cm depth at 0730 and 1430 hrs 1ST ha ve been collected ( 1998-2002). These hours of o bservation roughly correspond to minimum and max imum temperature epochs of air, respective l/ I. From these, weekly averages were worked out for each standard week of the year for the data at 0730 a nd 1430 hrs IST for air temperature and so il te mperature at 5 cm and 10 cm depth s. Wee kl y average of so i I te mpe rature at 0730 (Tns) and 1430 hrs rST (TxJ have bee n averaged again to obtain weekly mean so il temperature ( TillS) for the above depth s. Similarly, weekly average of minimum (Til) and maximum (Tx) air te mperatures are averaged to get th e weekly mea n air te mpe rature (Till)' The weekly averages of sub-so il temperature at the two depth s have been cOIl'elated with the weekly mea n a ir te mperatures over the 4-year peri od . Regress io n mode ls for es tin .atin g mean so il tem pe rature and so il temperature at 0730 hrs rST at 5 em and 10 c m depth were develo ped usin g th e mean air temperature of the 4-years data ( 1998-2001 ). Us ing the mode l equati o ns, sub- so il te mperatures were estimated fo r th e next year, i.e. 2002. These es timated temperatures ha ve been compared with the o bserved values.
3 Results and discussion Th e variation s in mean weekly so il te mperature at 5 and 10 cm depth s at Veil ani kara and Ve llaya ni for the period 1998-200 1 are show n in Figs L and 2. T he week ly mean so il te mperatures show an in c rease in th e month s of March and April in both th e station s. The o bserved so iI te mperat ure dec reases as th e depth in c reases . A sudden uepress ion in . o il te mpe ratures at these depth s is observed due to th e pre- mo nsoo n showers. Soi I te mperature dec reases and reaches th e minim um value durin g th e S-W mo nsoo n pe ri od. At Ve ll anikara, the o bserved maximum and mini mum val ues or so i I te mpe rature at 5 and I () c m de pth s are
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Fi g. 2- Wee kl y variation of sub-soil tempe rature at Ve ll ayan i Table I- Co rre lat io n coe fll c ie nt be twee n soil te mpe rature and air tempemture Te mpe rat ure
Co rre lat io n coeffi c ie nt for Vellanikara Ve ll ayani
°C
at so il de pth (cm)
at so il de pth (em )
5
10
5
10
Minimum
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0.902
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0.95 3
0.973
O.H44
39.9°C, 37.2°C and 28. l o C, 27.8°C, respective ly. In the case of Ve ll ayani , th ese are 39.8°C , 36 .6°C and 30.3°C , 28.4°C , respec tiv e ly. To study th e influ ence of a ir te mperature on so il te mperature, cotTe latio n coeffic ie nts (r) we re co mputed usin g mea n air te mperatures and sub-so il tem pe ratures (Tils and TillS)' There is sign ifica nt positi ve corre lat io n betwee n th e a ir te mpe rature a nd soi l te mpe ratures (Tab le I). The va ri ati ons in a ir te mperatures are statistically s ig nificant at 0. 1% level. T he,. va lu e obtained at 5 cm depth in Ve ll a nikara is lower than that at 10 cm depth. This ca n be ex pl ained by th e loose ness of th e up per layer of so il ane! ae ration provided to deeper layers. T he te mperature
TESSY & REN U KA: REG RESS ION MOD ELS FOR SUB-SO IL TEM PERAT UR ES
range at 5 and ] 0 c m depth at Ve lla nikara is highe r than that at Ve ll ayani. Fo r the so il at Ve ll ayani th e r va lue dec reases as the depth increases. It is interes tin g to no te that this so il shows mo re in sul atin g effect than the so il In Ve ll anikara . For es timating soil te mperature from a ir tem per - 27 .X 8 Tms = 1.84 Tm - 19. 5 Vc llayan i
5 10
2 3 4
S 10
T", = 1.1 7Tm- 3.85 Tn, = 1.39 Tm- 8.Tl T111 , = 2.19 Tm- 24.8 7;ns = 1.17 Tm- 14.7
Note: Tm = Mean air te mpe rature Tms = Mea n soille mperature T,,, = Soiltempcrature at 0730 hI'S 1ST 34 . ---------------------- - - - - - - - - ,
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Fig. 4-0bserved and model led sub-soil temperature at Vellayani
temperature of the so il at thi s station, the error is found to be more than ± 2°C in some occasions. Such occasions are associated wi th local thunder storms, low-pressure area and de pressions causing rainfall at the station or neighbourhood. The observed and estimated sub-soil te mperatures at Vellayani are presented in Fig. 4[(a)-(d)]. The model predictions against observations are obtained with an acc uracy of ± 2°C in about all the cases in model s 1 and 2. The model predictio ns are dev ia ted from actual measurements more in the case of mean soil te mperature at 5 cm depth [Fig. 4(c»). As the depth of the soil ii; : reases the deviation decreases. Soil moisture has much influence in controlling the so il te mperature. In the year 2002, a rem arkabl e shortage in rainfall was observed in Vell ayani compared to Vellanikara. Vellayani station in South Kerala normally receives about 25-30% of the ann ual rainfall during the pre-monsoon and north-east monsoon season and about 50% during south-west l2 monsoon season , The pre-monsoon showers , which
have maj or influe nce on the air temperature of the months of April and May , were comparatively very low and the post-mo nsoon shower in October was very liigh in 2002. These variations affected the so il temperature model of thi s station and showed maximum deviations from the observations in these seasons. The root mean square error (RMSE) pro vides information on the performance of a mode l by allow ing a term-by-term comparison of the actual difference between the predicted value and the observed val ue, The RMSE for soil temperature at the two station s are given in Table 2 . The small e r RMSE value is indicative of better model performance.
4 Conclusion The model predictions are co mparable with the observations. The easi ly avai lab le air temperature data can be used to predict sub-soil temperatures. The mean air temperature is found to be a significan t parameter for the es timation of sub-so il temperatures. It is noticed th at the variation between observed and
TESS Y & RENUKA: REG RESS ION MODELS FOR SU B-SOIL TEMPERAT URES
es ti mated so il te mperatures decreases as the depth in c reases. By introduc in g o ther me teo rolog ical para meters, th e mode l predi cti o ns can be improv ed .
Acknowledgements T he auth o rs are hi g hl y th a nkful to the Directo r and staff me mbers of the De partment o f Meteoro logy, Co ll ege of Ho rti c ulture, Ve ll a nikara. T ri chur and Co ll ege of Agri culture. Ve ll ay:mi , Thirll van a nth apllra m, fo r prov iding the data required fo r thi s wo rk .
References La mba B S & Kh ambe te N , Analysis o/" subso il tem pera ture nt va ri ous depth s by Fourier technique: M al/sam (1IIdia ). 42 ( 199 1) 269. 2 Ma njusha Kul shresth a & Shekh A M. Estimati on of soil tem perallire by Harmon ic analys is: M uu.lw lI (1IIdia) . 52 (200 1) 379. 3 Retnakum ari K. Renuka G & Prasada Rao G S L H V. Ana lys is of soil temperature in iso-hyperth ermi c temperature regi me using Fourier technique: Illdiall J Radio Space Phys. 27 (19%) 207. 4 Mathan K K. Effec ts of soil temperature and certain weather parameters on th e grow th and yield of ragi; Mausam (1IIdia), 47 ( 1996) 28 1. I
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5 Juli a Bilbao. Argimiro H de Mi guel & HmTy D Mi guel, Air temperature mode l evaluati on in the north Medi terranean belt area: J Appl Me/eorol ( USA), 41 (2002) 872. 6 Pal S, Sengupta P & Banelj ee S K. Short lerm pred iction of atm ospheri c temperallire using neural networks; Mall.l·alll (I ndi a), 53 (2002) 47 1. 7 Ananthakri shnan R. Parth asarath y B & Pat han J M. Meteorology of Kera la. COlllrib ll /iol/s TO Ma rille Sci (Pri va/e cOIIIII/IIll icwiol/). Cochin Uni versit y. Kerala. 1979. p. 123. 8 Tessy Chacko P & Renuk a G. Temperatu re mapping. th erma l di ffusivit y and subsoil heat tlu x at Kariavatto m of Kera la: Ea rTh & Plalle/ Sci (ll/dia ). 111 (2002) 79 . 9 Ramdas L A, & Mallik A K. The thick ness of the sur face layer of th e so il exchanging moisllIre wi lh the adj acent air laye rs duri ng th e clear seaso.-, ai Fool'.:: Cllr r Sci (ll/dia ). 8 ( 1939) 164. 10 Patel H R, Meth a A N. Venkatesh H, Shek h A M & Pate l J R. Quanti ficat ion of soil temperature in sole and intercropped pigeo n pea using meteorological parameters: Ma llsam ( II/dia ). 50 ( 1999) 77. II Raman C R Y, Soil thermometers for shallow depths; Ag r i Me/erl/' Techl/iw l Circula r, Meteorologica l Office. Pune. Vo l. 6, 1970. 12 Anu Simon & K Mohan Kum ar, Spatial vari ability and rainfall characteri stics of Kerala; Ear/h & Plane/ Sci (ll/ dia ). I 13 (2004) 2 11.
