ISPRS Archives XXXVIII-8/W3 Workshop Proceedings: Impact of Climate Change on Agriculture

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1 ENERGY BUDGET OVER SEMI-ARID AGRO-ECOSYSTEM USING SATELLITE DATA K.K. Dakhore, B.K. Bhattacharya b, K. Mallick c, R. Nigam b, N.K. Patel b, V. Pandey a, B.I. Karande a and A.M. Shekh a a Deptt. of Agril. Meteorology, Anand Agricultural Univerity, Anand , India b Agriculture-Foretry and Environment Group, Space Application Centre (ISRO), Ahmedabad , India c Deptt. of Environmental Science, Lancater Environment Centre Lancater Univerity, LA1 YQ *Deptt. of Natural Reource, International Intitute for Geo-Information Science and Earth Obervation, P.O. Box 6, 7500 AA Enchede, The Netherland KEYWORDS: Wheat, Semi-arid, ET, NDVI, LAS, MODIS. ABSTRACT: Energy-water-carbon cycle i the central theme of biophere-atmophere interaction to undertand the repone and feedback procee in a climate ytem. The crop repone in a emi-arid climate i frequently encountered by water carcity due to malditribution of rainfall or lack of irrigation water leading to low agricultural productivity. The characterization of water tre over large agriculture uing thermal regime over crop urface in periodic manner throughout crop growth cycle i thu extremely important. Though, conventional meaurement with Lyimeter keep record of field and oil pecific water balance in root zone but cannot be extrapolated to larger area. The combination of optical band data in term of albedo, NDVI threhold baed emiivity and thermal band data in term of Land Surface Temperature can repreent evapotranpiration (ET) through energy balance approach over a patial domain. Energy balance etimate derived from MODIS AQUA were validated with in itu attented and unattended obervation for two wheat eaon, and , within a 5km x 5km wheat growing region of Kheda ditrict in Gujarat. Daily AQUA evapotranpiration etimate in term of daytime latent heat fluxe were found to have root mean quare error (RMSE) 29 Wm-2 (r = 0.72) and 26 Wm-2 (r = 0.8), repectively a compared to attended meaurement during both the eaon. Correlation wa ubtantially more and RMSE were le when etimate were compared with in itu meaurement uing Large Aperture Scintillometer (LAS) enible heat fluxe. 1. INTRODUCTION The crop repone in emi-arid agro-ecoytem i frequently encountered by water carcity due to malditribution of rainfall or lack of irrigation water or it timely availability. It give low productivity to overall growth in agriculture. The characterization of water tre over large area agriculture uing thermal and radiation regime over crop urface in periodic manner throughout crop growth cycle i extremely important. Though, conventional meaurement with lyimeter keep record of field and oil pecific water balance in root zone but cannot be extrapolated to larger area. The ground baed micrometeorological or Bowen ratio tower having net radiometer at one height and temperature, humidity, wind peed enor at multiple height are alo ued to compute energy budget component to derive tre behavior over relatively larger area depending on the fetch (Shekh et al., 2001). Mot of the time, the ground baed equipment loe the proper calibration and become non-functional in the long run. Thee are rather more ueful to derive crop and oil pecific parameter and coefficient required to calibrate oil-atmophere-vegetation-tranfer (SVAT) model to imulate energy balance component. The concomitant ue of atellite optical and thermal data directly or their aimilation through SVAT i ueful to repreent water tre on pixel-by-pixel bai uing energy balance approach for regular monitoring on a regional cale (tate level). Land urface temperature (LST) i a good indicator of the evaporation from the Earth urface and the o-called greenhoue effect becaue it i one of the key parameter in the phyic of landurface procee on a regional a well a global cale. It act a a link for urface-atmophere interaction and energy fluxe between the atmophere and the ground (Manntein, 1987; Seller et al., 1988). Therefore, it i required for a wide variety of climatic, hydrological, ecological and biogeochemical tudie (Camillo, 1991; Schmugge and Becker, 1991; Running, 1991; Running et al., 199; Zhang et al., 1995). Another important urface parameter i expreed in term of the Normalized Difference Vegetation Index (NDVI), derived from the red and near infrared (NIR) channel in electromagnetic pectrum (Tucker et al., 198). The NDVI repreent amount of greenne and fractional vegetation cover (Tucker et al., 198; Tucker et al., 1986) and i an indicator of leaf area index and energy partitioning between canopy and oil below. Thu, the combination of optical band data in term of albedo, NDVI threhold baed emiivity and thermal ignature in term of LST from atellite platform can generate evapotranpiration on patial cale through energy balance approach. Regional evapotranpiration i, therefore, important to undertand energy, water and carbon cycle and their relevance to climate change tudie. A a ubet of that, the preent tudy wa undertaken with the following objective : To implement a implified ingle-ource energy balance cheme with MODIS TERRA (11.00hr) and AQUA (13.30 hr) optical and thermal data. dakhorekk@yahoo.com 293

2 To ae the accuracy of energy balance etimate with repect to meaurement over emi-arid wheat 2. MATERIALS AND METHODS Intenive diurnal meaurement on energy balance component were made in itu portable Bowen ratio obervation in replicate a well a area averaged unattended obervation from Large Aperture Scintillometer (LAS) within a 5km x 5km wheat growing region of Kheda ditrict in Gujarat. Thi validation experiment wa carried out for two conecutive wheat eaon, and Approach for Etimating Surface Energy Balance Component and Evapotranpiration Actual evapotranpiration (AET) or ET can be etimated from latent heat fluxe (λe) and latent heat (L) of evaporation. Latent heat flux (λe) i generally computed a a reidual of urface energy balance. A ingle (oil-vegetation complex a ingle unit) ource urface energy balance can be written a λ E= Rn G H M (1) The energy component for metabolic activitie (M) i very mall and hence can be neglected. The equation 1 can be rewritten a λ E = Rn G H (2) Rn = net radiation, H = enible heat flux, G = ground heat flux, Q = net available energy Q. Auming energy balance cloure at any tance during a day, equation 2 can alo be written a λe ( Rn G) Λ = Q. Λ =. (3) Where Λ = tantaneou evaporative fraction 2.2 Etimation of Λ In the preent tudy, Λ wa etimated from LST- albedo two dimenional catterogram uing techqnique given by Roerink et al. (2000) and further ued by Vertraeten et al. (2005) over European foret with NOAA AVHRR data and by Mallick et al. (2009) over Indian agroecoytem uing MODIS AQUA data. LST and albedo from MODIS TERRA and AQUA obervation correponding to overpa timing, 11: 00-11:30hr and 13:00-13:30hr Local Mean Time (LMT) repectively wa ued to derive Λ. 2.3 Etimation of Q Intantaneou net available energy Q = Rn G () G = tantaneou ground heat flux Clear ky tantaneou net radiation ( Rn urface radiation budget. Rn ) wa computed from = Rn in Rnl (5) Rn = tantaneou net hortwave radiation (Wm -2 ) = Rnin R (1 α) (6) = α = urface albedo which wa computed by converting even narrow-band optical reflectance uing coefficient given by Liang (2002) R = tantaneou olation (Wm -2 ) computed uing WMO clear ky model b R a. I 0. ε.( SINγ ) = (7) The coefficient, a and b were worked out to be 0.75 and 1.28 over Indian ub-continent by Mallick et al (2009) Where I 0 = olar contant = 1367 Wm -2 = un-earth ditance correction factor γ = un elevation angle Rnl = tantaneou net longwave radiation (Wm -2 ) = downwelling longwave outgoing longwave a = ε ε σt ε σt a T = land urface temperature (K). Here, MODIS land urface temperature (LST) product from TERRA and AQUA were ued. σ = Stephan-Boltzmann contant (5.67 X 10-8 Wm -2 S -1 K - ) ε = urface emiivity wa etimated from NDVI baed method given by Van de Griend and Owe (1993) ε a air emiivity etimated from air temperature uing empirical model given by Campbell and Norman (1998). Air temperature ( T a ) at atellite overpa wa etimated from NDVI-LST 2D triangular catter by extracting LST at maximum NDVI within a patial domain of 20 x 20 pixel. Intantaneou ground heat flux ( G ) wa etimated a: 2 [(0.0032α α )( NDVI )] ( T ) Rn + α Batiaanen et al. (1998) (8) The converion of tantaneou net radiation and net available energy to daytime etimate wa made uing a inuoidal integral 29

3 function. The daytime average latent heat fluxe were computed from day time net available energy and contant evaporative fraction hypothei. 3.1 Net Radiation 3. RESULTS AND DISCUSSION The etimate of tantaneou and day time Rn uing MODIS TERRA and AQUA LST, NDVI, albedo were compared with in itu meaurement. Error tatitic compriing of bia, MAE, RMSE are preented in Table 1 (a) for tantaneou and Table 1 (b) for daytime etimate. Thee varied between 12-1 Wm -2, Wm -2, 38-8 Wm -2, repectively at TERRA overpa. MODIS Overpae Error Statitic (Wm -2 ) Correlation Coefficient ( r ) Bia MAE RMSE At TERRA Pooled At AQUA Pooled bia, MAE and RMSE for daytime Rn etimate from AQUA were found to be Wm -2, 31- Wm -2 and 35-9 Wm -2, repectively with r varying between The RMSE over AQUA pooled etimate wa 0 Wm -2 (12% of mean). Though the error of tantaneou net radiation at TERRA overpae were le than thoe at AQUA overpae, but the error from daytime etimate were le in AQUA than thoe from TERRA. The 1:1 validation plot of AQUA etimated daytime Rn and meaured Rn i hown in Figure 1b. The daytime etimate were generated from ingle clear ky tantaneou etimate from either TERRA and AQUA uing inuoidal integration. It wa found by earlier worker that daytime net radiation etimate from ingle morning (10.30 to AM) and afternoon (1.00 to hr) etimate howed overetimation (Biht et al., 2005) and underetimation (Nihida et al., 2003). Table 1a: Error Statitic of Intantaneou Net Radiation Etimate MODIS Overpae Error Statitic (Wm -2 ) Correlation Coefficient (r) Bia MAE RMSE At TERRA Pooled At AQUA Pooled TERRA +AQUA Table 1b: Error Statitic of MODIS Baed Daytime Average Net Radiation Etimate The overall correlation coefficient (r) wa found to be 0.86 with dataet pooled over both the eaon for TERRA overpae producing RMSE of the order of 2 Wm -2. At AQUA overpae, the bia, MAE, RMSE varied from 8-19 Wm -2, 9-86 Wm -2 and 5-92 Wm -2, repectively with overall r = The RMSE over pooled dataet wa 71 Wm -2. The RMSE over pooled etimate of Rn at TERRA and AQUA overpae were found to be 58 Wm -2 with r = 0.93 Figure 1a. The daytime Rn etimate from TERRA howed Wm -2 bia, 1-71 Wm -2 MAE and Wm -2 RMSE with r varying between The RMSE for pooled etimate with two year TERRA dataet wa 67 Wm -2. The Figure 1. Validation Plot of Etimated and Meaured (a) Intantaneou (TERRA+AQUA) and (b) Daytime Net Radiation (AQUA only) Only, noon time etimate (at to hr) can lead to better repreentation of daytime net radiation when atmopheric boundary layer become fully developed. The reulting error from AQUA daytime etimate i little higher than obtained over homogenou agro-ecoytem (Mallick et al., 2009) in India. Though it i higher than the error over pare and heterogeneou agro-ecoytem in LASPEX region with NOAA AVHRR data, the overall RMSE i lower than globally reported error in daytime net radiation 295

4 etimate (Gupta et al., 2001) obtained by everal worker with NOAA AVHRR (Hurtado and Sobrino, 2001) and MODIS TERRA (Biht et al., 2005) optical and thermal data. 3.2 Uing Area Integrated Senible Heat Fluxe from LAS Senible heat flux i very critical in properly repreenting Bowen ratio to compare λe etimate at moderate patial cale (~1km). Moreover, frequent change in enible heat fluxe over 1 q. km area due to frequently changing tability, tability condition pecially in emi-arid climate (Chehbouni et al., 2000) may not be ufficiently repreented through one or two Bowen ratio ample within the tudy area at each hour. Rather area averaged enible heat flux meaurement over MODIS 1 q. km pixel along with meaurement on Rn and G can repreent better in itu λe while validating it atellite baed etimate. A a follow up to that, tantaneou and daytime λe etimate were compared with limited λe generated through area averaged H from large aperture cintillometer (LAS). The percent error wa dratically reduced to 9% of mean having abolute RMSE 29 Wm -2 with increae in r (= 0.97) for TERRA and AQUA Figure 2. Figure 3. Temporal Comparion of Intantaneou λe etimate from TERRA and AQUA and LAS Meaurement The comparion of daytime AQUA λe etimate and area averaged λe uing LAS H howed a good match in Figure. Figure 2. Validation of TERRA and AQUA λe Etimation with in itu Meaurement when area Averaged LAS H wa ued The eaonal variation of tantaneou etimate from TERRA- AQUA and LAS meaurement howed cloe reemblance in Figure 3. The daytime etimate alo howed increment in accuracy (RMSE = 27 Wm -2 ) even with limited LAS dataet (n=8) and correlation coefficient wa alo increaed to Figure. Comparion of Etimated and Meaured Daytime Latent Heat Flux with AQUA and LAS CONCLUSION The accuracy of MODIS AQUA baed energy balance etimate wa better than TERRA for daytime average. Accuracy of λe etimate wa improved (9 % of meaured mean) when compared with in itu meaurement with large area averaged enible heat 296

5 fluxe from LAS. The correlation coefficient between etimate and meaurement wa alo improved (r = 0.97). Partitioning of ytem evapotranpiration need to be attempted in future uing two-ource energy balance formulation to derive oil evaporation and canopy tranpiration. ACKNOWLEDGEMENTS The author are grateful to Director, Deputy Director, Remote Sening Application Area (RESA) and Group Director, Agriculture-Foretry and Environment Group (AFEG), Space Application Centre (ISRO), Ahmedabad for their immene help during the tudy period in term of LAS trumentation, radiation meaurement and digital image proceing. REFERENCES Batiaanen, W.G. M., Menenti, M., Fedde, R.A. and Holtlag, A.A.M A remote ening urface energy balance algorithm for land (SEBAL). Formulation 1. Journal of.hydrology, , pp Biht, G.; Venturini, V., Ilam, S. and Jiang, L Etimation of net radiation uing MODIS (Moderate Reolution Imaging Spectroradiometer) data for clear ky day. 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