TRACKING NDVI IN TIME AND SPACE: KNOW THE PHYSIOLOGY BEHIND THE NUMBER. Jacob Nederend, B.Sc. (Agr.)

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1 TRACKING NDVI IN TIME AND SPACE: KNOW THE PHYSIOLOGY BEHIND THE NUMBER Jco Nederend, B.Sc. (Agr.)

2 1 TRACKING NDVI IN TIME AND SPACE: KNOW THE PHYSIOLOGY BEHIND THE NUMBER Jco Nederend, B.Sc. (Agr.) Reserch Agronomist, UAS Agriculture Mnging frm tsks round the wether is perpetul struggle, nd s UAVs integrte into the cropping toolox, the wether poses similr issues due to vrile conditions. Differences in sunlight levels re prticulrly concerning ecuse sensors respond to the mount of light striking the imging rry. Deveron uses the ltest sensor technology to eliminte this vriility y clirting imges ginst incoming light nd trget of known reflectivity. This clirted imgery enles growers to monitor their crops using mesurements tht re stle through time. By stndrdizing ech flight to the conditions on tht dy, the resulting mps cn e compred t regulr intervls or trgeted timings during the growing seson. The Normlized Difference Vegettion Index (NDVI) is one representtion of the collected dt tht provides n overview of the size nd greenness of the crop cnopy. NDVI= NIR-RED NIR+RED Eqution 1 The NDVI formul (Eqution 1) tkes the difference of ner infrred nd red reflectnces nd divides it y their sum to provide n index scled from -1 to 1. But wht do these vlues relly men? The Micsense Rededge is one of the clirted sensors we use. The ottom-mounted sensor mesures reflectnce, while the top-mounted sensor simultneously records the incoming sunlight in the sme wvends. NDVI IS BOTH A LEAFINESS AND GREENNESS INDEX Reflectnce vlues in griculturl fields hve distinct sesonl ptterns (Fig. 1) tht follow the growth nd development of crop. Since red nd lue light provide the energy to drive photosynthesis, the reflectnce of those wvelengths decreses s the lef cnopy ecomes lrger nd more chlorophyll sors the intercepted light. At the sme time, NIR reflectnce lrgely depends on the size of the crop cnopy, nd continues to increse until round the time of flowering in mny crops. The result of these two fctors on NDVI is incresed sensitivity to lef re erly in the seson, nd shift to sensitivity to chlorophyll content s the cnopy mtures.

3 2 c Figure 1: Sesonl reflectnce vlues for corn (), winter whet (DOY < 18) nd doule-crop soyen (DOY > 18) () nd spring cnol (c) collected in Iow in 27 for n experiment conducted y Htfield nd Prueger (21) TIMING UAV SURVERYS EARLY SEASON IMAGERY SHOWS YOU WHAT S THERE (AND WHAT ISN T) When plnning UAV opertions for the seson, think out how crop growth nd development will ffect the gols you cn chieve. Erly in the seson, lck of lef re mkes NDVI poor cndidte for prolems relted to greenness. In ddition to hving less vegettion thn t more developed stges, most young crops do not express nutrient deficiencies until they enter the steep prts of their uptke curves (Fig. 2). Differences in nutrient sttus will lso e msked y strter fertilizer. Insted, use erly-seson imgery to ssess issues relted to the presence nd uniformity of vegettion identified y the NDVI. These opportunities re numerous ut some of the common use cses include: Assessing winter survivl of whet nd forges Emergence counts Determining post-emergent stnd reductions Grin Grin Tssel, Co, Husk Leves Flowers, Pods N Uptke, l N/A Stlk nd Led Sheths Lef Bldes Percent of totl % Stem, Petioles Leves N Uptke, (kg N h -1 ) , 1,5 2, 2,5 GDD F Dys After Plnting VE V2 V4 V6 V1 V14 VT/R1 R2 R4 R5 R6 Growth Stge PlntingV 2V 4V 7 R2 R4 R5 R6 R8 Growth Stge Figure 2: Nitrogen deficiency is common reson for reduced lef greenness. The uptke curves for corn 2 () nd soyen 3 () show tht very little nitrogen is tken up in the erly stges, ut tht rpidly chnges s the slope of the curves ecomes steeper

4 3 The ove pplictions rely on the NDVI s ility to identify if there is (or isn t) crop where you would expect it. The erlier tht growers cn identify prolems in their crop stnds, the greter the opportunity to rectify the prolem. The eril perspective nd high detil of UAV imgery is idel for supporting re-plnt decisions while there is still enough growing seson left to produce profitle crop. MID-SEASON IMAGERY The crop will rech point where lef re will e lrge nd uniform enough to e n insignificnt contriutor to vrition in the NDVI. At the sme time, the crop will e in n explosive stge of growth where stress fctors egin to influence the reflectnce of leves. Using NDVI to delinete zones of stress, nd on-the-ground dt to identify the cuses, growers cn pply vrile rte prescriptions of fertilizer or pesticides without the need for historicl yield dt. Surveys cn e trgeted for specific mngement gols, like sidedressing nitrogen into corn, or conducted t regulr intervls to monitor for sudden chnges in crop growth tht my wrrnt intervention outside of the usul product ppliction timings. Clirted imgery is importnt t this timing ecuse it ensures tht the vriility oserved y the NDVI is relted to iophysicl prmeters like nutrient deficiency or disese development. Unclirted imgery is suitle for detecting generl vrition, ut without ccounting for the incoming light, the mesured NDVI might e ised up or down, which mens ny reltionships etween the index nd ctul crop iophysicl chrcteristics will e inccurte. TRACKING NDVI WITHIN AND BETWEEN FIELDS experiences to support decisions in the current yer remins importnt. With clirted imgery collected y Deveron, growers cn trck oservtions of every cre nd compre the progress of crops oth pst nd present. NDVI cross comprisons it s not just the numer Using NDVI to compre the helth of severl fields in the sme yer, or of the sme field over mny yers, is not s sic s looking t the pixel vlues. For the former, e wre tht vrietl differences in reflectnce do exist. This mens tht, ll else eing equl, the sme growth ptterns would e identified y different mgnitudes of NDVI for different vrieties. Alwys keep the two underlying fctors in mind when compring NDVI s well. Discrepncies in NDVI etween fields or yers will e explined y one or oth fctors, mking unised comprisons chllenge if no dditionl informtion ccompnies the index. For exmple, two corn plnts cn hve low NDVIs, ut in one plnt the low NDVI is relted to stunted growth due to erly-seson frost dmge, nd in the other plnt it is due to nitrogen deficiency. Extrpolting this sitution to the field scle demonstrtes tht comprisons of the generl progress of crop re vlid, ut supporting informtion is mndtory to drw ny deeper conclusions. Aeril imgery s digitl noteook Record keeping is key prt of lerning nd improving s frmer. Pen nd pper my hve turned to spredsheets nd word processors, ut using pst Vriility of NDVI in poorly performing crops will e driven mostly y the mount of lef re (), wheres greenness is likely to e more significnt contriutor in helthier prts of the field ()

5 4 Figure 3: The ove NDVI mps of soyen field were cquired on 217 on August 2 () nd Septemer 19 (). The symology emphsizes shift in the distriution of NDVI over 4 weeks. Ech colour intervl on the scles covers n equl re of the field, rther thn defining intervls y fixed vlues, to mximize visiility of vriility. Scles re visul, not numericl When compring NDVI mps, rememer tht the colours re just visul representtion of the dt nd do not impct the ctul vlues. Altering the visuliztion (known s symology in the remote sensing world) cn revel or hide vriility in NDVI. If the gol is to find the top performing regions of field over time, using uniform scle might eliminte the ility to see vrition ecuse the distriution of NDVI vlues erly in the seson will e drsticlly different from tht of mid-seson imgery [Fig. 3]. Scling the colours y the mximum nd minimum NDVIs cross ll the surveys in time series will provide the est compromise etween visiility of vrition nd ese of comprisons etween dtes.

6 5 REFERENCES 1. Htfield, J.L., nd J.H. Prueger. 21. Vlue of using different vegettive indices to quntify griculturl crop chrcteristics t different growth stges under vrying mngement prctices. Remote Sens. 2(2): Bender, R.R., J.W. Hegele, M.L. Ruffo, nd F.R. Below Modern Corn Hyrids Nutrient Uptke Ptterns. Better Crops. 97(1): Bender, R.R., J.W. Hegele, nd F.E. Below Nutrient uptke, prtitioning, nd remoiliztion in modern soyen vrieties. Agron. J. 17(2): For more informtion or to discuss this rticle, plese contct: Jco Nederend jnederend@deveronus.com