Estimating spatiotemporal variability of maize crop evapotranspiration in the Mooi River catchment, South Africa: a crop coefficient approach using satellite remote sensing-based vegetation indices

Abstract

In an area experiencing semi-arid conditions, such as the Mooi River catchment, it is crucial to ascertain the evapotranspiration rate accurately. Evaluating evapotranspiration is beneficial in areas where the water demand surpasses its availability. This is particularly true in semi-arid and arid areas where the lack of water is increasingly impacting economic well-being and obstructing sustainable development. Developed by the Food and Agriculture Organization of the United Nations (FAO) in 1998, the crop coefficient approach estimates actual evapotranspiration (ETc) of crops using the equation ETc = ETo ∗ Kc, where ETo is reference evapotranspiration, and Kc is the crop coefficient. However, this approach, assuming uniform crop conditions, does not account for spatial variations in crop growth conditions, leading to limitations in reflecting the actual water use of crops. Using satellite remote sensing and vegetation indices, such as the Normalised Difference Vegetation Index (NDVI) and the Soil Adjusted Vegetation Index (SAVI), can overcome these limitations by accounting for the spatial and temporal variability in crop conditions and growth. NDVI was found to have a superior correlation with the FAO tabulated crop coefficients compared to SAVI. It was therefore chosen to compute the new Kc values to be used in the ETc crop coefficient equation as it accurately reflects real-time crop growth conditions and environmental impacts. The findings of this study underscore significant discrepancies in the spatial variation of ETc across the Mooi River catchment, revealing notable variation in evapotranspiration, particularly the lower rates in the southwest during the growing season. The study evidenced temporal variations between ETc values calculated from FAO tabulated Kc and those derived from NDVI-adjusted Kc throughout different maize cultivation phases of the year, emphasising, in particular, the need for precise assessment methods during the initial maize growth stages. Time-series analysis revealed that the average ETc in the Mooi River catchment begins at a low level, marking its minimum at 1.614 mm during the maize's initial growth stage. It then undergoes a steady ascent, reaching a peak of 8.83 mm at 85 days after planting before gradually decreasing to 2.89 mm as it approaches harvest. The significant ETc variations revealed in the maize growth cycle emphasise the need to consider several factors in agricultural assessments, such as climate, soil, and human activities. The developed maps and models offer critical tools for stakeholders, promoting efficient water use and sustainability in semi-arid areas, thereby increasing food security. The research also highlights the potential of this approach for different crops and regions, underscoring its significance for water allocation management and yield evaluations, particularly in arid zones. The insights gained, necessary for understanding drought impacts and water-related crop stresses, pave the path for sustainable water management and increased food security across similar agricultural settings.

In 'n area met semi-droë toestande soos die Mooirivier-opvanggebied, is die akkurate bepaling van die tempo van evapotranspirasie van kardinale belang. Evaluering van evapotranspirasie is voordelig in gebiede waar die wateraanvraag die beskikbaarheid oorskry. Dit is veral belangrik in semi-droë en droë areas waar waterskaarste die ekonomie beïnvloed en volhoubare ontwikkeling belemmer. Die Voedsel- en Landbou-organisasie van die Verenigde Nasies (FAO) het die gewaskoëffisiëntbenadering in 1998 ontwikkel om werklike evapotranspirasie (ETc) van gewasse te skat met die vergelyking ETc = ETo ∗ Kc, waar ETo verwysings evapotranspirasie is, en Kc die gewaskoëffisiënt. Hierdie benadering neem egter nie ruimtelike variasies in gewasgroeitoestande in ag nie, wat die vermoë daarvan beperk om werklike watergebruik te weerspieël. Om hierdie beperkings te oorkom, kan satellietafstandwaarneming en plantegroei-indekse soos die Genormaliseerde verskil plantegroei-indeks (NDVI) en die Grondaangepaste plantegroei-indeks (SAVI) gebruik word om ruimtelike en tydelike variasies in gewastoestande en groei in ag te neem. NDVI is bevind om 'n beter korrelasie te hê met die FAO se getabuleerde gewaskoëffisiënte in vergelyking met SAVI. Dit is dus gekies om die nuwe Kc-waardes te bereken wat gebruik sou word in die ETc gewaskoëffisiëntvergelyking, aangesien dit werklike tyd gewasgroeiomstandighede en omgewingsimpakte akkuraat weerspieël. Die bevindings van hierdie studie beklemtoon beduidende teenstrydighede in die ruimtelike variasie van ETc oor die Mooirivier-opvanggebied, en toon opmerklike variasie in evapotranspirasie aan, veral laer tellings in die suidweste gedurende die groeiseisoen. Die studie het tydelike variasies tussen ETc waardes wat bereken is vanaf FAO getabuleerde Kc en dié wat afgelei is van NDVI-aangepaste Kc deurheen verskillende tye van die jaar, getoon, en beklemtoon veral die behoefte aan presiese assesseringsmetodes gedurende die aanvanklike groei stadiums. Tydreeksanalise het aan die lig gebring dat die gemiddelde ETc in die Mooirivier-opvanggebied begin by 'n lae vlak, met sy minimum op 1.614 mm gedurende die aanvanklike groeifase van mielies. Dit ondergaan dan 'n gestadige styging, en bereik 'n piek by 8.83 mm 85 dae na plant tyd, voor dit geleidelik afneem tot 2.89 mm soos dit nader aan oes tyd kom. Die studie se beduidende ETc-variasies wat in die mieliegroeisiklus aan die lig gebring is, beklemtoon die noodsaaklikheid om verskeie faktore, soos klimaat, grond en menslike aktiwiteite, in landbouevaluasies in ag te neem. Die ontwikkelde kaarte en modelle bied kritieke hulpmiddels vir belanghebbendes, wat doeltreffende watergebruik en volhoubaarheid in semi-droë gebiede bevorder, en sodoende voedselsekerheid verhoog. Die navorsing beklemtoon ook die potensiaal van hierdie benadering vir verskillende gewasse en streke, en beklemtoon die belangrikheid daarvan vir watertoewysingsbestuur en opbrengs-evaluasies, veral in droë sones. Die insigte wat verkry is, wat nodig is om droogte-impakte en waterverwante gewasstremmings te verstaan, baan die pad vir volhoubare waterbestuur en verhoogde voedselsekuriteit oor soortgelyke landbouomgewings.

Endaweni enezimo ezinogwadule, njengendawo egcina amanzi e-Mooi River, kubalulekile ukuqinisekisa izinga lokuphefumula kokuhwamuka kwamanzi ngokunembile. Ukuhlola ukuhwamuka kokuphefumula kunenzuzo ezindaweni lapho isidingo samanzi sidlula ukutholakala kwawo. Lokhu kuyiqiniso ikakhulukazi ezindaweni ezingelona ugwadule ngokugcwele nalezo eziwugwadule lapho ukuntuleka kwamanzi kuba nomthelela omkhulu empilweni yezomnotho futhi kuvimbe intuthuko esimeme. Ukwakhiwa kwayo yiNhlangano Yezokudla Nezolimo yeNhlangano Yezizwe Ezihlangene (i-FAO) ngo-1998, indlela yokusebenza kahle yezitshalo ilinganisela kuyizinga lokuphefumula kokuhwamuka (LK) kwezitshalo kusetshenziswa ngokulinganayo LK = NKo ∗ Kc, lapho NKo kuyireferensi yokuphefumula kokuhwamuka, futhi i-Kc iesebenza kahle yezitshalo . Kodwa-ke, le ndlela, kucatshangelwa izimo ezifanayo zezitshalo, ayibali ukwehluka kwezindawo ezimweni zokukhula kwezitshalo, okuholela emikhawulweni ekuboniseni ukusetshenziswa kwamanzi kwangempela kwezitshalo. Ukusebenzisa inzwa eyinkomba neyirimothi yesathelayithi nezimila, njenge i-Normalized Difference Vegetation Index (NDVI) kanye ne-Soil Adjusted Vegetation Index (SAVI), kunganqoba le mikhawulo ngokubala ukuhlukahluka kwendawo kanye nesikhashana ezimeni zezitshalo nokukhula. I-NDVI itholwe inokuhlobana okuphakeme okuphathelene nokusebenza kahle kusivuno esifakwe kuthebula ye- FAO uma kuqhathaniswa ne-SAVI. Ngakho-ke kwakhethwa ukubala amanani amasha e-Kc azosetshenziswa ku-ETc crop coefficient equation njengoba ibonisa ngokunembile izimo zokukhula kwezitshalo zesikhathi sangempela nomthelela wemvelo. Okutholwe kulolu cwaningo kugcizelela umehluko omkhulu ekuhlukeni kwendawo NKc ngaphesheya kwendawo egcina amanzi e-Mooi River, okuveza ukuhluka okuphawulekayo kokuphefumulela kokuhwamuka, ikakhulukazi amazinga aphansi eningizimu-ntshonalanga ngesikhathi sokukhula. Ucwaningo lwafakazela ukwehluka kwesikhashana phakathi kwamanani NKc abalwe ku-FAO yethebula i-Kc kanye nalawo atholakala ku-NDVI elungiswe i-Kc kuzo zonke izigaba zokutshala ummbila ezihlukene zonyaka, okugcizelela ikakhulukazi isidingo sezindlela zokuhlola ezinembayo phakathi nezigaba zokuqala zokukhula kommbila. Ukuhlaziywa kochungechunge lwesikhathi kwaveza ukuthi isilinganiso so -NKc endaweni egcina amanzi e-Mooi River siqala ezingeni eliphansi, siphawula ubuncane baso ku-1.614 mm ngesikhathi sokukhula kommbila. Ibe seyikhuphuka kancane, ifike ku-8.83 mm ezinsukwini ezingama-85 ngemuva kokutshala ngaphambi kokwehla kancane kancane ibe ngama-2.89 mm njengoba sekusondele ukuvuna. Ukwehluka okubalulekile ko-NKc okuvezwe emjikelezweni wokukhula kommbila kugcizelela isidingo sokucabangela izici ezimbalwa, njengesimo sezulu, inhlabathi, kanye nemisebenzi yabantu, ekuhloleni kwezolimo. Amabalazwe athuthukisiwe namamodeli ahlinzeka ngamathuluzi abalulekile kubabambiqhaza, ekhuthaza ukusetshenziswa kwamanzi ngendlela efanele kanye nokusimama ezindaweni ezisagwadule, ngaleyo ndlela kwandise ukuvikeleka kokudla. Ucwaningo luphinde luveze amandla ale ndlela ezitshalweni nasezifundeni ezahlukene, lugcizelela ukubaluleka kwayo ekulawuleni ukwabiwa kwamanzi kanye nokuhlolwa kwesivuno, ikakhulukazi ezindaweni ezomile. Ukuqonda okuzuziwe, okudingekayo ukuze kuqondwe imithelela yesomiso kanye nengcindezi yezitshalo ehlobene namanzi, kuhlahla indlela yokulawulwa kwamanzi okusimeme kanye nokwanda kokuvikeleka kokudla kuzo zonke izimo zezolimo ezifanayo.