Water-removed spectra increase the retrieval accuracy when estimating savanna grass nitrogen and phosphorus concentrations |
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| Authors: | Abel Ramoelo Andrew K SkidmoreMartin Schlerf Renaud MathieuIgnas MA Heitkönig |
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| Institution: | a Council for Scientific and Industrial Research (CSIR), Natural Resource and Environment, Ecosystem-Earth Observation Research Group, P.O. Box 395, Pretoria, 0001, South Africab University of Twente, Faculty for Geoinformation Science and Earth Observation (UT-ITC), P.O. Box 217, Enschede, 7500 AE, The Netherlandsc Wageningen University, Resource Ecology Group, Droevendaalsesteeg 3a, 6708 PB Wageningen, The Netherlands |
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| Abstract: | Information about the distribution of grass foliar nitrogen (N) and phosphorus (P) is important for understanding rangeland vitality and for facilitating the effective management of wildlife and livestock. Water absorption effects in the near-infrared (NIR) and shortwave-infrared (SWIR) regions pose a challenge for nutrient estimation using remote sensing. The aim of this study was to test the utility of water-removed (WR) spectra in combination with partial least-squares regression (PLSR) and stepwise multiple linear regression (SMLR) to estimate foliar N and P, compared to spectral transformation techniques such as first derivative, continuum removal and log-transformed (Log(1/R)) spectra. The study was based on a greenhouse experiment with a savanna grass species (Digitaria eriantha). Spectral measurements were made using a spectrometer. The D. eriantha was cut, dried and chemically analyzed for foliar N and P concentrations. WR spectra were determined by calculating the residual from the modelled leaf water spectra using a nonlinear spectral matching technique and observed leaf spectra. Results indicated that the WR spectra yielded a higher N retrieval accuracy than a traditional first derivative transformation (R2=0.84, RMSE = 0.28) compared to R2=0.59, RMSE = 0.45 for PLSR. Similar trends were observed for SMLR. The highest P retrieval accuracy was derived from WR spectra using SMLR (R2=0.64, RMSE = 0.067), while the traditional first derivative and continuum removal resulted in lower accuracy. Only when using PLSR did the first derivative result in a higher P retrieval accuracy (R2=0.47, RMSE = 0.07) than the WR spectra (R2=0.43, RMSE = 0.070). It was concluded that the water removal technique is a promising technique to minimize the perturbing effect of foliar water content when estimating grass nutrient concentrations. |
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| Keywords: | Nitrogen concentration Phosphorus concentration Water removal Continuum removal Bootstrapping |
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