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Remote Sensing 2014, 6(2), 1171-1190; doi:10.3390/rs6021171

Article
An Extended Fourier Approach to Improve the Retrieved Leaf Area Index (LAI) in a Time Series from an Alpine Wetland
Xingwen Quan 1, Binbin He 1,*, Yong Wang 1,2, Zhi Tang 1 and Xing Li 1
1
School of Resources and Environment, University of Electronic Science and Technology of China, Chengdu 611731, China; E-Mails: quanxw7@gmail.com (X.Q.); wangy2012@uestc.edu.cn (Y.W.); 13980476627@163.com (Z.T.); zxwlxty@163.com (X.L.)
2
Department of Geography, Planning and Environment, East Carolina University, Greenville, NC 27858, USA
*
Author to whom correspondence should be addressed; E-Mail:binbinhe@uestc.edu.cn; Tel.: +86-28-6183-1586.
Received: 14 November 2013; in revised form: 5 January 2014 / Accepted: 17 January 2014 /
Published: 29 January 2014

Abstract

: An extended Fourier approach was presented to improve the retrieved leaf area index (LAIr) of herbaceous vegetation in a time series from an alpine wetland. The retrieval was performed from the Aqua MODIS 8-day composite surface reflectance product (MYD09Q1) from day of year (DOY) 97 to 297 using a look-up table (LUT) based inversion of a two-layer canopy reflectance model (ACRM). To reduce the uncertainty (the ACRM inversion is ill-posed), we used NDVI and NIR images to reduce the influence of the soil background and the priori information to constrain the range of sensitive ACRM parameters determined using the Sobol’s method. Even so the uncertainty caused the LAIr versus time curve to oscillate. To further reduce the uncertainty, a Fourier model was fitted using the periodically LAIr results, obtaining LAIF. We note that the level of precision of the LAIF potentially may increase through removing singular points or decrease if the LAIr data were too noisy. To further improve the precision level of the LAIr, the Fourier model was extended by considering the LAIr uncertainty. The LAIr, the LAI simulated using the Fourier model, and the LAI simulated using the extended Fourier approach (LAIeF) were validated through comparisons with the field measured LAI. The R2 values were 0.68, 0.67 and 0.72, the residual sums of squares (RSS) were 3.47, 3.42 and 3.15, and the root-mean-square errors (RMSE) were 0.31, 0.30 and 0.29, respectively, on DOY 177(early July 2011). In late August (DOY 233), the R2 values were 0.73, 0.77 and 0.79, the RSS values were 38.96, 29.25 and 27.48, and the RMSE values were 0.94, 0.81 and 0.78, respectively. The results demonstrate that the extended Fourier approach has the potential to increase the level of precision of estimates of the time varying LAI.
Keywords:
alpine wetland; extended Fourier approach; ill-posed inversion problem; leaf area index; uncertainty

1. Introduction

The use of remotely sensed optical data over large areas to quantitatively infer key biophysical and biochemical parameters of vegetation, such as the leaf area index (LAI), has been a popular application of remote sensing [13]. The LAI is a critical structural characteristic that affects the biophysical and biochemical processes of vegetation canopies and the exchange processes of energy and matter between the land surface and the lower atmosphere [46]. The approaches used to retrieve LAI generally include combined empirical and statistical approaches and the inversion of a physical canopy reflectance model. In the former, a technique is used to link the LAI to the associated sensitive spectral reflectance or to vegetation indices (VI) based on field data. These approaches are simple and computationally efficient but are regarded as sensor-specific, site-dependent, and vary significantly overtime and space [79]. The inversion approach is promising because it is based on physical models [1,4,7]. Unfortunately, the ill-posed inversion problem [10] is prevalent in the model-based retrieval procedure and results in uncertainty in the retrieved LAI. The use of a priori information has been demonstrated to alleviate the ill-posed inversion problem [1,6,9].

Vegetation parameters, including LAI, that are retrieved simultaneously may not meet the requirements of specific applications, such as the classification of vegetation types [11], vegetation growth changes and phenology [1215]. However, due to cloud interference and image-specific problems that result in missing data at aperiodic times of the year, remotely sensed imagery may not form high-quality time series [16]. Because of the poor-quality data and ill-posed inversion problems, the uncertainty in the values of parameters retrieved at a particular time can be large. When the parameters are plotted versus time, the curves have discrete and oscillating features. Thus, a reliable method is needed to smooth the curves.

One way to smooth these curves is the data assimilation technique [3,17], which has been used to integrate observed data with models in hydrology [18], crop science [19], oceanography [20], morphodynamics [21], and air quality modeling [22,23]. The data assimilation technique requires a dynamic model. Models such as WOFOST [24,25], DSSAT [26], and SWAP [27] have been widely used to simulate and forecast vegetation growth or the growth state and yield of crops. Because the uncertainty in the observed data, the dynamic model, and the background parameters of the model inputs are all considered, the technique can be used to reliably smooth curves of target parameters[28]. Unfortunately, the large set of input parameters required by the technique is difficult to obtain. Different types of vegetation usually require different inputs and models [24]. In addition, the technique can be time-consuming [28].

If the dataset is periodic, another way to smooth multi-temporal curves or solve missing data problems is the Fourier model [16]. The Fourier model is a specific infinite series and can be tailored to any period, baseline, and amplitude. As the number of harmonics used increases, the Fourier model can converge to any smooth periodic function. The model has been successfully used in the classification of vegetation types [15] and photosynthetic activity over the growing season [1214,29,30]. However, the aforementioned studies are primarily focused on smoothing the normalized difference vegetation index (NDVI) [31] and seldom on smoothing parameters retrieved through physically based models. The Fourier model has the ability to smooth multi-temporal curves and overcome missing data problems. However, the model cannot improve the precision of retrieved parameters, whereas the assimilation technique potentially can. Thus, this study considers an extended Fourier approach in which the advantage of the assimilation technique that is able to deal with the uncertainty in the retrieved parameters. In particular, we focus on the improvement of the retrieved LAI (LAIr) of herbaceous vegetation in a one-year time series from an alpine wetland. The study includes three major steps. The LAIs from different time periods are retrieved using a two-layer canopy reflectance model (ACRM), the Aqua MODIS 8-day composite surface reflectance product (MYD09Q1) from DOY 97 to 297, and a look-up table (LUT) algorithm. The Fourier model is then fitted to simulate the variation of LAI values in the time series using the LAIr and the least squares method. Finally, the Fourier model is extended to quantify the uncertainty in the LAIr. Therefore, the precision level of the fitted LAI curve is improved. The LAIr, fitted LAI, and the improved LAI are validated using the LAI values measured on DOY 177 (early July 2011) and 233 (late August). Details about the method are given next.

2. Method

2.1. The Schemes of LAI Retrieval

The inversion is by nature ill-posed [10]. Three strategies are implemented in the alleviation of the ill-posed problem [9]. (I) To reduce the influence of the soil background, we use the normalized difference vegetation index (NDVI)to retrieve the LAI for sparse vegetation and the near-infrared (NIR) reflectance for dense vegetation;(II) A sensitivity analysis of the ACRM at red (RED) and NIR wavelengths is conducted using Sobol’s method [32] to determine the sensitive parameters; (III) The priori information collected from field work is used to constrain the range of each sensitive parameter. The LAI is then retrieved from the time series by inverting the ACRM using the LUT algorithm and the MYD09Q1 data from DOY 97 to 297.

2.1.1. Radiative Transfer Model

The ACRM radiative transfer model is used [33]. The model assumes that the vegetation canopy consists of a main homogeneous layer of vegetation and a thin layer of vegetation on the ground surface. Both vegetation layers are characterized by a similar set of phytometric parameters that control the optical properties of the leaves. The model operates in the spectral region of 400–2,500 nm, and the spectral resolution is 1 nm, which is appropriate for direct and inverse problems [33]. The model input parameters are briefly discussed below.

With the Ǻith the turbidity equation [34], the turbidity coefficient (β) is used to compute the diffuse fraction of incoming radiation. The Markov clumping parameter (Sz) ranges from 0.4, which represents a significantly clumped plant canopy, to 1.0, which represents a homogeneous canopy of randomly positioned leaves [6]. The soil background reflectance ρsoil(λ) can be described by a function that is the combination of four basis vectors, φi(λ) (i =1, 2, 3, and 4). However, φ1(λ) and φ2(λ) are sufficient to describe the spectral variability of the reflectance because both account for nearly 94.2% of the spectral variability [35]. Thus,

ρ soil ( λ ) s 1 φ 1 ( λ ) + s 2 φ 2 ( λ )

In addition, weight s2 is related to s1 [6,36]. The angular distribution of leaves, which is described by the leaf inclination (θm) and the eccentricity (el), is an elliptically distributed function [37]. The parameter θm represents the inclination angle, with θm= 0 indicating a planophile canopy and θm= 90 denoting an erectophile canopy. The parameter el characterizes the leaf orientation, and 0 ≤ el≤1;el= 0 represents a spherical orientation, and el= 1 represents a fixed inclination angle [38]. The field measured leaf inclination angle (θl) is used to fit θm and el by utilizing a model that is based on the elliptical distribution function gl(θl) [36,39]

g l ( θ l ) = b 1 e l 2 cos 2 ( θ l θ m )
where b can be determined as
b = e l [ cos θ m ln ( cos η + sin ν cos ν sin η ) sin θ m ( η ν ) ]
with
η = sin 1 ( e l cos θ m )
ν = sin 1 ( e l sin θ m )

The leaf optics model PROSPECT [40] is added to the ACRM and used to calculate the leaf reflectance and transmittance from 400 nm to 2,500 nm at a spectral resolution of 1 nm. The biochemical parameters used as model inputs are expressed as multiple fractions of the dry matter of leaves [33]. The five key parameters included in the model are the leaf mesophyll structure parameter (N), the chlorophyll a and b content (Cab), the equivalent water thickness (Cw), the dry matter content (Cm), and the leaf brown pigment (Cbp). Of these, Cw has a large influence on the shortwave infrared but only a slight influence at the NIR and RED wavebands. Cbp represents the degree of leaf senescence and the light being absorbed by non-chlorophyll pigments and may vary from 0, representing no light absorption, to 6, which indicates maximum light absorption [6].

2.1.2. Sensitivity of NDVI and NIR Reflectance to LAI

The NDVI is generally sensitive to LAIs between 0 and 4.0 (Figure 1a) and becomes saturated as the LAI continues to increase. Thus, using NDVI images can cause significant uncertainty in retrieving high LAI values. The NIR reflectance is sensitive even at LAI values greater than 4.0 (Figure 1b). Figure 1, the influence of soil background (s1) exists, but the influence differs. Thus, to alleviate the ill-posed inversion and to reduce the uncertainty in LAIr, one can use NDVI images to retrieve the LAI when LAI < 4.0, and NIR images to retrieve the LAI of high values. Based on field measurement, the NDVI = 0.8 is selected as the threshold. When 0 < NDVI < 0.8, the LAI is retrieved by the NDVI, while 0.8≤NDVI < 1.0 the LAI is retrieved by NIR.

2.1.3. Sensitivity Analysis of ACRM

The sensitivities of different model parameters are different at RED and NIR wavelengths. To mitigate the ill-posed inversion problem, we set the sensitive parameters as free variables and the insensitive parameters as empirical values. Sobol’s method [32] is used in the sensitivity analysis. The method is based on variance decomposition [41] and is a global and model-independent sensitivity analysis. The method is superior to traditional local methods that examine sensitive parameters one at a time and the method is robust [42].

The main idea behind Sobol’s method for the computation of sensitivity indices is to decompose the function or model of f(x1, …, xn) into summands with increasing dimensionality as

f ( x 1 , , x n ) = f 0 + i = 1 n f i ( x i ) + i = 1 n j = i + 1 n f i j ( x i , x j ) + + f 1 , 2 , , n ( x 1 , , x n )
where x1, …, xn (n = 2, 3, 4…) are model input parameters. To hold f0 be a constant, the integrals of every summand over any of its own variables must be zero. The individual summands are then mutually orthogonal [41]. Additionally, the decomposition is unique. The total variance D of f(X) is defined as
D = K n f 2 ( X ) dX f 0 2
where Kn (n = 1, 2, 3, …) represents the n-dimensional unit hyperspace. Assuming that the parameters are mutually orthogonal, D is decomposed as
D = i = 1 n D i + i = 1 n j = i + 1 n D ij + + D 1 , 2 , , n
Therefore, the variance of the individual parameters and the interactions between the parameters that contribute to the total variance in the output can be determined. The contributions are characterized by Sobol’s sensitivity indices (Si) [43] as
First order : S i , S i = D i / D
Total : S Ti , S Ti = 1 D ~ i / D
The first order index Si is a measurement of the individual parameter xi to the total model variance. The partial variance of Di in Equation (9) is called the “main effect” of xi on f(x1, …, xn). STi is the total sensitivity indices (TSI) of xi, and “∼” means “complementary”.

2.1.4. LAI Retrieval Using the LUT Algorithm

The LUT algorithmis generally computationally efficient [44]. It contains three steps. The first is to set up the table by running the ACRM. In the table, different combinations of input parameters are linked to the model outputs. If the range of the sensitive parameters is too wide, the table will be very large. The uncertainty in the LAIr increases because some cases may not be true. For example, the LAI is always positive for vegetated surfaces. A zero or negative value of LAI in the LUT can cause large uncertainty in the LAIr. The priori information has been shown to be useful in constraining the range of sensitive parameters.

The cost function that links the simulated and observed values is then established. The function is expressed as

χ i = ( ρ j * ρ i ) 2 ɛ
where ρj* is the jth simulated value in the LUT, ρi is the ith observed value, and ε is a threshold value. The final step is to retrieve the target parameters based on the satellite images, the constructed LUT, and the cost function. If χiε, the combination of model input parameters can be regarded as the retrieved result. However, the ill-posed inversion problem still exists because the condition can be met in many cases for a simulated ρj*. The retrieved result is generally in a range of LAI values with different frequencies. The mean LAI or the LAI with the maximum frequency can be used to represent the LAIr if the retrieved LAI range is normally distributed. However, for the case of a non-normal distribution, it is more reliable to use the LAI with the maximum frequency (LAImax-fre) to represent the LAIr. It should be noted that more combinations of model parameters meet Equation (11) for the middle LAIr than for the LAIr value closest to the given boundary. This indicates that the scheme of LAI retrieval used in this study should have higher precision for low and high LAI values than values in the middle of the range.

2.2. The Extended Fourier Approach

Due to the periodicity in the LAI chronological series, the LAI versus time curves can be fitted using the Fourier model. The model is generally expressed as

LAI ( a 0 , a i , b i ) = a 0 + i = 1 n [ a i × sin ( i × t × 2 π T ) + b i × cos ( i × t × 2 π T ) ]
where a0, ai, and bi (i = 1, 2, 3, …, n) are unknown and should be fitted, n is the number of free variables and ranges from 3 to 5, t is the DOY, and T is the period.

The Fourier model coupled with polynomials (non-classical harmonic methods) can reduce the “roughness” in the fitting procedure [16,30]. Thus, the Fourier model is modified to improve its ability to smooth the “rough” LAI versus time curve. The modification is

LAI p ( a 0 , a i , b i , c j ) = a 0 + i = 1 n [ a i × sin ( i × t × 2 π T ) + b i × cos ( i × t × 2 π T ) ] + j = 1 k [ c j × ( t T ) j ]
where cj (with j = 1, 2, …, k) is unknown and needs to be fitted, and k is similar to n. Based on experience with the fitting procedure, n equals 4, and k is equal to 2. Because the LAI chronological series has a period of one year and the data used are MYD09Q1, T = 46. In particular, because the DOYs range from 13 to 42 in this experiment, the period is not one year. The Fourier model can be normalized as
LAI p , n ( a 0 , a i , b i , c j ) = a 0 + i = 1 n [ a i × sin ( i × t × 2 π T ) + b i × cos ( i × t × 2 π T ) ] + j = 1 k [ c j × ( t 13 42 13 ) j ]
Because each DOY is related to one Equation (14), the number of equations will be much greater than the number of unknown parameters. The parameters will be overdetermined. Thus, the optimal solution can be obtained using the method of least squares
J ( u ) = 1 2 t ( LAI p , n ( u ) s , t LAI r , t ) T ( LAI p , n ( u ) s , t LAI r , t )
where J(u) is the cost function, u is the vector of variables that should be fitted, LAIp,n(u)s,t is the LAI fitted using the Fourier model (LAIF), and LAIr,t is the dataset of LAIr on the tth DOY. Due to the ill-posed inversion problem, the LAIr dataset normally contains singular points. The fitting procedure can remove these points and improve the level of precision of the LAIr. However, if the dataset contains large uncertainty, the fitting procedure may reduce the level of precision of the LAIr.

Using the data assimilation technique, the uncertainty in the observed data can be taken into account to improve the simulation results. Thus, the uncertainty can be included in the fitting procedure to possibly improve the precision level of the LAIr in the time series. Based on this consideration, J(u) (15) is further extended by taking into account the uncertainty in LAIr as J(u)e

J ( u ) e = 1 2 t ( LAI p , n , u ( u ) s , t LAI r , t ) T LAI uncerta int y , t 2 ( LAI p , n ( u ) s , t LAI r , t )
where LAIp,n,u(u)s,t is the LAIr updated through the extension (LAIeF), LAIuncertainty,t is the uncertainty in the retrieved LAIs,t, and n is the total number of LAIr values in one year. Because the LAImax-fre is set as the LAIr (Section 2.1.4), the LAIuncertainty can be defined as
LAI uncertainty = t ( LAI r , t LAI max fre ) 2 f ( LAI r , t )
where f(LAIr,i) is the tth f requency of the LAIr,t. Therefore, every term in Equation (16) is defined, and the equation is solved by the method of least squares.

3. Study Area and Data

3.1. Study Area

The study area is located in the Wutumeiren prairie (latitude 36°46′ to 37°30′N, longitude 92°18′ to 93°24′E), Qinghai Province, China (Figure 2). It covers an area of ∼1,174 km2 and has an average elevation of 2900 m. The prairie is surrounded by the Gobi Desert. The Wutumeiren River is the main water source in the area. The climate includes annual rainy and dry seasons; the rainy season is normally from July to August, and the rest of the year is dry. The soil is characterized by high salinity and alkalinity. Thus, only a few types of adaptable and strong vegetation can grow. The vegetation is primarily composed of reeds. Due to variations in the soil moisture content, the reeds are clumped in some areas and are homogenous in others. Mixtures of grasses, shrubs, and trees are located at the edges of the prairie. They were planted to prevent the spread of the desert into the prairie. The edges of the prairie are not included in the study.

3.2. Satellite Images and Field Measurements

Thirty MYD09Q1 images( http://glovis.usgs.gov/) with a spatial resolution of 250 m were collected from DOY 97 to 329. The images were converted to surface reflectance. Each image was geometrically rectified into Universal Transverse Mercator (UTM) coordinates (zone 46) with the WGS-84 datum. The fieldwork was conducted from 7 to 9 July 2011 and from 26 to 29 August 2011 (Figure 2). An LAI-2000 instrument ( http://www.licor.com) was used to measure the canopy LAI under clear sky conditions at low solar elevation angles and at approximately the same local time each day. To prevent direct sunlight from shining on the sensor, a 45° view restrictor was used, and the Sun was kept behind the operator. Sample plots (30 m × 30m) were selected in homogenous reed areas (Figure 2), and their locations were recorded using global positioning system (GPS) units. Ninety-two plots were sampled. Within each sample plot, which was smaller than a single pixel, three subplots (1 m × 1 m) were randomly selected and measured. The mean LAI value of the subplots was used to represent the measured LAI. It should be noted that of eight plots, they were not apart far enough from each other. Three plots were within one 250 m × 250 m pixel, and another three plots were within a second pixel. In these two cases, the LAI was measured nine times per pixel. The LAI values of the nine subplots within each pixel were very similar. In four cases, two plots were within one pixel. Six LAIs were measured per pixel, and the LAIs were nearly the same. The similarity of the measured LAI values in each pixel indicated the homogeneity of the sampled area.

4. Results and Analysis

4.1. Sensitivity of ACRM Inputs to RED and NIR Reflectance and Parameterization of the Inputs

The parameters LAI, SL, N, Sz, s1, n, and θm were sensitive to NIR, and the parameters LAI, SL, N, Sz, s1, n, θm, and Cab were sensitive to RED (Figure 3). The hot spot parameter SL was approximately parameterized as a function of the LAI according to [6,45]. The parameter n was sensitive to both NIR reflectance and RED reflectance, but their values were fixed because only the reed was considered. The parameters θm and el were parameterized based on the field measurements of the leaf angle distribution. Therefore, the parameters LAI, SL, N, Sz, and s1 were set as free parameters for the NIR reflectance-based LAI retrieval procedure, and LAI, SL, N, Sz, s1, and Cab were set as free parameters for the NDVI-based LAI retrieval procedure.

Based on the qualitative and quantitative priori information acquired from the field measurements, empirical values were used to determine the parameters that were insensitive to NIR and RED wavelengths (Table 1). The range and increment (dx) of the sensitive parameters (LAI, N, s1, Sz, and Cab) for the NDVI- and NIR-based LAI retrieval schemes are listed in Table 2. The LAI ranged from 0 to 4 for the NDVI-based LAI retrieval procedure and from 2 to 7 for the NIR-based LAI procedure (Section 2.1.2). As stated in Sections 2.1.1 and 2.4, Sz was set from 0.4, which represents a significantly clumped plant canopy, to 1.0, which represents a homogeneous, randomly positioned canopy. Soil with higher moisture content normally has low reflectance; thus, s1 was set from 0.25 to 0.5 for sparse vegetation, where the soil typically has low soil moisture contents, and from 0.05 to 0.25 for dense vegetation, where the soil usually has high soil moisture contents. The range of N was determined based on the priori information. Because Cab had no effect on NIR, Cab was treated as a free parameter for NDVI. Based on the priori information, Cab ranged from 30 to 90 based on priori information. The sets of LUTs for the NDVI and NIR image-based retrievals were 18,144 and 8,088, respectively. The cost function (Equation (11)) was used to access the optimal target parameters. Each symbol “/” in Table 1 means that the related parameter was sensitive and that its variability was confined (Table 2). Finally, the Sun zenith angle (θs), view zenith angle (θv), and relative azimuth angle (θraz) parameters differed between pixels and were given in the MYD13 composite product.

4.2. Retrieved LAI on Each DOY

The NDVI images from DOY 97 to 177 and from DOY 265 to 329 (the arid periods) were used to retrieve relatively lower LAI values, and both the NIR and NDVI images from DOY 185 to 257 were used to retrieve relatively higher LAI values to alleviate the influence of the soil background (Section 2.1.2). As stated in Section 2.1.4, LAImax-fre was used as the LAIr, but it may be not reliable at low values of ε. For example, in Figure 4b, LAImean was 0.49, whereas LAImax-fre was 0. This singular case was avoided in Figure 4c, where ε = 0.05. However, if the value of ε was set too high, the retrieval scheme would be meaningless because the uncertainty in the LAIr values would increase as well (Equation (11)). In Figure 4d–f, as ε increases, the LAI distribution approached a normal distribution. In the case shown in Figure 4f, LAImean or LAImax-fre can be regarded as the LAIr. However, if the LAI was near the boundaries of the given range, LAImean and LAImax-fre might differ greatly. For example, in Figure 4i, LAImean = 6.14, whereas LAImax-fre = 7.0 because the LAI distribution was no longer normally distributed. In this case, it was more reliable to set LAImax-fre as the LAIr, as was assumed in Section 2.1.4. Therefore, in the study, ε = 0.05, and LAImax-fre was set as the LAIr.

Figure 5a compares the LAIr and ground-measured LAI (LAIm) values from July. The dashed line is the 1:1 line, and the solid line is the least-squares regression line. Of 38 points, the residual sum of squares (RSS), coefficient of determination(R2), and root-mean-square error (RMSE) values were 3.47, 0.68, and 0.31, respectively. Because the points were clustered at low LAI values (≤1.0), the RMSE value of 0.31 could signal a low level of precision of the retrieval. In August (Figure 5b), the LAIr and LAIm points at high and low LAI values were less scattered than those in the middle range of LAI values. In other words, the LAI retrieval procedure might be more sensitive at the low and high ends of the LAI values. Of 46 points, the RSS, R2, and RMSE values were 38.96, 0.73, and 0.94, respectively. The higher RSS and RMSE values in August compared to those in July could be caused by the increased LAI values as the reeds grew during this part of the year. This interpretation was supported by the high measured LAI values (Figure 5a,b). In summary, the LAIr values had large uncertainty in the retrieval; this is illustrated in Figure 6, where the peak LAIr values in the study period and from three pixels (locations) decrease from (a) to (b) to (c). Therefore, the LAIr values were smoothed to attempt to improve the precision level of LAIr.

4.3. Smoothing the LAI Curves Using the Fourier Model

As previously discussed, the Fourier model can remove singular points in time series. For example, in Figure 6a,b, the LAI should not decrease from DOY 209 to 241 because the reeds were in the growth stage. In Figure 6c, the decrease in LAI value at DOY 209 could be another singular point. The LAIr values were then smoothed using the polynomial included in the Fourier model (Equation (14)). Figure 7a–c shows the data from Figure 6a–c after the smoothing that was performed. Clearly, the singular points identified in Figure 6a–c were removed. The LAI fitted by the Fourier model (LAIF) and LAIm were then compared again. Of the 38 points from July (Figure 8a), the RSS, R2, and RMSE values were 3.42, 0.67, and 0.3, respectively. Of the 46 points from August (Figure 8b), the RSS, R2, and RMSE values were 29.25, 0.77, and 0.81, respectively. Compared to the July data, the precision of the LAIr assessed by the RSS, R2, and RMSE values were similar before and after the Fourier model smoothing. No meaningful improvement in the precision was achieved for the LAIr values. The precision level of the August LAIr data might have improved after the Fourier smoothing based on the decrease in the RSS and RSME values and the increase in R2 value compared with the values before the smoothing, but the improvement was slight. The extended Fourier approach was investigated next to determine whether the approach could increase the precision level in a meaningful way.

4.4. Improving the LAI Curves Using the Extended Fourier Approach

As shown in Figure 9, the LAIr curves were also smoothed using the extended Fourier approach (LAIeF), but the shapes of the LAIF and LAIeF curves were different. The LAIeF curves were farther away from the LAIr points with higher uncertainty and were closer to the LAIr points with lower uncertainty. For example, in Figure 9a, the LAIr value had a much lower uncertainty on DOY 217 than on DOY 225. The LAIeF curve was similar to the LAIr value on DOY 217 but far from that on DOY 225. Figure 9b,c had similar characteristics, but the LAIF curves did not. The LAIeF and LAIm values were then compared again (Figure 10). For the 38 points in July (Figure 10a), the RSS, R2, and RMSE values were 3.15, 0.72, and 0.29, respectively. For the 46 points in August (Figure 10b), the RSS, R2, and RMSE values were 27.48, 0.79, and 0.78, respectively. The precision level of the LAIeF values was improved compared with the LAIr and LAIF (Table 3). Therefore, the extended Fourier approach not only smoothed the LAIr curves in the time series but also improved the precision level of the LAIr values by taking into account the uncertainty in the LAIr.

5. Discussion

The Fourier model can be used to simulate the variation of LAI values in a time series. The Fourier model includes 11 free variables, which are fitted by the LAIr at different periods and the least squares method. However, the set of free variables in the Fourier model is much smaller than that in physical dynamic models, such as crop growth models. Physical dynamic models can better simulate the growth of crops because they are structured based on the physical mechanisms of vegetation growth. However, more work needs to be done on the selection of the appropriate model to simulate or approximate the growth of wild plants in alpine wetlands and to determine the large sets of input parameters. Therefore, it is infeasible to use a physical dynamic model to improve LAI estimates.

The Fourier model has been widely used to smooth the curves of parameters that are periodic. However, the model might not improve the precision level of the dataset. In this study, the Fourier model used in the fitting process was extended by taking into account the uncertainty in the LAIr values. As explained in Section 4.4, the improved fitted curves will be close to the points that have lower uncertainty and avoid points with larger uncertainty. The LAIr will have the best results and the precision level should be improved. As in the Fourier model, the extended Fourier approach can be used if the dataset is periodic, but the model will improve the precision of the dataset only if the uncertainty in the dataset is expressed correctly.

The LAIuncertainty imported in Equation (17) is regarded as the uncertainty caused by the ill-posed inversion problem. It can be used as an indicator to represent the uncertainty in the retrieval results as analyzed in Section 2.3. However, it may not describe the absolute uncertainty in the LAIr values because the uncertainty is affected by many factors, such as cloud cover, the LAI retrieval algorithm, and the ill-posed inversion problem.

6. Concluding Remarks

Characterized by high altitude and cold and dry weather, the alpine wetland is extremely fragile and vulnerable. The vegetation is typically composed of a single type of annual herb. In this study, an extended Fourier approach was presented to improve the LAIr values in a 2011 time series from an alpine wetland located in western China. The LAI was retrieved on DOYs based on the ACRM, the MYD09Q1 product from DOY 97 to 297, and the LUT algorithm. To alleviate the ill-posed inversion problem, three strategies were implemented. (I) To reduce the influence of the soil background, the NDVI and NIR reflectance of wavebands were used to retrieve the LAI for sparse vegetation and dense vegetation, respectively;(II) A sensitivity analysis of the ACRM in the RED and NIR wavelengths was performed to determine the free parameters using Sobol’s method;(III) The priori information was imported to constrain the range of free parameters in the ACRM. The LAIr in the time series did not form smooth curves due to the effects of the ill-posed inversion problem. A normalized Fourier model with two additional polynomials was used to smooth the LAIr curves in the time series because of the periodicity of the vegetation in the study area. The Fourier model is able toimprove the level of precision of the LAIr if singular points are present, but it may also reduce the level of precision if the datasets that the model is based on contain large uncertainty. An extended Fourier approach was presented by taking into account the uncertainty in the LAIr in the Fourier model to improve the level of precision of the LAIr. The LAIr, LAIF, and LAIeF values were validated through a comparison with field measured LAI (LAIm) values, which resulted in R2 values of 0.68, 0.67 and 0.72, RSS values of 3.47, 3.42 and 3.15, and RMSE values of 0.31, 0.30 and 0.29, respectively, on DOY 177 (early July). On DOY 233 (late August), the R2 values were 0.73, 0.77 and 0.79, the RSS values were 38.96, 29.25 and 27.48, and the RMSE values were 0.94, 0.81 and 0.78, respectively. The results demonstrated that this approach has the potential to improve the level of precision of periodically varying parameters in a time series.

This work was funded by the Fundamental Research Funds for the Central Universities (No. ZYGX2012Z005) and the National High-Tech Research and Development Program of China (863 Program, No. 2013AA12A302). The authors are grateful to Yongshuai Yan, Bo Zhang, Hongzhang Dai, Xiaojing Bai and Ningning Wang for their assistance during the field campaigns.

Author Contributions

XingwenQuan designed and performed experiments, analysed data, and wrote the manuscript; Binbin He gave comments and suggestions to the manuscript and performed the experiments; Yong Wang commented the manuscript and checked the writing; Zhi Tang and Xing Li performed part of the experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Sensitivity of normalized difference vegetation index (NDVI) to leaf area index (LAI) (a) and of NIR reflectance to LAI (b) by running two-layer canopy reflectance model (ACRM) forward at different values of s1.

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Figure 1. Sensitivity of normalized difference vegetation index (NDVI) to leaf area index (LAI) (a) and of NIR reflectance to LAI (b) by running two-layer canopy reflectance model (ACRM) forward at different values of s1.
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Figure 2. The study area (Wutumeiren prairie). The color composite Landsat5 image is TM4 (red), TM3 (green), and TM2 (blue). Green points represent the sampling plots from 6 to 9 July 2011, and the yellow points represent sampling plots between 26 and 29 August 2011.

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Figure 2. The study area (Wutumeiren prairie). The color composite Landsat5 image is TM4 (red), TM3 (green), and TM2 (blue). Green points represent the sampling plots from 6 to 9 July 2011, and the yellow points represent sampling plots between 26 and 29 August 2011.
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Figure 3. Sensitivity analysis of key input parameters of ACRM at near-infrared (NIR) and RED wavebands using Sobol’s method. TSI stands for total sensitivity index.

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Figure 3. Sensitivity analysis of key input parameters of ACRM at near-infrared (NIR) and RED wavebands using Sobol’s method. TSI stands for total sensitivity index.
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Figure 4. Distribution of LAIr. Because of the ill-posed inversion problem, LAIr is not a single value but a wide range of different frequencies.

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Figure 4. Distribution of LAIr. Because of the ill-posed inversion problem, LAIr is not a single value but a wide range of different frequencies.
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Figure 5. LAIr compared to the field-measured LAI on DOY 177 (July) (a) and DOY 233 (August) (b) in 2011.

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Figure 5. LAIr compared to the field-measured LAI on DOY 177 (July) (a) and DOY 233 (August) (b) in 2011.
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Figure 6. LAIr versus day of year (DOY) for three pixels. The solid line is the LAIr, and the error bars on each DOY represent LAIuncertainty. (a) and (b) are the LAIr of vegetation near Wutumeiren river; (c) is the LAIr of vegetation in drought area. The peak LAIr values in the study period decrease from (a) to (b) to (c).

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Figure 6. LAIr versus day of year (DOY) for three pixels. The solid line is the LAIr, and the error bars on each DOY represent LAIuncertainty. (a) and (b) are the LAIr of vegetation near Wutumeiren river; (c) is the LAIr of vegetation in drought area. The peak LAIr values in the study period decrease from (a) to (b) to (c).
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Figure 7. Comparison of the LAIr and LAIF for three pixels from dense vegetation to sparse vegetation in the time series. (a) and (b) are the LAIr of vegetation near Wutumeiren river; (c) is the LAIr of vegetation in drought area.

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Figure 7. Comparison of the LAIr and LAIF for three pixels from dense vegetation to sparse vegetation in the time series. (a) and (b) are the LAIr of vegetation near Wutumeiren river; (c) is the LAIr of vegetation in drought area.
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Figure 8. Results of LAIF compared to LAIr in early July (a) and late August (b) 2011.

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Figure 8. Results of LAIF compared to LAIr in early July (a) and late August (b) 2011.
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Figure 9. Comparison between the LAIr, LAIF, and LAIeF values of three pixels from dense vegetation to sparse vegetation in the time series. (a) and (b) are the LAIr of vegetation near Wutumeiren river; (c) is the LAIr of vegetation in drought area.

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Figure 9. Comparison between the LAIr, LAIF, and LAIeF values of three pixels from dense vegetation to sparse vegetation in the time series. (a) and (b) are the LAIr of vegetation near Wutumeiren river; (c) is the LAIr of vegetation in drought area.
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Figure 10. Results of the LAIeF values compared to the LAIm values in early July (a) and late August (b) 2011.

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Figure 10. Results of the LAIeF values compared to the LAIm values in early July (a) and late August (b) 2011.
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Table Table 1. Inputs to the ACRM.

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Table 1. Inputs to the ACRM.
ParametersUnitsSymbolValue
Sun zenith angle(°)θs-
View zenith angle(°)θv-
Relative azimuth angle(°)θraz-
Ǻngström turbidity coefficientβ0.12
Leaf area indexm2/m2LAI/
LAI of ground levelm2/m2LAIg0.05
Mean leaf angle of Elliptical LAD(°)θl60.0
Hot spot parameterSL0.5/LAI
Markov clumping parameterSz/
Refractive indexn0.9
Weight of the first basis functions1/
Leaf mesophyll structureN/
Chlorophyll a and b contentμg·cm−2Cab/
Leaf equivalent water thicknesscmCW0.015
Dry matter contentg·m−2Cm90
Brown pigmentCbp0.4
Table Table 2. Ranges and steps (dx) of sensitive parameters in the ACRM for the NDVI and NIR image-based retrieval schemes.

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Table 2. Ranges and steps (dx) of sensitive parameters in the ACRM for the NDVI and NIR image-based retrieval schemes.
SymbolRange for NDVI-Based Retrieval SchemedxRange for NIR-Based Retrieval Schemedx
LAI0–40.22–70.2
Sz0.4–1.00.20.4–1.00.2
s10.25–0.50.030.05–0.250.03
N1.0–2.00.21.5–2.50.1
Cab30–9020600
Table Table 3. Levels of precision of LAIr, LAIF, and LAIeF compared to LAIm on DOY 177 and 233.

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Table 3. Levels of precision of LAIr, LAIF, and LAIeF compared to LAIm on DOY 177 and 233.
RSSR-SquareRMSE
DOY 177LAIr3.470.680.31
LAIF3.420.670.30
LAIeF3.150.720.29

DOY 233LAIr38.960.730.94
LAIF29.250.770.81
LAIeF27.480.790.78
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