Peijuan Wang

Yield estimation of winter wheat in the North China Plain using the remote-sensing–photosynthesis–yield estimation for crops (RS–P–YEC) model

The accurate prediction of crop yield is of great help for grain policy making. By assuming a horizontally homogeneous, vertically laminar structure and introducing a multilayer-two-big-leaf model, we develop a radiative-transfer equation for winter-wheat canopy and a model, named the remote-sensing–photosynthesis–yield estimation for crops (RS–P–YEC) model, for winter-wheat yield estimation. The yield is calculated by multiplying the net primary productivity (NPP) by the harvest index (HI). In this study, the yield of winter wheat in the North China Plain in 2006 is estimated using the RS–P–YEC model. The simulated yield is consistent with observations from 17 agro-meteorological stations, and the mean relative error is 4.6%. The results demonstrate that the RS–P–YEC model is a useful tool for winter-wheat yield estimation in the North China Plain with widely available remotely sensed imageries.

Influences of Leaf-Specular Reflection on Canopy BRF Characteristics: A Case Study of Real Maize Canopies With a 3-D Scene BRDF Model

The diffuse and specular components of leaf reflection are both important to determine the leaf optical properties as well as to describe the leaf bidirectional reflectance distribution function (BRDF). However, the specular component is usually ignored in practice in numerous canopy reflectance models that describe the interaction between solar light and vegetated scene components. To evaluate the impact of leaf-specular reflection on canopy bidirectional reflectance factor (BRF) characteristics, we introduce a leaf BRDF model into the radiosity-graphics combined model (RGM; a 3-D scene model) to calculate canopy BRFs with nondiffuse leaves. The modified RGM is validated by comparing simulated BRFs against in situ measured BRFs over real maize canopies. The results show that ignorance of leaf-specular reflection can result in up to 50% of relative error in the blue band (435.8 nm). A series of maize canopies with different leaf angle distributions (LADs) is reconstructed to investigate the effect of five major biophysical/geometrical parameters such as leaf area index, LAD, leaf surface property, view direction, and solar zenith angle on leaf-specular reflection contributions to the canopy BRF. It is demonstrated that increasing the incident solar zenith angle and decreasing the mean leaf angle impact the angular distribution of the canopy BRF more significantly than other factors. The cumulative hemispherical relative and absolute errors of canopy BRF caused by the leaf-specular reflection are often too large to be ignored, even for canopies with rough surface leaves. Moreover, the relative error of BRF in visible waveband shows that, in general, leaf-specular reflection has a large impact than that in near-infrared waveband. However, such impact can be sufficiently accounted for by even just consideration of the first-order leaf-specular reflection in canopy reflectance calculation, leading to a substantial improvement in simulation accuracy for most vegetation canopies.

Research on PAR and FPAR of crop canopies based on RGM

PAR and FPAR are two important variables in agricultural field. Some researches show that many factors, such as LAI(leaf area index), LAD(leaf ange distribution) and the heterogeneity of vegetation will affect the distribution of PAR and FPAR. In order to understanding the exchange process of material and energy, Radiosity-Graphics combined Model (RGM) (Qin et al., 2000) is used to simulate the distribution of PAR and FPAR in canopy and some effect factors, such as the structure of canopy and sun zenith angle, can be analyze carefully. PAR and FPAR of a typical winter wheat canopy is simulated and the results are validated with the measured data. They agreed well. Next work is to simulate and analyze several factors of the distribution of PAR and FPAR, including sun incident angle, LAD, LAI, special for the heterogeneous canopies such as that crop with width and narrow ridges which can direct cropping patterns and remote sensing inversion.

Monitoring growth condition of spring maize in Northeast China using a process-based model

Abstract Early and accurate assessment of the growth condition of spring maize, a major crop in China, is important for the national food security. This study used a process-based Remote-Sensing-Photosynthesis-Yield Estimation for Crops (RS-P-YEC) model, driven by satellite-derived leaf area index and ground-based meteorological observations, to simulate net primary productivity (NPP) of spring maize in Northeast China from the first ten-day (FTD) of May to the second ten-day (STD) of August during 2001–2014. The growth condition of spring maize in 2014 in Northeast China was monitored and evaluated spatially and temporally by comparison with 5- and 13-year averages, as well as 2009 and 2013. Results showed that NPP simulated by the RS-P-YEC model, with consideration of multi-scattered radiation inside the crop canopy, could reveal the growth condition of spring maize more reasonably than the Boreal Ecosystem Productivity Simulator. Moreover, NPP outperformed other commonly used vegetation indices (e.g., Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI)) for monitoring and evaluating the growth condition of spring maize. Compared with the 5- and 13-year averages, the growth condition of spring maize in 2014 was worse before the STD of June and after the FTD of August, and it was better from the third ten-day (TTD) of June to the TTD of July across Northeast China. Spatially, regions with slightly worse and worse growth conditions in the STD of August 2014 were concentrated mainly in central Northeast China, and they accounted for about half of the production area of spring maize in Northeast China. This study confirms that NPP is a good indicator for monitoring and evaluating growth condition because of its capacity to reflect the physiological characteristics of crops. Meanwhile, the RS-P-YEC model, driven by remote sensing and ground-based meteorological data, is effective for monitoring crop growth condition over large areas in a near real time.

Research on the polarized characteristics of leaves

The distributions of polarized reflectance from several leaves surfaces are measured by the multi-direction instrument, including corn tender leaf, corn mature leaf, and lilac leaf. The degrees of polarization corresponding to different incident zenith angle and view zenith angle are calculated. Some results can be concluded by comparing the degree of polarization: the degree of polarization will increase with incident zenith angle and view zenith angle. These indicate that non-Lambertian of leaf surface will be distinct with the increasing of incident zenith angle.

Spatial-temporal characteristics of NPP based on processed model from 2002 to 2010 in Gansu Province, Northwest China

It is a complex ecological structure with grassland, forest and agriculture in arid and semi-arid regions in Northwest China. It is important for understanding and evaluating ecological efficiency to study net primary productivity of vegetation in Northwest China. In this paper, Gansu Province was selected in Northwest China to study vegetation NPP based on adjusted boreal ecosystem productivity simulator (A-BEPS) model in arid and semi-arid regions. Net primary productivity of vegetation was simulated in Gansu Province from the year of 2002 to 2010 with moderate resolution remote sensing imageries and meteorological data. And then, spatial-temporal distribution patterns of average NPP were analyzed in Gansu Province from the year of 2002 to 2010. The results show that it is high in the south and low in the north for spatial distribution, and obviously seasonal characteristics are got in Gansu Province. Finally, the trends of averaged NPP for three vegetation types and relative meteorological factors were analyzed for the past nine years. The results of a little decreasing NPP and precipitation are got for Gansu province.

The relationship between canopy parameters and spectrum of winter wheat under different irrigations in Hebei Province

Drought is the first place in all the natural disasters in the world. It is especially serious in North China Plain. In this paper, different soil water content control levels at winter wheat growth stages are performed on Gucheng Ecological-Meteorological Integrated Observation Experiment Station of CAMS, China. Some canopy parameters, including growth conditions, dry weight, physiological parameters and hyperspectral reflectance, are measured from erecting stage to milk stage for winter wheat in 2009. The relationship between canopy parameters and soil relative moisture, canopy water content and water indices of winter wheat are established. The results show that some parameters, such as SPAD and dry weight of leaves, decrease with the increasing of soil relative moisture, while other parameters, including dry weight of caudexes, above ground dry weight, height, photosynthesis rate, intercellular CO2 concentration, stomatal conductance and transpiration rate, increase corresponding to the soil relative moisture. Obvious linear relationship between stomatal conductance and transpiration rate is established with 45 samples, which R2 reaches to 0.6152. Finally, the fitting equations between canopy water content and water indices are regressed with b5, b6 and b7 of MODIS bands. The equations are best with b7 and worst with b5. So the fitting equations with b7 can be used to inverse the canopy water content of winter wheat using MODIS or other remote sensing images with similar bands range to MODIS in Hebei Province.

Simulation for NPP of grassland ecosystem in Qinghai-Tibetan Plateau based on the process model

Qinghai-Tibetan Plateau plays an important role in estimating net primary productivity of grassland ecosystem for global carbon cycling research. In this paper, boreal ecosystem productivity simulator (BEPS) model was modified according to the characteristics of grassland canopy. A hypothesis of horizontal homogeneity and vertical layer was put forward for grassland canopy and BEPS was modified to GEPS (grassland ecosystem productivity simulator) to simulate the NPP of grassland ecosystem. With MODIS products (MOD15A2 and MOD12Q1) and routine meteorological data, net primary productivity of grassland ecosystem was simulated in Qinghai-Tibetan Plateau in 2006 based on GEPS. The result shows that NPP of grassland ecosystem in Qinghai-Tibetan Plateau is between 20 and 500 gC/m2·a, which is close to the other studies. The spatial distribution of NPP of grassland ecosystem in Qinghai-Tibetan Plateau has the trend of decreasing from east to west. Finally the seasonal change of NPP was investigated based on monthly NPP, which has good coherence with the seasonal changes of temperature. This study suggests that the process model - GEPS - is suitable to simulate NPP of grassland ecosystem in Qinghai-Tibetan Plateau.