Joint Assimilation of Leaf Area Index and Soil Moisture from Sentinel-1 and Sentinel-2 Data into the WOFOST Model for Winter Wheat Yield Estimation

Abstract

It is well known that timely crop growth monitoring and accurate crop yield estimation at a fine scale is of vital importance for agricultural monitoring and crop management. Crop growth models have been widely used for crop growth process description and yield prediction. In particular, the accurate simulation of important state variables, such as leaf area index (LAI) and root zone soil moisture (SM), is of great importance for yield estimation. Data assimilation is a useful tool that combines a crop model and external observations (often derived from remote sensing data) to improve the simulated crop state variables and consequently model outputs like crop total biomass, water use and grain yield. In spite of its effectiveness, applying data assimilation for monitoring crop growth at the regional scale in China remains challenging, due to the lack of high spatiotemporal resolution satellite data that can match the small field sizes which are typical for agriculture in China. With the accessibility of freely available images acquired by Sentinel satellites, it becomes possible to acquire data at high spatiotemporal resolution (10–30 m, 5–6 days), which offers attractive opportunities to characterize crop growth. In this study, we assimilated remotely sensed LAI and SM into the Word Food Studies (WOFOST) model to estimate winter wheat yield using an ensemble Kalman filter (EnKF) algorithm. The LAI was calculated from Sentinel-2 using a lookup table method, and the SM was calculated from Sentinel-1 and Sentinel-2 based on a change detection approach. Through validation with field data, the inverse error was 10% and 35% for LAI and SM, respectively. The open-loop wheat yield estimation, independent assimilations of LAI and SM, and a joint assimilation of LAI + SM were tested and validated using field measurement observation in the city of Hengshui, China, during the 2016–2017 winter wheat growing season. The results indicated that the accuracy of wheat yield simulated by WOFOST was significantly improved after joint assimilation at the field scale. Compared to the open-loop estimation, the yield root mean square error (RMSE) with field observations was decreased by 69 kg/ha for the LAI assimilation, 39 kg/ha for the SM assimilation and 167 kg/ha for the joint LAI + SM assimilation. Yield coefficients of determination (R2) of 0.41, 0.65, 0.50, and 0.76 and mean relative errors (MRE) of 4.87%, 4.32%, 4.45% and 3.17% were obtained for open-loop, LAI assimilation alone, SM assimilation alone and joint LAI + SM assimilation, respectively. The results suggest that LAI was the first-choice variable for crop data assimilation over SM, and when both LAI and SM satellite data are available, the joint data assimilation has a better performance because LAI and SM have interacting effects. Hence, joint assimilation of LAI and SM from Sentinel-1 and Sentinel-2 at a 20 m resolution into the WOFOST provides a robust method to improve crop yield estimations. However, there is still bias between the key soil moisture in the root zone and the Sentinel-1 C band retrieved SM, especially when the vegetation cover is high. By active and passive microwave data fusion, it may be possible to offer a higher accuracy SM for crop yield prediction.

Coupling remote sensing and crop growth model to estimate national wheat yield in Ethiopia

“…Our findings are in agreement with previous studies (e.g. Pan et al, 2019;Ines, Das, Hansen, & Njoku, 2013). Pan et al (2019) assimilated the Soil Moisture (SM) and LAI from sentinel-1 and 2 into the WOFOST and the result showed that the RMSE indices decreased by 69, 39 and 169 kg ha −1 after assimilating LAI, SM and the combination of them, respectively.…”

“…Pan et al, 2019;Ines, Das, Hansen, & Njoku, 2013). Pan et al (2019) assimilated the Soil Moisture (SM) and LAI from sentinel-1 and 2 into the WOFOST and the result showed that the RMSE indices decreased by 69, 39 and 169 kg ha −1 after assimilating LAI, SM and the combination of them, respectively. Similar studies in other countries such as China and the United States have reported an increase in the estimation accuracy of yield by assimilating MODIS LAI into the Decision Support System for the Agrotechnology Transfer-cropping System (DSSAT-CSM)-Maize model and improved the accuracy from 0.47 to 0.51 (Ines et al, 2013).…”

New Approaches for the Assimilation of LAI Measurements into a Crop Model Ensemble to Improve Wheat Biomass Estimations

“…Based on these measurements, the model can be modified and used to make predictions about future states of the crop [9]. A range of different observations, either field measurements or derived from remote sensing, have been assimilated into crop models: phenology [10,11], soil moisture content [12][13][14][15][16][17], canopy cover [18,19], and, most-frequently used, leaf area index (LAI) [10,[14][15][16]18,[20][21][22][23][24][25][26][27][28]. Defined as the total one-sided area of leaf tissue per unit of ground surface area (provided in m 2 m −2 ), LAI is one of the key parameters in crop growth analysis due to its influence on light interception, biomass production, plant growth and ultimately on crop yield, and it is critical to understand the functioning of many crop management practices [29,30].…”

Rice growth vegetation index 2 for improving estimation of rice plant phenology in costal ecosystems

“…Sentinel-2 has a 5 day revisit cycle with high resolution, which is appropriate for crop mapping at field level. The Sentinel-2 improved spatial and temporal resolution that is beneficial for farmers and academic researchers focusing on agricultural development [24,25,26]. The China agriculture field sizes are typically less than 1 hectare, so high spatial resolution acquaintance is essential for evaluating crop information for decisionmaking.…”