Evaluation Of Paddy Rice Disaster In Post-Typhoon Period Based On Google Earth Engine

As one of the countries that most severely affected by typhoons,paddy field in coastal area of China has long been influenced by typhoons.The strong winds,excessive rainfall brought by typhoons can cause the rice plants lodging or be flooded,which has a serious impact on rice yield.Thereby,it is imperative to obtain accurate,rapid disaster information of paddy fields after the passage of typhoon.It is difficult to obtain comprehensive information of rice disaster after typhoons,because of the sudden occurrence and wide impact of typhoons.At present,there are many studies on typhoon disaster assessment methods,but they mainly focus on the risk assessment of typhoon disasters and the extraction of flooded areas.They can't obtain detailed information on rice disaster status,such as rice growth status(lodging or upright),flooded depth and so on.In addition,previous studies lacked an effective assessment of rice plants recovery after disasters,therefore it was difficult to reflect the real situation of rice disasters.In this paper,based on the platform of Google Earth Engine(GEE),the real-time inversion of rice flooding depth and the identification of rice disaster types after the passage of typhoons are realized by combining the field experiment of rice flooding on the ground and multi-temporal Moderate Resolution Imaging Spectroradiometer(MODIS)remote sensing data.Specific research contents and results are as follows:(1)In this study,the flooding and lodging simulation experiments of rice were carried out to simulate different disaster scenarios of rice.And the hyperspectral reflectance of erect and lodged rice under different flooding depths was measured.With the experiments results,the response of various spectral derivative indices,such as Normalized Difference Vegetation Index(NDVI),Enhanced Vegetation Index(EVI)and Modified Land Surface Water Index(MLSWI),of different flooding depth were analyzed.Finally,the index changes sensitive to flooding depth were selected as factors to establish the estimation models of vertical and lodging rice flooding depth,and the flooding model was further applied to MODIS reflectance products to realize post-typhoon research.Furthermore,nearly real-time mapping of rice flooding depth in the study area were achieved.The rice flooding depth model has been tested by Typhoon Fitow the 23th typhoon in 2013.The results show that the flooding depth retrieved by remote sensing is significantly related to the accumulated precipitation during the typhoon(R~2=0.58),which proves the reliability of the proposed flooding model in this paper.(2)For post disaster evolution,Rice Recovery Growth Index(RGI)was designed using multi-temporal NDVI time series to identify and quantitatively evaluate the recovery degree of post-disaster restored growing rice.At the same time,combining the fragility Curves(FCs)model of rice vulnerability curve,RGI,and NDVI fluctuation characteristics before,during and after the passage of typhoon,the rice disaster was classified and identified,and the disaster area of rice was further obtained.This method has passed the test of four typhoons,including Typhoon Fitow in 2013,Typhoon Kalmaegi in 2014,Typhoon Dujuan in 2015 and Typhoon Mujigae in 2015.Finally,by analyzing the distribution correlation between the maximum wind speed,cumulative precipitation and RGI values during typhoon,the results show that there is a significant correlation between the recovery growth degree of rice after disaster and the disaster-causing factors in four cases(R~2=0.58,R~2=0.53,R~2=0.46,R~2=0.43).In addition,there is a significant correlation between the disaster area of typhoon rice and the actual disaster area of rice(R~2=0.7)in four cases estimated by the study,which further demonstrates the reliability of the rice disaster assessment method proposed in this paper.