| Dry-hot wind is the main disaster weather during wheat flowering stage in north China, and its frequency is high and damaged area is large. Dry-hot wind leads to a serious produce of wheat production and threats to the national food security in China. Extensive research about dry-hot wind which focused on dry-hot wind index has been conducted. The index was only meteorological indicator and it could not fully reflect the damage effect on wheat growth or development process from dry-hot wind without reflecting wheat growth or soil moisture conditions. Satellite remote sensing has wide monitoring range and time phases, which has been widely applied to monitor the crop growth and soil moisture. Due to the high temporal resolution and high spatial resolution of MODIS data, as well as its advantages of free access, a number of studies used temporal vegetation index NDVI/EVI data to monitor crop growth condition. In addition, based on the principle that evaporation and vegetation transpiration decrease surface temperature, TVDI(Temperature Vegetation drought Index) can reflect the humidity of the soil. The method is accurate, simple and easy to implement, which has been mature and widely used at home and abroad. Therefore, we can quickly monitor winter wheat growth and soil moisture conditions during dry-hot wind based on MODIS data in large area.In this paper, MODIS EVI was applied to estimate the winter wheat area on HHH in 2010 using a model built according to the seasonal change pattern of winter wheat EVI. Considering the influences on winter wheat from latitude increasing with and without monsoon climate, we divided the study area in four zones paralleling and inclining to latitude lines and extracted winter wheat of these zones respectively. By comparing the extraction of no-zoning, parallel-zoning and inclining-zoning, we found inclining-zoning extraction has highest accuracy, with overall accuracy 91.0%, and deviation 4.1. So the accuracy was enough for winter wheat extraction.Then the accumulated EVI of every zone from reviving stage to mature stage was used to reflect winter wheat growth in 2010 and it was divided into three levels including the better, the normal and the worse. By analyzing spatial distribution of accumulated precipitation of every site during the winter wheat growth period, we found it was consistent with spatial distribution of winter wheat growth in 2010, and they had the obvious correlation(R2 = 0.3407), which verified growth monitoring results from a certain degree.Then TVDI was used to assess the moisture of every zone during dry-hot wind period, and the monitoring was compared with no-zoning assessment. The result was verified by the measured soil relative humidity data and it showed that the relationship between zoning TVDI and the soil relative humidity in depth of 10cm、20cm and 50cm( R2 was 0.4674,0.3238 and 0.2263, respectively) was much better than that of no-zoning(R2 was 0.3067、0.2365、0.1394, respectively). Then the condition of soil moisture was divided into three levels including the wet, the normal and the dry.Finally we analyzed meteorological data of each site of HHH plain in 2010 during the dry-hot wind to obtain the distribution of light and heavy dry-hot wind according to yearly indicator. Because the winterwheat growth and soil moisture reflected the influence on winter wheat from dry-hot wind together, they were overlapped. Then we did the damage assessment of dry-hot wind on winter wheat combining the overlap data with the dry-hot wind data, and divided the influence on winter wheat from dry-hot wind into "free" "very mild" "mild" "heavier" "severe " "heaviest " six degrees. And we found that mild and heavier influence was most, slight and severe impact was less, and free and heaviest influence was least. Because soil moisture was dry in a wide range, corresponding to worse winter wheat growth, some light dry-hot wind has heavier effect on winter wheat.So we assessed the effect on winter wheat from dry-hot wind effectively, which could provide help for dry-hot wind warning and prevention. |
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