| Agricultural information is the basis for agricultural production and agricultural drought relief management, in the wake of introduction of remote sensing technology, the monitoring for agriculture situation become more real-time, comprehensiveness and reliability. To explore the applicability and advantages of remote sensing technology in crops information extraction and agricultural drought research, the winter wheat of Beijing-Tianjin-Hebei region was selected for the study in this paper. The MODIS data, meteorological data, wheat growth indicators, traditional classification and mixed-pixel decomposition method was used to extract and estimate winter wheat area in the region. Then, we selected vegetation index and land surface temperature to build TVDI drought index, and to monitor winter wheat drought situation in different growth stage and 11 consecutive years respectively. The main conclusions are:(1) It is feasible for crop area monitoring over a large area with MODIS data. We identified a combination of the best time to extract winter wheat area. These are October (Seeding), November (Tillering) and next year from May to June (Heading to Maturity). The results of the winter wheat planting area estimation accuracy showed that the winter wheat distribution trend is consistent with the actual planting distribution trend. And the accuracy of the test results are reached 82% which was compared with the actural winter wheat field data, TM image and Google earth image, obviously meet the accuracy requirements, and the method has a strong practical value in estimating the area of crop;(2) We extracted the winter wheat area in 2002 and 2012 respectively. The results showed that the total winter wheat area decreased of study area in nearly 10 years, area decreased by about 0.7%. The planting area were changed differently in all cities. Cornfield distribution of winter wheat showed a gradual centralization trend. Although some cornfields are reduced, but for most winter wheat areas, especially in central and southern regions, the wheat area showed a large area of contiguous planting phenomenon;(3) Vegetation index and land surface temperature were used to build the two-dimensional feature spaces of NDVI and LST and draw dry edge and wet edge. TVDI which was calculated according NDVI-LST feature space has significant negative correlation with surface soil relative humidity and the correlation coefficient got through 0.05 level test in April 22,2012; Meanwhile, by comparing the monitoring results and rainfall data in whole growth stage, we concluded that the TVDI can monitor actual drought changes well in long time series, and TVDI can be greatly used in evaluating the drought condition in the study area. From early October (Seeding) to early April (Jointing), drought conditions were continued severe in Beijing-Tianjin-Hebei region. The drought and water shortage area were more than 60% for the whole area. From early April (Jointing) to early June (Maturity), the drought have eased progressively, and soil moisture was gradually improved. The drought overall showing gradually increasing trend from north to south in Beijing-Tianjin-Hebei region;(4) Through adjusting drought grading standards, we obtained the level of distribution of winter wheat drought. From tillering to wintering of winter wheat, the planting area almost fully rendered water shortage and arid status. From entering the jointing stage, wheat field drought situation was mitigated from north to south, the water shortage and arid area decreased. From heading to milk-ripe stage, most winter wheat plots’s drought degree were normal and humid, arid land area were rare, and the drought was significantly improved;(5) The degree of matching is poor for precipitation and water demand in all growth stages of winter wheat, especially in heading, the gap is significantly increased, and it showed a severe water shortage. The soil moisture of main winter wheat producing areas could achieve appropriate or moist, much better than non-winter wheat area. To some extent, it reflected the local focus on strengthening agricultural irrigation and drought resistance management. From 2009, the cornfield drought situation was more serious than that in previous years. For the spatial distribution, soil moisture in winter wheat area of piedmont of Taihang Mountain sustained significantly better than that in Hebei Hebei Low Plain winter wheat area. The existence of the phenomenon was that catcher irrigation was inadequate and unbalanced. Through the correlation analysis between TVDI and winter wheat yield, TVDI was significant negative correlation with winter wheat yield. The temporal and spatial distribution of winter wheat yield was also showed based on TVDI. It provides a scientific basis for effective and timely forecast of winter wheat production situation in the region. Meanwhile, it provided a new idea for the winter wheat yield estimation. |
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