| Food security and water security are the prerequisites for national socio-economic development.Accurately monitoring spatiotemporal distribution and change of crops assists in a better decision-making for regional agricultural production management and food security.The method of crop classification using remote sensing images,combined with decision tree classification,has been widely used due to its advantages including clear physical mechanism,easy implementation,high accuracy and efficiency.Consequently,it is important to reveal the influence of spatiotemporal changes of crop planting scale on crop water demand and water resources,which is critical for sustainable agriculture development and efficient utilization of water resources.This study selected MODIS-based NDVI,EVI,LSWI and statistical yearbook data to constructed a decision tree model,the aim of which is to extract the spatial distribution of rice,maize and soybean in the Songhua River Basin.Furthermore,this study analyzed the spatial variation characteristics of major crops in the basin;calculated the water requirements of different crops based on the Penman-Monteith model in combination with the crop coefficient method;calculated the water deficit and water deficit index of different crops in the basin based on effective precipitation,and finally analyzed the changes of water requirements and water deficit of crops under different cropping structures.The results are expected to provide the scientific guidance for the adjustment of cropping structure and the rational allocation of water resources in the basin.The main conclusions are as follows:(1)MODIS data was used to extract the spatial distribution of crops in the Songhua River basin at 5-year intervals from 2000 to 2020,and the crop classification results were verified by combining statistical yearbook data and field sampling data.The overall classification accuracy was>80%,80%,87%and 71%for rice,maize and soybean,respectively.(2)From 2000 to 2020,the area of crop planting in the Songhua River basin has doubled and the planting structure changed greatly.The sown area of rice increased significantly,with an increase of 209.2%;from 2000 to 2015,the proportion of maize planting continued to increase,and in 2015 the positive policy was proposed to adjust the planting structure.After that,the maize is fluctuating at a downward trend,but maize sown area was the greatest;soybean sown area was dominated by first fluctuating increase and then decrease during 2000-2015.The national policy encouraged the planting of soybean and results in its increase in the early period.In terms of spatial distribution,the rice is mainly distributed near rivers such as the Songhua River,Second Songhua River and Nenjiang River;the maize is mainly distributed in the south of the basin;the soybean is mostly planted in the north of the basin.The spatial distribution pattern of crops in the basin acted as the characteristic of"soybean in the north,maize in the south,and rice near rivers".(3)The increase of sown area and the change of planting structure caused a significant change of total water demand in the Songhua River Basin from 2000 to 2020 in the basin,and the total water demand in the basin increased from 2336.29×10~8 m~3 in 2000 to 2874.26×10~8 m~3in 2020.The water demand in the maize planting area in the south of the Songhua River Basin is significantly higher than that in the north,and the water demand near the river is significantly higher than that in other areas.The water demand in the southern part of the Songhua River Basin is significantly higher than that in the north.The annual average fertility water demand of rice,maize and soybean over 21 years is 573 mm,466 mm and 400 mm,respectively.The water demand of rice,maize and soybean in the basin is overall decreasing from 2000 to 2020.(4)The total water deficit of crop in the Songhua River Basin from 2000 to 2020 is high heterogeneous spatially and varies greatly from year to year,with a huge water deficit of166.42×10~8 m~3 in the basin in 2007,and without water deficit in most of the Songhua River Basin in 2019 and 2020.The precipitation in the basin is not synchronized with the timing of water demand for soybean growth,thus resulting in insufficient water supply in the middle of soybean growth period.The phase with high water demand for rice and maize are synchronized with rainy season,but the effective precipitation is far from sufficient to meet the water required for crop growth. |
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