Spatial-Temporal Dynamics Of The Northern Limit Of Paddy Rice In China,and Its Driving Forces And Impacts On Water Resource

Clearing the pattern of crop spatio-temporal shifts and its driving mechanism is the frontier and hotspot in global change and sustainable development fields.Rice is one of the three staple grains in China.In 2021,rice planting area and production are 29.92 million hectares and 212.84 million tonnes,accounting for 25.4%and 31.2%of the total grain planting area and production in China,respectively.In the past decades,the spatial and temporal distribution of rice in China shifts dramatically with a significant increase in rice area in the north and a decrease in the rice area in the south due to the interaction of climate change and socioeconomic factors such as the rice variety,effective irrigation area,and agricultural policies.Therefore,it is valuable and essential for ensuring national food security,achieving carbon peaking and carbon neutrality in agriculture,and ensuring regional water safety to further analyze the process and pattern of the dynamics of rice distribution in time and space,quantify the mechanism of such dynamics,and evaluate the environmental effects of the spatio-temporal dynamics of rice distribution.This thesis mainly focuses on the spatio-temporal shifts of the northern limit of paddy rice(NLPR)in China and its effects on regional water safety,taking Heilongjiang province as a case.Firstly,based on the spatio-temporal distribution of rice interpreted from Landsat images,we mapped the NLPR in China from 1984 through 2013 using kernel density estimation,and then analyzed the latitudinal and elevational dynamics of the NLPR.Secondly,the spatial panel model was used to quantify the driving mechanism of the shifts of the NLPR in time and space including both natural factors such as accumulated temperature above 10℃and precipitation during the rice-growing season,and socioeconomic factors such as per capita agricultural GDP,rice yield,effective irrigation area ratio,mulch consumption and total power of agricultural machinery input per cropland.Finally,we used the daily soil water balance model to evaluate the water consumption of rice and the effect of total water consumption of rice on regional water resources from 1984 through 2013.Our results showed that:(1)In the past three decades,the average latitude of the NLPR moved northward from 47.34°N to 47.64°N,and the NLPR moved towards a higher latitude and altitude.Along the latitudinal gradient,the NLPR shifted 24.93 km northward(the greatest movement was 88.01 km occurring at approximately 133°E)from 1984 through 2013.The Sanjiang Plain showed the most notable northward shifts in the NLPR.Along the altitudinal gradient,elevational limits increased by 39.15 m(the greatest movement was 117.08 m occurring at approximately 129°E,south of the Lesser Khingan Mountains)in the past three decades.The largest movement occurred during the 1990s along the latitude and altitude.(2)Increased temperature made the suitable rice planting area in Heilongjiang province expand to higher latitude regions,while economic motivation and technological improvement made rice planting expand northward and eastward.That led to the northward shifts of the NLPR so that rice planting can adapt to climate change.Among all natural and socioeconomic factors,the temperature had the strongest positive association with the changes in rice area across the NLPR region.Each 1%increase in the temperature of the previous growing season was expected to increase the rice planting area by2.24%.Socioeconomic factors played significantly positive roles in rice expansion,in which the improvement of irrigation facilities played the most significant role,followed by the incentive of related agricultural policies,the increase of rice yield,and the advancement of greenhouse nursery technology.(3)In the past 30 years,the total water consumption per m~2 of rice planted in the NLPR region showed the spatial characteristics of more water consumption per m~2 in the west and north but less in the east and south,and unnoticeable temporal change.For blue water consumption per m~2 of rice planted,it showed the same spatial characteristics as the total water consumption per m~2,while the temporal change of it from 1984 to 2013 was little even though it increased first and then decreased.For green water consumption per m~2 of rice planted,it was less in the west than in the east,but no significant difference in the south and north.The temporal variation is little even though it decreased first and then increased.For the high blue water consumption ratio,it increased first and then decreased in time,and showed spatial characteristics of more in the west than in the east,but with no obvious difference between the north and south.(4)In the past 30 years,the total water consumption of rice planted in the NLPR region showed a significantly increasing trend,which mainly resulted from rice expansion.Rice expansion even led to the consequence of the overexploitation of groundwater in regions where rice planting is concentrated.From 1984 through 2013,the total water consumption of rice in the NLPR region increased by about13.40 billion m~3,while the total consumption of blue water increased by 8.17 billion m~3.The most significant increase in total water and blue water consumption happened in Sanjiang Plain from 2000 to2013.The increase in rice area contributed 99.69%to such an increase in total water consumption,while the increase in total water consumption per m~2 of rice planted only contributed 0.31%.In the Sanjiang Plain,rice cultivation is highly concentrated with a high proportion of groundwater pumping,which resulted in the overexploitation of groundwater in the parts of Sanjiang Plain.