Spatial And Temporal Evolution Of Land Use In The Mekong Delta Of Vietnam In The Past 40 Years

As the third largest delta in the world,the Mekong delta is a globally recognized hotspot owing to agricultural production and biodiversity.Land use/Land cover(LULC)in the Mekong delta has undergone dramatic changes in recent decades due to increased human activity and rapid economic development.Understanding the temporal and spatial variation of land use and land cover change(LUCC)in the Mekong delta is helpful to explore the relationship between human activities and land use change.However,there is a lack of accurate land use data for a long time series throughout the delta owing to limitations of geographical environment and climatic conditions.Considering the shortcomings of the current research,this study used Landsat and Chinese HJ-1B satellite images over the past 40 years to draw the LULC maps of the Mekong delta in 7 years from 1979 to 2015 with classification regression tree(CART).With the help of the Google Earth Engine(GEE)cloud platform,time series classification method based on MODIS images was used to complete more detailed classification of large-scale rice planting areas in the past 20 years.Based on the LULC results,the main driving factors of the change were analyzed.The main research results and conclusions of this paper are as follows:(1)The land use types of the Mekong delta in this study mainly include planting land,aquaculture,mangrove,other forests,residential land,waste land and unused land.From 1979 to 2015,the seven major LULC categories have undergone varying degrees of changes,with an average annual change of 2.1%.The area of aquaculture and residential land in the Mekong delta as a whole increased,while the area of mangrove,planting land,forest,wasteland and unused land decreased.The spatial and temporal variations of various features have obvious regional characteristics,except that the residential land and planting land were distributed in the whole Mekong delta,other features mainly took the Mekong river as the boundary.In the south of the Mekong river,land use conversion mainly took place among abandoned land,planting land,aquaculture and mangroves,while in the south of the Mekong river,it mainly occurred between planting land,unused land and other forests.(2)There are four main types of rice-crop in the Mekong Delta: single-season rice,double-season rice,triple-season rice and aquaculture-rice complex culture,which account for more than 70% of all land use types and about 60% of the whole delta.From 2005 to 2015,the area of rice-crop increased slightly and then decreased,among which the area of double-season rice and triple-season rice accounted for a larger proportion,and the conversion between these two land uses was more significant.The triple-season rice expanded outward from the Mekong river and the Bassac river after2009,while the area of single-season rice and complex cultivation decreased significantly.By estimating the heading dates and growth period,the results showed that the closer to the upper reaches of the Mekong Delta the more the heading dates occurred in the dry season and the average growth period of each year was decreased after the increase,with 100 days of the overall average growth period from 2005 to 2015.(3)The rapid changes of LULC in the Mekong Delta are mainly caused by economic and policy factors,hydrological system factors,defoliant in Vietnam War and other factors.Driven by economic and policy,agricultural activities including aquaculture and agricultural cultivation in the Mekong Delta occupied part of the forest resources and grow rapidly.Flood,saltwater intrusion,water conservancy infrastructure were the main reasons for the land use change of rice in the inland area of the delta.The negative effects of defoliant on land use decreased gradually,and the strip wasteland was almost completely recovered.In addition,natural environmental factors such as sea level rise,extreme weather,acid sulfate soil and acid water are also influencing the Mekong Delta.The results of this study will provide valuable insights into the LUCC and the impact of economic development,policy regulation and environmental factors on the entire Mekong delta over the past 40 years.With climate change and the intensification of human activities in future,the sustainable development,ecology and food security of the delta region will face more severe challenges.This study will provide basic data and reference for the sustainable development planning and ecological evaluation of the Mekong delta region.