Spatiotemporal Change Of Cultivated Land And Its Impact On Agricultural Water Resources In The Upper Reaches Of The Yellow River

Water resources are important for human survival and development,and the rational utilization of water resources is a universally important issue all around the world.China,with extremely uneven temporal and spatial distribution of water resources,is one of the countries with serious issue of water resources shortage in the world.And the economic and social development of China is constrained by the water resource shortage.Cultivated land is the basis of human survival.Irrigation cultivated land produces about 40%of the total food production,with the area 20%of global cultivated land.The distribution and evolution of cultivated land has an important impact on the utilization of regional water resources and the development of ecological environment,Unreasonable utilization of cultivated land will lead to a series of environmental problems.The Yellow River Basin is an important ecological barrier area in China,but the water resource shortage has seriously restricted the high-quality development of this region.The upper reaches of the Yellow River are the main water producing areas of the Yellow River Basin,which produces 60.3%of the whole water resources,while agriculture consumed 40%of the produced water resource.The low output and the low utilization efficiency of water resources in agriculture further aggravate the regional water environment problems.Therefore,it is of great significance to study the spatiotemporal changes of cultivated land and their impacts on water resources in the upper reaches of the Yellow River in the past decades for the rational management of water and soil resources,the formulation of reasonable water resources utilization and scientific planting structure adjustment planning.In order to explore the influence of main crop planting area change on water demand in the region from Longyangxia to Hekou town(Longhe region),the distribution of cultivated land in Longhe region was extracted based on satellite remote sensing data combined with crop phenological data.And the water requirement of main crops was calculated based on Penman-Monteith formula and meteorological data.According to the statistical data about main crop planting structure,the water demand of main crops in Longhe region was comprehensively analyzed.The main conclusions are as follows:(1)The spatial-temporal variation distribution map of cultivated land in Longhe region was plotted,based on the compiled rules of remote sensing data using the decision tree classification method.The confusion matrix verification was carried out by using the real ground object samples obtained based on field investigation,and the extraction accuracy of cultivated land was above 75%.Further,the county-level statistical data was used to verify the extraction results,with the R~2 0.85.The cultivated land in Longhe region is mainly distributed along the river and valley.The cultivated land in Longhe region increased from 2002 to 2018,with increase of 8821 km~2 in the total region,1889 km~2 in Gansu,2993 km~2 in Ningxia,and4474 km~2in Inner Mongolia,and with decrease of 536 km~2 in Qinghai,respectively.The average annual planting area of main crops in the Longhe region is 6327 km~2,showing an increasing trend at a rate of 299 km~2/a.Wheat and rape planting area showed a downward trend,while other crops showed an upward trend.(2)The average annual water requirements of wheat,maize,rape,rice and sunflower were505.6mm,543.5mm,478.8mm,716.2mm and 534.7mm,respectively.The spatial distribution of water requirements within wheat and maize was similar,mainly higher in the north and lower in the south.The distribution of sunflower's water requirement presents a north-south band,decreasing from west to east.The water requirement of rape is highest in the Guide-Minhe Line,gradually decreasing to north or south,while the water requirement of rice decreased in a circular way,highest in Hongsibu.The average annual evapotranspiration and increasing rates of wheat,maize,sunflower,rice and rape were 295.4mm,311.1mm,251.3mm,342.7mm and380.3mm,and 2.1mm/a,2.3mm/a,1.8mm/a,4.9mm/a and 1.6mm/a,respectively.The annual average water requirement of all crops in Longhe region is 555.8mm,with an increasing trend at the rate of 0.5mm/a.whereas they are 316.2mm and 0.5mm/a in terms of evapotranspiration.(3)On the whole,the average annual irrigation water demand in the Longhe region is 13.3billion m~3/a,with an increasing trend.Agricultural water consumption was affected by the cultivated land evolution.The agricultural water consumption decreased by 1.8 billion m~3and0.07 billion m~3 caused by the planting area decrease of wheat and rape,respectively.It increased by 2.71 billion m~3 and 0.8 billion m~3 caused by the planting area increase of maize and combined rice and sunflower,respectively.