| The middle reaches of the Heihe River Basin is located in northwest arid areas,with rare precipitation and large evapotranspiration.In recent years,with the increase of population and the area of arable land,agricultural irrigation water has seriously occupied the ecological water use,and due to the downstream ecological conservation and restoration projects,the ecological environment of Heihe river began to degenerate.It has the great significance to study the ecological water demand at the basin scale for rational water allocation and the sustainable development of the ecological environmental system in the middle reaches of Heihe river.Based on the remote sensing satellite image data,combined with the contemporaneous land cover types and ground data,this paper analyzed the change characteristic of the normalized difference vegetation index(NDVI)at spatial and temporal scale of recent 11 years.According to the spatial position of water demand,the ecological environmental water requirement(EWR)can be divided into EWR of inner river and EWR of outer river.And this paper adopted the following methods to estimate EWR,including the vegetation index method for vegetation EWR,the area quota method for the saline-alkali soil EWR,the maximum average monthly sediment concentration method for the river sediment transport EWR,the water evaporation method for the water surface of river,lake and wetland EWR,water balance method for river leakage EWR and minimum average monthly flow method for river basic EWR.The main results are as follows:(1)From 2001 to 2011,the average value of NDVI in the middle reaches of the Heihe River was between 0.17 and 0.23,and an observable increasing trend exists.From the view of spatial distribution,the area of covered by vegetation accounted for 65.53% of the total study area,no vegetation coverage area accounted for 34.47% of the total area.According to the analysis results of land cover types,the area of unused land in the middle reaches was 56% of total area,the proportion of vegetation and cultivated land was 20.26% and 19.33% respectively,and the area of water was only 3.12%.NDVI showed a strong seasonal variation throughout the year,and also there was a difference between various vegetation types.In general,the NDVI of the Arbor forest land was the highest during the year,and the NDVI values were decreased from Shrub forest,Sparse woodland,High cover grassland,Middle cover grassland,Other forest to Low cover grassland.From April to June,the NDVI sharply increased,and reaching the highest level in July.From the spatial distribution of NDVI of forest land and grassland,the forest land had higher NDVI value in Zhangye and Linze,the grassland were well developed along the riverine and the boundary of the middle reaches.(2)Based on the NDVI,the results of the estimation showed that the ecological environmental water requirement of the vegetation was 577 million ?.The results showed that the total ET vary from different land cover types,with the high ET value in Arbor forest,about 342.4mm,in contrast to low ET value in Low cover grassland,about 211.2mm,among them were Shrub forest,Sparse woodland,High cover grassland,Middle cover grassland,Other forest,and the evapotranspiration was 330.0mm,313.9mm,270.6mm,219.0mm,214.2mm,respectively.The EWR was 4.49 million ? of Arbor forest land,22.21 million ? of Shrub forest land,19.64 million ? of Sparse forest,0.72 million ? of Other forest land,9.31 million ? of High cover grassland,68.79 million ? of Middle cover grassland,451.44 million ? of Low cover grassland.The ecological environmental water requirement of different types of vegetation was mainly concentrated in June to August,accounting for about 65% of the total vegetation EWR during the growing period.(3)In the study area,the EWR of lakes-wetlands,river surface evaporation was 206 million ?,144 million ? respectively.The water requirement for the prevention and control of cultivated land salinization was 20 million ?.The basic ecological water demand of the river is 415 million ?,which accounts for about 26% of the total amount of water in the Ying Luoxia Hydrological Station.The water demand of sediment transport was 413 million ?,and it showed that the water demand in the flood seasons are higher than those in non-flood seasons,the water demand in flood seasons(from June to September)was 287.5 million ?,accounting for 69.6% of the total water demand for transporting sediment.The leakage of river way in the study area was 219 million ?,accounting for 11.90% of the total amount of water in the Ying Luoxia Hydrological Station.(4)According to the calculation of EWR,in the middle reaches of the Heihe River,the total inner river EWR was 778 million ?,the EWR outside the river was 803 million ?.By the different conservation objectives,the total amount of EWR can be estimated according to three scenarios: the water demand of ensure the natural ecosystem was 1.560 billion ? water,to satisfied the overall development of ecosystem require 1.607 billion ? water,the EWR of maintain the growth of vegetation require 577 million ?.In the monthly distribution of water requirement,the peak of the water demand was during May to September.In July,the water demand reach the highest level.Take the runoff of Ying Luoxia station in 2011 as an example,in the period of May to July,the monthly runoff can not meet the requirements of the ecological environmental water demand.More strict water saving or diversion measures are needed in the middle reaches of Heihe river basin. |
PDF Full Text Request