Land-use And Land-cover Change And Its Impacts On Evapotranspiration In Inland River Basin Of Arid Region

As the second largest inland river in China,Heihe river basin(HRB)has been paid much attention as a hot spot region of remote sensing,hydrology,ecology and land-cover research field.The quantitative detection of land-use,land-cover status and its change could be of great reference to the allocation of future land resources.Evapotranspiration(ET)plays a key role in the water balance and energy balance,especially in arid river basin.Previous studies had took substantial attention to detecting land-cover and simulating or estimating ET for different ecosystems,but little was focused on the impacts of land-cover change on the ET.The response of ET to land-cover change in HRB remains unclear.This study aims at quantitatively estimating the change of ET derived by land-use and land-cover change in HRB.Firstly,the land-use and land-cover changes from 1990 to 2016 were investigated by using remote sensing technique.Then,the water,heat and energy characteristics of different land covers were simulated by using EALCO model.Further comparison and analysis of ET for these different land covers were also conducted at different time scale.Thirdly,the spatial ET in 2000 and 2010 over the whole region were modeled respectively.Finally,ET response to the land-cover change from 2000 to 2010 was estimated.The results show that:(1)The 'hard' and 'soft' methods are proposed to classify the complex land-cover schemes in HRB.The overall accuracy of hard classification in 2016 is 89.87%.The area of cultivated land and impervious surface area(ISA)had been continuously increasing driving by the process of land reclamation and urbanization respectively.The area of permanent ice was substantially decreased because of the warmer climate in the up reach.There is a significant difference for the land-cover change between the period before the start year of water allocation(2000)and the period after 2000.The area of desert vegetation,populous forest and lake was dramatically decreased during 1990 to 2000 while their areas were generally recovering after 2000.(2)A promising method is proposed in detecting irrigation attributes such as frequency and timing which have not been explored in previous research.The overall accuracy of water supplement stage using the proposed method is 87%.Validation of the number of water supplement stages and possible dates of water supply with a "strict"(or "loose")assessment method show an overall accuracy of 55%(94%)and 59%(89%),respectively.The good accuracy of an additional independent validations for different years and sites demonstrates the robustness of the proposed method,suggesting the general applicability to other arid agricultural regions.(3)The good accuracy of simulated net radiation,latent heat and sensible heat at both half-hour scale and daily scale indicates EALCO model can well simulated the water,energy and heat exchange in HRB.The peak value of daily ET of non-desert vegetation is much higher than that of desert vegetation and non-vegetation.The length of ET peak days in the sites which does not suffer the stress of cold temperature or drought is longer than that in the sites which suffers those stress.The water consumption of crop and wetland through ET is much larger than that of other land covers which are slightly or merely affected by human activities.The difference of monthly ET among all the sites is also obvious.For example,wetland basically ranks the largest ET in each month.July has the largest ET at all the sites.For the yearly scale,wetland ranks the top position with 796 mm.Following with wetland,the cultivated site-Daman has yearly ET of 619 mm.The sand desert site,Bajitan,has the lowest ET with only 113 mm in the whole year.(4)Evaporation dominates before planting while transpiration dominates after planting in the irrigation mode.On the contrary,evaporation almost dominates the entire year in the non-irrigation mode,with only slight transpiration in the summer.The ET during the growing period in the igation mode are both much larger than that in the non-irrigation mode.The ET is 619 mm in the whole year with irrigation mode,which is 361 mm larger than that in the non-irrigation mode.(5)The spatial heterogeneity of yearly and seasonal ET in 2010 is obvious,characterized by high ET in the up and middle reach while low ET in the low reach.Conversions from tundra to grass in the up reach,baresoil and desert grass to cropland in the middle reach and baresoil to desert vegetation in the low reach caused an increased ET from 2000 to 2010 while conversions from rangeland to desert grass in the up reach,baresoil,desert vegetation and cropland to ISA in the middle reach resulted in a decreased ET during the same period.The conversion from baresoil vegetation to waterbody has the most spatial-averaged increased-ET with 900 mm,followed by the conversion from desert vegetation to waterbody.The spatial-averaged increased-ET from baresoil to cropland,from desert vegetation to cropland and from baresoil to desert vegetation is approximately 390 mm,350 mm and 30 mm,respectively.The conversion from cropland to ISA has the most spatial-averaged decreased-ET with around 500 mm.The spatial-averaged decreased-ET from baresoil to ISA is around 20 mm.(6)The ET water consumption increased around 0.95 billion m3 from all the land cover changes during 2000-2010.The conversion from desert vegetation to crop,accounting for 58.57%of the total increased ET water consumption,is the most ET water consumed conversion type.The conversion from baresoil to desert vegetation follows behind with 0.25 billion m3 ET water consumption.It is worth noting that the ET water consumption from baresoil and desert vegetation to crop in the middle reach is approximately 0.36 million m3/km2 while that from baresoil to desert vegetation in the low reach only needs 0.03 million m3/km2.It means that the ET water consumption from a square meter increased cropland from baresoil and desert can be used to recover twelve square meter desert vegetation in the low reach.Therefore,in order to guarantee water consumption of desert ecosystem in the low reach,the expansion of cropland in the middle reach should be controlled and limited within a reasonable extent in the future's land allocation.