Effects Of Land Use/cover Change And Climatic Change On Evapotranspiration In The Western Songnen Plain Of Northeast China

Evapotranspiration (ET) and energy fluxes are two key processes of water cycle and energy balance in ecosystem. It is critical for understanding hydrological processes and energy transfere in the Western Songnen Plain (WSP) of Northeast China.The estimation of ET and energy components from 2001-2012 in the growing season was conducted via the Surface Energy Balance Algorithm (SEBAL) model based on MODIS data. Futhermore, the effects of climate change and human activity on ET and trends of ET and energy components were anaylized on annual, month and 16-day scale. The results showed that:(1) The temperature and precipitation of the growing seasons ascended distinctly inthe WSP, whereas the ET was on the decline; the ET was positively correlated with temperature and precipitation in the 16-day scales, while negatively correlated with precipitation in the inter-annual scales; the increase of ET in the early growing season (May) was attributed to increasing precipitation in non-growing season (from November to March) and rising temperature in April. However, increased precipitation in late spring (April-May) was likely to reduce ET during the peak growing season (July-august);(2) The average daily ET of multi-year growing seasons under different land uses followed descending order of bare land, cropland and grassland in WSP where transforms occurred mainly from grassland to cropland and bare land to grassland. The cropland which was once grassland had lower ET in the early stage of growing season in 2001-2005, and rose to the level of cropland in 2006-2012. After returning farmland to pastures, the ET was higher in 2001-2005, and fell below the grassland ET level in 2006-2012. The degraded grassland had lower ET in 2001-2005 and approximate bare land ET level in 2006-2012 when retrograded into bare land; the ET of grassland after restoration from bare land was low in 2001-2012, and 0.2mm-d below the raw grassland level later;(3)Both climate change and land use change had impacts on ET. Furthermore, climate change had same effect on grassland with land use change, whereas less on cropland and bare land.(4)Energy allocation during the growing seasons on 16d scale followed a descending order of latent heat flux (λET), sensible heat flux (H) and soil heat flux (G). The ratio of each in total net radiation flux (Rn)is 75.08%,16.61% and 8.93%.There was significantly positive correlation between ET and Rn(r=0.887, P<0.01) and negitive between H and G(r=-0.452, P<0.01).(5)The fluctuation of Rn, λ, ET and G is small, while the H fluctuation is comparatively large. There is lower Rn,λ ET and H in dry year (2001) than wet year (2012) except G. Precipitation is the key constraint factor of energy fluxes for annual variation. The seasonal variance of Rn and λ ET descended in the order of summer, spring and autunm while G was from spring, summer and autunm.