Analysis of Spatiotemporal Variations in Evapotranspiration and Its Influencing Factors over the Songnen Plain in the Growing Season during the Period 2000-2008
2000 年至2008 年松嫩平原生长季蒸散量时空格局及影响因素分析

Evapotranspiration (ET) is a critical component of the land surface energy balance system and hydrologic processes. Analysis of spatiotemporal variations and influencing factors of ET in the growing season is of great importance to evaluate the growing environment for crops and to effectively use water resources across the Songnen Plain, a critical base for grain production in China. The authors made use of the Surface Energy Balance Algorithm for Land (SEBAL) and the Penman-Monteith equation (P-M) in conjunction with Moderate-resolution Imaging Spectroradiometer (MODIS) products (i.e., MOD11A1, MOD11A2, MOD13A2, MOD43B3, and MCD43B3) to estimate land surface actual ET in the growing season generally from May to September during the period 2000-2008. Furthermore, spatial patterns and temporal trends in the ET estimates were comprehensively analyzed, with investigating correlation of five key climatic factors (i.e., precipitation, air temperature, relative humidity, wind speed, and sunshine hours) with ET estimates. In addition to large water bodies and wetlands, the average ET in the growing season from 2000 to 2008 changed obviously, increasing gradually from the southwest to the east and northeast of the study region. It was suggested that the average ET of the whole Songnen Plain exhibited a significant decreasing trend during the period 2000-2008, with a maximum ET value of 669.31 mm occurring in 2000 and a minimum ET value of 570.79 mm in 2005. The mean annual ET was found to be roughly 612.63 mm. Under a combined effect of weather conditions, soil water supply status, and fractional vegetation cover, the monthly ET from May to September showed a marked spatiotemporal variation, with a maximum monthly ET value of 141.60 mm in July and a minimum ET value of 81.35 mm in September. This could be ascribed largely to relatively less precipitation and lower air temperatures. By examining responses of the ET estimates to climatic variables, it was found that ET was positively correlated with precipitation and average air temperature over most regions of the Songnen Plain, indicating that the two climatic variables may be the major factors affecting ET for the growing season. Additionally, average relative humidity was positively correlated with ET in areas covered primarily by forests and water bodies, whereas showed negative correspondence with ET over other regions. The ET estimates showed insignificant correlation with climatic factors, such as wind speed and sunshine hours. This work would provide a basis for the assessment of water supply status during the crop growth stage over the Songnen Plain, particularly over the arid and semi-arid environments. It can also facilitate assessing ecological water requirement and irrigation system functions, further strengthening water resources management.