中国西北潜在蒸散时空演变特征及其定量化成因

Abstract

Potential evapotranspiration represents the maximum possible evaporation and is the rate that would occur under given meteorological conditions from a continuously saturated surface.As a main component of the hydrological cycle,potential evapotranspiration(ET0) is essential for understanding regional moisture conditions,estimating crop water requirements and managing water resources.In recent years,change trends in ET0 and its dominant factors across the different regions of the world have been studied by many researchers.Despite global warming,decreasing trends in ET0 have been detected in several countries including United States,Russia,India,China,Australia and New Zealand.Decreasing sunshine duration,declining wind speed and increasing relative humidity have been considered to be the main causes for the decreasing ET0. Northwest China in the hinterland of Eurasia and far away from oceans is one of the driest areas in the world.Water shortage is a serious problem for agriculture in this area.So far,detailed investigation on the change trend in ET0 over Northwest China has been lacking. In this paper,daily ET0 was computed with the recommended FAO-56 Penman-Monteith equation for the arid Northwest China using data collected 126 weather stations during 1961 to 2009 and its spatial variations were characterized.Temporal variations in ET0 were also investigated using the nonparametric Sen′s method and the Mann-Kendall test,and determining factors in ET0 trends were inquired into through partial derivative quantification analysis for the study region.Results showed that the mean annual ET0 was 981mm over the whole region during the study period.ET0 peaked in summer and decreased to a minimum in winter.Higher ET0 was found over areas with high solar radiation,while lower values occurred over mountain areas with low air temperatures.The spatial distribution of spring,summer and autumn ET0 had roughly similar patterns compared to that of the annual ET0.Great differences either in the mean annual ET0 or in its determining factors existed from area to area due to the diverse topographies and climates in the Northwest China.Both wind speed and air temperature were the dominant factors contributing to its interannual change,with less contribution from relative humidity or sunshine duration.The annual ET0 showed a general decline at a rate of-4.6mm/decade owing to a more negative contribution of falling wind speed than a positive contribution of rising air temperature.The decrease occurred mainly from 1974 to 1993.After reaching the lowest value in 1993,ET0 began to increase slowly.ET0 increased slightly in both spring and winter and decreased in both summer and autumn.The rate of the increase in the winter was twice that in the spring,and ET0 decreased more significantly in the summer than in the autumn.The main impacting factor was wind speed in the spring,summer and autumn,and but it was air temperature in the winter.Decreases in ET0 in the western part of the region and the southern part of Shannxi Province arose from both declining wind speed and falling sunshine duration.Rising air temperature was the determining factor in ET0 increase over other areas of the region.