中国西北潜在蒸散时空演变特征及其定量化成因

潜在蒸散是区域干湿状况评价、作物需水量估算和水资源合理规划的关键因子。基于FAO推荐的Penman-Monteith公式和126个台站1961—2009年逐日气象观测资料估算西北干旱半干旱区的潜在蒸散量ET0,并对其空间分布特征进行了讨论。通过非参数化Sen趋势分析法和M-K统计检验法方法分析潜在蒸散的时间演变规律,并定量探讨了西北地区影响ET0变化的主导因素。结果表明:49 a来,西北地区ET0的年平均值约为980.63 mm,其中夏季的值最大,冬季的值最小。年平均ET0的大值区位于西北日照时数的高值中心,低值区主要位于海拔高,气温低的山区。西北地形和气候的多样性导致多年平均潜在蒸散的变化及其原因具有明显的时空差异。ET0的变化主要归因于风速和气温,而相对湿度和日照时数的作用较小。由于风速的负贡献超过气温的正贡献,导致年平均ET0整体上呈下降趋势。四季中,春冬两季的ET0缓慢上升,冬季的变化率是春季的两倍;夏秋两季的ET0有所下降,但只有夏季的变化趋势显著。春、夏、秋三季ET0变化的首要主导因子是风速,而冬季的首要主导因子是气温。空间上,西风带气候区ET0降低主要归因于风速的减小,陕南地区ET0下降主要归因于日照时数的减少,其它地区ET0上升的主要原因是气温的增加。

Temporal-spatial variations of potential evapotranspiration and quantification of the causes in Northwest China

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.