公路自然区划Ⅱ1区路基湿度指数优化算法

Optimal calculation method of subgrade moisture index based on FAO-PM method in highway climatic zone Ⅱ1

摘要:

为了提高公路自然区划Ⅱ₁区路基湿度指数计算精度,建立一种路基湿度指数优化计算方法,其采用FAO-PM法取代Thornthwaite法来估算路基土月均潜在蒸发蒸腾量(potential evapotranspiration, PE),以解决后者对路基土在负温条件下蒸发蒸腾量计算值为零的问题。在分析了各气象参数对路基土月均潜在蒸发蒸腾量的影响规律的基础上,验证了新方法的适用性并简化了路基湿度平衡计算的流程。针对路基湿度的季节性变化特征,提出了阶段湿度指数计算方法,并细化了Ⅱ₁区三级划分区域的路基湿度指数取值范围。结果表明:在负温条件下,路基的月均潜在蒸发蒸腾量在与平均日照时数、平均气温和平均风速正相关的同时,与平均相对湿度负相关,并且总潜在蒸发蒸腾量占全年总量的9.8%~15.7%；路基湿度平衡后的状态可简化为有径流、无径流不干旱和无径流干旱3种,并可采用4阶段湿度指数对其最不利季节进行判别；对于所研究的7个气象站,不同土组的湿度指数计算最小值为-16.5,最大值为33.2,这与现行规范推荐值（-8.1~35.1）相比,不仅数值整体偏小、路基湿度状态更偏干旱,而且自西南向东北逐渐降低。采用该方法可有效计算Ⅱ₁区负温下土壤蒸发蒸腾量,并进而确保对路基湿度指数估算的精度。

Abstract:

In order to improve the calculation accuracy of moisture index of subgrade in highway climatic zone Ⅱ₁, an optimal calculation method of subgrade moisture index was established by employing the Food and Agriculture Organization Penman-Monteith (FAO-PM) method instead of the Thornthwaite method to estimate the average monthly potential evapotranspiration (PE) of soil subgrade, which considers the evapotranspiration of soil subgrade as zero under negative temperature conditions. After analyzing the influence of meteorological parameters on the monthly PE of soil subgrade, the applicability of the proposed method was verified and the calculation process of subgrade moisture balance was simplified. Based on the seasonal change characteristics of subgrade moisture, a staged moisture index calculation method was proposed to refine the range of subgrade moisture index in climate zone Ⅱ₁. Results show that when the temperature was below zero, the average monthly PE of the subgrade was positively correlated with average sunshine hours, average temperature, and average wind speed, but negatively correlated with average relative humidity. Moreover, the total PE accounted for 9.8%-15.7% of the total evapotranspiration throughout the year. The subgrade moisture equilibrium states were classified into three types: runoff, no runoff with or without drought, for which the most unfavorable seasons could be distinguished by the four-stage statistical analysis of the moisture index. For the seven meteorological stations studied, the minimum and maximum moisture indexes of different soil groups were calculated as -16.5 and 33.2 respectively, which are less than the current standard recommendation (-8.1-35.1), indicating that the subgrade moisture in these regions are more arid than expected and gradually decreases from southwest to northeast. The method can not only be used to effectively calculate the evapotranspiration of soil under negative temperature, but also ensure the accuracy of subgrade moisture index estimation in climatic zone Ⅱ₁.