Evaporation between saturated bare soil and water – an analysis based on field observations and energy balance consideration
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摘要:
准确计算裸土蒸发量对研究旱区地下水文循环过程具有重要意义。潜在蒸发量是计算裸土实际蒸发量的重要指标，由于饱和裸土蒸发量（PEs）难以获取，因此常用水面蒸发量（PEw）替代，但该方法的有效性有待验证。本研究基于蒸渗仪实测蒸发量、气象要素等野外观测数据，对比了2种饱和砂土与水面蒸发量昼夜变化特征及其差异。实测结果表明，年内平均饱和裸土蒸发量大于水面蒸发量；春夏两季蒸发强烈，两者差异最为显著；在天尺度上，水面蒸发量曲线滞后于饱和裸土。有效能量（Rn−Gs/Nw）是决定潜在蒸发量差异的主要因素。与纯水相比，饱和裸土中固相颗粒的存在，削弱了短波辐射的穿透能力，影响净辐射量（Rn），并导致土体热容降低，影响土面总热通量（Gs）。计算结果表明，饱和裸土可用于蒸发的有效能量大于水面（Rn,s−Gs>Rn,w−Nw），因此饱和裸土蒸发量较大；由于饱和裸土剖面升温更快，水体储热变化量（Nw）曲线滞后于土面总热通量（Gs），因此饱和裸土日蒸发量峰值也早于水面出现。该研究为准确计算实际蒸发量、提升地下水资源估测精度提供了理论依据。
Abstract:Accurate estimation of evaporation rate in bare soil is of great significance for hydrogeological processes in arid regions. Potential evaporation has been regard as a standard to estimate the actual evaporation rate. In applications, evaporation rate in the saturated bare soil (PEs) is often replaced by water evaporation rate (PEw). Whether this simplification is adequate needs to be verified. This research is based on the measured potential evaporation rates (by lysimeters) and meteorological elements. The results show that PEs is higher than PEw on a yearly scale, and the differences are more obvious especially in spring and summer. In summer, PEs is greater than PEw at day but smaller at night. Besides, the curve of PEw lags behind PEs. Detailed analyses of evaporation dynamics over fully saturated bare sandy soils and water surface are provided by energy balance considerations. PE dynamics are mainly governed by available energy (Rn−Gs/Nw). Compared with water, the existence of solid particles in the saturated bare soil results in a smaller albedo and heat capacity, which has a further influence on Rn and Gs. Available energy for the saturated bare sandy soil is higher than that of water (Rn,s−Gs>Rn,w−Nw), resulting in a higher rate for PEs. The peak value of Gs exists earlier than Nw, leading to lag of PEw behind PEs. This research provides a theoretical basis for accurate calculation of the actual evaporation rate and groundwater resources.
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Key words:
- potential evaporation /
- lysimeter /
- saturated bare soil evaporation /
- water evaporation
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表 1 砂样颗粒分析
Table 1. Particle compositions of sands in the lysimeter
粒径 /mm 粒径组成/% ≥2.00 2.00～0.50 0.50～0.25 0.25～0.075 <0.075 细砂 0 0 32.7 57.5 9.8 粗砂 21.5 43.7 24.8 8.9 1.1 
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表 2 砂样水热参数
Table 2. Hydraulic and thermal parameters of sands in the lysimeter
砂样 饱和含水率/（cm3·cm−3） 热容/（MJ·m−3·K−1） 细砂 0.39 2.2 粗砂 0.37 2.1
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