基于野外观测与能量守恒原理分析饱和裸土与水面蒸发量的差异

Evaporation between saturated bare soil and water-an analysis based on field observations and energy balance consideration

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(PE_(s))难以获取,因此常用水面蒸发量(PE_(w))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(R_(n)−G_(s)/N_(w))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(R_(n)),并导致土体热容降低,影响土面总热通量(G_(s))。计算结果表明,饱和裸土可用于蒸发的有效能量大于水面(R_(n),s−G_(s)>R_(n),w−N_(w)),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(N_(w))曲线滞后于土面总热通量(G_(s)),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。

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(PE_(s))is often replaced by water evaporation rate(PE_(w)).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 PE_(s) is higher than PE_(w) on a yearly scale,and the differences are more obvious especially in spring and summer.In summer,PE_(s) is greater than PE_(w) at day but smaller at night.Besides,the curve of PE_(w) lags behind PE_(s).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(R_(n)−G_(s)/N_(w)).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 R_(n) and G_(s).Available energy for the saturated bare sandy soil is higher than that of water(R_(n),s−G_(s)>R_(n),w−N_(w)),resulting in a higher rate for PE_(s).The peak value of G_(s) exists earlier than N_(w),leading to lag of PE_(w) behind PE_(s).This research provides a theoretical basis for accurate calculation of the actual evaporation rate and groundwater resources.