Bare soils and grasslands in arid and semi-arid conditions constitute a large portion of the earth surface. Evaporation, which is the main component of the water balance in these conditions, often takes place through a dry soil layer (DSL). There is no scientific agreement yet on the DSL effects on evaporation rates. The implementations of three conceptual models of DSL-evaporation were tested for the simulation of evaporation rates in a semi-arid study area in Central Spain: (i) the daily-average model, based on the assumption that the daily average vapour transport in a DSL can be represented in analogy to isothermal liquid flow; (ii) the numerical model solving the Richards equation, in this case HYDRUS1D was used; and (iii) the pore-scale model, based on soil column experiments in laboratory conditions. The evaporation rates estimated by the three conceptual models for semi-arid field conditions were compared with the evaporation rates measured by an eddy covariance tower in the same area. The results indicate that the daily-average conceptual model assumption, in which the DSL has no effects on evaporation, does not hold in very dry conditions. The numerical model solving the Richards equation was not able to simulate the effects of the DSL on evaporation rates. The evaporation estimates obtained by the pore-scale conceptual model were closest to the eddy covariance measurements during the dry season, however this model was applicable only to the relatively steady evaporation conditions during afternoons and only assuming spatially constant DSL thickness.

中文翻译：

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测试通过半干旱地区干燥土壤层估算土壤蒸发的三种方法

干旱和半干旱条件下的裸露土壤和草地构成了地球表面的很大一部分。蒸发是这些条件下水平衡的主要组成部分，通常通过干土层 (DSL) 进行。关于 DSL 对蒸发率的影响尚无科学共识。为了模拟西班牙中部半干旱研究区的蒸发率，测试了三个 DSL 蒸发概念模型的实施：(i) 日平均模型，基于以下假设： DSL 可以用等温液体流来表示；(ii) 求解理查兹方程的数值模型，在这种情况下使用 HYDRUS1D；(iii) 孔隙尺度模型，基于实验室条件下的土柱实验。将三个概念模型估计的半干旱田间条件的蒸发率与同一地区涡流协方差塔测量的蒸发率进行了比较。结果表明，DSL 对蒸发没有影响的日平均概念模型假设在非常干燥的条件下不成立。求解理查兹方程的数值模型无法模拟 DSL 对蒸发率的影响。孔隙尺度概念模型获得的蒸发估计值最接近旱季涡度协方差测量值，但该模型仅适用于下午相对稳定的蒸发条件，并且仅假设 DSL 厚度空间恒定。