Knowledge of the parameterization of the dry surface layer (DSL) is essential for evaluating near-surface water flow and water balance in arid and semi-arid areas. Existing studies have parameterized DSL thickness and vapor flow as functions of the soil moisture content (SMC) in the surface layer to predict soil evaporation. However, hydrochemical processes related to DSL development have been ignored, including changes in hydrochemistry, the underlying hydrochemical mechanism, and the role of dissolved substances in the DSL development. Herein, we performed a series of soil evaporation experiments for 260 days and explored the factors influencing DSL development (e.g., soil texture, atmospheric temperature, SMC, solutes). Evaporation experiments were performed using silty loess, sandy loess, and fine sand with a 60-cm water table. Results showed that the cumulative evaporation of silty loess, sandy loess, and fine sand over the experimental period were 1,391.52, 460.10, and 185.53 mm, respectively, which determined by the maximum height of liquid flow continuity. The content of total dissolved solids (TDS) and major ions at the surface soil were significantly higher than the values at deep depths of 5‒55 cm, which largely depend on evaporative water loss. Evolutionary trends of chemical facies in sand media along the liquid water migration were from HCO-Ca type to SO·Cl-Na type. This was attributed to mineral dissolution at a depth of 5–55 cm and their transport with liquid water, resulting in the precipitation of salt crystals at the surface soil. Furthermore, a consolidated DSL with a thickness of 3.0–3.5 cm in the sandy loess and a loose DSL with a thickness of 1.5–2.0 cm in fine sand were observed at the end of the experiments. The accumulation of solutes at the surface leads to a reduction in effective porosity and the aggregation of soil particles during continuous drying, which facilitates the consolidation of DSL in sandy loess. This overestimated the DSL thickness, resulting in a difference between the experimental and predicted evaporation rates by Fick's law. Overall, these results highlight the limitations of considering DSL thickness as a function of SMC only, providing new insights into hydrochemical processes and dissolved solutes involving DSL parameterization during continuous soil drying.

中文翻译：

---

对干燥表层参数化及其水文地球化学机制的新见解：实验研究


了解干燥表层 (DSL) 的参数化对于评估干旱和半干旱地区的近地表水流和水平衡至关重要。现有研究将 DSL 厚度和蒸汽流量参数化为表层土壤含水量 (SMC) 的函数，以预测土壤蒸发。然而，与DSL发展相关的水化学过程却被忽视了，包括水化学的变化、潜在的水化学机制以及溶解物质在DSL发展中的作用。在此，我们进行了260天的一系列土壤蒸发实验，探讨了影响DSL发展的因素（例如土壤质地、大气温度、SMC、溶质）。蒸发实验采用粉质黄土、砂质黄土、细砂进行，地下水位60cm。结果表明，试验期间粉质黄土、砂质黄土和细砂的累计蒸发量分别为1391.52、460.10和185.53mm，这是由液流连续性最大高度决定的。表层土壤的总溶解固体（TDS）和主要离子含量显着高于5～55 cm深处的值，这在很大程度上取决于蒸发水损失。砂介质化学相沿液态水运移的演化趋势为从HCO-Ca型向SO·Cl-Na型演化。这是由于5-55厘米深度的矿物质溶解及其随液态水的输送，导致盐晶体在表层土壤沉淀。此外，在实验结束时还观察到沙质黄土中厚度为3.0-3.5 cm的固结DSL和细砂中厚度为1.5-2.0 cm的松散DSL。 溶质在表面的积累导致有效孔隙度的降低以及持续干燥过程中土壤颗粒的聚集，有利于沙质黄土中DSL的固结。这高估了 DSL 厚度，导致实验蒸发率与根据菲克定律预测的蒸发率之间存在差异。总体而言，这些结果凸显了仅将 DSL 厚度视为 SMC 函数的局限性，为连续土壤干燥过程中涉及 DSL 参数化的水化学过程和溶解溶质提供了新的见解。