The Namib Sand Sea is a hyper-arid coastal fog desert, with limited vegetation scattered across the extensive sand dunes. Minimal and unpredictable rainfall, along with proximity to the ocean, make non-rainfall water inputs (NRWIs), i.e. fog, dew and water vapor adsorption, an important part of the ecohydrology. We studied water accumulation across an elevated hummock formed around a !Nara thicket (Acanthosicyos horridus) in the Namib dunes during February 2020. Continuous measurements of temperature and relative humidity (RH) gradients between the soil and the air were complemented with three nights of intensive NRWI observations using 14 micro-lysimeters. While the overall objective was to study total NRWI accumulation, micro-lysimeter placement was designed to address additional questions about how NRWI accumulation was affected by hummock micro-topography and the position of the !Nara thicket. Water vapor adsorption was the primary form of NRWI, occurring nightly and accounting for at least half of the total water accumulation, even on nights with fog. This was confirmed by the measured water vapor pressure gradients between the air and the soil, which indicated nightly downward vapor fluxes. Actual vapor pressures in the air were similar on nights with and without fog, indicating fog was mostly determined by temperature. Soil RH was used as a proxy for soil water content and was found to be strongly correlated with air RH. Average nighttime air RH was correlated with total nighttime water accumulation with an R² of 0.69-0.78. Average nighttime air RH was correlated with total nighttime water accumulation with a coefficient of determination (R²) of 0.69-0.78. Even though measurements were limited to the soil, comparison between micro-lysimeters placed on the hummock inside and outside the !Nara thicket indicated consistent, but not significantly, lower water contents inside the thicket, suggesting interception of water vapor by the spines and stems of the !Nara. We conclude that water vapor adsorption, though not always appreciated as such, is a large contributor to total NRWI. The fact that water vapor adsorption was the primary NRWI in sand, which has a much lower surface area than most other soil types, and in a place where fog occurs regularly, has implications for deserts worldwide.

纳米布沙海是一块高干旱的沿海雾沙漠，植被稀疏，散布在宽广的沙丘上。最小且不可预测的降雨，以及靠近海洋，使得非降雨水输入（NRWI），即雾，露水和水蒸气的吸附，是生态水文学的重要组成部分。我们研究了在奈良灌木丛（Acanthosicyos horridus）于2020年2月在纳米布沙丘上进行。连续进行的三夜的密集NRWI观测（使用14台微型测微计）补充了土壤和空气之间温度和相对湿度（RH）梯度的连续测量。虽然总体目标是研究总的NRWI积累，但设计了微型测渗仪以解决有关NRWI积累如何受到山岗微地形和！Nara灌木丛的位置影响的其他问题。水蒸气吸附是NRWI的主要形式，夜间发生，即使在有雾的夜晚也至少占总蓄水量的一半。空气和土壤之间测得的水蒸气压梯度证实了这一点，该梯度指示了夜间向下的蒸气通量。在有雾和无雾的夜晚，空气中的实际蒸气压相似，这表明雾主要由温度决定。土壤相对湿度被用作土壤水分的替代物，并被发现与空气相对湿度密切相关。夜间平均空气相对湿度与夜间总蓄水量相关，R0.69-0.78中的²。夜间平均空气RH与夜间总蓄水量相关，确定系数（R ²）为0.69-0.78。尽管测量仅限于土壤，但在奈拉灌木丛内外的小孔上放置的微测厚仪之间的比较表明，灌木丛内的水分含量一致但不是很明显，这表明刺槐的茎和茎截留了水蒸气。奈良 我们得出的结论是，尽管并非总是如此，但水蒸气吸附是造成总NRWI的主要因素。水蒸气吸附是沙子中主要的NRWI，其表面积比大多数其他土壤类型低得多，并且在经常发生雾的地方，这一事实对全世界的沙漠都有影响。