Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78701;
Department of Earth and Planetary Science, University of California, Berkeley, CA 94720
Proc Natl Acad Sci U S A. 2018 Mar 13;115(11):2664-2669. doi: 10.1073/pnas.1800141115. Epub 2018 Feb 28.
Recent theory and field observations suggest that a systematically varying weathering zone, that can be tens of meters thick, commonly develops in the bedrock underlying hillslopes. Weathering turns otherwise poorly conductive bedrock into a dynamic water storage reservoir. Infiltrating precipitation typically will pass through unsaturated weathered bedrock before reaching groundwater and running off to streams. This invisible and difficult to access unsaturated zone is virtually unexplored compared with the surface soil mantle. We have proposed the term "rock moisture" to describe the exchangeable water stored in the unsaturated zone in weathered bedrock, purposely choosing a term parallel to, but distinct from, soil moisture, because weathered bedrock is a distinctly different material that is distributed across landscapes independently of soil thickness. Here, we report a multiyear intensive campaign of quantifying rock moisture across a hillslope underlain by a thick weathered bedrock zone using repeat neutron probe measurements in a suite of boreholes. Rock moisture storage accumulates in the wet season, reaches a characteristic upper value, and rapidly passes any additional rainfall downward to groundwater. Hence, rock moisture storage mediates the initiation and magnitude of recharge and runoff. In the dry season, rock moisture storage is gradually depleted by trees for transpiration, leading to a common lower value at the end of the dry season. Up to 27% of the annual rainfall is seasonally stored as rock moisture. Significant rock moisture storage is likely common, and yet it is missing from hydrologic and land-surface models used to predict regional and global climate.
最近的理论和实地观测表明，在山坡下的基岩中通常会形成一个厚度可达数十米的系统变化的风化带。风化作用将原本导电性差的基岩变成了一个动态的储水水库。渗透降水通常会在到达地下水并流到溪流之前穿过不饱和风化基岩。与表土覆盖层相比，这个看不见且难以进入的不饱和带几乎没有被探索过。我们提出了“岩石水分”一词来描述风化基岩中不饱和带中储存的可交换水，故意选择了一个与土壤水分平行但又不同的术语，因为风化基岩是一种截然不同的物质，它分布在景观中，独立于土壤厚度。在这里，我们报告了一项多年的密集活动，即在一个厚风化基岩带下的山坡上使用一系列钻孔中的重复中子探针测量来量化岩石水分。岩石水分储存量在雨季积累，达到一个特征的上限值，并迅速将任何额外的降雨向下传递到地下水。因此，岩石水分储存量调节了补给和径流的起始和大小。在旱季，树木通过蒸腾作用逐渐耗尽岩石水分储存量，导致旱季结束时达到常见的下限值。多达 27%的年降雨量被季节性地储存为岩石水分。大量的岩石水分储存可能很常见，但在用于预测区域和全球气候的水文和陆面模型中却没有考虑到这一点。
