Direct observations of rock moisture, a hidden component of the hydrologic cycle
Direct observations of rock moisture, a hidden component of the hydrologic cycle
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DOI：
10.1073/pnas.1800141115
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发表时间：
2018-02
期刊：
影响因子：
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通讯作者：
D. Rempe;W. Dietrich
中科院分区：
文献类型：
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作者：
D. Rempe;W. Dietrich
意义长期以来，土壤水分一直被认为是水文循环的关键组成部分。在这里，我们量化了土壤下风化基岩中的重要可交换水，这些水调节植物可用水和径流。我们称之为岩石水分-一个术语平行于土壤水分，但适用于不同的材料。深层风化的基岩能够储存植物可用的水分是常见的，但这种岩石水分的水库-不同于土壤和地下水-基本上是无法量化的。在我们的研究地点，岩石水分的体积超过土壤水分，是干旱年份植物的关键和稳定的水源。我们的观察表明，岩石水分现在需要纳入水文和气候模型。最近的理论和野外观察表明，系统变化的风化带，可以是几十米厚，通常在基岩下的山坡上发展。风化作用使原本导电性差的基岩变成了动态蓄水池。渗透性降水通常会通过不饱和的风化基岩，然后到达地下水并流入河流。与表层土壤覆盖层相比，这一看不见的、难以接近的非饱和带实际上是未经勘探的。我们提出了术语“岩石水分”来描述储存在风化基岩中的非饱和带中的可交换水，故意选择一个与土壤水分平行但不同的术语，因为风化基岩是一种明显不同的材料，分布在景观中，与土壤厚度无关。在这里，我们报告了一个多年密集的运动量化岩石水分在山坡下的厚风化基岩区使用重复中子探头测量一套钻孔。岩石水分储存在雨季积累，达到一个特征上限值，并迅速将任何额外的降雨向下传递到地下水。因此，岩石水分储存介导的补给和径流的启动和大小。在旱季，岩石水分储存逐渐耗尽树木蒸腾，导致一个共同的较低的值在旱季结束。高达27%的年降雨量是季节性储存的岩石水分。大量的岩石水分储存可能是常见的，但它是用于预测区域和全球气候的水文和陆地表面模型失踪。
Significance Soil moisture has long been recognized as a key component of the hydrologic cycle. Here, we quantify significant exchangeable water held in weathered bedrock, beneath the soil, that regulates plant-available water and streamflow. We refer to this as rock moisture—a term parallel to soil moisture, but applied to different material. Deep weathered bedrock capable of storing plant-available moisture is common, yet this reservoir of rock moisture—distinct from soil and groundwater—is essentially unquantified. At our study site, the volume of rock moisture exceeds soil moisture and is a critical and stable source of water to plants in drought years. Our observations indicate that rock moisture now needs to be incorporated into hydrologic and climate models. 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.