Thermal Methods for Investigating Ground-Water Recharge
Thermal Methods for Investigating Ground-Water Recharge
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研究地下水补给的热学方法
DOI：
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发表时间：
2007
期刊：
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通讯作者：
D. Stonestrom
中科院分区：
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作者：
K. Blasch;J. Constantz;D. Stonestrom
在干旱和半干旱环境中，含水层的补给被定义为水在区域地下水位的向下通量。在陆地表面引入补给水可以发生在不显眼的位置，例如在河道中，或者分布在整个景观上，例如在冲积地下水盆地内的广泛的阿罗约间地区。在离散位置发生的补给被称为集中补给，而在广泛区域发生的补给被称为弥漫性补给。这项工作的主要兴趣是集中补给，但无论补给类型如何，向下通量的估计对其量化至关重要。直接测量地下的水通量是困难的，这促使研究人员寻求间接的方法。利用热量和水输运之间的耦合关系的地球物理方法提供了一类有吸引力的方法，这些方法已变得越来越可靠，并广泛用于研究补给，例如，本卷的D-H、J和K章。本附录回顾了热在充值估计问题中的应用。除热能以外的地球物理方法见附录2。关于测量和模拟热量作为水文过程示踪剂的教程已经在其他地方出现（Stonestrom和Blasch, 2003； Niswonger和Prudic， 2003）。本附录的目的是提供一个相当完整的理论基础的帐户连同热方法在实践中的全面审查。像化学示踪剂一样，热量可以来自自然来源，也可以被有意引入以推断输送特性和含水层补给。来自自然过程（如日照、入渗和地热活动）的热量的进入和再分配可用于量化地下流动情况（图1）。热非常适合作为地下水示踪剂，因为它提供自然存在的动态信号，并且在诱导扰动的有用范围内相对无害；然而，人工地与地下水系统交换热量会改变水力特性和流体通量。取决于温度的特定性能包括粘度、密度和表面张力，所有这些都会影响水力导电性和流体流动。由于几个原因，热方法已被证明是有价值的。首先，水热耦合输运的理论描述可用于实践中最常遇到的水文过程。这些机制包括地表机制，如来自太阳的辐射加热、进入太空的辐射冷却和蒸发蒸腾，此外还有通常在深处占主导地位的平流和传导机制。其次，温度在理论上有很好的定义，而且很容易测量。第三，从地表到数百米深度的热方法基于相同的物理原理。最后，模拟热和水输运的数值代码是广泛可用的。在20世纪初，研究人员开始利用地下温度来划定补给区，并推断地下水的流动方向。在20世纪60年代，简化的热和流体流动问题的解析和数值解决方案成为可能。这些解决方案虽然是一维的，而且受到其他方面的限制，但却为将热方法应用于从土壤到地热储层等系统中的液体和蒸汽运动问题提供了强大的推动力。快速处理器、海量数据存储和高效矩阵技术的结合，现在为复杂的三维传输问题提供了数值解决方案。这些解决方案使研究人员能够利用高精度温度工作中常规可实现的大量信息内容。
Recharge of aquifers within arid and semiarid environments is defined as the downward flux of water across the regional water table. The introduction of recharging water at the land surface can occur at discreet locations, such as in stream channels, or be distributed over the landscape, such as across broad interarroyo areas within an alluvial groundwater basin. The occurrence of recharge at discreet locations is referred to as focused recharge, whereas the occurrence of recharge over broad regions is referred to as diffuse recharge. The primary interest of this work is focused recharge, but regardless of the type of recharge, estimation of downward fluxes is essential to its quantification. Direct measurement of water flux in the subsurface is difficult, prompting investigators to pursue indirect methods. Geophysical approaches that exploit the coupled relation between heat and water transport provide an attractive class of methods that have become increasingly reliable and widely used in investigations of recharge—for example, in chapters D–H, J, and K of this volume. This appendix reviews the application of heat to the problem of recharge estimation. Geophysical methods other than heat are presented in appendix 2. Tutorials on measuring and modeling heat as a tracer of hydrologic processes have appeared elsewhere (Stonestrom and Blasch, 2003; Niswonger and Prudic, 2003). The objective of this appendix is to provide a fairly complete account of the theoretical underpinnings together with a comprehensive review of thermal methods in practice. Like chemical tracers, heat can come from natural sources or be intentionally introduced to infer transport properties and aquifer recharge. The admission and redistribution of heat from natural processes such as insolation, infiltration, and geothermal activity can be used to quantify subsurface flow regimes (fig. 1). Heat is well suited as a ground-water tracer because it provides a naturally present dynamic signal and is relatively harmless over a useful range of induced perturbations; however, artificially exchanging heat with ground-water systems can change the hydraulic properties and fluid fluxes of interest. Specific properties that depend on temperature include viscosity, density, and surface tension, all of which affect hydraulic conductivity and fluid flow. Thermal methods have proven valuable for recharge investigations for several reasons. First, theoretical descriptions of coupled water-and-heat transport are available for hydrologic processes most often encountered in practice. These include land-surface mechanisms such as radiant heating from the sun, radiant cooling into space, and evapotranspiration, in addition to the advective and conductive mechanisms that usually dominate at depth. Second, temperature is theoretically well defined and readily measured. Third, thermal methods for depths ranging from the ground surface to depths of hundreds of meters are based on the same physical principles. Finally, numerical codes for simulating heat and water transport are widely available. Investigators began using subsurface temperatures to delineate recharge areas and infer directions of ground-water flow about the turn of the 20th century. During the 1960s, analytical and numerical solutions for simplified heatand fluid-flow problems became available. These solutions, though one-dimensional and otherwise restricted, provided a strong impetus for applying thermal methods to problems of liquid and vapor movement in systems ranging from soils to geothermal reservoirs. The combination of fast processors, massive data storage, and efficient matrix techniques now provide numerical solutions to complex, three-dimensional transport problems. These solutions allow researchers to take advantage of the considerable information content routinely achievable in high-accuracy temperature work.