Convective Groundwater Flow Induced by Climatic Gradients inthe Great Basin: A Computational Approach to System Dynamics

当前的环境流体流动问题通常涉及在具有稀疏场数据的系统中进行大空间和时间尺度的预测。经典方法将特征长度和时间尺度确定为求解驱动流体循环的基本力或通量的手段。在对封闭盆地的水文研究中，Duffy和Al-Hassan（1988）应用尺度不变地形的概念，构建了犹他州西部Pilot Valley Playa附近区域的经验地形分布。利用区域平均降水与海拔高度的相关性构建了模拟山前补给的尺度输入。在干盐湖或短暂湖泊，观察到高蒸发速率在浅层地下水中产生浓盐水，导致不稳定的分层和自由对流的可能性。这些观测结果指导了在一系列封闭盆地几何形状上进行的数值实验，并导致了由山地降水和湖泊蒸发驱动的地下环流的力或通量平衡的发展。结果表明，盐湖边缘成为山前上升水补给（强制对流）和中部盐湖盐水回流（自由对流）交汇的关键水文带，产生淡水和（或）微咸泉。这项工作的前提是淡水-咸水界面的位置代表了盆地中水文气候力的平衡。利用封闭盆地的特征长度和速度，盐度的瑞利数与代表界面稳态位置的无量纲群呈线性关系。本研究将尝试扩展以往的数值实验，以包含复杂的时空变异性，即：(1)山区补给和盐湖蒸发瞬态波动引起的地下对流的动态响应；(2)确定性和随机异质性对有效流域参数估计的影响；(3)不同基准面多级联盆地的区域流；(4)地表水文过程（径流）的作用；(5)不饱和流与大气-陆地表面边界条件的意义。

Present-day environmental fluid flow problems are routinely concerned with making predictions over large space and time scales in systems with sparse field data. The classical approach identifies characteristic length and time scales as a means of resolving the basic forces or fluxes driving fluid circulation. In a study of the hydrology of closed basins, Duffy and Al-Hassan (1988) applied the concept of scale invariant topography to construct an empirical topographic distribution for the range bordering the Pilot Valley Playa in western Utah. Regional correlation of mean precipitation with altitude was used to construct scaled inputs for modeling mountain-front recharge. At the playa or ephemeral lake, high evaporation rates were observed to produce concentrated brines in the shallow groundwater, resulting in unstable stratification and the potential for free convection. These observations guided numerical experiments conducted over a range of closed -basin geometries, and led to the development of a force or flux balance for subsurface circulation driven by mountain precipitation and playa-lake evaporation. The results suggest that the playa margin becomes a critical hyhdrologic zone where upwelling groundwaters from mountain-front recharge (forced convection) and brine recirculation from the central playa (free convection) converge, producing fresh and/or brackish springs. The premise of this work is that the position of the freshwater-saltwater interface represents a balance of hydro-climatic forces in the basin. Using characteristic lengths and velocities of the closed basin, the Rayleigh number for salinity was shown to be linearly related to a dimensionless group representing the steady-state position of the interface. The proposed research will attempt to extend previous numerical experiments to include complexities of space and time variability, namely: (1) the dynamic response of subsurface convection resulting from transient fluctuations in mountain recharge and playa evaporation; (2) the implications of deterministic and stochastic heterogeneity to the estimation of effective basin parameters; (3) regional flow from multiple- cascading basins of varying base level; (4) the role of surficial hydrologic processes (runoff); and (5) the significance of unsaturated flow and the atmosphere-land surface boundary condition.