GroMoPo Metadata for Santa Barbara and Foothill Basins USGS model

Groundwater has been a part of the city of Santa Barbara's water-supply portfolio since the 1800s; however, since the 1960s, the majority of the city's water has come from local surface water, and the remainder has come from groundwater, State Water Project, recycled water, increased water conservation, and as needed, seawater desalination. Although groundwater from the Santa Barbara and Foothill groundwater basins only accounts for a small percentage of the long-term supply, it is an important source of supplemental water during times of surface-water shortages. During the late 1980s and early 1990s, production wells extracted additional groundwater to compensate for drought related water-delivery shortfalls from other sources; in response, water levels declined substantially in the Santa Barbara and Foothill groundwater basins (below sea level in the Santa Barbara groundwater basin). In coastal basins that have groundwater extraction near shore, seawater intrusion is often a problem. Seawater intrusion in the Santa Barbara groundwater basin is thought to be more limited than in other coastal basins because of an offshore fault that acts as a partial barrier to groundwater flow. During the late 1980s and early 1990s, seawater intrusion was observed in the Santa Barbara groundwater basin, as indicated by increased chloride concentrations at several monitoring wells that ranged from 200 ft to 1,300 ft from the ocean and as close as 2,900 ft to the nearest pumping well. This demonstrated that seawater can intrude into the Santa Barbara groundwater basin when groundwater levels fall below sea level near the coast. The city of Santa Barbara is interested in developing a better understanding of the sustainability of its groundwater supplies. In 2014, California adopted historic legislation to manage its groundwater: the Sustainable Groundwater Management Act (SGMA). The SGMA requires the development and implementation of Groundwater Sustainability Plans in 127 priority groundwater basins; although Santa Barbara was not a designated priority basin, the city is taking steps to achieve sustainability. Sustainability was defined in the SGMA in terms of avoiding undesirable results: significant and unreasonable groundwater-level declines, reduction in groundwater storage, seawater intrusion, water-quality degradation, land subsidence, and surface-water depletion. In this project, a cooperative study between the U.S. Geological Survey (USGS) and the city of Santa Barbara, sustainable yield is defined as the volume of groundwater that can be pumped from storage without causing water-level drawdowns and the associated increases in seawater intrusion (as indicated by increases in measured chloride concentrations) at selected wells. In order to estimate the sustainability of Santa Barbara's groundwater basins, a three-dimensional density-dependent groundwater-flow and solute-transport model (the Santa Barbara Flow and Transport Model, or SBFTM) was developed on the basis of an existing groundwater-flow model. To simulate seawater intrusion to the Santa Barbara Basin under various management strategies, the SBFTM uses the USGS code SEAWAT to simulate salinity transport and variable-density flow. The completed SBFTM was coupled with a management optimization tool, in this case a multi-objective evolutionary algorithm, to determine optimal pumping strategies that maximize the sustainable yield and at the same time satisfy user-defined drawdown and chloride-concentration constraints. As part of this study, a three-dimensional hydrogeologic framework model was developed to quantify the extent and hydrogeologic characteristics of the Santa Barbara and Foothill groundwater basins and to help define the discretization and hydraulic properties used in the SBFTM. The development of the hydrogeologic framework model required the collection and reconciliation of geologic and geophysical data from existing maps, reports, and databases, along with geologic and hydrologic data from recently drilled wells. These data were integrated into a three-dimensional hydrogeologic framework model that defines the stratigraphy and geometry of the aquifer zones and the major geologic structures in the basin. The hydrogeologic framework model also quantifies the variation in sediment grain size within each aquifer zone as the percentage of coarse-grained sediment. Previous studies indicated that there are two principal water-producing zones in the Santa Barbara groundwater basin, the upper and lower producing zones; an additional thin, productive zone was identified as part of this study. This middle producing zone is not as areally extensive as the upper and lower producing zones and only exists in the coastal part of Storage Unit I. These producing zones are bounded at depth by less productive shallow, middle, and deep zones. Two versions of the SBFTM were constructed: an initial-condition model and a modern transient model. The initial-condition model is a long-term transient model that simulates flow and solute-transport conditions during a period with limited anthropogenic influences preceeding the modern transient model. The simulation-transient model simulates flow and transport conditions from 1929 through 2013; however, because of data availability, the focus of the model calibration was 1972-2013. The SBFTM was calibrated to measured groundwater levels and drawdown, as well as measured chloride concentrations and change in concentrations, using a combination of automated and trial-and-error parameter-estimation techniques. A sensitivity analysis indicated that, in general, the SBFTM was most sensitive to recharge- and pumping-distribution parameters, specifically those controlling the amount of small-catchment recharge and the distribution of water extraction by hydrogeologic layer for production wells. The model was also sensitive to parameters controlling stream-recharge rates, horizontal and vertical hydraulic conductivity, and porosity. From 1929 to 1971, most of the water entering the area represented by the SBFTM was from creek and small-catchment recharge, and the majority of water leaving the SBFTM area was from pumping, discharge to creeks, and drains. In addition, about 37 percent of the total pumpage came from a net reduction in groundwater storage. From 1972 to 2013, the amount of water entering and leaving the SBFTM was fairly similar as that from 1929 to 1971, except the reduction in pumpage added about 17,000 acre-ft of water to storage. During this later period, there were also times of storage loss. For example, during July 1990, a month when approximately 705 acre-ft of groundwater was pumped in the study area, the pumpage was much greater than all sources of recharge combined, and about 382 acre-ft of water was removed from groundwater storage. Simulated hydraulic heads replicated the observed data to an acceptable matching of the measured water-level, flow direction, and vertical gradients. Simulated hydrographs for selected wells were in good agreement with the measured data, with an average residual of -2.7 ft and a standard deviation of 14.5 ft, indicating that the simulated heads, on average, underestimated the observed water levels. An examination of the model fit indicated that most of the discrepancies were lower simulated heads at wells proximal to production well sites. The simulated chloride concentrations reasonably matched the rising limbs of the measured breakthrough curves in terms of timing and magnitude; however, the simulation overestimated the chloride concentrations on the falling limbs. The overestimation of low chloride concentrations was attributed to the model overestimating the advance of the chloride front during periods of heavy pumping and underestimating the retreat of the chloride front during periods of low pumping. These simulation errors would result in a conservative response by local water managers to seawater intrusion. The SBFTM was used to develop a collection of predictive simulations optimized to produce pumping schedules that maximize yield, subject to a set of constraints and competing objectives. The simulations were grouped as scenarios that differed in their time horizon, initial conditions for groundwater levels and chloride concentrations, as well as precipitation, which was incorporated into the model through simulated recharge. Overall, five scenarios were developed in a multi-objective framework to obtain optimal pumping rates for all of the wells managed by the city, while minimizing excessive drawdown and seawater intrusion. For the current study, complexities in the simulation model and the optimization formulation required additional considerations. Incorporating the solute-transport equations to simulate chloride transport added a highly nonlinear process that is solved iteratively in each time step of the groundwater-flow model. These nonlinearities, coupled with the highly refined grid in the current model, creates challenges for many traditional optimization methods. Therefore, an optimization method was needed that could address nonlinear relationships as well as a very large problem size. Lastly, the optimization problem was reformulated to include multiple objectives without requiring convergence to a single solution. This approach, guided by the city's objectives, allowed the maximum extraction of information from the complex simulation. Borg, a multi-objective evolutionary algorithm, was chosen as the optimization algorithm for this study for several reasons: (1) it is very computationally efficient; (2) it can run in parallel; (3) it requires little user input; and (4) it can solve for multiple competing objectives. The first three points allow the algorithm to proceed toward the optimal solutions at the fastest possible rate. The fourth point is advantageous for large, complex optimization problems because it is difficult to formulate the optimization problem in a way that produces only one optimal solution. The problem formulation consisted of four competing objectives and a constraint set in accordance with the main concerns of the city. The objectives were maximizing total pumpage, minimizing seawater intrusion, minimizing total drawdown in production wells, and minimizing the maximum drawdown. The constraints were pump capacity, meeting drinking-water standards for chloride, maintaining a specified minimum flowrate to a groundwater treatment plant, and maintaining minimum water levels in pumping wells. The decision variables either were quarterly pumpage by well or total pumpage by basin. Five optimization scenarios were developed that allow the decision makers to evaluate a range of optimal solutions for a variety of water levels and chloride concentrations as well as potential future climatic conditions. Three scenarios (1, 2, and 5) were multi-objective optimization formulations that allowed for variations in management preferences and climatic conditions. The other two scenarios (3 and 4) were designed to examine the optimization results to answer specific questions. Scenario 1 described the best-case sustainable yield assuming a full basin (that is, high initial water levels) and typical climate conditions for 10 years. Scenario 2 also started with a full basin; however, this was followed by a 10-year drought. Scenario 3 determined if an empty basin (that is, low initial water levels) would recover to full conditions (1998 conditions) given climate assumptions and optimal pumping schedules from scenarios 1 and 2. Scenario 4 was designed to produce decision rules that can be used by water managers to help choose an optimal pumping schedule based on measured water-level or chloride data. Scenario 5 identified future pumping schedules based on short-term climate variations during a 2-year management horizon. The results from scenarios 1 and 2 described the differences in maximum pumpage in the basin under typical and dry long-term climate projections, respectively. The scenario 1 results indicated the maximum 10-year pumpage of the basin was about 31,300 acre-ft under typical conditions and controlling simulated seawater intrusion and drawdowns. For scenario 2, less recharge over the 10-year dry climate produced a maximum pumpage estimate of 30,000 acre-ft to control seawater intrusion and drawdowns. The larger pumpage for scenario 1 resulted in more seawater intrusion, but less total drawdown, compared to that of scenario 2. Results for scenarios 3 and 4 showed the basin's response to management actions combined with climate projections. Both scenarios used the optimal pumping schedules and the 10-year climates from scenarios 1 and 2. The scenario 3 results showed that under minimal pumping, the basin did not fully recover to 1998 water levels within 10 years under either climate scenario. The relatively larger recharge from the typical climate resulted in less drawdown at coastal monitoring wells after the 10-year recovery period than that from the dry climate. The location of the seawater intrusion front was not appreciably different between the scenarios, however. Scenario 4 used the optimal results from scenarios 1 and 2 to produce decision-rule curves that illustrated the pumpage for each basin, given measured levels of chloride concentration or drawdown. This allowed the use of additional measurements at monitoring wells to assess future management decisions on the basis of the sensitivity of observations of drawdown and seawater intrusion to various pumping rates. Scenario 5 allowed managers to investigate the effects of short-term climate variations on optimal pumping schedules. Three specific 2-year simulations were optimized: typical-to-dry (scenario 5A), dry-to-typical (scenario 5B), and dry-to-dry (scenario 5C). The most noteable result from scenario 5 was the overall reduction in optimal pumpage for most schedules in scenario 5C, when the climate is simulated as dry-to-dry. There are also many optimal pumping schedules that produced an overall increase in waterlevels over the two-year simulation period, regardless of climatic condition. Similar to scenario 2, the scenario 5C results represents conservative yield estimates under a minimal-precipitation climatic condition.自19世纪以来，地下水一直是圣巴巴拉市供水组合的组成部分；然而自20世纪60年代起，该市绝大多数供水来自本地地表水，剩余部分则来自地下水、州水资源工程（State Water Project）、再生水、强化节水措施，以及按需启用的海水淡化技术。 尽管圣巴巴拉和山麓地下水盆地的地下水仅占长期供水的一小部分，但在地表水短缺时期，它是重要的补充水源。20世纪80年代末至90年代初，为弥补其他水源因干旱导致的输水不足，生产井抽取了更多地下水，圣巴巴拉和山麓地下水盆地的水位大幅下降（圣巴巴拉地下水盆地的水位甚至低于海平面）。 在近岸抽取地下水的沿海盆地中，海水入侵通常是一个难题。圣巴巴拉地下水盆地的海水入侵被认为比其他沿海盆地更为有限，因为存在一条近海断层，可作为地下水流的部分屏障。20世纪80年代末至90年代初，圣巴巴拉地下水盆地观测到海水入侵现象：距离海洋200英尺至1300英尺、距离最近抽水井仅2900英尺的多口监测井中，氯离子浓度均有所上升。这表明，当海岸附近的地下水位低于海平面时，海水可入侵圣巴巴拉地下水盆地。 圣巴巴拉市希望更好地了解其地下水供应的可持续性。2014年，加利福尼亚州通过了具有历史意义的地下水管理立法：《可持续地下水管理法案》（Sustainable Groundwater Management Act, SGMA）。该法案要求在127个优先地下水盆地制定并实施地下水可持续性计划；尽管圣巴巴拉盆地未被列为优先盆地，但该市正采取措施实现可持续性。SGMA将可持续性定义为避免以下不良结果：地下水水位显著且不合理的下降、地下水储量减少、海水入侵、水质退化、地面沉降以及地表水枯竭。 本项目为美国地质调查局（U.S. Geological Survey, USGS）与圣巴巴拉市的合作研究，其中可持续产量被定义为：在选定井中，可从储层抽取而不会导致水位下降以及相关的海水入侵加剧（以实测氯离子浓度升高为指标）的地下水量。为评估圣巴巴拉地下水盆地的可持续性，研究团队在现有地下水流动模型的基础上，开发了三维密度依赖地下水流动与溶质运移模型——圣巴巴拉流动与运移模型（Santa Barbara Flow and Transport Model, SBFTM）。 为模拟不同管理策略下圣巴巴拉盆地的海水入侵情况，SBFTM采用USGS的SEAWAT代码模拟盐分运移与变密度流动。完成后的SBFTM与管理优化工具（本研究中为多目标进化算法）耦合，以确定最优抽水策略：在最大化可持续产量的同时，满足用户定义的水位下降与氯离子浓度约束。 作为本研究的一部分，研究团队开发了三维水文地质结构模型，以量化圣巴巴拉和山麓地下水盆地的范围与水文地质特征，并帮助确定SBFTM中的离散化方案与水力参数。水文地质结构模型的开发需要收集并整合现有地图、报告、数据库中的地质与地球物理数据，以及近期钻井获取的地质与水文数据。这些数据被整合为三维水文地质结构模型，该模型定义了含水层带的地层与几何形态，以及盆地内的主要地质构造。此外，该模型还以粗颗粒沉积物占比的形式，量化了每个含水层带内沉积物粒径的变化。 以往研究表明，圣巴巴拉地下水盆地存在两个主要产水带：上部产水带与下部产水带；本研究还识别出一个额外的薄产水带。该中部产水带的平面分布范围不及上下产水带，仅存在于储层单元I的沿海区域。上述产水带在深部被生产力较低的浅层、中层与深层带所限制。 研究团队构建了两个版本的SBFTM：初始条件模型与现代瞬态模型。初始条件模型为长期瞬态模型，用于模拟现代瞬态模型之前受人为影响有限时期的流动与溶质运移条件。瞬态模拟模型模拟了1929年至2013年的流动与运移条件；但受数据可得性限制，模型校准的重点时段为1972年至2013年。SBFTM采用自动与试错参数估计相结合的技术，以实测地下水位与水位下降量、实测氯离子浓度及其变化量作为校准依据。 敏感性分析表明，总体而言，SBFTM对补给与抽水分布参数最为敏感，具体包括控制小集水区补给量的参数，以及控制生产井按水文地质分层抽水分布的参数。此外，模型对控制溪流补给速率、水平与垂直水力传导率以及孔隙度的参数也较为敏感。 1929年至1971年，进入SBFTM代表区域的水源主要为溪流与小集水区补给，而离开该区域的水主要来自抽水、向溪流排泄以及排水。此外，总抽水量中约37%来自地下水储量的净减少。1972年至2013年，进入与离开SBFTM区域的水量与1929年至1971年基本相当，只是抽水减少为储层补充了约17000英亩-英尺的水量。在这一后期时段，也曾出现过储量损失的情况。例如，1990年7月，研究区域内约抽取了705英亩-英尺的地下水，此时抽水量远大于所有补给源的总和，约382英亩-英尺的水量从地下水储层中被抽取。 模拟的水力水头与实测数据匹配良好，符合实测水位、流动方向与垂直梯度的要求。选定井的模拟水文过程线与实测数据吻合度较高，平均残差为-2.7英尺，标准差为14.5英尺，表明模拟水头平均低估了实测水位。对模型拟合情况的分析表明，大部分偏差出现在靠近生产井的监测井中，模拟水头偏低。模拟的氯离子浓度在时间与量级上均较好地匹配了实测穿透曲线的上升段；但在下降段，模拟结果高估了氯离子浓度。这种低氯离子浓度下的高估现象，归因于模型在强抽水时期高估了氯离子锋面的推进，而在低抽水时期低估了氯离子锋面的后退。这类模拟误差会导致当地水资源管理者对海水入侵采取保守的应对措施。 SBFTM被用于开发一系列优化预测模拟，以生成抽水计划：在满足一系列约束与竞争目标的前提下最大化产量。这些模拟被分组为不同情景，情景间的差异体现在时间跨度、地下水位与氯离子浓度的初始条件，以及通过模拟补给纳入模型的降水情况。总体而言，本研究在多目标框架下开发了5个情景，以确定该市管理的所有供水井的最优抽水速率，同时最大限度减少过度水位下降与海水入侵。 针对本研究，模拟模型与优化公式的复杂性需要额外考量。纳入溶质运移方程以模拟氯离子输运，新增了一个高度非线性的过程，该过程需在地下水流动模型的每个时间步长中迭代求解。这些非线性特性，结合当前模型中高度精细化的网格，给许多传统优化方法带来了挑战。因此，需要一种能够处理非线性关系且可应对超大问题规模的优化方法。最后，优化问题被重构为包含多目标的形式，无需收敛至单一解。这种以该市目标为导向的方法，能够从复杂的模拟中最大化提取信息。 本研究选择多目标进化算法Borg作为优化算法，原因如下：（1）计算效率极高；（2）支持并行运行；（3）所需用户输入极少；（4）可求解多个竞争目标。前三点可使算法以尽可能快的速度趋近最优解，第四点则适用于大型复杂优化问题，因为很难通过单一方式构建仅产生单个最优解的优化问题。 问题公式包含四个竞争目标与一组约束，符合该市的主要关切。目标分别为：最大化总抽水量、最小化海水入侵、最小化生产井的总水位下降量、最小化最大水位下降量。约束条件包括：抽水能力、符合饮用水氯离子标准、维持地下水处理厂的指定最小流量，以及维持抽水井的最低水位。决策变量为单井季度抽水量或盆地总抽水量。 研究团队开发了5个优化情景，使决策者能够评估一系列最优解，涵盖不同水位、氯离子浓度以及潜在未来气候条件。其中情景1、2、5为多目标优化公式，允许管理偏好与气候条件发生变化；情景3、4则用于检验优化结果以回答特定问题。情景1假设盆地处于满水状态（即初始水位较高）且气候条件典型，模拟10年的最优可持续产量。情景2同样以满水状态为初始条件，但随后经历10年干旱。情景3旨在确定：在采用情景1与情景2的最优抽水计划且给定气候假设的前提下，空盆地（即初始水位较低）能否在10年内恢复至1998年的水位水平。情景4旨在生成决策规则，帮助水资源管理者根据实测水位或氯离子数据选择最优抽水计划。情景5则基于2年管理时段内的短期气候变异，确定未来抽水计划。 情景1与情景2的结果分别描述了典型与干旱长期气候预测下，盆地最大抽水量的差异。情景1的结果表明，在典型气候条件下，控制海水入侵与水位下降的前提下，盆地10年最大抽水量约为31300英亩-英尺。情景2中，10年干旱气候下补给量更少，为控制海水入侵与水位下降，最大抽水量估算为30000英亩-英尺。与情景2相比，情景1的抽水量更大，导致更多海水入侵，但总水位下降量更小。 情景3与情景4的结果展示了盆地在管理措施与气候预测共同作用下的响应。两个情景均采用了情景1与情景2的最优抽水计划以及10年气候条件。情景3的结果表明，在最小抽水量的情况下，无论采用哪种气候情景，盆地均无法在10年内完全恢复至1998年的水位水平。典型气候下的补给量相对更大，因此在10年恢复期后，沿海监测井的水位下降程度比干旱气候下更小。不过，两个情景下海水入侵锋面的位置并无显著差异。情景4利用情景1与情景2的最优结果生成了决策规则曲线，展示了在给定实测氯离子浓度或水位下降量的情况下，每个盆地的抽水量。这使得水资源管理者可以利用监测井的额外实测数据，根据水位下降与海水入侵对不同抽水速率的敏感性，评估未来管理决策。 情景5允许管理者研究短期气候变异对最优抽水计划的影响。研究团队优化了三个特定的2年模拟情景：典型转干旱（情景5A）、干旱转典型（情景5B）以及干旱转干旱（情景5C）。情景5最显著的结果是，在气候模拟为干旱转干旱的情景5C中，多数抽水计划的最优抽水量整体下降。此外，无论气候条件如何，均存在许多最优抽水计划可使两年模拟期内的水位整体上升。与情景2类似，情景5C的结果代表了最低降水气候条件下的保守产量估算。