Map 02: ArcGIS layer showing contours of the mean of October water levels during the 2000—2009 water years (feet)

Statistical analyses and maps representing mean, high, and low water-level conditions in the surface water and groundwater of Miami-Dade County were made by the U.S. Geological Survey, in cooperation with the Miami-Dade County Department of Regulatory and Economic Resources, to help inform decisions necessary for urban planning and development. Sixteen maps were created that show contours of (1) the mean of daily water levels at each site during October and May for the 2000-2009 water years; (2) the 25th, 50th, and 75th percentiles of the daily water levels at each site during October and May and for all months during 2000-2009; and (3) the differences between mean October and May water levels, as well as the differences in the percentiles of water levels for all months, between 1990-1999 and 2000-2009. The 80th, 90th, and 96th percentiles of the annual maximums of daily groundwater levels during 1974-2009 (a 35-year period) were computed to provide an indication of unusually high groundwater-level conditions. These maps and statistics provide a generalized understanding of the variations of water levels in the aquifer, rather than a survey of concurrent water levels. Water-level measurements from 473 sites in Miami-Dade County and surrounding counties were analyzed to generate statistical analyses. The monitored water levels included surface-water levels in canals and wetland areas and groundwater levels in the Biscayne aquifer. Maps were created by importing site coordinates, summary water-level statistics, and completeness of record statistics into a geographic information system, and by interpolating between water levels at monitoring sites in the canals and water levels along the coastline. Raster surfaces were created from these data by using the triangular irregular network interpolation method. The raster surfaces were contoured by using geographic information system software. These contours were imprecise in some areas because the software could not fully evaluate the hydrology given available information; therefore, contours were manually modified where necessary. The ability to evaluate differences in water levels between 1990-1999 and 2000-2009 is limited in some areas because most of the monitoring sites did not have 80 percent complete records for one or both of these periods. The quality of the analyses was limited by (1) deficiencies in spatial coverage; (2) the combination of pre- and post-construction water levels in areas where canals, levees, retention basins, detention basins, or water-control structures were installed or removed; (3) an inability to address the potential effects of the vertical hydraulic head gradient on water levels in wells of different depths; and (4) an inability to correct for the differences between daily water-level statistics. Contours are dashed in areas where the locations of contours have been approximated because of the uncertainty caused by these limitations. Although the ability of the maps to depict differences in water levels between 1990-1999 and 2000-2009 was limited by missing data, results indicate that near the coast water levels were generally higher in May during 2000-2009 than during 1990-1999; and that inland water levels were generally lower during 2000-2009 than during 1990-1999. Generally, the 25th, 50th, and 75th percentiles of water levels from all months were also higher near the coast and lower inland during 2000–2009 than during 1990-1999. Mean October water levels during 2000-2009 were generally higher than during 1990-1999 in much of western Miami-Dade County, but were lower in a large part of eastern Miami-Dade County.

本数据集由美国地质调查局（U.S. Geological Survey）联合迈阿密-戴德县监管与经济资源部（Miami-Dade County Department of Regulatory and Economic Resources）完成，包含表征迈阿密-戴德县地表水与地下水平均、最高及最低水位状态的统计分析结果与专题地图，旨在为城市规划与开发相关决策提供支撑。
本次共编制16幅等值线专题地图，分别展示：（1）2000-2009水文年期间，10月与5月各监测站点日水位的平均值；（2）2000-2009年期间，10月、5月及全月度各监测站点日水位的25th、50th、75th百分位数；（3）1990-1999年与2000-2009年两个时段间，10月与5月平均水位的差值，以及全月度水位百分位数的差值。
另针对1974-2009年（共35年）的逐日地下水最高水位序列，计算其80th、90th、96th百分位数，用以表征异常偏高的地下水水位状态。本数据集及专题地图仅用于概括性认知含水层水位的变化规律，而非同步开展的水位普查成果。
研究团队对迈阿密-戴德县及周边县域内473个监测站点的水位数据进行统计分析，监测水位类型涵盖运河与湿地的地表水位，以及比斯坎含水层（Biscayne aquifer）的地下水水位。
专题地图的编制流程如下：将监测站点坐标、水位统计汇总结果及记录完整性统计数据导入地理信息系统（Geographic Information System, GIS），并对运河监测站点与海岸线处的水位数据进行空间插值；采用不规则三角网（Triangular Irregular Network, TIN）插值法，基于上述数据生成栅格表面，再通过地理信息系统软件对栅格表面生成等值线。
由于现有信息不足以完整评估区域水文条件，部分区域的等值线精度不足，因此需在必要时进行人工修正。此外，由于多数监测站点在1990-1999年、2000-2009年其中一个或两个时段内的记录完整率未达80%，对两个时段水位差异的评估能力在部分区域受到限制。
本次分析的质量还受限于以下因素：（1）空间监测覆盖存在缺陷；（2）在运河、堤坝、滞留池、调蓄池或水控设施新建或拆除的区域，将施工前后的水位数据合并分析；（3）无法考量垂向水力梯度对不同深度监测井水位的潜在影响；（4）无法对不同时段的日水位统计结果进行偏差校正。因上述局限性带来的不确定性，在通过近似方法确定等值线位置的区域，等值线以虚线形式标注。
尽管缺失数据限制了1990-1999年与2000-2009年水位差异的制图表现力，但分析结果显示：2000-2009年期间，海岸附近5月的水位整体高于1990-1999年同期；而内陆区域的水位整体低于1990-1999年同期。整体而言，2000-2009年全月度水位的25th、50th、75th百分位数同样呈现沿海偏高、内陆偏低的特征。2000-2009年10月的平均水位，在迈阿密-戴德县西部大部分区域整体高于1990-1999年同期，但在东部大部分区域则整体低于此前时段。