Map 03 - Contours of the 25 percentile of May 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.

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），针对迈阿密-戴德县地表水与地下水的平均、最高及最低水位状况开展了统计分析并编制专题地图，旨在为城市规划与开发所需的决策提供支撑依据。

本次研究共生成16幅等值线地图，涵盖三类内容：（1）2000-2009水文年（water years）内，各监测站点10月与5月的日水位平均值；（2）2000-2009水文年的10月、5月及全年度内，各监测站点日水位的25分位数、50分位数与75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，以及全年度水位分位数的差值。

此外，研究计算了1974-2009年（共35年）内各年度日地下水位最大值的80分位数、90分位数与96分位数，用以表征极端偏高的地下水位状况。

本次统计分析基于迈阿密-戴德县及周边县域内473个监测站点的水位监测数据，监测内容涵盖运河与湿地的地表水水位，以及比斯坎含水层（Biscayne aquifer）中的地下水水位。本套地图与统计数据旨在概览含水层（aquifer）内水位的变化特征，而非同步水位的实地调查结果。

地图编制流程如下：将监测站点坐标、水位统计摘要及记录完整性统计数据导入地理信息系统（Geographic Information System, GIS），对运河监测站点水位与海岸沿线水位进行空间插值；随后采用不规则三角网（Triangular Irregular Network, TIN）插值法生成栅格表面，再通过GIS软件对栅格表面进行等值线绘制。

由于现有信息不足以完整评估区域水文条件，部分区域的等值线存在精度缺陷，因此研究人员在必要时对等值线进行了人工修正。

部分区域无法有效对比1990-1999年与2000-2009年的水位差异，原因是多数监测站点在其中一个或两个时段内的记录完整率未达到80%。本次分析的质量受到以下因素限制：（1）空间监测覆盖范围存在缺陷；（2）在运河、堤坝、滞留塘、调蓄池或水控设施加装或拆除的区域，将建设前后的水位数据合并分析；（3）无法考虑垂直水力梯度对不同深度井孔水位的潜在影响；（4）无法对日水位统计数据的差异进行校正。由于上述局限性带来的不确定性，部分区域的等值线采用虚线绘制，以表示其位置为近似估算结果。

尽管因数据缺失限制了1990-1999年与2000-2009年水位差异的展示效果，分析结果仍显示：2000-2009年期间，海岸附近5月的水位普遍高于1990-1999年同期，而内陆地区水位则普遍低于此前时段；全年度水位的25分位数、50分位数与75分位数同样呈现出近海偏高、内陆偏低的特征，且该趋势在2000-2009年较1990-1999年更为显著。2000-2009年10月的平均水位在迈阿密-戴德县西部大部分区域普遍高于1990-1999年同期，但在该县东部大片区域则普遍低于此前时段。