Map 16: ArcGIS layer showing contours of the difference in the 75th percentile of all water levels from the water-year periods 1990 to 1999 and 2000 to 2009 (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，旨在为城市规划与开发相关决策提供科学支撑，内容涵盖迈阿密-戴德县地表水与地下水的平均、最高及最低水位条件的统计分析与专题地图。
研究共编制16幅专题地图，分别展示三类内容：①2000-2009水年（water year）期间10月与5月各监测站点日水位的平均值；②2000-2009水年期间10月、5月及全月度各监测站点日水位的25分位数、50分位数与75分位数；③1990-1999年与2000-2009年两个时段内，10月与5月平均水位的差值，以及全月度水位分位数的差值。
此外，研究计算了1974-2009年（共35年）内各监测站点日地下水位年最大值的80分位数、90分位数与96分位数，用于表征极端偏高地下水位状况。本数据集与相关地图提供了对含水层水位变化的广义认知，而非同步水位普查结果。
本研究对迈阿密-戴德县及周边县域内473个监测站点的水位观测数据开展分析，监测对象包括运河与湿地的地表水水位，以及比斯坎含水层（Biscayne aquifer）的地下水水位。
专题地图的制作流程为：将监测站点坐标、水位统计汇总结果与观测记录完整性统计结果导入地理信息系统（Geographic Information System, GIS），对运河监测站点水位与沿海岸线水位进行空间插值，采用不规则三角网（triangular irregular network）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。
由于现有水文数据有限，部分区域的等值线绘制存在精度不足的问题，因此需针对该类区域进行人工修正。同时，部分区域的1990-1999年与2000-2009年水位差值分析存在局限性，原因是多数监测站点在其中一个或两个时段内的观测记录完整率未达到80%。本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在修建/拆除运河、堤防、滞留塘、调蓄池或水控设施的区域，将设施建设前后的水位数据合并分析；③无法考量不同深度监测井内垂向水力梯度对水位的潜在影响；④无法对不同时段的日水位统计结果进行一致性校正。因上述局限性带来的不确定性，当等值线位置为近似估算所得时，将采用虚线表示。
尽管因数据缺失导致1990-1999年与2000-2009年水位差值的制图能力受限，但分析结果显示：沿海区域2000-2009年5月的水位普遍高于1990-1999年同期，而内陆区域同期水位则普遍低于1990-1999年；全月度水位的25分位数、50分位数与75分位数也呈现出沿海偏高、内陆偏低的2000-2009年时段特征。2000-2009年10月平均水位在迈阿密-戴德县西部大部分区域普遍高于1990-1999年同期，但在东部大部分区域则普遍低于后者。