Grid Cells and Modeled Groundwater Levels to Characterize Hydrologic Conditions for Public-supply Aquifers in California's Central Valley, 1990-2020

This data release documents grid cells and modeled groundwater levels for hydrologic characterization of aquifer areas used for public drinking-water supply in California’s Central Valley from 1990 to 2020. Observations of spring groundwater level (median depth to groundwater measured from January through April) were calculated for selected wells from the California Department of Water Resources (DWR) periodic groundwater level measurements dataset (DWR, 2021) that had data for less than 50% of years from 1990 to 2020. Spring groundwater levels for years without data were imputed by multiple linear regression on the top five most correlated records within the same Central Valley subregion (Sacramento Valley, San Joaquin Basin, or Tulare Basin) using established methods described by Evans and others (2020). This resulted in annual time series for spring groundwater level at 3,239 locations throughout the Central Valley from 1990 to 2020. Observed and imputed spring groundwater level data were used to interpolate spring groundwater levels throughout the Central Valley at a five-kilometer resolution for individual water years from 1990 to 2020 using ordinary kriging. Annual time series for spring groundwater levels and associated kriging error were extracted from kriging model output at the locations of 6031 public-supply wells (PSWs). Spring groundwater levels interpolated to the locations of PSWs are not intended to approximate measured groundwater head values for specific geologic strata, but rather to assess the general direction and magnitude of interannual groundwater level changes in areas used for public-drinking water supply. The area used for public-drinking water supply was discretized into 187 hexagonal grid cells for geostatistical characterization of the groundwater resource. This data release contains five tables formatted as tab-delimited text files and one geospatial shapefile containing hexagonal grid cells. Table 1 contains the location (coordinates and associated subregion) and observed spring groundwater levels (as feet below ground surface) for 3,239 wells in the DWR dataset, with years with no data represented as NAs. Table 2 is identical to Table 1 but contains imputed groundwater levels in place of NAs. Table 3 contains kriging parameters (nugget, sill, range) for each water year from 1990 to 2020. Table 4 contains depth to groundwater (in feet) interpolated to the locations of 6031 PSWs in the Central Valley. Table 5 contains kriging errors (in feet) for the corresponding interpolated groundwater levels presented in table 4. Depth measurements are presented in feet for consistency with DWR source data. These data support the following publication: Levy, Z.F., Jurgens, B.C., Burow, K.R., Voss, S.A., Faulkner, K.E., Arroyo-Lopez, J.A., Fram, M.S., In-review, Critical aquifer overdraft accelerates groundwater-quality degradation in California's Central Valley during drought: Geophysical Research Letters. References California Department of Water Resources [DWR], 2021, Periodic Groundwater Level Measurements: https://data.ca.gov/en/dataset/periodic-groundwater-level-measurements. Accessed online: January, 2021. Evans, S.W., Jones, N.L., Williams, G.P., Ames, D.P., and Nelson, E.J., 2020, Groundwater Level Mapping Tool: An open source web application for assessing groundwater sustainability: Environmental Modelling and Software, 131, 104782. https://doi.org/10.1016/j.envsoft.2020.104782.

本数据集发布了1990年至2020年期间，美国加利福尼亚州中央谷（California's Central Valley）用于公共饮用水供水的含水层区域的水文特征表征所需的网格单元与模拟地下水位数据。
针对加利福尼亚州水资源部（California Department of Water Resources, DWR）2021年发布的周期性地下水位观测数据集（DWR, 2021），本研究从中选取观测井，针对1990-2020年间有效观测年限不足总年限50%的井，计算其春季地下水位——即1月至4月期间测定的地下水位埋深中位数。
针对无观测数据的年份，采用Evans等人（2020）提出的成熟方法，通过对同一中央谷子区域（萨克拉门托谷、圣华金盆地或图莱里盆地）内相关性排名前五的观测记录进行多元线性回归，插补缺失的春季地下水位数据。
最终得到1990-2020年期间，中央谷全域3239个点位的春季地下水位年时序序列。
基于观测与插补得到的春季地下水位数据，采用普通克里金法（ordinary kriging），对1990-2020年每个水文年的中央谷全域开展5公里分辨率的春季地下水位空间插值。
从克里金模型输出结果中，提取6031口公共供水井（public-supply wells, PSWs）点位处的春季地下水位年时序序列及其对应克里金插值误差。
插值得到的公共供水井点位春季地下水位，并非用于近似特定地质地层的实测地下水头值，而是用于评估公共饮用水供水区域内地下水位年际变化的总体趋势与幅度。
为开展地下水资源的地统计特征表征，将公共饮用水供水区域离散为187个六边形网格单元。
本数据集包含5个制表符分隔格式的文本文件表格，以及1个存储六边形网格单元的地理空间形状文件（shapefile）。
表格1包含DWR数据集中3239口观测井的位置信息（坐标及所属子区域）与实测春季地下水位（单位为英尺，以地下表面为基准面），无数据年份以NAs标识。
表格2与表格1内容一致，但将NAs替换为插补得到的地下水位数据。
表格3包含1990-2020年每个水文年的克里金插值参数（块金值、基台值、变程）。
表格4包含插值得到的中央谷6031口公共供水井点位处的地下水位埋深数据（单位为英尺）。
表格5包含与表格4中插值地下水位对应的克里金插值误差（单位为英尺）。
为与DWR源数据保持一致，所有埋深数据均以英尺为单位。
本数据集支撑以下已投稿论文：Levy, Z.F.、Jurgens, B.C.、Burow, K.R.、Voss, S.A.、Faulkner, K.E.、Arroyo-Lopez, J.A.、Fram, M.S.（审稿中），《干旱期间加利福尼亚州中央谷关键含水层超采加剧地下水水质退化》，发表于《Geophysical Research Letters》。
参考文献：
1. 加利福尼亚州水资源部（California Department of Water Resources, DWR）, 2021, 《周期性地下水位观测数据集》：https://data.ca.gov/en/dataset/periodic-groundwater-level-measurements. 在线访问时间：2021年1月。
2. Evans, S.W.等人, 2020, 《地下水位绘图工具：用于评估地下水可持续性的开源Web应用》, 《Environmental Modelling and Software》, 第131卷, 第104782号. https://doi.org/10.1016/j.envsoft.2020.104782.