Using previously published (Robinson and others, 2019) no-flow fractions and L-moments of nonzero streamflow from decadal streamflow flow-duration analysis (daily mean streamflow), probability distributions were fit to provide 27 estimated quantiles of decadal flow-duration curves, and hence the probability distributions are a form of parametric modeling that ensures monotonicity of the quantiles by non-exceedance probability (NEP). For both U.S. Geological Survey streamflow-gaging stations (streamgages) and level-12 hydrologic unit code (HUC12) catchments, as defined by Crowley-Ornelas and others (2019), the 27 quantiles were estimated and tabulated in this data release. Three probability distributions were used and are summarized by Asquith and others (2017): the asymmetric exponential power (AEP4) (4-parameter), generalized normal (GNO) (3-parameter log-normal), and kappa (KAP) (4-parameter). A summary of the mathematics for these distributions is provided in the README files within this data release and close consultation of the mathematical discussion in Asquith and others (2017) also is suggested. The lmomco R package (Asquith, 2020) was used for distribution fitting and the technically-demanding implementation for a single location is archived in the RESTORE/fdclmrpplo software release within file fdclmrpplo/scripts/pred_fdc_ref/pred_fdc_ref.R (Asquith and others, 2020). The implementation for the streamgages is archived in the RESTORE/fdclmrpplo software release within file fdclmrpplo/scripts/pred_fdc_gage/pred_fdc_gage.R, and the implementation for the HUC12s is archived file fdclmrpplo/scripts/pred_fdc_huc12/pred_fdc_huc12.R and README files therein. For a given data set of no-flow fraction and L-moments, the three distributions will have similar results in the central parts of NEP and differences will be largest in the far left (low flow) and far right (flood flow) tails. No opinion that a particular distribution is more suitable than another is provided with exception that the GNO is fit to the first three L-moments and the AEP4 and KAP are fit to the first four L-moments. As a result, it is logical to state that more information on the distribution of streamflow is retained by the AEP4 and KAP distributions than the GNO. The availability of three distributions with the data release is considered a feature because a semi-quantitative assessment of model error (uncertainty attributed to choice of model) can be made. Asquith, W.H., 2020, lmomco—L-moments, censored L-moments, trimmed L-moments, L-comoments, and many distributions: R package version 2.3.6, https://CRAN.R-project.org/package=lmomco. Asquith, W.H., Kiang, J.E., and Cohn, T.A., 2017, Application of at-site peak-streamflow frequency analyses for very low annual exceedance probabilities: U.S. Geological Survey Scientific Investigation Report 2017–5038, 93 p., https://doi.org/10.3133/sir20175038. Asquith, W.H., Knight, R.R., and Crowley-Ornelas, E.R., 2020, RESTORE/fdclmrpplo—Source code for estimation of L-moments and percent no-flow conditions for decadal flow-duration curves and estimation at level-12 hydrologic unit codes along with other statistical computations: U.S. Geological Survey software release, Reston, Va., https://doi.org/10.5066/P93CKH92. Crowley-Ornelas, E.R., Worland, S.C., Wieczorek, M.E., Asquith, W.H., Knight, R.R., 2019, Summary of basin characteristics for National Hydrography Dataset, version 2 catchments in the Southeastern United States, 1950–2010: U.S. Geological Survey data release, https://doi.org/10.5066/P9KXTDU4. Robinson, A.L., Asquith, W.H., and Knight, R.R., 2019, Summary of decadal no-flow fractions and decadal L-moments of nonzero streamflow flow-duration curves for National Hydrography Dataset, version 2 catchments in the southeastern United States, 1950–2010: U.S. Geological Survey data release, https://doi.org/10.5066/P9Z4PM55.

Censored and uncensored generalized additive models (GAMs) are developed from 955 U.S. Geological Survey streamflow-gaging stations (streamgages) to predict decadal statistics of streamflow. The streamgages are located on streams draining to the Gulf of Mexico. Decadal statistics include no-flow fractions and selected L-moments of nonzero streamflow for six decades (1950s—2000s). These statistics represent metrics of decadal flow-duration curves (dFDCs) derived from about 10 million daily mean streamflows. The L-moments include the mean, coefficient of L-variation, and the third through fifth L-moment ratios. The models are fit using watershed properties such as basin area and slope, decadal precipitation and temperature and decadal values of flood storage and urban development percentages. The GAMs then estimate decadal statistics for 8,988 prediction locations (stream reaches) coincident with outlets of level-12 hydrologic unit codes. Both the entire dataset (whole model) and leave-one-watershed-out model results are reported. No-flow fractions are censored data and Tobit extensions to GAMs are effective in estimation of ephemeral streamflow conditions. Uncensored GAMs conversely were used for estimation of the L-moments. The R language was used to pull and process the streamflow data, and the scripts can be found online at https://code.usgs.gov/water/restore/fdclmrpplo.

Censored and uncensored generalized additive models (GAMs) are developed from 955 U.S. Geological Survey streamflow-gaging stations (streamgages) to predict decadal statistics of streamflow. The streamgages are located on streams draining to the Gulf of Mexico. Decadal statistics include no-flow fractions and selected L-moments of nonzero streamflow for six decades (1950s—2000s). These statistics represent metrics of decadal flow-duration curves (dFDCs) derived from about 10 million daily mean streamflows. The L-moments include the mean, coefficient of L-variation, and the third through fifth L-moment ratios. The models are fit using watershed properties such as basin area and slope, decadal precipitation and temperature and decadal values of flood storage and urban development percentages. The GAMs then estimate decadal statistics for 8,988 prediction locations (stream reaches) coincident with outlets of level-12 hydrologic unit codes. Both the entire dataset (whole model) and leave-one-watershed-out model results are reported. No-flow fractions are censored data and Tobit extensions to GAMs are effective in estimation of ephemeral streamflow conditions. Uncensored GAMs conversely were used for estimation of the L-moments. The R language was used to pull and process the streamflow data, and the scripts can be found online at https://code.usgs.gov/water/restore/fdclmrpplo.

Two methods of calculating hydrologic alteration were applied to modeled daily streamflow data for 9,201 12-digit hydrologic unit code (HUC12) pour points draining to the Gulf of Mexico (Robinson and others, 2020). The first method is a new modified method of calculating ecosurplus and ecodeficit called hydro change. For this project, ecosurplus and ecodeficit have been combined to assess overall hydrologic regime change. The second method is the confidence interval hypothesis test (Kroll and others, 2015). The first method is a means of quantifying hydrologic alteration while the second is a hypothesis test to simply determine if statistically significant alteration has occurred. Both methods are employed to determine which is best at analyzing alteration of the hydrologic regime in the Gulf Coast Ecosystem Restoration Council (RESTORE) study area. Statistical analysis was done in RStudio (2020). The data release includes four attached files: (1) metadata .xml file, (2) csv with the p-values for each HUC12, (3) csv with results from the hydrologic change analysis, and (4) the shapefile of the pour point locations for the HUC12s used in the analyses.

This data release contains estimated daily streamflow values for 9,203 level-12 hydrologic unit codes (HUC12) in the southeastern United States for the period 1950-2009. The files are provided in text different format, which can be used in R (https://www.r-project.org/) and in other programs. The 'Site Info' files contain the streamgage information and the 'Daily Values' files contain the predicted daily streamflow value for a given HUC12. Each of the datasets contains 500, 501, or 704 HUC12s, respectively. Data preparation and analysis of the predicted streamflow values were scripted in RStudio (version 3.5.3). Results of the analysis will be used to quantify streamflow alterations across the study area and will be published in a separate product.

Daily streamflow discharge data from 139 streamgages located on tributaries and streams flowing to the Gulf of Mexico were used to calculate mean monthly, mean seasonal, and decile values. Streamgages used to calculate trends required a minimum of 65 years of continuous daily streamflow data. These values were used to analyze trends in streamflow using the Mann-Kendall trend test in the R package entitled “Trends” and a new methodology created by Robert M. Hirsch known as a “Quantile-Kendall” plot. Data were analyzed based on water year using the Mann-Kendall trend test and by climate year using the Quantile-Kendall methodology to: (1) identify regions which are statistically similar for estimating streamflow characteristics; (2) identify trends related to changing streamflow and streamflow alteration over time; and (3) to identify possible correlations with estuary health in the Gulf of Mexico.

This dataset provides numerical and categorical descriptions of 48 basin characteristics for 9,314 ungaged basins coinciding with 12-digit hydrologic unit code (HUC12) pour points that drain to the Gulf of Mexico. Characteristics are indexed by National Hydrography Dataset (NHD) version 2 COMID (integer that uniquely identifies each feature in the NHD) and HUC12 identifying number. The variables represent mutable and immutable basin characteristics and are organized by characteristic type: physical (5), hydrologic (6), categorical (12), climate (6), landscape alteration (7), and land cover (12). Mutable characteristics such as climate, land cover, and landscape alteration variables are reported in decadal increments (for example, average percent forest for the decade 1950-1959, 1960-1969, etc). The majority of basin characteristics in this dataset were calculated using divergence-routing methods and are often referred to as “network-accumulated”. This method uses a modified routing database to navigate the NHDPlus reach network to aggregate (accumulate) the values derived from the reach catchment scale (Schwarz, G.E., and Wieczorek, M.E., 2018, Database of modified routing for NHDPlus version 2.1 flowlines: ENHDPlusV2_us: U.S. Geological Survey data release, https://doi.org/10.5066/P9PA63SM ). In four instances, values are also provided for the entire catchment above a site and area designated using the “CAT_” prefix.