Hydrology - Subsurfaceflow
NLDAS Surface Baseflow
The North American Land Data Assimilation System (NLDAS) combines North American radar data and satellite data from CMORPH (https://www.cpc.ncep.noaa.gov/products/janowiak/cmorph_description.html). NLDAS has a one-hour time step on a 0.125-degree grid of North America, with an average time delay of four days for data retrieval. NLDAS has data coverage from January 1, 1979 to the present. NLDAS data are reported in UTC (GMT).
GLDAS Surface Baseflow
The Global Land Data Assimilation System (GLDAS) combines satellite data and ground-based observational data to provide baseflow and other hydrologic parameters. GLDAS has a three-hour time step on a global 0.25-degree grid. GLDAS-2.1 provides data coverage from January 1, 2000 to present, with an average time delay of one month for data retrieval. GLDAS data are reported in UTC (GMT).
Curve Number
Curve Number algorithm in HMS is implemented using Normalized Difference Vegetation Index (NDVI). 16-day interval NDVI satellite data from MODIS (https://modis.gsfc.nasa.gov/data/dataprod/mod13.php) at 250 meter resolution for the period of 2001-2018 along with soil hydrological group data was used to calculate CN for each NHDPlus catchment. The CN calculation methodology was based on an earlier published study ( https://www.sciencedirect.com/science/article/pii/S0301479718315433). More details on the HMS CN calculation methodology are available at (Curve Number Development using Normalized Difference Vegetation Index). CN Subsurface flow is calculated as a percentage of surface runoff calculated by Curve Number. StreamCat is used to determine percentage of subsurface flow.
Temporal Aggregations
The available temporal aggregations are dependent upon the native timestep size of the data source. Possible options include 'daily', for those sources which are not by default daily, and 'monthly'. Aggregated surface baseflow data are the totals over these time periods and provided in the aggregated timeseries. Monthly aggregations correspond to the calendar month, and require the entire month to be specified in the date time span.