Workflow - Streamflow
Using Streamflow
Currently, this workflow has three different stream flow algorithms: NLDAS, GLDAS, and Curve Number (CN).To calculate Streamflow for each NHDPlus catchment in the selected stream network, data from 3 separatedatasets is downloaded simultaneously when NLDAS or GLDAS algorithm. Data for precipitation, surfacerunoff, and subsurface flow is downloaded from the user-specified source in the GMT time-zone localization.Only Precipitation data in GMT is downloaded when CN is selected. Data cannot be retrieved for the user'slocal time-zone.
Handling Missing Data
Depending on the size of the specified timespan and area of interest, many requests to external serverswill need to be initiated in parallel. Occasionally, these servers will become overloaded by the volumeof these requests and return error codes. The streamflow workflow will attempt to retry failed requestsseveral times in a staggered manner. However, if they continue to fail, data for that particular segmentwill be replaced with a -9999 value.
Total Flow
The algorithm for calculating streamflow involves adding up the total flow (surface and subsurface) perarea of the stream, then checking for boundary conditions and adding in existing flow from upstream. Theworkflow currently supports only the constant volume stream hydrology algorithm, but changing volume andkinematic wave algorithms are being developed.
Constant Volume Routing
Flow Routing Techniques for Environmental Modeling is a simple method using only the continuity equation where flow going out of the segment is equal tothe flow going into the segment. Volume, velocity, and depth remain constant with changing flows in this scheme.
NLDAS and GLDAS
NLDAS uses Noah Land Surface Model L4 (NLDAS Noah Land Surface Model Summary) to calculate hourly surface and subsurface runoff hourly at 0.125X0.125 degree resolution for North America. GLDAS also uses Noah Land Surface Model L4 (GLDAS Noad Land Surface Model Summary) to calculate 3-hourly surface and subsurface runoff at 0.25X0.25 degree resolution for most of the globe. HMS streamflow workflow downloads NLDAS and GLDAS precipitation, surface runoff, and subsurface runoff data from NASA Hydrology Data Rods and performs temporal and flow aggregations and stream network flow routing.
DAYMET Precipitation
DAYMET is a daily dataset of rain gauge data that has been interpolated and extrapolated. DAYMET uses ground station data with their model algorithm to produce gridded estimates of daily weather parameters. The interpolated spatial resolution is about a 0.009-degree grid over North America. Data is accessible since 1980 to the latest full year. DAYMET discards values for December 31 from leap years to maintain a 365-day year. DAYMET data are reported in UTC (GMT).
PRISM Precipitation
The Parameter-elevation Relationship on Independent Slopes Model (PRISM) is a combined dataset consisting of ground gauge station and RADAR products. The data is on a 4km grid resolution covering the contiguous United States. Data is available from 1981 to present.PRISM data are reported in GMT (UTC).
Curve Number
Curve Number algorithm in HMS is implemented using Normalized Difference Vegetation Index (NDVI). 16-day interval NDVI satellite data from MODIS (MODIS Vegetation Index Products (NDVI and EVI)) 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 (Phenology-adjusted dynamic curve number). More details on the HMS CN calculation methodology are available at (Curve Number Development using Normalized Difference Vegetation Index). CN algorithm in the streamflow workflow uses average curve number for the catchment being modeled.