To decide which watersheds were best for processing with VELMA, our first step was to select a pour point. The United States Geological Survey maintains a website called WaterWatch which provides and displays data for real-time and past stream flow conditions across the United States. For our research, we utilized the website’s map to locate and select each gauge station. Once you select a gauge station, you can view and download a variety of information including daily flow rate, water temperature, and statistics.
The four watersheds chosen for this project (the Noyo, Eel, Trinity, and Russian) were selected based of their location within Northern California, overall drainage area, and availability of data. After selecting the watersheds and the pour points, the bulk of the processing/work was in collecting, editing, and converting data into the correct format to be input into VELMA.
Once the pour point for a watershed was selected, the daily gauge station flow data from WaterWatch was downloaded and then converted from cubic feet per second to millimeters per day.
Next digital elevation models (DEM) were prepared, starting by downloading and mosaicking together multiple DEMs to encompass the entire watershed areas. In order for VELMA to run, it is essential for the entire watershed to fall within the DEM with about a 10% buffer. We chose to use ASTER 30 meter DEMs for our analysis because of its wide availability and high resolution. The DEM then needs to be flat processed and formatted properly. We chose to use the GIS software BlueSpray to prepare the digital elevation models for VELMA. BlueSpray has a built in water transform that can flat process and convert DEMs into the correct format for VELMA in 7 steps.
Next daily air temperature and precipitation were downloaded from NOAA weather stations. Since NOAA data can sometime be patchy and inconsistent, Daymet data was also downloaded to fill in the missing values. Daymet data is more generalized in area but is very consistent with daily readings. These two parameters and data sources were then merged and converted into the proper formatting for VELMA.
The subset DEMs processed in BlueSpray were then used to produce the data on tree age, tree type, and stream chemistry using the software LandTrendr. Finally, the Soil Web Survey can be used to produce the soils layer. It is important to note that the soil layer is considered the least significant parameter during VELMA analysis, and any detailed soil data files should be produced last for your project (if at all). For our project we chose to create a generalized soil data file, classify all pixels as sandy loam.
Once all the data was collected and properly formatted, the VELMA simulation can be run. Our watersheds were run on the newest version of VELMA, which is available for download. Simulations must run uninterrupted until completion, and run time will vary depending mostly on the size of the watershed and the DEM resolution. For our watersheds, the simulation took approximately 5 1/2 hours for Noyo, 11 1/2 hours for Trinity, and 12 hours for Eel. We were unable to successfully run the Russian River VELMA simulation to completion.