Hydrographs lab preparation

The most robust methods for calculating temporal summary statistics of stream flow starts with knowing how to calculate the total volume of flow over a given period of time. This calculation is an exercise in numerical integration. Here is a detailed description of editing a spreadsheet to integrate the total volume of flow over a day from an instantaneous flow hydrograph. The steps of numerical integration should look familiar relative to the dilution gauging lab activities. The dimensions being integrated are different, but the numerical approaches are identical (8:57 min).

The USGS is modernizing the web interface to the National Water Information Service, though the changes made to the interface so far will not yet affect the exercises outlined in this lab. Here is an reminder from lecture of what you might see when you come across the new interface first, and how to get to the old interface (2:02 min).

To start exploring your own spreadsheet calculations on hydrographs from watersheds of your choice in the National Water Information System, you first need to know how to download data in a form that can be imported to Excel.

Note that the following video uses the old NWIS interface. The primary focus of this video is to download daily mean flow for a gauge and water year of choice, which to date still requires use of the old interface. The newer interface is currently only for the instantaneous flows. If you select the "Daily Data" option from the menu on the new interface, you will be directed to the tool for viewing and downloading daily mean data using the old interface.

Finally note that a couple times in the following video I say that NWIS graphs the "mean daily flow" (see about 4:54), but that is not quite correct because the graphing tool provides the "median daily flow". Medians and means are technically different statistics but have similar interpretations of representing the flow you might expect for a given day based on the history of the gauge. The median is just less affected by extreme values than the mean (14:02 min).

Once you have data in a tabular text file format, you will need to import those data into Excel for analysis. Here is a description of how to import text data using the newer Excel "power query" tool. The "power query" is more flexible and has more features that the older method of importing data, but that comes with some additional complexity in learning how to use it (16:13 min).

The older feature used for importing software is still available in Excel (at the time of the last update of this material). Here is a description of how to turn on the legacy features and use the legacy tools to import data. Unless you are curious, there is no need to study this video if you have successfully learned how to use the power query tool. While this tool is considerably simpler, it is probably better to start building proficiency with the more powerful tools of data management (9:50 min).

Once you have watershed data, you are ready to start playing with the typical approaches to watershed analysis for any hydrograph you choose! For this lab activity, you are asked to perform annual flow duration curve analyses, comparing and contrasting watersheds with different "flashiness" and storage capacity. Here is a detailed description of building an Excel spreadsheet to calculate the exceedance probabilities necessary for a flow duration curve analysis (14:47 min).

Remaining lab activities will ask you to do a risk analysis based on a flood frequency curve. The following two lecture videos are especially relevant to this lab. No need to study these again if you are comfortable with the concepts after study of the lecture materials. You should be particularly sure you understand the lecture materials regarding reading flood frequency curve graphs... (7:13 min)

and compounding exceedance probabilities over multiple years (9:57 min).