Have you wondered how to calculate water level elevations in Hydro GeoAnalyst (HGA) if your Top of Casing (TOC) changes over time? You don’t want old water levels now referencing your new TOC. Here is how you would set up things up in HGA to have your water level elevations calculated when the TOC changes over time. I am using the Demo project provided within HGA to demonstrate this.
First lets take a look at Station W-05 on the Station Data tab – we want to check out the water levels that have been measured (select the Monitoring Event category and the Water Level table). There are five measurements from January to August of 1996. These measurements are depth to water level and are in reference to the TOC.
Now lets add some information to the Well History category TOC History table. Add one record for the original TOC which spans from January 1 to June 1 of 1996. Then add a second record to reflect the new TOC elevation starting June 1 and going to some point in the future (e.g. January 2018). This means the August water level measurement should reference the new TOC elevation.
And now we set up a query to do the calculation for us – its fairly straightforward (TOC elevation minus depth to water level will give us water level elevations) However, we need to make sure the correct TOC elevation is used based on the date (this is done with the conditions).
Here is how to set up the Display Fields in the Query Builder:
And here is how you would set up the Conditions in the Query Builder:
Now when you run this query and review the results you will see that the water level elevation for the measurement taken in August is in fact in reference to the new TOC elevation.
By setting up your database this way – you store the depth to water level as they are measured in the field (so field techs aren’t doing the calculations) and you are storing the changing TOC in one place (TOC History table) and letting the Query Build take care of the calculations on the fly.