“Water Discharge Using Salt Tracer and Area-Velocity” Catherine Riihimaki, Drew University

Area-Velocity Discharge Salt Tracer Discharge 1. Dump salt upstream 2. Measure electrical conductivity downstream

Goals of assignment: Students learn basic plotting in KaleidaGraph — Excel can work too, but K-Graph can integrate curve automatically — We use K-Graph later in course because it has more options for complex curve-fitting Students learn some calculation tools in K-Graph (or return to visual integration) Students have opportunity to observe stream characteristics Students can compare strengths and weaknesses of two field techniques Students think about conservation of mass

The background Reach of stream of length L and discharge Q M s : added mass of salt at upstream end C 0 : background concentration C 1 : new concentration due to the addition of M s QC 0 : background rate of mass transport of salt (in kg/s) QC 1: background plus additional mass transport due to the added tracer Over a time of observation T: Incoming mass of salt naturally plus added mass of salt equals the mass leaving the reach Rearrange…

The background Convert concentration to electrical conductivity E using conversion factor F Solve for Q

The background

Procedures: Introduce conservation of mass Introduce salt tracer method Go into field for measurements (1 50-minute class) — Need 1 salt dumper — 2-3 people per measurement team (measurer, timer, recorder) — Record every seconds (background through peak back to background) — All or some do area-velocity measurements Students work independently out of class on calculations Students complete a lab write-up with graphs

Why this activity: Many of the students have done area-velocity measurements, but not salt tracer Wading is not necessary unless area-velocity calculations are done concurrently Good opportunity for methodology comparison The students will have some measure of skepticism that the procedure will work until they do the calculations Portability: Need electrical conductivity meter(s), bucket, salt Can use both methods or just replace area-velocity method in your discharge lab Try it yourself first!