Q FE 430 Problem Set #6: hydrograph separation submit on Canvas by Sunday of Week 7 at 11:59pm Pacific Time Objectives: 1. Use stable isotope data to separate a hydrograph into event and pre-event flow. Grading Rubric: Criteria Points Possible Paragraph describing hydrograph 15 separation Total volumes of Qe and Qp 10 Percentage of precip that ends up as 5 runoff Hydrograph 10 Paragraph about impressions 10 Total points for assignment 50 Background Information and Resources: Helpful Resources: • This document • Instructor overview The purpose of this lab is to evaluate the amount of event and pre-event runoff, produced by a 3-day storm covering the Green River basin in Washington. In order to determine the event flow contribution to the hydrograph, you must first separate out the pre-event portion. If the discharge components are defined in the simple manner shown above, then: Qt = Qe – Qp Where Qt is the total stream flow, Qe is the event flow and Qp is the pre-event flow. (1) Dividing the total volume of event flow by the drainage area, and adjusting for the appropriate units, gives the depth of runoff. In most cases, only a small part of the precipitation (< 50%) ends up as event flow. You will be provided with an Excel file containing hourly measurements of discharge and ?18O. You will use these data to compute the total volume of event flow using a 2-end member mixing model. You will also be provided with the drainage area of the Green River, and the precipitation data from a nearby rain gauge. Directions • Open the Excel file for this problem set. You will see that some cells are colored blue. These blue cells will be the ones that you’ll eventually fill with your calculations. You will also notice that there is a box contain-ing your isotopic information and watershed area information. I have already filled this out for you. • You will find 3 columns listing the date, time (in hours), and Qt discharge (in cubic feet per second, cfs) plus information regarding isotopic concentrations. • Convert the values of discharge from cubic feet per second to cubic meters per second, cms (remember 1 cfs has 0.02832 cms). • Note that I have estimated the pre-event water isotopic composition of the streamwater (Pre-event ?18O ‰ (Cp):) by taking the average of the 3 pre-event water samples taken before the beginning of the storm. • The event ?18O was measured as -2.5 ‰ • Recall the mass balance and mixing equations from this week’s overview: Qt = Qp + Qe Ct Qt = Cp Qp + Ce Qe Where, Qt = streamflow, Qp= pre-event, Qe = event and Ct , Cp and Ce are the isotopic concentrations of the stream the pre-event water and the event water, respectively • Next we will use both equations solve for Qe/Qt. This will require basic algebra that is outlined for you in the instructor overview. I have set up the equation for this part for you in cell F21. • Qe/Qt is the fraction of the total discharge seen as event water. Compute this fraction for every time step during the storm (cells F21 to F73). • Next, we will compute Qe for every time step (Qe/Qt x Qt ). I have set up this equation H21. Compute this value for all time steps during the storm. • Using a similar process to the one described in the three bullets above, solve for Qp/Qt in cells G21 to G73. If you are unsure about the algebra for this step, revisit the instructor overview. Then solve for Qp. You can do this either by multiplying Qp/Qt by Qt or with the equation Qt = Qp + Qe for each time step during the storm. • Next, we’ll numerically integrate the area under the hydrograph and determine the total volume of event flow. The average volume of flow contributed in each time period (ti to ti+1 = 1 hr) is: • Enter a formula corresponding to equation above in cell I22 and do a fill down to t = 69 hr. The total vol-ume contributed between t = 17 hr and t = 69 hr is the sum of the computed hourly event flows. Cell I79 will display the sum of all of the event flow during the storm as I have already entered the SUM equation for you. • Follow this same procedure for your Qp estimates for column K. • I have set up the hydrograph of the storm to automatically populate, showing the pre-event water volume calculated by the 2-end mixing model method. Notice how much of the total discharge is composed of water coming from pre-event sources (area under the red line). • Lastly, you want to compare the amount of runoff to the amount of precipitation. At this point, the total runoff is expressed as a volume. To convert the runoff volume (in cubic meters) to a depth (in meters), you must divide by the drainage area. Do this in cells I82 and I83. Watershed area is provided at the top of the spreadsheet. Make sure your units of discharge and area are compatible! • As your end result, calculate the ratio of the depth of event flow (cm) to the depth of precipitation and the percent of precipitation that becomes runoff. (cells I88 and I89). What to Turn In Please write out your paragraphs and turn them in as a .doc or .pdf file. You can include your numerical an-swers and your graph in the same document, or you can send me your .xlsx file. (I can only give partial credit for math mistakes if I can see your math). 1. Describe in a paragraph and in your own words the method used to separate the hydrograph. Include all equations used (i.e., the derivation of the equation to compute Qe/Qt and or Qp/Qt). (15 points) 2. The total volumes of Qe and Qp for the storm in m3. (10 points) 3. The percentage of precipitation that ends up as runoff. (5 points) 4. A graph with the labeled hydrograph showing the separation between even and pre-event. (10 points). 5. In a few sentences describe if you were expecting the results you received. Was the pre-event contribution higher or less than you were anticipating? Imagine you have a catchment that has the ability to store a lot of water in the subsurface. Now imagine you have another catchment that is unable to store as much water in the subsurface due to its soil type and underlying geology. Which catchment would you expect to have a higher pre-event contribution to its streams during a storm event? (10 points).