← Module 1 · OpenHydroQual Exercise 1

Your first groundwater model: an aquifer feeding a stream

About 1.5 hours · submit a short report (PDF) to Brightspace → Assignments

What you will do

You will open a ready-made model of a 1 km² aquifer that drains into a stream, run it for one year with a recharge record, and check its results by hand. You won't need any equations beyond this module: porosity, water content, the water balance, and residence time.

The model has three objects:
  • Aquifer (a Groundwater cell block): 1 km² in plan, 10 m thick, porosity 0.30, initial volumetric water content 0.22. Recharge enters through the time series in recharge.csv.
  • Stream (a fixed_head block): the receiving stream, with its water level held at 6 m above the aquifer base. Its Storage starts at zero and accumulates all the water it receives, so it works as a running total of baseflow.
  • Baseflow (a groundwater_to_fixedhead link): the groundwater discharge from the aquifer to the stream. Its rate grows with the height of the water table above the stream. (You will learn the equation behind it, Darcy's law, in Module 3.)

Recharge (recharge.csv): none for the first 60 days, 3000 m³/day (3 mm/day) from day 61 to 120, none until day 200, a 5-day pulse of 10,000 m³/day (10 mm/day), then none to the end of the year.

Step 0 · Install and download

  1. Install OpenHydroQual from openhydroqual.com/download (Windows installer; Linux .deb packages are on GitHub). Watch the 1-minute GUI demo on the module page first.
  2. Download both files into the same folder. The model reads recharge.csv from the folder it is opened from.
    aquifer_stream.ohq recharge.csv

Part 1 · Explore the model and predict (before running)

  1. Start OpenHydroQual and choose File → Open, then select aquifer_stream.ohq. You should see two blocks joined by one link.
  2. Click each object and read its properties in the properties panel. Record in a table: aquifer area, depth (thickness), porosity, moisture content, hydraulic conductivity; stream head; link length and interface area.
  3. Hand calculations (show your work):
    1. Total volume of the aquifer cell, \(V_t\).
    2. Pore volume, \(nV_t\): the most water the cell could hold.
    3. Initial volume of water stored, \(\theta V_t\).
    4. Initial water-table height above the aquifer base. In this model the water table sits at \(h = \theta b / n\), where \(b\) is the thickness. Explain in one sentence why dividing by the porosity makes sense: if all the water were collected in the pores at the bottom of the cell, how high would it reach?
    5. The stream level is 6 m. The water table cannot drain below the stream, so water below 6 m stays in the aquifer. How much water (m³) is above the stream level at the start? This is the water available to drain.
  4. Predict: sketch what you expect the baseflow to do over the year, given the recharge pattern. Keep your sketch to compare with the results.

Part 2 · Run and read the results

  1. Click Run. The model simulates 365 days; it takes a few seconds.
  2. Right-click the Aquifer block and choose Results → Storage, then Results → head. Right-click the Baseflow link and choose Results → flow. Right-click the Stream block and choose Results → Storage (cumulative baseflow).
  3. To get numbers, right-click a plot and use Copy Curve, then paste into Excel or another spreadsheet. Fill in this table:
    dayAquifer storage (m³)water table (m)baseflow (m³/day)stream storage (m³)
    0
    60
    120
    200
    365
  4. Do the day-0 storage and water table match your hand calculations from Part 1?

Part 3 · Check the water balance

  1. Compute the total recharge for the year from recharge.csv: the area under the recharge curve. Treat the one-day ramps as straight lines. Express it both in m³ and as a depth in mm over the 1 km² aquifer.
  2. Write the water balance for the aquifer over the year: \(S_{\text{end}} = S_0 + \text{recharge} - \text{baseflow}\). Use it to compute the total baseflow, and compare with the stream storage at day 365. What percentage difference do you get?
  3. What fraction of the year's recharge left the aquifer as baseflow? Where did the rest go, or did more leave than came in? Explain using your table.

Part 4 · Residence time

  1. During the first 60 days there is no recharge, and baseflow simply recedes. Using the baseflow at day 0 and day 60, compute the recession constant \(k = \ln(Q_0/Q_{60})/60\) (1/day).
  2. The residence time of the drainable water is \(1/k\). Compare it with "drainable storage ÷ baseflow" at day 0, using your answer to Part 1(e). Are they close?
  3. How long does it take baseflow to halve? (\(t_{1/2} = \ln 2 / k\))
  4. Look at the 5-day recharge pulse around day 200. Recharge jumps to 10,000 m³/day, but how high does baseflow go? Use the residence time to explain why the aquifer smooths out the pulse.

Part 5 · Change one thing

  1. Predict first: if the aquifer's hydraulic conductivity were doubled (a more permeable sand), what would happen to the baseflow at day 0, the residence time, and the baseflow at the end of the year?
  2. Click the Aquifer, change hydraulic_conductivity from 20 to 40 m/day, and run again. Record the same quantities as in Part 2.
  3. Were your predictions right? Explain in 3–4 sentences. Does doubling K change the total water that could ever drain to the stream (the storage above stream level)?

Part 6 · Think about real aquifers

  1. In this model, adding 1 mm of water over the aquifer raises the water table by \(1/n = 3.3\) mm. In a real unconfined sand aquifer, only the drainable pore space fills and empties as the water table moves. Which property from this module should replace porosity, and what would the water-table rise be for a sand with \(S_y = 0.25\)?
  2. Name one real-world process that this model leaves out. Which term of the hydrologic balance would it affect?

What to submit

  • The property table and hand calculations from Part 1, with your prediction sketch.
  • The results table (Part 2) for the base case and the doubled-K case.
  • Two plots: baseflow vs. time (both cases on one plot) and water-table height vs. time.
  • Answers to Parts 3–6, each in a few sentences with your numbers.
  • At most 3 pages, as a single PDF, uploaded to Brightspace → Assignments → OHQ Exercise 1.

Model adapted from the "Linear-Reservoir Baseflow" example in the OpenHydroQual example catalog (examples.openhydroqual.com), rebuilt with the Darcy-type groundwater-to-fixed-head connector included in the standard OpenHydroQual installation.