Water Measurement

1. Water Measurement Brady S. McElroy, P.E. USDA-NRCS Lamar, Colorado

2. Objectives • Why is water measurement important to IWM? • Explain some of the mathematics of water measurement • Discuss some of the common measuring devices encountered in NRCS work • Discuss other opportunities for measurement • Work some example problems

3. Why is water measurement important? • Difficult to effectively manage irrigation without measurement • Positive aspects • Maximize use of available water supply • Reduced cost due to leached nutrients • Reduced environmental impact from over-irrigation

4. Why is water measurement important? • Some measurement may have a negative connotation • Regulatory (mandated by state, etc.) • Billing

5. KANSAS

6. Why is water measurement important? • Water is one of the most precious resources in the West • Increased competition among water users

7. “Whiskey is for drinking. Water is for fighting over.” Mark Twain

8. References Primary reference for NRCS is Chapter 9 of Part 623 (Irrigation) of the National Engineering Handbook • States that NRCS’ reference shall be the Bureau of Reclamation’s Water Measurement Manual, 3rd edition, published in 1997 • Available online at http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/

9. References Other useful references • Other NRCS documents • Irrigator’s Guides • Extension publications • Hydraulic texts • King’s Handbook of Hydraulics

10. Definitions Volume: length3 Flow Rate (Q): volume/time Velocity: length/time Area: length2

11. Definitions Head- measurement of the energy in a fluid. Units are typically length. • Total head at a given point is the sum of three components • Elevation head, which is equal to the elevation of the point above a datum • Pressure head, which is the height of a column of static water that can be supported by the static pressure at the point • Velocity head, which is the height to which the kinetic energy of the liquid is capable of lifting the liquid

12. Definitions Pressure- measurement of the force acting on a surface. Units are force/length2 Often convenient to express in terms of feet of fluid (pressure head) h=p/γ (multiply psi x 2.31 for feet of H20)

13. Units • Typically in U.S. Customary units for irrigation work. • Units vary depending on type of measurement • Q vs. volume • Open channel vs. pipe flow

14. Units Flow rate units expressed in volume/time • Open channel flow • Cubic feet per second (cfs) • second-feet • Pipe flow • Gallons per minute (gpm)

15. Units Handy Conversion Factor 1 cfs = 448.8 gpm or 1 cfs ≈ 450 gpm

16. Units May also vary regionally • Shares • Some canals refer to a head of water as a delivery unit • Not the same as energy measurement • Miner’s inches • 38.4 miner’s inches = 1 cfs (Colorado) • 40 miner's inches = 1 cfs (California, et al.) • 50 miner’s inches = 1 cfs (New Mexico, et al.)

17. Units A share is not a share is not a share CanalAllocation/share (cfs) Bessemer 0.0150 Colorado 0.0125 Rocky Ford Highline 0.180 Oxford 0.0960 Otero 0.050 Holbrook 0.0250 Catlin 0.0180 Rocky Ford 0.140 Fort Lyon 0.0150 Amity 5 cfs at 0.6 hr/share Lamar 0.0100

18. Units Volume units are often expressed in units of area x depth or depth Acre-foot = volume of water that would cover 1 acre to a depth of 1 foot • 12 acre-inches • 43,560 cubic feet • 325,851 gallons

19. Units Handy Conversion Factor 1 cfs for 24 hours ≈ 2 acre-feet or 1 cfs ≈ 1 ac-in/hr

20. Water Measurement Mathematics

21. Water Measurement Mathematics

22. Water Measurement Mathematics Continuity Equation Q=vA Irrigator’s Equation Qt=Ad

23. Qin Qout v1 v2 A1 A2 Continuity Equation Q=vA Q = flow rate v = velocity A = area

24. Continuity Equation Q=vA v=Q/A A=Q/v

25. 12 in. Q v=2.5 ft/s Continuity Equation Given: d=12 inches v=2.5 ft/s Find: Q in cfs

26. 12 in. Q v=2.5 ft/s Continuity Equation Solution: Q = vA A = 0.785 ft2 Q = 2.5 ft/s x 0.785 ft2 = 1.96 ft3/s

27. Irrigator’s Equation Qt = Ad Q = flow rate t = time A = area D = depth

28. Irrigator’s Equation d = Qt/A Q = Ad/t t = Ad/Q A = Qt/d

29. Irrigator’s Equation Given: d = 3 inches A = 50 acres Q = 2 cfs Find: Time required to apply d

30. Irrigator’s Equation Solution: t = dA/Q 1 cfs ≈ 1 ac-in/hr t = 75 hours

31. Irrigator’s Equation Given: t = 36 hours A = 20 acres Q = 2 cfs Find: Depth of applied water, d

32. Irrigator’s Equation Solution: d = Qt/A 1 cfs ≈ 1 ac-in/hr d = 3.6 inches

33. Water Measurement Devices Most water measurement devices either sense or measure velocity, or measure either pressure or head. Tables, charts, or equations are then used to calculate the corresponding discharge

34. Water Measurement Devices Devices that sample or sense velocity • Current meters • Propeller meters • Vane deflection meters • Float and stopwatch

35. Water Measurement Devices Devices that measure head or pressure • Open channel devices commonly use h • Pipeline devices may use p • Flumes • Orifices • Venturi meters • Weirs • Velocity is computed from h, so weirs are classifed as head measuring devices

36. Open Channel Devices • Weirs • Flumes • Submerged Orifices • Other devices

37. Weirs A weir is an overflow structure installed perpendicular to open channel flow • Has a unique depth of water at an upstream measuring point for each discharge • If the water springs clear of downstream face, acts as sharp-crested weir • A long, raised channel control crest is a broad-crested weir

38. Weirs • Usually named for the shape of the overflow opening • Rectangular • Triangular • Cipolletti • Lowest elevation on overflow is zero reference elevation for measuring h

39. Weirs Rectangular weirs can be either contracted or suppressed • Suppressed weirs use side of flow channel for weir ends • No side contraction occurs • Often used in divide boxes • Canal overshot gates can act as weirs

40. Weirs

41. Weirs Cipolletti Weir

42. Weirs Weir Box Turnout with Cipolletti Weir

43. Weirs Compound Weir 90 degree triangular and suppressed rectangular

44. Weirs Advantages • Simple to construct • Fairly good at passing trash • 1 head measurement Disadvantages • High head loss • Susceptible to sedimentation problems • Sensitive to approach and exit conditions

45. Weirs Conditions needed for sharp-crested weirs • Upstream face should be plumb, smooth, normal to axis of channel • Entire crest should be level for rectangular and Cipolletti. Bisector of V-notch angles should be plumb for triangular. • Plate should be thin enough to act as a sharp-crested weir • Chamfer downstream edge if necessary • Upstream edge must be straight and sharp • Thickness should be uniform for entire length

46. Weirs • Maximum downstream elevation should be at least 0.2 ft below crest • Head measurement should be greater than 0.2 ft for optimal elevation • Head is measured upstream 4 X maximum head on crest • Approach must be kept free of sediment deposits

47. Weirs Given: Standard Contracted Rectangular Weir L = 2 feet h = 0.40 feet Find: Q, in cfs Solution: Refer to Table A7-2 in BoR Water Measurement Manual, 3rd edition

48. Weirs

49. Weirs Inspection of Existing Structures • Approach flow • Turbulence • Rough water surface at staff gage • Velocity head • Exit flow conditions • Worn equipment • Poor installation • Crest must be correctly installed

50. Weirs Poor approach condition