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How To Estimate Non-Agricultural Water Withdrawal

The following describes four methods to estimate a water withdrawal:

1. Flow-through a calibrated pump.
2. Flow-through a calibrated pipe.
3. Flow-through a calibrated rectangular weir.
4. Flow-through a calibrated V-notch weir.

1. Flow-through a calibrated pump

The amount of water pumped from groundwater or surface water depends on two factors: the horsepower of the pump and the height (or head) the water is pumped. Table I shows water discharge estimates from various pumps at various heads. Pump rating tables which may accompany your pump show the horsepower. The tables also show discharges associated with different heads.

Table 1 Description

The first column in Table I gives various pump horsepowers ranging from 0.33 to 10.0 horsepower. The second column shows the amount of water (gallons per minute) that would be pumped to a head of 5 feet. The third column shows the time (hours) needed to pump 3 million gallons of water. The time that the pump is running multiplied by the amount of water discharged gives the gallons of water pumped. Once 3 million gallons of water has been pumped in any consecutive 30-day period the water withdrawal must be registered. Example:

A 5 horsepower motor (see bold figures) pumping water to a head of 5 feet will pump 2,970 gallons per minute. In 16.84 hours (16 hours and 51 minutes) more than 3 million gallons of water is used and registration is required. The same information is provided for seven examples for heads ranging from 10 feet to 250 feet.

Table 1: Estimate of Pumping Rates
Motor*
Horse-
power
(gpm)
5 feet
Head
Hours to
Pump 3
Million
Gallons
(gpm)
10 feet
Head
Hours to
Pump 3
Million
Gallons
(gpm)
20 feet
Head
Hours to
Pump 3
Million
Gallons
(gpm)
30 feet
Head
Hours to
Pump 3
Million
Gallons
0.33 196 255.10 98 510.20 49 1020.41 32 1562.50
0.50 297 168.35 148 337.84 74 675.68 49 1020.41
0.75 445 112.36 223 224.22 111 450.45 74 675.68
1.00 594 84.18 297 168.35 148 337.84 99 505.05
2.00 1188 42.09 594 84.18 297 168.35 198 252.53
5.00 2970 16.84 1485 33.67 742 67.39 495 101.01
10.00 5940 8.42 2970 16.84 1480 33.78 990 50.50
Motor*
Horse-
power
(gpm)
50 feet
Head
Hours to
Pump 3
Million
Gallons
(gpm)
100 feet
Head
Hours to
Pump 3
Million
Gallons
(gpm)
200 feet
Head
Hours to
Pump 3
Million
Gallons
(gpm)
300 feet
Head
Hours to
Pump 3
Million
Gallons
0.33 19.8 2525.3 9.8 5102.0 4.9 10204.1 3.9 12820.5
0.50 29.7 1683.5 14.8 3378.4 7.4 6756.8 5.9 8474.6
0.75 44.6 1121.1 22.3 2242.2 11.1 4504.5 8.9 5617.7
1.00 59.4 841.8 29.7 1683.5 14.8 3378.4 11.9 4201.7
2.00 118.9 420.5 59.4 841.8 29.7 1683.5 23.8 2100.5
5.00 297.0 168.4 148.5 336.7 74.2 673.9 59.4 841.8
10.00 594.0 84.2 297.0 168.4 148.5 336.7 118.8 420.9

* 0.75 efficiency

Diagram of Water Discharged Through a Pump
Diagram of Water Discharged Through a Pump

2. Flow-through a calibrated pipe Estimated withdrawal may be based on the diameter of the pipe. To use this procedure the following information is needed:

  • Make and model number of the pump(s).
  • Diameter of the discharge port or main line leaving the pump(s).
  • Rated capacity of the pump(s), if available. (If the rated capacity of the pump is in gallons per minute, one of the other techniques can be used.).
  • Hours of operation.

Table 2 Description

The first column of Table 2 shows various diameters (inches) of discharge ports or mains leaving the pump(s). The second column shows the amount of water discharged (gallons per minute) for each diameter. The third column gives the flow for one hour of pipe operation. The fourth and fifth columns show the time (in hours and days) of pipe operation needed to reach the 3 million gallon threshold that requires registration.

Example

A pump with a 4-inch discharge, or a main line 4 inches in diameter (see bold figures) has a discharge of 188 gallons per minute. A continuous discharge for 11 or more days reaches the 3 million gallon threshold, requiring registration.

For multiple pumps on the site, the estimate of the total water withdrawal must be calculated by adding the discharge amounts (in gallons per minute or per hour). Do not add diameters of separate pipes because water withdrawal may be overestimated. (Two 2-inch pipes do not equal one 4-inch pipe. Two 2-inch pipes will discharge 120 gallons per minute (2X60); one 4-inch pipe will discharge 188 gallons per minute.)

Table 2: Water Withdrawal Estimation by Pipe Diameters
Pipe
Diameter
in inches
Gallons
per Minute
Discharged
Gallons
accumulated
after 1 Hour
Hours
To Reach
3 Million
Gallons
Days
2 60 3,600 833.3 34.7
3 93 5,580 537.6 22.4
4 188 11,280 266.0 11.1
5 325 19,500 153.8 6.4
6 500 30,000 100.0 4.2
8 750 45,000 67.7 2.8
10 1,200 72,000 42.7 1.7

If a pump is not used to withdraw water, a rectangular or a V-notched weir can be used to estimate water withdrawal. A weir is a dam. A gauge calibrated to hundredths of a foot will be needed for these methods.

3. Flow-through a V-notched Weir

The flow through a V-notched weir can be estimated by measuring the head (height of the water) flowing through a precise angled notch in a weir. As Table 3 shows, the amount of water withdrawn (gallons per day) increases with the head and decreases with the acuteness (smaller angle) of the notch in the weir.

Table 3 Description

The first column shows the head (in tenths of a foot) of water flow. The second column shows the amount of water (gallons per day) that flows through a 45 degree angle notch in a weir. The third column gives the time (days) needed to discharge 3 million gallons of water through the notch in the weir. The same information is provided for weir angle notches of 60 degrees and 90 degrees.

Example

Five tenths of a foot (0.5) of water (see bold figures) flowing through a 45 degree angle withdraws 118,000 gallons daily. The threshold of 3 million gallons will be reached in 25.42 days.

This table shows that with a withdrawal head less than 0.3 feet, the number of days required to withdraw 3 million gallons is always more than 30 days and, therefore, no registration is required.

Table 3: Discharge Through a V-Notched Weir
Head
in ft.
Discharge
Gallons
Per Day
45 Degree Angle
Number of
Days to Use
3 Million Gallons
Discharge
Gallons
Per Day
60 Degree Angle
Number of
Days to Use
3 Million Gallons
Discharge
Gallons
Per Day
90 Degree Angle
Number of
Days to Use
3 Million Gallons
0.1 2,000 1500.00 3,000 1000.00 5,000 600.00
0.2 12,000 250.00 17,000 176.47 29,000 103.45
0.3 33,000 90.91 46.000 65.22 79,000 37.97
0.4 68,000 44.12 94,000 31.91 164,000 18.29
0.5 118,000 25.42 165,000 18.18 286,000 10.49
0.6 187,000 16.04 260,000 11.54 450,000 6.67
0.7 274,000 10.95 383,000 7.83 662,000 4.53
0.8 383,000 7.83 534,000 5.62 925,000 3.24
0.9 574,000 5.23 717,000 4.18 1,240,000 2.42
1.0 669,000 4.48 933,000 3.22 1,620,000 1.85
1.1 849,000 3.53 1,180,000 2.54 2,050.000 1.46
1.2 1,050,000 2.86 1,470,000 2.04 2,550,000 1.18
1.3 1,290,000 2.33 1,800,000 1.67 3,120,000 0.96

diagram of water discharge through a V-Notched Weir
Diagram of a Water Discharge Through a V-Notched Weir

4. Flow-through a rectangular weir

The procedure for estimating flow through a rectangular notched weir is similar to that for a V-notched weir. The head of the water flowing through a rectangular notch must be measured. Table 4 shows that the amount of water withdrawn increases with the head of water flow and increases with the width of the notch in the weir.

Table 4 Description

The first column in Table 4 lists various heads of water flow (tenths of a foot). The second column shows how much water (gallons per day) discharges over the weir for each head. The third column shows the time (days) needed to discharge 3 million gallons of water through the notch in the weir. The same information is provided for six openings, ranging from 1 to 5 feet, in the weir crest.

Example

One tenth of a foot of water (see bold figures) flowing through a rectangular notch 2 feet in length withdraws 135,000 gallons of water daily. The threshold of 3 million gallons is reached in 22.22 days.

This table shows that for a withdrawal head less than 0.04 feet, the number of days needed to withdraw 3 million gallons is always more than 30 days and, therefore, no registration is required.

Table 4: Discharge through a Rectangular Weir
Head
in Feet
Gallons
Per Day
1 Ft. Weir Crest
Days to Discharge
3 Million
Gallons
Gallons
Per Day
1.5 Ft. Weir Crest
Days to
Discharge
3 Million
Gallons
Gallons
Per Day
2 Ft. Weir Crest
Days to
Discharge
3 Million
Gallons
0.04 17,000 176.47 26,000 115.38 34,000 88.24
0.05 24,000 125.00 36.000 83.33 48,000 62.50
0.06 31,000 96.77 47,000 63.83 63,000 47.62
0.08 48,000 62.50 72,000 41.67 96,000 31.25
0.10 67,000 44.78 101,000 29.70 135,000 22.22
0.12 87,000 34.48 132,000 22.73 177,000 16.95
0.14 110,000 27.27 166,000 18.07 222,000 13.51
0.15 122,000 24.59 184,000 16.30 246,000 12.20
0.20 185,000 16.22 281,000 10.68 377,000 7.96
0.25 255,000 11.76 390,000 7.69 525,000 5.71
0.30 332,000 9.04 509,000 5.89 686,000 4.37
Head
in Feet
Gallons
Per Day
3 Ft. Weir Crest
Days to
Discharge
3 Million
Gallons
Gallons
Per Day
4 Ft. Weir Crest
Days to
Discharge
3 Million
Gallons
Gallons
Per Day
5 Ft. Weir Crest
Days to
Discharge
3 Million
Gallons
0.04 52,000 57.69 69,000 43.48 86,000 34.88
0.05 72,000 41.67 96,000 31.25 120,000 25.00
0.06 94,000 31.91 126,000 23.81 158,000 18.99
0.08 145,000 20.69 194,000 15.46 243,000 12.35
0.10 203,000 14.78 271,000 11.07 339,000 8.85
0.12 226,000 13.27 335,000 8.96 445,000 6.74
0.14 335,000 8.96 448,000 6.70 560,000 5.36
0.15 372,000 8.06 496,000 6.05 622,000 4.82
0.20 570,000 5.26 762,000 3.94 955,000 3.14
0.25 794,000 3.78 1.063,000 2.82 1,331,000 2.25
0.30 1,040,000 2.88 1,393,000 2.15 1,747,000 1.72

Diagram of a Water Discharge Through a Rectangular Weir
Diagram of a Water Discharge Through a Rectangular Weir

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