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.
| 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
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.)
| 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.
| 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 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.
| 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
