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Determining Non-Agricultural Water Loss in the Great Lakes Basin

The Water Resources Development Act of 1986 (P.L. 99-662) prohibits any new interbasin diversion from the Great Lakes Basin unless it has been approved by the governors of each of the eight Great Lakes states. For purposes of administering this act, a consumptive water loss is not considered an interbasin diversion. A consumptive loss of more than 2 million gallons per day may have to be approved by the governors of the Great Lakes states and the premiers of the Great Lakes provinces.

The Great Lakes Charter requires that both interbasin diversions and consumptive losses of 5 million gallons of water per day averaged over any consecutive 30-day period are subject to review by the governors and premiers of the Great Lakes states and provinces. Because of this requirement, it is wise to establish historical water loss registration even if it is under the 5 million gallons per day threshold.

For Metered Withdrawals

Generally, if water withdrawal and effluent going to the sewage treatment plant are both metered, water loss can be calculated by subtracting the effluent from the water withdrawal. The following additional calculations may be necessary:

  • Groundwater inflow to the outgoing sewer lines should be subtracted from the water loss calculation.
  • A spray irrigator, used by a sewage treatment plant to discharge wastewater to ground, creates an evaporative loss that must be added to the water loss calculation.

For Unmetered Withdrawals

If water withdrawals and the wastewater return flows are not metered, the following parameters should be considered in estimating water loss:

  • Estimate of water withdrawal (refer to sections above to derive this figure).
  • Actual water use from public water supply.
  • Estimate of return flow to the treatment plant (should be the daily average flow).
  • Estimate of any extraneous water inflow to the sewage treatment plant.
  • Storm water overflow should be subtracted from the return flow total.
  • An estimate of the water lost through evaporation, (should be added to water loss) if treated wastewater is discharged to groundwater.

How to Estimate Non-agricultural Irrigation Water Withdrawals And Losses

When registering water withdrawal, irrigators may report a direct measurement of water use or an estimation of water use. While measurement is preferred, irrigators without meters may estimate water withdrawals by previously discussed methods or based on the number of acres irrigated and the depth of water applied (inches) per irrigation.

Non-agricultural irrigators, such as golf course operators, may also use these estimation methods. However, unlike agricultural irrigators, they must comply with the two year advance registration requirement.

Appendix C contains a record keeping and computation form to help keep track of water use. Use of this form is voluntary. Only the official water withdrawal form in Appendix B is required to be submitted to DEC.

The Acre-inches Method

The "acre-inches" method for estimating water use is designed to minimize record keeping and computation requirements. It requires measurement or estimation of only the land area irrigated and the depth of the water applied. The amount of water applied to one field during a given time period can be computed using Formula 1. Formula 1:

acre-inches applied = acres irrigated x depth of water in inches

Acre-inches can be converted into gallons using Formula 2.

Formula 2:

gallons = 27,150 x acre-inches

Registration is required if water use in any 30-day period exceeds 3 million gallons. This is equal to 110.5 acre-inches per 30 days or a daily average water use of 100,000 gallons (3.7 acre-inches). A 30-day running total record of the days that irrigation took place and the amount of water applied per acre will help determine the need for registration.

If water users are certain of registration, they can skip the 30-day running total and report the total water use by calendar month.

If irrigation lasts longer than one day, water applied must be divided into daily proportions for the following reasons:

The law requires reporting the amounts of water withdrawn and lost during each calendar month. If an irrigation period overlaps two months, the water used in the period must be pro-rated into two parts - one for each month.

If it is uncertain whether withdrawals will exceed three million gallons (110. 5 acre-inches) in any 30-day period, irrigators will need to keep a running total of water used in the most recent 30 days. Thus, daily water use must be calculated for the 30-day running total.

Table 5 shows how to calculate the number of irrigation periods needed to exceed the reporting limit for a 1-inch application depth on fields of various sizes. The first column shows acreage for five irrigated areas, ranging from 20 to 200 acres. Using Formula 1, the acre-inches of water applied per period can be calculated as column one times one inch application depth. These numbers appear in the second column. The third column converts acre-inches to gallons using Formula 2. The last column shows the number of time periods needed to reach the 3 million gallon threshold. If a facility has many irrigated areas, the water use must be calculated for each area. If the sum of water used on all areas exceeds 110.5 acre-inches in any 30-day period, registration is required.


Table 5 shows that an area of 100 acres (see boldface row) receiving 1.0 inch of water uses 100 acre-inches of water per period of use, or 2,715,000 gallons of water. The 1. 1 in the last column indicates that if more than one irrigation period falls within a 30-day time period, the irrigator will exceed the three million gallon threshold and, therefore, must register.

Table 5: Water withdrawal calculation for a 1.0 inch application depth.
Water applied
per period in
Gallons Period to reach 3
million gallons or
110.5 acre-inches
20 20 543,167 5.5
50 50 1,357,900 2.2
100 100 2,715,800 1.1

How To Estimate Irrigation Water Loss:

The Great Lakes Water Management and Conservation Act requires monthly reports of amounts of water "lost" as well as water withdrawn. Since the amount of lost water cannot be measured for outdoor water use, it must be estimated. DEC will estimate water loss based on the information supplied by the irrigator or the irrigator can estimate his own water loss. An acceptable way to estimate water loss is to use a method based on evapotranspiration (ET) rates during dry years. The method (see Table 6) is simplified in order to ease calculation and reporting. However, it is not precise enough to use in determining crop irrigation needs in a given year.

To estimate total monthly water losses, multiply the Great Lakes Basin ET values (Column 2) by the total acres irrigated (Column 3) and enter the result in Column 4. Compare these results to the total water withdrawals (Column 5) and report the values that are lower in Column 6. Enter these values in the "consumed" column of the Water Withdrawal Form. For example, the total maximum water loss for a 100-acre facility applying one inch of water in July would be 200 acre-inches (Column 4). Total monthly withdrawal (Column 5) is 100 acre-inches (see previous example). Since the number in Column 5 is less than the number in Column 4, estimated monthly water loss would be 100 acre-inches.

Table 6: Water Loss Calculation
Col. 1 Col. 2 Col. 3 Col. 4 Col. 5 Col. 6
Month Maximum
Area Irrigated
Total Maximum
Water Loss (col. 2 X col. 3)
Total Monthly Withdrawal
Estimated Monthly
Water Loss
May 0.5
June 1.0
July 2.0 100 200 100 100
August 2.0
September 1.0
October 0.5
Other months 0.0

Column 6: For each month, choose whichever value is lower, either the monthly water loss from column 4 or the monthly withdrawal from column 5.

For Additional Help

Additional help in estimating water withdrawals or completing registration forms can be obtained by contacting:

New York State Department of Environmental Conservation
Bureau of Water Resource Management
625 Broadway
Albany, New York 12233-3508

Telephone (518) 402-8182.

This web page was prepared from a handbook developed with the assistance of the New York State Water Resources Institute and Cornell Cooperative Extension. HANDBK (10/91)-4a

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