How To Estimate Agricultural Water Withdrawal

When registering water withdrawal, agricultural 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 based on the number of acres irrigated and the depth of water applied (inches) per irrigation. This page describes the "acre-inches" estimation method. Other methods for estimating water use described in the Great Lakes Basin Water Withdrawal Registration Handbook are also allowed.

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.

The Recordkeeping and Calculation for Water Withdrawal Using the Acre-Inches Method Worksheet (PDF file, 8 kb) 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 (and continuation sheet) 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The law requires reporting the amounts of water withdrawn and lost during each calendar month.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 3 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 1 shows how to calculate the number of irrigation periods needed to exceed the reporting limit for a one 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 farm has many irrigated fields, the water use must be calculated for each field. If the sum of water used on all fields exceeds 110.5 acre-inches in any 30-day period, registration is required.

Example

Table 1 shows that a farm of 100 acres (see bold row) applying 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 3 million gallon threshold and, therefore, must register.

Table 1: Water withdrawal calculation for a 1.0 inch application depth.
Acres Irrigated	Water applied per period in acre-inches	Gallons	Period to reach 3 million gallons or 110.5 acre-inches
20	10	271,500	11.0
50	25	678,750	4.4
100	50	1,357,500	2.2
150	75	2,036,250	1.4
200	100	2,715,000	1.2

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) 402-8109

This web page was prepared from a handbook developed with the assistance of the New York State Water Resources Institute and Cornell Cooperative Extension.