Water budgeting is a valuable tool to further water conservation efforts. It has been gaining in popularity for the last 15 years or so.

The purpose of this article is to better define what the water budget formula does, what goes into it and its improper use by many in the industry. By better understanding how the formula works, we can correct some of its misuses.

A water budget estimates how much water a site should be using. By taking into account the area, efficiency and plant water use, one can come up with a number of units of water that the site in question should be using. This can be calculated by day, week, month, season or year. Of course, caution should be used when defining a water budget as, “What a site should be using.” (What has happened in recent years is that some well meaning individuals and agencies have inserted their own variables into the water budget formula that give an answer of what they think a site should be using.)

Once this number is calculated, a water agency or other governing organization can then tell an end user how much water they should be using and base water rates on this number. Some states have actually written the water budget formula into laws and regulations. It can also be calculated by end users themselves to better manage their own water. Where regulations are not in place, it can be used as a guide to see how much a site is over-watering and serve as a baseline to work towards in conservation efforts.

While there are several different versions, they all contain the same basic information: Area, ET, Species Factor or Crop Coefficient and Efficiency.

The version that I prefer of the basic annual water budget calculation looks like this:

Where: SWB = Site Water Budget (adjusted) (CCF or Per 1000 gals.)

ET0 = Reference Evapotranspiration

(inches per year) Cwb = Water Budget Adjustment Factor (decimal) A = Total landscaped area of site (square feet or acres) Cu = Conversion Factor

I like this version because it allows you to choose your billing units (CCF or K Gal) and it also allows you to choose the area unit of measurement (square ft. or acres).

The most misunderstood part of the formula is the water budget adjustment factor. This is the “magic” number that is being hotly debated in the industry.

Water Budget Adjustment Factor: The formula for calculating the Cwb is:

Where: Cwb = Water Budget Adjustment Factor Ks = Species Factor (crop coefficient) IE = System Efficiency

As an example, if the result of the above calculation was .80, then the water budget formula would look like this:

It is important to note that the Water Budget Adjustment Factor asks for Efficiency, not Uniformity. Efficiency is arrived by the following: 

In regard to the “water applied,” let us ask ourselves, “What goes into the application of water?” When applying water to a site one has to use hardware and then schedule it correctly. As an example, if a system was 100% uniform (impossible, but let’s assume this) it would still be possible to waste water just by letting it run too long.

If the uniformity is 80%, we would now pick a management efficiency as a multiplier. This is because, as mentioned earlier, the budget formula asks for the system efficiency, not the system uniformity. As a reminder, efficiency is defined as the water used beneficially by the plant, divided by the water applied. For this example we will assume the person managing the system does so at a 75% management efficiency. We might then have an equation as follows:

This means that the system has a uniformity of 80% but an efficiency of 60%. The fact is that no matter what the system uniformity, the system efficiency will always be lower. This is because no human or machine is perfect in his/her management and scheduling of a system.

Before we go much further, I would like to deviate for a moment so as to clarify the issue of watering at more or less than 100% efficiency.

Technically, it would be possible to water at, or above, 100% efficiency. This could be done if one was watering at what is called deficit irrigation. Deficit irrigation is when one waters less than what the plants need to grow and be healthy.

What is happening in many areas is that people, professionals, and regulatory agencies are picking a Water Budget Adjustment Factor without really understanding what goes into it and how all of the numbers come together.

What is not being communicated to the end-user in these instances is what exactly is expected of them. In other words, if a water budget number is based on the desire of a regulatory agency to have a site be only 25% turf, with a system uniformity of 80% and 90% management efficiency, then that needs to be communicated up front.

If this is not communicated up front, then the end-user will struggle trying to make changes without really knowing what and why they are doing them, and spending thousands of dollars in the process; all the while probably not being successful.

Without communicating the details behind the math in the water budget, end-users will naturally install whatever landscape and irrigation system they are comfortable with or have the dollar budget for, and just assume that if they are careful and efficient they can reach a budget. The reality is that with many budgets, it would take a herculean effort and major advanced design steps for both the landscape and irrigation system to meet a budget. At least general, basic guidelines can be communicated.