Water Balance Studies Under 30 TAC §309.20: What They Require and Who Prepares Them

Where does §309.20 apply?

To land application of domestic wastewater effluent under a TCEQ permit. 30 TAC §309.20 sits in Chapter 309, Subchapter C, which covers land disposal of sewage effluent. That is the rule behind a Texas Land Application Permit (TLAP) that irrigates a cover crop.

Reuse under Chapter 210 has its own irrigation rule. §210.24(d) has the reclaimed water provider or user weigh a water balance too, and §210.56(f)(1) applies §210.24 to 210E industrial reuse. The ideas are close, but the citation is different.

What does the water balance itself require?

A monthly comparison of water in and water used, with the method documented. Under §309.20(b)(3)(A), the water balance determines the irrigation water requirement of the crop system, including a leaching requirement if needed. It "should generally follow" the example in Table 1 of the rule's figure.

The inputs the rule names:

• Precipitation: "the average yearly rainfall and the monthly precipitation distribution based on past rainfall records."
• Evapotranspiration: the crop's consumptive use, developed month by month, with the method documented in the study.
• Leaching: a leaching requirement when the effluent's total dissolved solids could build up salt in the soil over the life of the system.

The effluent side is the plant's monthly flow. The site must take it every month, whatever the weather.

Where does the 25-year rainfall come in?

In the storage study, not the water balance. Section 309.20(b)(3)(B) requires an effluent storage study based on "a design rainfall year with a return frequency of at least 25 years." The engineer may use the expected 25-year, one-year rainfall, or the highest annual rainfall in the last 25 years of record, with a normal monthly distribution.

The storage study also uses the application rate and cycle, the effluent available each month and evaporation losses. Table 3 in the rule's figure shows an example.

A common mix-up is running the whole analysis on the wet year. The rule uses average rainfall to set the irrigation rate and the 25-year design year to size storage. Using the wrong rainfall in either place can mis-size the field or the pond.

What else limits the application rate?

Nitrogen, soils and groundwater. Under §309.20(b)(3)(C), the annual liquid loading may not add more nitrogen than the crop needs each year plus 20 percent for volatilization, and the crop nitrogen values must be justified in the design report.

The rest of the technical report under §309.20(a) and (b) supports those numbers:

• a soil survey, using USDA Natural Resources Conservation Service surveys where available, with rates and runoff controls based on soil properties;
• every water well within half a mile of the site, with its use and construction details where available;
• the crop system, its annual nutrient uptake and how often it is harvested, at least once a year;
• preoperational soil tests for pH, total nitrogen, potassium, phosphorus and conductivity, then at least one test a year.

Where the land is accessible to the public, the effluent needs treatment equivalent to secondary standards before it is applied (§309.20(b)(1)).

How do the field size and the pond size trade off?

They are set together. The hydraulic rate and the nitrogen rate each imply a minimum irrigated area, and the more restrictive one governs. Storage then carries the months when that area cannot take the full flow, usually the cool months when crop water use is low.

More area can mean less storage, and the reverse, within the rule's limits. A site with too little land shows up as a monthly surplus the pond cannot hold in the design wet year. Our post on sizing a storage reservoir for a Texas reuse project walks through the storage side.

Who prepares the study?

The designing engineer, as part of the preliminary engineering report. Section 309.20(a) requires a report "outlining the design of the wastewater application system," with maps, basis of design and calculations. Section 309.20(b)(3) says the designing engineers base the design on the detailed analysis of hydraulic and nutrient rates and storage needs.

In Texas, that is engineering work. Texas Occupations Code §1001.301 bars practicing engineering without a license. Inputs come from others: soil surveys, an agronomist's crop and nutrient data, a well inventory and long-term rainfall records.

Before the study starts, answer each question yes or no. Any no is a gap to close with your engineer:

1. Is the system a TLAP under Chapter 309, or reuse under Chapter 210, so you know which rule governs?
2. Do you have the design flow by month for each phase?
3. Is the irrigated land identified, controlled and mapped with its legal description?
4. Is there a soil survey for every application area?
5. Have the water wells within half a mile been located?
6. Is the crop chosen, with its nitrogen need and harvest plan?
7. Do you know the effluent's total dissolved solids, for the leaching check?

This is general education on 30 TAC §309.20 and §210.24 as published in October 2026. TCEQ proposed amendments to Chapter 309 in May 2026, mainly to cover industrial wastewater including produced water, so check the current text before you file. TCEQ decides each application.

Frequently asked questions

Does a §309.20 water balance have to use 25-year worst-case rainfall?

No. The water balance uses average yearly rainfall with the monthly distribution from past records. The 25-year design rainfall year is required for the effluent storage study under §309.20(b)(3)(B).

Does a 210E authorization need a §309.20 water balance?

Not by citation. Section 210.56(f)(1) applies the Chapter 210 irrigation rule, §210.24, which has the provider or user consider a water balance with average monthly precipitation and documented monthly evapotranspiration. The provider or user keeps the study's records for possible TCEQ review.

I'm a land developer in Hays County planning a TLAP for a subdivision, and my engineer keeps mentioning 25-year rainfall. Is that for the irrigation field or the storage pond?

For the pond. Under §309.20, the storage study uses a design rainfall year of at least a 25-year return frequency. The irrigation rate comes from the water balance on average rainfall, then the nitrogen limit is checked against it.

I'm a Texas civil engineer preparing my first TLAP technical report. Which parts of §309.20 does the water balance feed, and what else must the report include?

It feeds the hydraulic application rate, which you check against the nitrogen rate and then use in the storage study. The report also needs site maps and drawings, geology, soils, the well inventory, the crop system and preoperational soil tests.

Can someone other than an engineer prepare the water balance?

Others can supply inputs, such as soil data and crop nitrogen values. The rule assigns the design analysis to the designing engineers, and Texas law requires a license to practice engineering.

Related resources

• Texas Land Application Permits Explained: Spray Irrigation, Subsurface Drip, and Evaporation Ponds
• Land Application Permits in Texas: A Faster Path to Wastewater Approval for Developers
• Water Reuse for Austin Area Golf Courses and HOA Irrigation: How Chapter 210 Creates a Revenue Stream
• Reuse vs. Discharge: Which Wastewater Strategy Fits Your Texas Development?

Need a water balance that holds up in TCEQ review?

We build the water balance, storage study and nitrogen check from site data, so the field and the pond are sized to the rule that actually governs your system.

We specialize in:

• TLAP applications and technical reports
• Water balance and effluent storage studies
• 210E and Chapter 210 reuse authorizations
• Reuse area, irrigation and storage design
• Wastewater treatment plant design
• Site feasibility and due diligence

Modern Engineering Solutions, McKinney, Texas. Contact: (214) 833-6748 or mod-eng.com