Irrigation on a Solar Site: Retention Ponds, Zones and Water Delivery
On a Texas solar site, seed in the ground is not the finish line. The state's construction stormwater permit is built around one number: 70 percent of the native vegetation around the site. Unless the site qualifies as dry, the permit does not close until the ground carries a uniform, perennial stand at that density, with no large bare areas, on every unpaved acre that is not under a structure. A dry site can close sooner, on seed and erosion control designed to reach the same 70 percent within three years. Either way, until the site meets its condition, the permit stays open and the operator stays responsible for it. In a dry stretch, rain alone may not get a new stand there on schedule. That is what irrigation on a solar site is for: getting a site's stand to the 70 percent the permit is built around, and holding it there.
The Target Is Written Down
The Texas Commission on Environmental Quality's Construction General Permit, TXR150000, defines final stabilization in plain terms. All soil-disturbing work is complete, and a uniform perennial vegetative cover, evenly distributed and without large bare areas, has been established at a density of at least 70 percent of the native background cover on all unpaved areas not covered by permanent structures. Two other routes in the definition apply to a site like this. One is equivalent permanent stabilization, such as riprap or gabions, which is not a realistic answer across an entire array field. The other applies in arid, semi-arid and drought-stricken areas only. Semi-arid in the permit means 10 to 20 inches of average annual rainfall, and drought-stricken means NOAA's Seasonal Drought Outlook shows drought conditions likely for the period of construction. There, the permit can close once temporary erosion control, such as a degradable rolled erosion control product, goes in with a seed base, designed to control erosion for at least three years without active maintenance and to reach 70 percent of the native background cover within three years. Either way, the number the site is built toward is 70 percent.
The permit is direct about what that cover is for. Final stabilization must be achieved before coverage can terminate, the Notice of Termination for a large construction site is due within 30 days after final stabilization is reached, and compliance with the permit is required until that notice is submitted and approved. The vegetation is not a finishing touch. Outside the dry-site route, it is the condition that ends the permit. Inside it, 70 percent is still the number the erosion control has to be designed to reach.
The permit also sets the clock. Stabilization has to begin no later than the end of the next workday after earth-disturbing work stops on an area for good, or for longer than 14 days, and temporary stabilization has to be complete within 14 calendar days of starting. It recognizes Texas conditions too: in arid, semi-arid and drought-stricken areas where starting vegetation immediately is not feasible, non-vegetative measures have to go in as soon as practicable. Those measures hold the ground in the meantime. Outside the dry-site route, the stand still has to come in before the permit closes. Inside it, the measures still have to be designed to reach 70 percent. In a dry year, rainfall alone may not bring it in on schedule.
How Much Water Establishment Takes
The clearest published Texas benchmark for establishment watering, and the one Running Beaver plans against, is the Texas Department of Transportation's. TxDOT's Dallas District vegetation establishment sheet, written to the 2024 Standard Specifications under Item 168, Vegetative Watering, sets the schedule for new seed. The statewide specification itself sets no rate: Item 168 requires water distributed at a uniform and controllable rate, without eroding soil or plantings, in the quantity shown on the plans, and measures it by the thousand gallons as applied. The district sheet is where the numbers are:
In spring and fall (March, April, May and October), 7,000 gallons per acre per working day. In summer (June through September), 12,000 gallons per acre per working day. Either way, for 60 consecutive working days, which comes to 420,000 gallons per acre in spring and fall and 720,000 gallons per acre in summer. In winter the rate drops to 1,000 gallons per acre per working day, for 15 working days, which comes to 15,000 gallons per acre.
The same sheet is specific about how the water goes on. Watering keeps the seed bed moist through germination. After drill seeding, it waits until the site has had at least half an inch of natural rain in a single day, and for warm-season grasses it waits until the soil is above 70 degrees. Every piece of watering equipment carries a metering device. No watering between noon and 5:00 p.m. when daytime temperatures are above 95 degrees. A quarter inch of rain on a working day, which the sheet notes equals 7,000 gallons per acre, counts as that day's water. Once the stand is up, watering continues at about an inch a week through the summer months until the end of the contract. And if the contractor falls short on water, the seed that fails gets replaced at the contractor's expense.
That is a highway roadside specification for one TxDOT district, and a solar site is not a roadside. It is still the best published Texas number for what a new stand drinks, and it makes the scale obvious. On the arithmetic, a 10,000-gallon tank covers about 1.4 acres for one spring working day at that rate, and less than one acre in summer. Multiply by the acres seeded and by 60 working days, and the water supply stops being a line item and becomes the plan.
The Pond Is a Supply, and a Permitting Question
A retention pond on site puts the water where the work is instead of on a truck route. It also loses water whether anyone draws from it or not. The Texas Water Development Board studied evaporation from the state's major reservoirs from 2001 through 2018. In the 2011 drought, net evaporation from the 188 major water supply reservoirs came to an estimated 5.83 million acre-feet, more than all municipal water use in Texas that year, 4.97 million acre-feet. That study covers large reservoirs, not site ponds, so the number does not scale down. The principle does: in a Texas summer the pond is losing water to the air at the same time it is being drawn down for the seed, and the water plan has to account for both.
The water source is also a permitting question. Texas Water Code Section 11.142 lets a person build a reservoir on their own property, holding up to 200 acre-feet, without a permit, for domestic and livestock purposes, and the statute states that the exemption does not apply to a commercial operation. Whether a particular pond on a particular site needs a water right is a question for the owner's permitting team. It is a question to answer before the pond is counted as the water supply, not after the seed is down.
Zones, the Nurse Tank and Delivery
Running Beaver's irrigation scope on a Texas solar site includes six zones fed from the site's large retention pond, with a 10,000-gallon nurse tank in the system. The site's grazing flock gets its water and troughs from the same job, which is the kind of addition that comes from a crew that already knows how the site is run.
Zones are how one water source covers a whole site. Running the site in sections, in rotation, is how each section gets its scheduled water without overrunning the supply, and it is what makes the permit's uniformity test reachable. A stand that is thick next to the pond and thin at the far zone does not meet a definition that says evenly distributed, without large bare areas.
The Months After Installation Are the Job
An irrigation system on a solar site earns its keep in the weeks after it is turned on, not the day it is finished. The scope above keeps crew on site for a few months after installation, because establishment is a daily operation: running zones on schedule, tracking water against the rate, standing down after a qualifying rain, staying off the hottest hours, watching for bare patches and washouts, and reseeding where the stand does not take.
The federal standard for planting disturbed ground is built the same way. USDA Natural Resources Conservation Service Conservation Practice Standard 342, Critical Area Planting, applies to construction areas among other highly disturbed sites. It calls for the plan to address the supplemental water needed for establishment and to describe what successful establishment means, such as a minimum percent of ground cover. It then has establishment observed at regular intervals until the planting meets that description, with reseeding or replanting where it is needed.
The construction side of the industry puts the same priority on the stand. Virginia Brown, SWPPP and vegetation senior manager in McCarthy Building Companies' Renewable Energy and Storage Group, told Solar Builder in 2021 that a healthy stand of vegetation is the most effective and sustainable stormwater control on a solar project, and named establishing vegetation early in the project timeline and phased seeding among the steps that get it there. Water is what makes that schedule hold in a Texas summer.
When the stand is established, the site does not need a new vendor. The crew that watered it in can be the crew that maintains it, which is the whole point of having the work sit with a vegetation company in the first place.
Questions to Settle Before the Water Goes On
What cover does the stormwater permit require, and who will certify that final stabilization has been reached?
What is the seeding window, and what is the water plan if it is dry when the seed goes in?
How many acres are being established, and what does that come to in gallons per working day at the benchmark rate?
Where does the water come from, and has the owner's permitting team confirmed it can be used for this?
How much will the pond lose to evaporation in July and August, and what makes up the difference?
How many zones, and how long does one full rotation take?
Who is on site after installation, for how long, and what do they record?
Once the stand holds, who takes it over as maintenance?
Running Beaver plans establishment watering against the 70 percent target and the published Texas benchmark, and keeps the crew on it until the stand holds. If irrigation is on the plan for a site you run, see Running Beaver's irrigation work and talk to us before it goes out to bid.
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