Basic Functions
If you’ve decided, or are still considering, using an irrigation pond in your operations, it’s a good time to take a good look at the elements involved in designing a pond that will meet your specific needs. It’s easier and cheaper to build it correctly from the start, rather than having to expand or modify it due to unexpected situations. It’s highly recommended (and sometimes mandated) to engage an experienced professional in the design of your pond. Consultations with local natural resource professionals and permitting bodies also must not be overlooked.
There are several basic functions that an irrigation reservoir should fulfill:
- Whether your irrigation pond will be fed by stormwater capture or tailwater return, your pond will inevitably collect sediment. Your reservoir design should integrate strategies to manage both. In this way, the reservoir design can be compared to that of stormwater detention ponds found in urban areas.
- The size of your irrigation reservoir depends on your intended use and specific volume needs. Since storage volume is critical and storage losses due to excessive evaporation is problematic, reservoirs are usually built to be deep rather than wide. An average depth of 10 feet is generally accepted as the minimum depth, but if properly engineered, reservoirs can be constructed 20 feet or deeper. A major benefit of this practical necessity is that greater depth results in a smaller footprint, and less potentially productive land taken out of service.
- When calculating the design and capacity of your pond, it’s important to check on historical weather data in your region (with special focus on more recent patterns) to calculate the amount of precipitation falling during storm events, including extreme examples. This will help you estimate how much water you are able to collect overall, as well as the maximum you’ll be able to accommodate if a series of excessive rainfall occurs over several days. Regions familiar with atmospheric rivers will understand the frustrating nature of these overwhelming gifts of nature.
- A proper reservoir design should take into consideration the volume of water that enters and leaves, which helps determine how long collected water will remain in residence before it drains out or is withdrawn for irrigation. This hydraulic retention time (HRT) is an important indicator in how effective the reservoir will be at removing contaminants, including sediment. When water quality is in question, longer HRTs will produce better quality parameters.
- HRT can be maximized during the design stage by maximizing storage volume and extending the distance water must travel between the reservoir inlet and outlet. Other measures, such as the use of baffles, can substantially improve HRT in situations where the choice of reservoir size or configuration is limited.
Calculating Reserve Volume
Reserve volume is a key element in the design and planning of an irrigation reservoir. In fact, determining the requirements and location of such a reservoir would ideally be in the initial farm planning stage, where it can be naturally integrated into the farm-wide irrigation and drainage plan from the very beginning. Even if the reservoir won’t be added until sometime in the future, redesigning and rebuilding those systems can be expensive. Leaving room for expansion saves money and ensures your options remain open.
The first consideration when calculating the necessary storage capacity for an irrigation pond is the effective precipitation that typically falls during the growing season. Keep in mind that weather patterns are changing and consider placing more weight on recent records than on historic examples. You’ll also want to make allowances if the rainfall is evenly spread across the growing season or if you typically experience a dry period amid it.
Next, consider the types of crops you’ll be growing – specifically acreage and water requirements. Look at your irrigation system and consider how efficient it is, and how much water is typically lost through evaporation, wind, and seepage. Using these numbers, you can prepare a month-to-month water budget that will determine the required capacity of the reservoir. At its most basic, the pond’s storage capacity should meet total irrigation requirements less the expected amount of natural rainfall. Be sure to include expected water losses due to evaporation or seepage to the total storage capacity.
Ideally, your storage reservoir will hold a minimum water supply of one to two years, to allow for the occasional dry year. This is good insurance, but keep in mind that even irrigation reservoirs won’t be able to completely cover all your needs for a severe multi-year drought. To be prepared for this possibility, it’s important to have a contingency plan for situations where water supplies are under extraordinary pressure. You may choose to sacrifice quality and yield to keep your crops in the ground for one more year in hopes the drought will break, or you may choose to let acres of lower-value crops go and focus your existing supplies on supporting smaller acreages of high value crops. Neither of these outcomes are ideal, but a full irrigation reservoir does present options that you may not have had otherwise.
When you’re calculating the amount of stormwater runoff you can expect to collect in a normal year, it’s safe to assume that you’ll need a drainage area of at least 10 acres for every acre of pond surface. You can consult local environmental or watershed management departments to help you identify drainage patterns in your vicinity. If your water storage needs and usage patterns are high, it’s likely that you’ll need more than 10 acres of drainage area. In cases like these, it may be worth considering constructing several smaller reservoirs that draw from different drainage areas to maximize how much you can capture.
