Tag Archives: irrigation

Designing a Commercial Rainwater Collection System for Irrigation

Designing a Commercial Rainwater Collection System for IrrigationWe recently discussed designing a residential rainwater collection storage for irrigation based on capacity and amount of irrigation needed. Now lets look into designing a commercial rainwater collection system for irrigation, based on the same theory.

Designing a Commercial Rainwater Collection System for IrrigationCommercial irrigation demands are significantly higher than residential, requiring much larger storage capacities. Landscaping of a commercial building can be extensive. The use of drought tolerant plants,  swales and rain gardens are becoming increasingly popular. New construction is required to infiltrate or use roof runoff. Rainwater collection can offset costs of expensive infiltration and cut municipal water supply usage.

Designing a Commercial Rainwater Collection System for IrrigationThe larger roof area of a typical commercial building will produce a large amount of water. If one square foot of surface area receives .623 gallons of water per 1″ of rainfall, a 20,000 square foot roof will yield approximately 12,460 gallons per 1″ of rain.  If 1″ of rainfall is required for the healthy growth of plants and 1 square foot of surface area receives .623 gallons per 1″ of rainfall, a 4,000 square foot area would require 2,492  gallons per week or 29,904 gallons stored for the 3 month dry period.

From these equations, you can see that careful thought to the landscaping will help bring the storage capacity down. As much as 50% reduction can be achieved by integrating both landscaping design and storage capacities. Generally, steel, bolted together “silo”, style tanks are best suited for commercial applications. Many architects are including these cisterns as focal points in their designs, adding a bit of an industrial look. Irrigation of landscaping uses most of its water during the summer months. By incorporating rain gardens or swales into the landscaping, considerable amounts of water can be infiltrated during the rainy season while stored water providing watering during the summer months.

Designing a Residential Rainwater Collection System for Irrigation

Designing Residential Rainwater Collection Systems for IrrigationWe recently discussed key factors in designing a rainwater collection system and briefly discussed intended uses such as irrigation, toilet and laundry facility, and whole house usage including potable. Let’s take a look at designing a residential rainwater collection system for irrigation based on storage capacity.

For rural applications with more available space for storage, this can be achieved with large tanks. However in an urban setting where space is limited, the designer must take into consideration not only the planting area, but also available space for storage.

Most  horticulturists will say that for the healthy growth of plants, one inch of rainfall per week is required. If one square foot of surface area receives .623 gallons per inch of rainfall, we can size usage and equate that to storage needed.

For example: 200 sq ft of planting area requires 124.6 gallons per week for irrigation or 1495.2 gallons stored for 12 weeks of dry season.

Now let’s look at space required for storage of 1495.2 gallons. Considering 7.48 gallons per cu ft, 1495.2 gallons = 199.89 cu ft.

Designing Residential Rainwater Collection Systems for IrrigationSizes and dimensions very from each manufacturer and most manufacturers offer cylindrical and slim line style tanks. Slim line designs are better suited for tight areas along side the house. While box tanks with their low profiles are best suited for under decks. A cylindrical tank needs some space, but offers the most bang for your buck with higher storage capacities, requiring fewer tanks for larger storage needs.

Sizing your storage for 3 months of dry season, like we experience in the Seattle area, is dependent on area of planting and available space for storage. Both need to be considered for a successful design.