4.9.3 Sizing

The basic rule for sizing any rain water harvesting system is that the volume of water that can be captured and stored (the supply) must equal or exceed the volume of water used (the demand) (Texas Water Development Board, 2005). Understanding the water balance will allow the designer to understand whether harvested rain water will be adequate to meet demands. Stormwater runoff from roof areas only should be directed to rain water harvesting systems.

The size of the roof, expressed as the catchment area, is equal to the width times the length of the area flowing to a gutter. The slope of the roof is not considered in the catchment area calculation (e.g., the horizontal projection of the area is used for sizing, which is smaller than the actual area for sloped roofs).

General guidelines for sizing the rain water collection system to the catchment area include:
  • One inch of rain falling on one square-foot of rooftop will produce 0.6233 gallons of water or approximately 600 gallons per 1,000 square feet of roof without inefficiencies.
  • The system will lose approximately 10 to 25 percent of the total rainfall due to evaporation, initial wetting of the collection material, and inefficiencies in the collection process (Texas Water Development Board, 2005). Precipitation loss increases with the roughness of the roofing material. Precipitation loss the least with metal, more with composition, and greatest with wood shake or shingle.

See the Texas Rainwater Harvesting Manual (Texas Water Development Board, 2005) and the 2004 SWMMEW for detailed discussion on water balance modeling for sizing of rain water harvesting systems. Where daily or sub-daily rainfall data are available, daily or sub-daily waterbalance modeling is recommended over monthly water balance modeling. The finer time scale will allow the designer to evaluate the timing and magnitude of system overflows, thereby allowing for more accurate sizing of downstream conveyance and flow control facilities where needed. Estimating Indoor Water Demand

Indoor water demand is largely unaffected by changes in weather, although changes in household occupancy rates depending upon seasons and very minor changes in consumption of water due to increases in temperature may be worth factoring in some instances. The results of a study of 1,200 single-family homes by the American Water Works Association (AWWA) in 1999 found that the average water conserving households used approximately 49.6 gallons per person per day (AWWA, 1999).

Many households use less than the average of 49.6 gallons per person found in the 1999 report by the
AWWA, Residential End Uses of Water. Overall demand in showers, baths, and faucet uses is a function of both time of use and rate of flow. Many people do not open the flow rate as high as it could be finding low or moderate flow rates more comfortable (WSU-PSP, 2012). In estimating demand, measuring flow rates and consumption in the household may be worth the effort to get more accurate estimates, but should verified against the records of historical use from a municipal water bill if available. Estimating Outdoor Water Demand

Outdoor water demand peaks during the summer months. The water demands of a large turfgrass area often exceed the volume of harvested rain water for irrigation. For planning purposes, historical evapotranspiration and evaporation should be used to project potential water demand. Additional resources related to the water demands for standard and drought tolerant landscapes by region are available.