Cistern Sizing Model

Description/Limitations

The cistern sizing model (CSM) is intended to help users determine the appropriate cistern storage size for rainwater harvesting by assessing water supply and water demand. The challenge in answering the deceptively simple question of how large to make the cistern is to identify what key variables are significant for the creation of an adaptable model that the user can customize to fit their needs.

The CSM uses zip codes linked to a local precipitation information database provided by www.rainlog.org, landscape area/type, and roof area to perform the calculations. The most prominent factor that the model is unable to consider is the human factor due to its level of complicity. In attempt to address this 3 Methods of determining cistern volume are utilized by the CSM. They provide a range of cisterns for the user to consider based on their goals for the harvested rainwater.

Method descriptions

Method 1 generates storage volume finding the landscape's water demand deficiencies from April, May and June to determine the volume of water necessary to get the landscape through the driest part of the year in this region of the southwest. Method 2 determines storage volume by calculating water that would be accrued over December to January, the months with the greatest net precipitation relative to plant demand, to allow for excess water supplies to be stored for unexpected dry periods. Method 3 generates storage volume with precipitation information and roof area and thus is a purely water supply calculation.

Download worksheets

The only differences are the examples

simple roof

moderately complex roof

complex roof

Landscape Information

Desert Landscape

Desert Landscape utilizes low water use desert adapted plants that may or may not be indigenous to Arizona. These plants, for example cacti and other succulents, can typically survive on precipitation.

Fruit Trees

Fruit Trees are trees that bare fruit such as citrus trees. These trees typically require high amounts of supplementary irrigation.

Moderate Water Use Trees

Many trees fall into this category. For example young Desert Landscape plants like mesquite trees require additional irrigation when they are young to establish property for the first few years. However, trees like olive trees require this level of irrigation throughout their lifetime.

Turf

Turf is commonly referred to as grass, of which numerous varieties exist. Bermuda Grass is an example of grass used locally in Arizona.

Turf
ft²

Desert Landscape
ft² 10% 50% 100%

Fruit Trees
Number of trees:
Avg. canopy area: ft²

Moderate Water Use Trees
Number of trees:
Avg. canopy area: ft²

Roof Area Information

Roof Material Samples (average or majority)

Built up tar or gravel

concrete or asphault

glazed tile

metal

Rooftop Estimator

Roof Areas

Calculation

Press the button to calculate