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2.2 Effective Average Annual Rainfall
To comply with the city’s Commercial
Harvesting Ordinance, average rainfall for Tucson should be adjusted to a lower effective average rainfall. Two adjustments should be made: reduce average rainfall by 25% to address localized variability and reduce average rainfall by an additional 25% to remove very light and very heavy rainfall events from monthly rainfall. Tucson’s average rainfall and the calculation of effective average rainfall are shown in Table A-5 month-by-month and totaled for the year.
RAINFALL ASSUMPTIONS | MONTHLY AVERAGE RAINFALL (Inches) | ANNUAL TOTAL (inches) | |||||||||||
Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | ||
Average rainfall | 0.9 9 | 0.8 8 | 0.81 | 0.28 | 0.24 | 0.24 | 2.07 | 2.3 | 1.45 | 1.21 | 0.67 | 1.03 | 12.17 |
Effective Average Rainfall | 0.5 0 | 0.4 4 | 0.41 | 0.14 | 0.12 | 0.12 | 1.04 | 1.15 | 0.73 | 0.61 | 0.34 | 0.52 | 6.09 |
2.3 Alternative Calculations
Applicants wishing to use alternative values and methods from those described above may do so. Along with the alternative calculations, they should provide justification for deviation from the assumptions and methods recommended above.
3.0 DETERMINING
CATCHMENT NEEDS FOR PASSIVE
3.1 Definition of
and
Because most commercial
should be able to accomplish the ordinance’s 50%
goal using
strategies alone, the following information focuses on data for
.
typically consist of an array of
, each served by a
that provides harvested water to support the plants within it. To meet the city ordinance requirements, the
needs to be designed so that 50% of annual plant water demand is met with harvested
as an average across the
. If the 50% goal cannot be met at some
due to
conditions, then other
should be designed to exceed the 50% goal in order to achieve 50% overall.
are locations at a
from which water is harvested for beneficial use. These locations include areas where rain falls directly into earthen basins and infiltrates into the ground (these are known as
.
also include locations where rain falls on rooftops, sidewalks, parking lots, driveways and other hard surfaces then flows toward
where the water infiltrates into the soil.
The
for any given
Infiltration Area is the ratio between the
serving it and the canopy area of the plants located within it. As one example, runoff from 100 square feet of sloped parking lot and soil drains to a
Infiltration Area that is planted with trees that have a canopy area of 20 square feet (as seen from a bird’s eye view). The
for this example is 100 to 20, which can be simplified as five to one.
3.2 Calculation of
In Tucson, different types of plants need different amounts of water each month because rainfall and temperature vary from month-to-month (Table A-4). Table A-6 shows the
needed to provide harvested water for each square foot of
for different plant types in different months. The data on Table A-6 was calculated using the following equation for each month and each plant type:
needed = Monthly water demand per square foot of plant type
Effective monthly rainfall
PLANT TYPE | SQUARE FEET OF
NEEDED TO MEET WATER DEMAND FOR EACH SQUARE FOOT OF CANOPY AREA | |||||||||||
Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
Very low water use | 0.8 | 1.0 | 1.8 | 7.1 | 10.3 | 10.9 | 1.1 | 0.9 | 1.2 | 1.2 | 1.3 | 0.6 |
Low water use | 1.5 | 2.1 | 3.7 | 14.2 | 20.6 | 21.8 | 2.2 | 1.7 | 2.5 | 2.4 | 2.7 | 1.3 |
Moderat e water use | 2.6 | 3.6 | 6.3 | 24.6 | 35.6 | 37.7 | 3.9 | 3.0 | 4.3 | 4.1 | 4.6 | 2.2 |
High water use | 3.8 | 5.2 | 9.2 | 35.6 | 51.4 | 54.5 | 5.6 | 4.3 | 6.1 | 5.9 | 6.7 | 3.2 |
3.3 Calculation of
In Tucson, plants need around 37% of their annual water supply in the hot dry months of April, May and June. The remainder of their annual water supply, about 63%, is needed in July through March. Figure A-1 illustrates the relationship between plant water demand and effective rainfall supply in Tucson. In April, May and June there is a large gap between plant water demand and effective rainfall supply. There is a smaller, but still substantial gap between demand and supply in March. The rest of the year,
supply lags behind demand, but not as much as in March through June.
By designing a
that has
sized to fully meet March plant water demand, the harvested water supply would typically exceed plant water demand in July through February (Figure A-2). This design strategy provides a cushion in meeting the 50% goal for the
and is recommended by the city as a general approach.
Based on Table A-6, a landscape composed of low water use plant types would need a
of 3.7 to 1 to meet March water demand. This means that for each square foot of
(as seen from a bird’s eye view), 3.7 square feet of
is needed to collect
from. This area includes the dirt
Infiltration Area the plants are located in, and any parking lot, roof-top, or other hard surface around the plant that drains to that area.
It is best to locate plants of the same type in a
Infiltration Area, though
may have different plant types in other
. Table A-6 can be used to determine appropriate
for the plant types used in Tucson.
Annual Water Use Report | ||||
Reporting period (month/day/year): from _______________ to _____________________ | ||||
Project Name | ||||
Project Address | ||||
Owner name | ||||
Report Preparer name Preparer title | ||||
Preparer address | ||||
Preparer email Preparer phone | ||||
Source of rain data (check all that apply): ____on raingage ___ www.rainlog.org station: note the closest cross streets ___________________________________________________ | ||||
Source of metered irrigation water (check all that apply): ___Tucson Water, acct # _______________ ____ water from another water utility ____well water ____ reclaimed water | ||||
MONTHS | RAINFALL (inches) | IRRIGATION WATER USE | ||
Projected landscape water demand shown in the Harvesting Plan (gallons) | Actual metered use (gallons) | Difference (gallons) | ||
January | ||||
February | ||||
March | ||||
April | ||||
May | ||||
June | ||||
July | ||||
August | ||||
September | ||||
October | ||||
November | ||||
December | ||||
ANNUAL TOTAL | ||||
Explanation for any exceedence of annual irrigation water use projected in the approved Harvesting Plan: | ||||
Changes to the landscape or irrigation system in the reporting year: | ||||
AREA BELOW FOR STAFF USE ONLY | ||||
Drought conditions exist at the : | ||||
Compliance with Harvesting Plan: | ||||
Audit required/date/outcome | ||||
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