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1.3 Plant Water Demand
Plant water demand is the water needed over a given period of time to support a landscape. The first step in determining plant water demand is to calculate the inches of water needed per year for each square foot of plant canopy (as seen from a bird’s eye view). This is calculated by multiplying Tucson’s monthly reference ETo by the plant coefficient for each plant type to be used at a
(Table A-4). Add monthly amounts to get the total annual plant demand per square foot of canopy (Table A-4).
PLANT TYPE | MONTHLY PLANT WATER DEMAND (Inches) | ANNUAL DEMAND (Inches) | |||||||||||
Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | ||
Very low water use | 0.39 | 0.48 | 0.78 | 1.05 | 1.29 | 1.37 | 1.22 | 1.03 | 0.94 | 0.75 | 0.47 | 0.34 | 10.11 |
Low water use | 0.78 | 0.97 | 1.56 | 2.09 | 2.59 | 2.75 | 2.43 | 2.06 | 1.87 | 1.50 | 0.94 | 0.69 | 20.22 |
Moderate water use | 1.35 | 1.67 | 2.70 | 3.62 | 4.48 | 4.75 | 4.21 | 3.56 | 3.24 | 2.59 | 1.62 | 1.19 | 34.99 |
High water use | 1.95 | 2.42 | 3.90 | 5.23 | 6.47 | 6.86 | 6.08 | 5.15 | 4.68 | 3.74 | 2.34 | 1.72 | 50.54 |
To calculate various plant water demands for large planted areas, the inches of water needed per square foot of one type of plant canopy is multiplied by the total canopy area for that plant type to get plant water demand. Plant Water Demand should be calculated for each individual
. These can then be added together to get total plant water demand for the
.
1.4 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.
2.0 RAINWATER SUPPLY ASSUMPTIONS
2.1 Factors Affecting Rainfall
Rainfall in the Sonoran Desert is highly variable. Between 1993 and 2008, Tucson’s annual rainfall ranged from 7.62 inches to 14.99 inches and averaged 12.17 inches per year. Tucson
experience localized differences in rainfall primarily due to widely spaced summer monsoon storms. Winter rains tend to cover larger areas with more even distribution of rainfall. The amount of water that can be harvested also depends on how much rain falls each time it rains. Very light rains might not create sufficient runoff to reach waterharvesting basins, while runoff from heavy rains might overflow basins.
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 |
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