1.2 Plant Coefficients

   Four categories of plant types are typically used in Tucson. These categories are established based on their water use characteristics (Table A-3). Plant coefficients are an estimate of the water needed by each plant type expressed as a fraction of the water needed for the reference high-water-use grass crop. Plant types and corresponding plant coefficients are shown in Table A-3.

Table A-3. Plant Coefficients for use in Tucson

 

PLANT TYPE	PLANT COEFFICIENT
Very low water use	0.13 times monthly Reference ETo
Low water use	0.26 times monthly Reference ETo
Moderate water use	0.45 times monthly Reference ETo
High water use	0.65 times monthly Reference ETo

 

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 site (Table A-4). Add monthly amounts to get the total annual plant demand per square foot of canopy (Table A-4).

Table A-4. Plant water demand per square foot of plant canopy for Tucson

 

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 Water Harvesting Infiltration Areas . These can then be added together to get total plant water demand for the site .

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 sites 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 Rainwater 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.

Table A-5. Effective average rainfall for Tucson for use in Site Water Budget calculations

 

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.