1.0 WATER DEMAND ASSUMPTIONS

   Multiple techniques can be used to determine how much water is needed to support plants at a site . For purposes of complying with city requirements, the methods and assumptions described below are recommended.

1.1 Reference Evapotranspiration

   To determine how much water plants need, a measurement of how much water plants consume through evapotranspiration is needed. Evapotranspiration is the transfer of water from land surface to the atmosphere through the combination of evaporation and plant transpiration. In the Reference Evapotranspiration Method (ETo Method), water use by plants is estimated for a high-water-use grass cover crop (such as alfalfa) for which evapotranspiration can be directly measured. Monthly Reference ETo values for a high-water-use grass crop in Tucson have been tabulated in Table A-2.

Table A-2. Reference Evapotranspiration (ETo) for Tucson

 

MONTHLY REFERENCE EVAPOTRANSPIRATION (Inches)
Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
3.0	3.72	6.0	8.04	9.96	10.56	9.36	7.92	7.20	5.76	3.60	2.64

 

Table A-1. Site Water Budget Format

Line	DESCRIPTIONS OF WATER BUDGET COMPONENT	SOURCES FOR EACH WATER HARVESTING INFILTRATION AREAS (WHIAs)	EXAMPLE WHIA
	PLANT WATER DEMAND
1	Individual WHIA identifier	APPLICANT PROVIDES	WHIA - 1
2	Plant water demand category for this WHIA	APPLICANT PROVIDE USING ADWR PLANT LIST	low
3	Plant canopy area (square feet) for this WHIA. Add the canopies of understory, midstory and overstory plant areas to get total canopy for each WHIA. Assume 60% of mature plant size	APPLICANT PROVIDE. CAN BE BASED ON LANDSCAPE PLAN OR PLANT LIST	1250
4	Plant water demand per year (vertical feet of water per year per square foot of canopy) for this WHIA	APPLICANT USES APPROPRIATE NUMBER BASED ON PLANT WATER DEMAND CATEGORY FOR THIS WHIA	1.7
5	CALCULATE: Annual plant water demand for this WHIA based on plant canopy area (gallons)	= line 3 times line 4 times 7.48 gallons/cubic foot of water	15,895
6	OVERALL WATER HARVESTING SUPPLY
7	WHIA area (square feet)	APPLICANT PROVIDE	1000
8	WHIA average depth (feet)	APPLICANT PROVIDES	0.5
9	CALCULATE: WHIA capacity (gallons)	line 7 times line 8 times 7.48 gallons/cubic foot of water	3,740
10	If Passive water harvesting is used:
11	Subwatershed identifier	APPLICANT	S-1
12	Ratio of subwatershed area to plant canopy area needed to meet plant water demand in this WHIA in July through March (use March plant water demand as the indicator month) (no units)	APPLICANT USES APPROPRIATE NUMBER BASED ON PLANT WATER DEMAND CATEGORY FOR EACH WHIA	3.85
13	CALCULATE: Total catchment area ideally needed to meet plant water demand in March (square feet)	= line 3 x line 12	4,813
14	Actual total catchment area designed for this WHIA including the WHIA area itself (square feet)	APPLICANT PROVIDE	1500
15	CALCULATE: Actual catchment ratio for this WHIA	= line 14 divided by line 3	1.20
16	CALCULATE: Actual percent of plant water demand that will be met for this WHIA	= 64% times line 15 divided by line 12	20%
17	If Active water harvesting is used:
18	Tank identifier	APPLICANT PROVIDES	T-1
19	above or below ground?	APPLICANT PROVIDES	above
20	tank height (feet)	APPLICANT PROVIDES	8
21	tank diameter (feet)	APPLICANT PROVIDES	6
22	tank capacity (gallons)	APPLICANT PROVIDES	1619
23	tank location	APPLICANT PROVIDES	East corner
24	CALCULATE: Percent of plant water demand for this WHIA met by this tank	(Assume tank will be filled and emptied 4 times per year). Total water provided = 4 times line 22 divided by line 5	41%
25	CALCULATE: Percent of plant water demand for this WHIA met using harvested rainwater from passive systems and active systems (as applicable)	= line 16 + line 24	61%
26	TOTAL SITE INFORMATION
27	Percent to total site annual landscape demand met using harvested water	Prorate percent of water harvesting supply based on area of each WHIA	61%
28	Water harvesting capacity offsetting retention basin size capacity	APPLICANT PROVIDES

 

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.