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Supporting Information
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Water-budget method to estimate recharge
A water budget was calculated to estimate total annual recharge beneath the infiltration
trench system and irrigated lawn site:
R  P  Roff  I  O f  ET
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(SI-1)
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where R is recharge [L3 T–1], P is precipitation [L3 T–1], Roff is runoff from the paved surfaces [L3
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T–1], I is irrigation [L3 T–1], Of is overflow out of the gravel trench [L3 T–1], and ET is
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evapotranspiration [L3 T–1], which are described below. Total annual recharge was calculated by
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summing all hourly intervals of recharge for each year of interest. In this study, the change in
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storage of the soil beneath the irrigated lawn site was negligible because water content remains
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relatively stable (Figure 8b) and the soil lacks a zero-flux plane (Figure 7b) that is typical of
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semi/arid-environments [Healy, 2010]. Hourly intervals were used for the infiltration trench
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water budgets to accurately capture the transient dynamics of the overflow out of the gravel
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trench and daily intervals were used for the irrigated lawn water budget.
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Runoff flowing into the infiltration trench was estimated using a curve number and runoff
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method. Runoff does not apply to the irrigated lawn site because there is no surface runoff
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draining to the irrigated lawn. Daily and hourly runoff to the infiltration trench was calculated
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using the TR-55 SCS curve number method [Natural Resources Conservation Service, 1986]. A
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curve number of 98 that represents impermeable surfaces such as roofs and pavement was
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applied to the impermeable surfaces draining to the infiltration trench. The runoff (Roff) is
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calculated as:
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Roff

 100

  P  0.2
 10 
 CN


2

 100

 P  0.8 CN  10 



(SI-2)
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where Roff is the runoff [L T–1], P is the precipitation [L T–1], and CN is the composite curve
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number (unitless) [Natural Resources Conservation Service, 1986]. We estimated an effective
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drainage area of 430 m2. It should be noted that large discrepancies can occur when using the
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runoff curve number method for hourly precipitation values < 1.0 mm [Natural Resources
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Conservation Service, 1986; Pitt, 1999].
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Irrigation (I, equation (SI-1)) for the lawn site was calculated as 3.1 cm3 hour–1 (or
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approximately 2.8 cm3 week–1 at 54 minutes of total irrigation week–1) using the methods
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outlined in a specialized irrigation auditor manual [Irrigation Association, 2010]. Irrigation rates
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for the infiltration trench were provided by SFSU Facilities personnel and estimated at
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approximately 2.4 cm3 week–1.
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Overflow (Of, equation (SI-1)) was calculated in two-steps. First, the maximum overflow
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rate from the 3-cm diameter weephole (Figure 3a) was calculated at 7.6 m3 hour–1 using
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Toricelli's Law and Bernoulli's Equation [Dingman, 2008]. Second, hourly overflow from the
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weephole was assumed to equal hourly water inflow (Roff and P) provided the water in the trench
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had exceeded the maximum storage capacity of approximately 2.0 m3 and the water inflow was
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less than the maximum outflow.
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Actual evapotranspiration (ET, equation (SI-1)) was estimated as the sum of soil potential
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evaporation (PE) and plant evapotranspiration (ETp). We estimate evaporation using simple
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meteorological variables and the modified version of the Penman-Monteith equation developed
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by Kay and Davies, [2008]:
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PE 
Re  T  5
for T  5 C
   w  100
(SI-3)
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where PE is the potential evaporation from the soil [m day–1], Re is the extraterrestrial radiation
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[J m–2 s–1], T is the mean daily air temperature [°C], λ is the latent heat of vaporization [2.45E6 J
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kg–1], and ρw is the density of water [kg m–3]. Equation SI-3 is a reasonable approximation of
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actual evaporation given that the system occasionally has standing water and is a water-rich
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system. Plant evapotranspiration was calculated for the irrigated lawn using an empirical-
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coefficient approach [California Department of Water Resources, 2000; Romero and Dukes,
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2009]:
ETp  K L  ET0
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(SI-4)
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where ETp is the plant evapotranspiration [m day–1], KL is the landscape coefficient (unitless),
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and ET0 is the reference evapotranspiration for San Francisco (m day–1). Reference values of KL
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range from 0.5 to 0.8. Reference values for ET0 in San Francisco range from 1.3 to 3.6 cm
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month–1 in July and are estimated to decrease six months prior to and after July to values of
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approximately 1.0 to 2.0 cm month–1 in December and January based on the coefficients
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provided [California Department of Water Resources, 2000; Romero and Dukes, 2009].
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References
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California Department of Water Resources (2000), A Guide to Estimating Irrigation Water
Needs of Landscape Plantings in California. The Landscape Coefficient Method and
WUCOLS III, [online] Available from:
http://www.water.ca.gov/wateruseefficiency/docs/wucols00.pdf (Accessed 15 April
2012)
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Dingman, S. L. (2008), Physical Hydrology, Waveland Press Inc., 656 p.
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Irrigation Association (2010), Landscape Irrigation Auditor, Falls Church, Virginia, 312 p.
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Kay, A. L., and H. N. Davies (2008), Calculating potential evaporation from climate model data:
A source of uncertainty for hydrological climate change impacts, J. Hydrol., 358(3-4),
221–239, doi:10.1016/j.jhydrol.2008.06.005.
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Natural Resources Conservation Service (1986), Urban Hydrology for Small Watersheds,
Technical Manual, United States Department of Agriculture-Conservation Engineering
Division.
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Pitt, R. (1999), Small storm hydrology ad why it is important for the desin of stormwater control
practices., in Advances in modeling the management of stormwater impacts, vol. 7, edited
by W. James, Computational Hydraulics International and Lewis Publishers/CRC Press,
Guelph, Ontario.
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Romero, C. C., and M. D. Dukes (2009), Turfgrass and Ornamental Plant Evapotranspiration and
Crop Coefficient Literature Review, University of Florida Agricultural and Biological
Engineering Department, UFAS Extension, Gainesville, FL, 55 p. [online] Available
from: http://abe.ufl.edu/mdukes/pdf/irrigationefficiency/Romero_Dukes_Turfgrass%20ET_Crop_%20Coefficient_%20Lit.pdf
(Accessed 5 June 2013)
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