Using ANSI/ASABE S623 & SLIDE to Estimate Landscape Water Requirements

SLIDE (Simplified Landscape Irrigation Demand Estimation) is the most accurate and scientific method currently available for estimating the water demand of established landscapes.  It provides a research-based set of plant factors for landscape plant materials (see Table 1) and rules for assigning and applying them to estimate landscape water demand (see below).

SLIDE serves as the basis for the ANSI/ASABE S623 Standard, Determining Landscape Plant Water Demands.  SLIDE and the ANSI/ASABE S623 Standard are the basis of a set of easy to use calculators that provide reliable water requirement estimates for lawns and common landscape scenarios. A small consortium of academic experts formulated SLIDE based on analysis and interpretation of published field research studies on landscape plant water requirements and on plant water-use physiology completed over the past several years.

For irrigated landscapes, SLIDE provides an accurate and simple method for estimating plant water demand when designing landscapes and when calculating effective irrigation schedules for them.  When designing non-irrigated landscapes, SLIDE enables one to estimate accurately whether or not a particular landscape will perform acceptably with anticipated precipitation.

• Simple to understand and use, and it replaces the need for a large database of Plant Factors for adjusting reference evapotranspiration (ET) data.
• Scientifically defensible, conceptually sound, and logical.
• Applicable nationally.
• Able to provide accurate plant factors immediately for new plant introductions.
• Effective in enabling water conservation.
• Reliable in providing landscape water requirement data needed for:
  • complying with California’s Model Water Efficient Landscape Ordinance.
  • designing landscapes that comply with water conservation ordinances and green building programs.
  • calculating landscape water budgets and irrigation schedules.
  • smart irrigation controller algorithms.

¹ Values do not apply to any plant production operations, such as nurseries, greenhouses, sod farms, or commercial farms.

² Plant Factor shown is the annual average Kc value; monthly or seasonal Kc's may be available if more precision is desired.

³ Plant Factor does not apply to sports fields, golf greens or tees.

Data adapted from: ANSI/ASABE Standard S623, Determining Landscape Plant Water Requirements. 2015.; Snyder, R.L. 2014.  Irrigation scheduling: Water balance method. http://biomet.ucdavis.edu/irrigation_scheduling/bis/ISWBM.pdf.; Meyer, J.L., V.A. Gibeault, and V.B. Youngner. 1985. Irrigation of turfgrass below replacement of evapotranspiration as a means of water conservation: Determining crop coefficient of turfgrasses. Proc. 5^th Intl. Turfgrass Research Conf.

SLIDE Rules

SLIDE is framed by the following four SLIDE Rules that integrate and apply the available science in plant environmental physiology and landscape plant water requirements:

• SLIDE Rule #1. Reference evapotranspiration (ET_o) accurately estimates water demand of lawns and other uniform turf areas, but it marginally represents water demand of non-turf, non-uniform, physically and biologically diverse landscapes.
• SLIDE Rule #2. Plant Factors (PFs) alone accurately adjust ET_o to estimate landscape water demand, and they are assigned by general plant type categories, not by individual species (see Table 1 below). 
• SLIDE Rule #3. A landscape area or zone controlled by one irrigation valve (hydrozone) is the smallest water management unit in a landscape; when plant types are mixed in a hydrozone, the water demand is governed by the plant type with the highest PF.
• SLIDE Rule #4. Water demand of dense plant cover (canopy covers =80%of the ground surface) comprised of mixed plant types is that of a single ‘big leaf’ governed by the plant type category in the mix with the highest PF; demand of sparse plant cover (canopy covers <80% of the ground surface) is that of individual plants and is governed by their leaf area and the PF of their plant type category.

 SLIDE Equation

The basic SLIDE equation uses only the Plant Factors from Table 1 to adjust reference ET and follows simple calculations to produce an estimate of the water required by a landscape area for a given period.  The basic SLIDE equation is:

Landscape Water Demand (gal.) = ET_o × PF × LA × 0.623    (Eq. 1)

where,

• ET_o is inches of historical average or real-time reference evapotranspiration data in inches for the period of interest.
• PF is the Plant Factor from Table 1.
• LA is the landscape area, in square feet.
• 0.623 is the factor to convert inches of water to gallons; omit this factor if the estimated water demand is desired in inches.

The only ET_o adjustment needed for estimating the water requirements of a landscape area is a Plant Factor (PF) which accounts sensibly and accurately for the water demand characteristics of the plant types present. As noted in the SLIDE Rules, non-turf landscape areas are biologically and physically complex such that their water demand characteristics are marginally estimated by ET_o, so attempting to make precise adjustments to ET_o for each plant species would do little to improve an estimate of a landscape’s water requirement. It is also ineffective to use a complicated equation with multiple ET_o adjustment factors for planting density, microclimate, and so on because, depending on the factor and planting, it may not meaningfully affect landscape water demand or it is impractical to determine a meaningful value to assign to it. Incorporating such factors provides a false sense of precision and unnecessarily complicates the calculations.  More information on making sense of ET adjustment factors is discussed here.

Using the SLIDE Equation

The basic SLIDE Equation (Equation 1 above) can be used to estimate a complex landscape's water requirement and irrigation demand by adding sub-equations that calculate the water demand for each hydrozone or distinct planting area within the larger landscape (See Equations 2 and 3 below). 

Following the SLIDE Rules, a landscape’s estimated water requirement is the sum of the water required by the areas planted with various types of plants found in Table 1. The estimated water demand will need to be met by precipitation, irrigation, or a combination in order for plants to perform acceptably. Irrigated landscapes should be designed so that each irrigation station is composed of plants with similar water requirements in what is known as a hydrozone. When plants of different water requirement categories are mixed in the same irrigation station, the water demand of the entire zone is that of the plant category with the highest PF.

Procedures

To estimate the water demand for an established landscape, use the PF values for plant material categories provided in Table 1 with historic or real-time ET_o and precipitation data from or another reliable source.

The SLIDE algorithm below calculates estimated water requirements for the landscape area occupied by the plant categories present for any period of interest. It subtracts effective monthly rainfall to derive estimated monthly irrigation demand for each plant category (if desired), sums the water or irrigation demands, adds an allowance for distribution uniformity of irrigation application, and converts units to gallons of irrigation or water demand per year. The equations within the algorithm can be converted to a spreadsheet format to automate the individual calculations.

Note in Table 1 the category “Mixed Plantings”, which represents situations where plants from various categories are interplanted and, if irrigation is used, they are watered simultaneously. “Mixed Plantings” also applies where plant categories are grouped in discrete beds or zones according to PF but there are no data on the square footage devoted to each plant category represented in the overall landscape mix. In all of these circumstances, it is simplest and safest to assume the PF is that of the plant type in the mix with highest PF.

In situations where shade trees are planted within a turf area, there is no need to factor in the water requirement of the trees. Their demand will be accounted for in the water demand estimate for the turf, as would occur for trees in any other mixed planting where the PF meets or exceeds that for trees. When overall plant canopy (turf, trees, shrubs, etc.) shades at least 80% of the soil, water use is at its maximum and at the rate of the plant creating most of the canopy cover in the area or zone. Adding layers of canopy does not significantly increase the area’s water use.

Importantly, the equations below can be used to develop a spreadsheet that automates the multiple calculations needed to derive estimated irrigation demand. An Excel spreadsheet using a layout with columns for each independent calculation step can work well. The basic calculations are:

1. Multiply ET_o for the period or year by the PF’s for the plant categories in the landscape and for each month of the year. This is the estimated water requirement in inches of the plant category for the period. Multiplying this value by the specific square planted gives the gallons of water required (see Equation 1).  
2. If precipitation is to be considered, calculate estimated effective precipitation by multiplying average monthly (or whatever period of interest) precipitation by 0.5 (i.e. 50% of precipitation) or other defensible percentage.
3. Subtract the effective precipitation from plant water requirement calculated in #1. Results that are negative values are recorded as zero. This is the estimated amount of irrigation (if available), in inches, each plant category will need in order to maintain acceptable landscape appearance and function.
4. For each plant category, multiply the inches of irrigation required by the square feet of each plant category. See #1 for guidance where plant-category specific square footage is not available.
5. If irrigation demand is needed, multiply the product from #4 by the inverse of the DU (1 ÷ DU).
6. Sum the products calculated in #4 or, if irrigation demand is needed, #5 and multiply it by 0.623; the result is the gallons of water required (see Equation 2).
7. When irrigation water quality is low due to soluble salt concentration, calculate a leaching requirement (LR) and use it to increase the irrigation water requirement as described in Equation 3. The result is the total irrigation demand when using water with elevated salinity.

The SLIDE equation to estimate a complex landscape's water demand is:

Landscape Water Demand (gal.) = ?{(ET_o × PF) × LA}_1-x × 0.623       (Eq. 2)

where,

• ET_o is historical average or real-time reference evapotranspiration data in inches for the period of interest.
• PF is the Plant Factor from Table 1 for the plant category represented in a hydrozone or a landscape area, 1 through x; when plant categories are mixed in a landscape or a hydrozone it is the highest PF among the plant categories represented.
• LA is the landscape area or hydrozone planted with the respective PF, in square feet.
• 0.623 is the factor to convert depth of water to volume (gal. ÷ [in. x sq. ft.]); omit this factor if the estimated water demand is desired in inches.

The SLIDE equation to estimate a complex landscape's irrigation requirement is:

Irrigation Demand (gal.) = ?{([ET_o × PF] - P)_J-D × LA × (1 ÷ DU)}_1-x × 0.623     (Eq. 3)

where,

• ET_o is historic or real-time annual or monthly average reference evapotranspiration data in inches for months January through December, or other period of interest.
• PF is the Plant Factor from Table 1 for the plant category represented in a hydrozone or occupying a portion of landscape area, 1 through x; when plant categories are mixed in a landscape or a hydrozone it is the highest PF among the plant categories represented.
• P is optional; it is the historical average or real-time effective precipitation in inches for months January-December, or other period of interest; 50% or similar percentage of precipitation is usually considered effective and is the amount used in the equation.
• LA is the landscape area or hydrozone, in square feet, devoted to the respective PF.
• 0.623 is the factor to convert depth of water to volume (gal. ÷ [in. x sq. ft.]); omit this factor if the estimated water demand is desired in inches.
• DU is the distribution uniformity of irrigation in the landscape area or hydrozone 1 through x (often mandated to be =0.7).

Results from the equation are converted to other units as follows:

• Hundred Cubic Feet (CCF) = Gallons ÷ 748
• Billing Units = Gallons ÷ 748 (note: Billing Unit conversion can vary, this is the most common).
• Acre-Feet = Gallons ÷ 325,853.
• Acre-Inches = Gallons ÷ 27,154.

When water quality is a concern due to elevated soluble salts, landscape irrigation requirements are estimated with the following equation:

Irrigation Demand (gal.) = ?{([ET_o × PF] - P)_J-D × LA × (1 ÷ DU)}_1-x × 0.623 × LR (Eq. 4)

where,

• ET_o is historic or real-time annual or monthly average reference evapotranspiration data in inches for months January through December, or other period of interest.
• PF is the Plant Factor from Table 1 for the plant category represented in a hydrozone or occupying a portion of landscape area, 1 through x; when plant categories are mixed in a landscape or a hydrozone it is the highest PF among the plant categories represented.
• P is optional; it is the historical average or real-time effective precipitation in inches for months January-December.
• LA is the landscape area, in square feet, devoted to the respective PF.
• 0.623 is the factor to convert depth of water to volume (gal. ÷ [in. x sq. ft.]); omit this factor if the estimated water demand is desired in inches.
• DU is the distribution uniformity of irrigation in the landscape area or hydrozone 1 through x (often mandated to be =0.7).
• LR is the leaching requirement needed if irrigation water contains soluble salts levels injurious to the plants present, calculated as follows (Ayers and Westcot, 1985; Harivandi, et al., 2008).