Irrigation scheduling is an essential part of conserving landscape water use. WIM pursues these and other steps to assure that watering is properly scheduled for each landscape’s specific conditions.
CALCULATE REFERENCE EVAPOTRANSPIRATION
The need for water changes with the weather. Evapotranspiration (ET) is the sum of the water lost from the soil surface (evaporation) and the water used by the plants (transpiration).
When calculating a base watering schedule, Reference Evapotranspiration (ETo) is often used. Reference ET can be estimated from a variety of sources, including local weather stations (such as WIM's), computer services, historical data, or simply measuring the depth of water evaporated from an open pan.
Peak ET Rates of Selected U.S. Cities
ET Connection - by state
APPLY EFFECTIVE PLANT COEFFICIENTS
This is used to determine the water needs for each type of plant material.
DETERMINE IRRIGATION APPLICATION EFFICIENCY
This helps identify the amount of applied water needed to overcome the inefficiencies involved in the application process.
A more efficient, site-specific irrigation system requires the application of less water applied to get the same result as system that’s designed to operate less efficiently.
CALCULATE PRECIPITATION RATES
Calculating Precipitation Rates (PR) helps determine how quickly the water is applied per unit of time. Precipitation Rate is a critical design factor, as sprinklers can easily apply water faster than the soil's intake rate.
Precipitation Rates can be calculated based on two different formulas.
- Gross Precipitation Rate: based on the total flow of the irrigation station (or zone), not accounting for water losses that occur between sprinklers and the landscape.
PRgross (in./h) = 96.3 x Q / A
PRgross = Net precipitation rate (in./h)
Q = Station flow rate (gpm)
A = Station area (ft2)
96.3 = Conversion factor - Net Precipitation Rate: the measure of the amount of water that reaches the landscape, accounting for the losses that occur between the sprinkler and the landscape.
PRnet (in./h) = CVavg x 3.66 / TR x CDA
PRnet = Net precipitation rate (in./h)
CVavg = Average catch can volume (milliliters)
3.66 = Constant that converts milliliters to in.3 and minutes to hours
TR = Testing run times (minutes)
CDA = Catch device throat area (square inches)
APPLY SOIL/PLANT/WATER RELATIONSHIPS
This helps identify the needed frequency of irrigation and cycle/soak. Final recommendations are based on the site-specific analysis of:
- Soil type
- Infiltration rate
- Water holding capacity
