Irrigation Scheduling
2007 Weekly Estimated Water-use
Alfalfa and Urban Landscape Water-use
To efficiently manage irrigations on crops and landscapes, three parameters must be known or estimated. These are:
- The water output and distribution patterns of the irrigation system.
- The soil's water holding capacity and intake rate.
- The crop's water requirements or evapotranspiration (ET).
The Irrigation System
Irrigations cannot be scheduled effectively without knowing the water application rates (depth per unit time) and water distribution patterns provided by the irrigation system. While gross application (precipitation) rate can be calculated with measurements of flow rates and wetted areas, or estimated using the design specifications of the irrigation system, the most effective and accurate method of determining precipitation rate, and distribution uniformity, is by conducting an irrigation system audit.
Detailed procedures for conducting an audit can be found at: http://aggie-horticulture.tamu.edu/greenhouse/hortgardens/conservation/agentdemo1.pdf. While this website is specific to landscapes, similar procedures are used for auditing agricultural irrigation systems.
The Soil
The soil type, specifically as it relates to soil texture (proportion of clay, silt, and sand particles) reflects both the water holding capacity (Table 1) and the water intake rate (Table 2) of the soil. This in turn determines the frequency and duration of irrigation events. To prevent deep water percolation, the depth of irrigation should not exceed the water holding capacity of the soil in the crop's root zone (for irrigation management purposes, 0-18 inches for turfgrass and 0-48 inches for alfalfa). To prevent runoff, water should not be applied at a rate (depth per unit time) faster than the soil's intake rate.
Your soil type with depth, along with specific water characteristics, can be determined by sending soil samples to a qualified laboratory for textural analyses. Consult your local Cooperative Extensive Service for details. In lieu of actual sampling, a county soil survey (prepared by the Natural Resources Conservation Service) may be consulted for this information.
Table 1. Approximate available soil water in variously textured soils.
Table 2. Water intake rates of variously textured soils.
Crop Water-Use (ET)
Assuming plants are in reasonably good health, crop water-use or evapotranspiration (ET) is primarily related to the plant species, the plant’s growth stage or size, and daily weather factors (temperature, solar radiation, humidity, and wind). By correlating these factors with actual measurements of crop ET using soil-water balance techniques, the Agricultural Science Center at Farmington (ASCF) has developed crop-coefficients that can be used to estimate crop ET during any growing season using local weather data provided by two regional weather stations
Daily/weekly ET estimates for established alfalfa, cool season turf (Kentucky bluegrass, fescue, and perennial ryegrass), warm season turf (bermudagrass, buffalograss, and blue gramagrass) and xeric (low water-use) landscape plants during 2006 are posted every Friday. Table 3 is an example of the chart. Estimates are shown for the previous week (based on calculations using that week's weather data) and for the upcoming week (based on historical data). The table is updated weekly.
These estimates are designed to serve as a guide only. Actual crop ET at the irrigated site may be different due to variability in microclimate (i.e. shading, slope, reflected light and heat, etc.) and management (soil fertility, weed growth and pest damage, etc.). Additionally, these estimates are for established crops (2-3 years old). Actual ET for new crops, whose root systems have not yet developed, will be less.
Also, the irrigation requirement (IR) to satisfy crop ET will depend on irrigation efficiency (IE). Since IE is normally less than 100% using flood and sprinkler systems, IR will be greater than ET and adjustments should be made accordingly. For example, if estimated weekly ET is 2.0 inches and the estimated IE is 80% (or 0.80), IR will equal 2.5 inches (2.0 divided by 0.80).
Table 3. Example of Weekly/daily Water-use or evapotranspiration (ET) estimates for alfalfa, cool season turf, warm season turf, and xeric plants in the Farmington, NM vicinity during 2006.
1These are baseline estimates of crop water-use. Actual ET at site may be different due to microclimate, plant health, etc. Actual irrigation requirement to satisfy ET depends on irrigation efficiency.
2To convert inches to gallons, multiply inches x 0.62 x the plant canopy area in sq. ft.
'Calculated' water-use is based on the current week's weather. The ‘Predicted’ weekly water-use estimate is based on historical averages. The ‘Predicted’ data is provided for irrigation planning and controller-based irrigation systems. The data will be updated weekly every Friday, during the irrigation season, for irrigator convenience. The '2006 Archived Water-use' data remains accessible for reference.