Agricultural Water Management

FAQ's

Q: When should I start irrigating?
A: WSU’s AgWeatherNet has new weather stations in Whatcom County, which now include soil water (kPa) measures at 2” and 8” depths to help determine when to start irrigation.
Look at how fast your moisture levels are changing to determine when to irrigate. The drier the soil gets, the faster the soil moisture readings will change.
Look at readings from both sensors to determine how much to irrigate. If the top (shallow) sensor is dry but the bottom (deep) sensor has adequate moisture, your field only needs a shorter irrigation cycle. If the top and bottom sensors are dry, your field needs a longer irrigation cycle to replenish the entire root profile.
Link: www.weather.wsu.edu

Q: How much should I irrigate?
A: Start with getting an Irrigation Water Management (IWM) Plan. You can also use WSU’s Irrigation Scheduler Tool, which integrates with AgWeatherNet to build a specific irrigation plan for your fields.
Link (mobile version): www.weather.wsu.edu/ism

Q: How can I efficiently irrigate?
A: Use soil water sensors to execute your IWM plan and download your water threshold curve to optimize water use by soil type throughout the year. Use the field's predominant soil texture to determine your soil's "trigger level" or value above field capacity to start and stop irrigation (see table below).

Irrigate when shallow soil sensor reaches the irrigation threshold "trigger level" for soil type

Soil Texture

Loamy Sand

Sandy Loam

Loam

Silt Loam

Clay Loam

Irrigation Threshold (kPa)

-25 to -30

-30 to -40

-35 to -50

-40 to -60

-60 to -80

 

How to use Soil Water Potential (kPa) Data for Irrigation Management

Soil water potential is a measure of the amount of water available to plants at any given time. It relates to soil water content when you take into account the soil and crop type. The figure below gives you an idea of how to generally interpret soil water potential levels for use in irrigation water management. Values should be tailored based on your soil type and crop grown.

  • Saturation is which is typically reached between 0 and -10 kPa when all the pore spaces in a soil are filled with water and water cannot infiltrate into the soil.
  • Field Capacity is the amount of water that can be held in the soil and is a start/stop point for irrigation. This is the max/min value you want to reach when irrigating and is close to soil saturation.
  • Available Water is the amount of water held in the soil between field capacity and permanent wilting point. This is the range you want to maintain when irrigating.
  • Allowable Depletion (readily available) is the point where plants begin to experience drought stress. For most crops, the amount of allowable depletion, or the readily available water represents about 50% of the total available water in the soil. This is the top range you want to maintain when irrigating.
  • Permanent Wilting Point is the point at which the water remaining in the soil is not available for uptake by plant roots.  When the soil water content reaches this point, plants die.

Additional Irrigation Resources

Irrigation in the Pacific Northwest - Good irrigation planning and management are truly among the few win-win practices that increase grower profitability while protecting and improving our environment. This website was developed by the extension irrigation specialists from the land grant universities of the Pacific Northwest.

 

 

This page was last modified on 06/02/20 - 12:20