Allan Fulton, UCCE Irrigation and Water Resources Advisor, Tehama, Colusa, Glenn & Shasta Counties; Luke Milliron, UCCE Orchards Advisor, Butte, Tehama & Glenn Counties

The dormant winter season is a time when evapotranspiration (ET) is low and it is an opportunity to refill the soil profile in your orchards prior to the next season. Some benefits include:

• Moist soils tend to be warmer and reduce the risk of winter kill from severe cold snaps.
• A full profile before leaf-out should delay the need for the first crop irrigation (except for frost protection) and provide better aeration to discourage diseases as the trees grow.
• Banking water for use later in the season which may help lessen summer pumping. It is particularly important in orchard soils that have very slow infiltration (silt and clay soils).

Tracking rainfall to gauge need for winter irrigation

Weather forecasts give an 80% chance of a weak El Niño during winter 2018-2019, that may bring above average rainfall. Although the need to irrigate to replenish soil moisture has been abated for the time being and the forecast is promising, it is wise to continue tracking rainfall levels and the resulting soil moisture profile in your orchards. By tracking rainfall, it is possible to substitute irrigation for the shortage in rainfall on a monthly basis beginning in December until enough rainfall in combination with irrigation has been received to refill the soil profile at least three feet deep. Tables 1-4 demonstrate the idea of tracking rainfall to help gauge if and how much winter irrigation might be needed.

* Precipitation reported is for December 1-17, 2018.

The tables show average monthly rainfall and total rainfall for four different areas in the Sacramento Valley and compares rainfall measured in the past two fall and winter seasons at four area CIMIS weather stations. It highlights that total rainfall in 2017/18 generally lagged well behind average rainfall amounts from October through February, one notable exception was rainfall in November 2017 in the Chico (Durham) area. As a result, winter irrigation was a fairly common practice in 2017/18.

Focusing on the 2018/19 fall and winter season, rainfall in October was below average.  Late November rains brought that month’s total closer to the average in Red Bluff and Williams and above average in Woodland. Like the previous year, rainfall in November in Chico (Durham) area was the outlier, almost doubling the historic average and bringing localized flooding following the Camp Fire. So far, December rainfall through the 17^th is lagging below average. As always, using site specific rain gauges rather than depending upon regional weather stations that are some distance away will improve accuracy. Also, different lengths in the orchard dormancy period for almond, prune, and walnut will also influence decisions.

How Much Winter Irrigation is Needed?

Winter irrigation is only given consideration when rainfall is in short supply. How much irrigation water is needed is determined by how much water the soil profile can hold. Most orchard soils will hold between 1 to 2 inches of available water per foot of depth. Sandy soils will hold less water than silt and clay soils. So, when rainfall is very low, 5 inches or more of winter irrigation should go a long ways towards refilling at least a three-foot soil profile, where the vast majority of roots in mature almond and walnuts trees are found. More or less irrigation may be needed depending upon how deep the root system is thought to reach.

Most micro and mini sprinkler systems will apply between 1.25 and 2.0 inches of water per 18 to 24 hours of irrigation. So two or three winter irrigations, at most, should help. Drip irrigation applies water at lower rates and wets a smaller fraction of the orchard floor so this needs to be considered.

Leftover moisture from last season’s post-harvest irrigations and effectively stored winter rainfall will reduce the amount of winter irrigation needed. Not all rainfall will be effectively stored in the soil profile as it depends upon the soil conditions and intensity of the storm. Relatively wet soils and particularly those with rolling topography will be prone to more runoff, especially if it is an intense storm. Light showers may be prone to evaporation, especially if they are followed by warmer, sunny weather. An orchard floor with vigorous winter vegetation will also transpire water and reduce stored moisture. As a general rule of thumb, on average only 50 to 60 percent of the actual rainfall will likely be effectively stored in the soil profile.

Eliminate the Guess Work

Given the extended forecast, December rainfall in 2018 is likely to far-exceed levels in 2017 (precipitation was virtually absent). Total rainfall in 2018/19 may go a long way towards the objective of refilling the soil profile to a depth of five feet or more. Soil moisture monitoring is the best way to measure the extent of soil profile refilling. Checking soil moisture by hand is a very basic method to evaluate soil moisture conditions. There are many online stores where soil augers can be purchased (try: JMC Backsaver, AMS samplers; Forestry Suppliers; and Ben Meadows). The USDA, NRCS also offers a nicely prepared publication with color pictures titled Estimate soil moisture by feel and appearance.

There are also a wide variety of soil moisture sensors that can also be used. Refer to the article Soil Moisture Sensor Selection is Confusing for more insight.