Written by Allan Fulton, UCCE Farm Advisor Tehama, Glenn, Colusa and Shasta counties and Richard P. Buchner, UCCE Farm Advisor Emeritus, Tehama, Glenn and Butte counties.

Overly aggressive, early irrigation can saturate soils and deprive roots of necessary oxygen. Without oxygen, roots often suffocate and die compromising tree performance. In addition, soil fungal pathogens such as Phytophthora are favored in saturated soils increasing the risk of infection and tree damage or death. On the dry side , delaying the start of irrigation too long can result in tree water stress also compromising tree and crop performance. Impacts may include smaller fruit and reduced yield. Water stress during the season  can reduce flower quality, influence carbohydrate reserves and increase fruit drop, sunburn and limb dieback.

Monitoring Tree water use (ET) and Rainfall
One relatively simple technique to help decide when to begin irrigating is to estimate soil moisture depletion  by comparing estimates of evapotranspiration (ET) to effective rainfall received since leaf out. Not all in-season rainfall is effective. In longer, more intensive storms some water runs off, some may infiltrate into an already full soil profile and cause earlier stored water to drain out of the primary root zone. Short, intermittent showers may evaporate quickly if followed by sunshine and not add to stored moisture in the root zone. Anecdotal experience suggests that about 50 percent of the total in-season rainfall might, on average, effectively be stored in the soil to supply ET. The first irrigation is suggested when the cumulative ET exceeds the amount of spring rainfall received since leaf out by at least the amount of water that will be applied in a typical irrigation event. This requires knowledge of the hourly water application rate of the drip or microsprinker system. If there is concern about tree damage from root pathogens and/or poor aeration, the first irrigation can be delayed even more. Shovels or soil augers are useful for visual soil evaluation and/or soil moisture monitoring can  confirm any decisions to delay the first irrigation. Real-time ET reports are posted weekly during the growing season on this website, or can also be subscribed to be received by email.

Measuring Soil Moisture
There are numerous manufacturers and providers of soil moisture sensing equipment. Some detect volumetric soil moisture content and some measure soil moisture tension. Soil moisture levels can be measured manually or automatically with data loggers and delivered on demand via cellular and internet services. An important aspect of monitoring soil moisture depletion is placement of the soil sensors to achieve good representation of the root zone and soil variability. The decision to begin the irrigation season can be determined by comparing the amount of soil moisture depletion to the amount of irrigation that will be applied and balancing them.

Measuring Orchard Water Status
The pressure chamber and midday stem water potential (MSWP) has been the state of the art for monitoring tree water stress for some time. Pressure chambers are used to measure actual tree water status usually midday when trees are experiencing the most amount of water stress. Low to mild stress levels in the -8 to -10 bar range would be reasonable threshold to begin irrigation for prune. A free on-line UC ANR Publication 8503 describes in detail how to use the pressure chamber to guide water management decisions in prune, almond and walnut.