Decisions about irrigation water management in tree nut crops can make or break a growing season. Effective water management, said University of California Cooperative Extension (UCCE) Fresno County nut crops advisor Mae Culumber, should support high yield potential and favor desirable nut quality.
Effective Water Management
In her presentation at the South Valley Nut Conference, Culumber noted that effective water management can extend orchard life, assist in pest management, use water and energy efficiently, contribute to nitrogen management and mitigate salinity problems. Using irrigation management tools can help growers achieve those goals.
Irrigation water management is applying water according to crop needs in an amount that can be stored in the plant root zone of the soil.

The most common questions about irrigation management in tree nut orchards include:

• When should irrigation begin?
• How frequently should irrigations occur?
• How long should irrigation systems be run?
• When should irrigation end for the season?
• How should young, developing trees be irrigated?

There are tools available to give growers and managers answers to these questions.

Water Budget
Culumber said having a water budget can help with irrigation decisions. A water budget is an accounting of water used by the crop and supplied to it.
Culumber said the purpose of a water budget is to maintain a balance between soil moisture depletion and applied water. Using current weather data for irrigation scheduling improves irrigation effectiveness compared to the less precise method of irrigating by calendar. A water budget is a method of estimating soil moisture depletion or anticipating crop water stress.

Evapotranspiration Rates
Monitoring evapotranspiration (ET) rates beginning at leaf out and keeping track during the growing season shows how much water trees need. The information can help avoid over watering or deficit. Culumber said monitoring could be conducted on a daily, weekly, monthly, seasonally or annual basis, but for precise irrigation management, weekly or more frequent monitoring is necessary.
Rainfall can influence how much irrigation is needed to meet ET rates for tree crops in different growing regions. Culumber said in the north valley, rain supplies 22 percent of water needs, 16 percent in the central valley and only nine percent in the south.
There are online resources for water budgets and coming next year will be available on the San Joaquin Valley Tree and Vine website. http:// www.sjvtandv.com/irrigation.
The weekly report will provide information and advice on balancing a water budget.
Other sites include http://www.waterright.net/WaterBalancetutorial. For a mobile irrigation scheduler, go to http://weather.wsu.edu/is/. Crop manage for tree crops is http://v3.cropmanage.ucanr.edu/account/login.
Other factors to consider in irrigation management are orchard density, age, and soil types. Estimating ET for young trees can be done by doubling the estimated percentage of canopy cover and multiplying by estimated ETc for a mature orchard. For example: a third leaf orchard covering 35 percent of the orchard floor will use 70 percent of the water that a mature orchard would use: ETc= (2.1 inches) (70 percent)= 1.47 inches.

Know the Irrigation System
Culumber said that water budgeting requires knowing the irrigation system water application rate, and an estimation of ET for the crop. It also helps, she said, to know root zone and soil water storage, track in-season rainfall and know irrigation distribution uniformity. The ‘feel and appearance’ of soil is another irrigation scheduling method used to determine when to irrigate, amount and duration. How to judge soils response to irrigation can be found at the United States Department of Agriculture (USDA) site under Estimating Soil Moisture.
Knowing available water in relation to soil texture can help with water budgeting. Soil moisture depletion is the amount of water needed to raise the soil-water content of the crop root zone to field capacity. An example is fine sandy loam at field capacity= 1.3 inches/feet times 5 rooting depth= 6.5 inches available water to tree. Allowable depletion (50 percent)= 3.25 inches.
It is easy to over-irrigate and lose water to deep percolation below the root zone, Culumber said. In water management, the percentage of wetted area impacted by the irrigation system and the soil type needs to be considered.

Soil Moisture Depletion Method
Using the soil moisture depletion method can lead to improved decisions on irrigation frequency and duration. There are a number of tools available to help with those decisions, Culumber said. Soil moisture sensors coupled with radio telemetry can deliver continuous information and also deliver more detailed information than manual measurements. They are also useful for measuring effective rainfall during the dormant season.
She noted that acquiring representative data could be a challenge due to orchard and soil variability, depth of profile to monitor, root distribution and density, and distribution of applied water. Small volumes of soil monitored can also be a challenge.

Pressure Chamber
A pressure chamber is another useful tool for irrigation scheduling. Midday stem water potential integrates and quantifies how an orchard is responding to soil, water and climatic conditions. It can help confirm data used with the soil moisture depletion method. There are tutorials on the web for using pressure chambers, Culumber said. Determining plant water status with this tool is a good way to evaluate irrigation timing and the effectiveness of irrigation scheduling to meet tree water demand.
Midday stem water potential uniquely integrates and quantifies how an orchard is responding to soil, water and climatic conditions. Culumber said it can also confirm and adjust assumptions that are used with soil moisture depletion methods or when using a water budget.
Limitations include frequent use of this tool in the orchard. Use can also be labor intensive and only limited acreage can be monitored in one day with one instrument. Use of the pressure chamber does encourage routine observation of the orchard.
Culumber recommends a combination of at least two of soil, plant or ET monitoring to make informed and effective water management decisions. Factors in the decision include long and short-term production goals and motivation to invest in irrigation scheduling. Farm operation and conditions will affect the ability to adopt management tools.