No orchard in Montana can survive let alone thrive without some form of irrigation.  Annual precipitation often hovers around 14 inches, much of this falling during winter months when trees are dormant.  Apples require around 20 inches (1.7 ac-ft) of water during the growing season meaning supplemental water from an irrigation ditch or a well must be applied (Handley, 2018).  How much and how often that water is applied is also important to the overall health of the trees.  The sometimes drastic day to day changes in weather unique to the mountainous west result in different rates of water use by trees in a short period of time.  These conditions make irrigation scheduling extremely important to ensure trees are not over or under watered, particularly during times of the season where risk for diseases like fire blight may be high or at crucial stages of fruit development and nutritional uptake.

Below are resources for understanding your water resource, apple tree water use and irrigation in arid Rocky Mountain orchards:

Understanding your water resource

Growers must first understand their water rights including annual volume limits, flow rates and where water can be used (place of use) to ensure their water resource is appropriate to the location and size of the orchard desired.  If you are unsure of your water right contact the Montana Department of Natural Resources and ConservationA Buyer's Guide to Montana Water Rights also contains several resources for landowners to assess the water right connected to their property and evaluate their legal right to use it.  MSU's Extension Water Quality program also provides additional resources for Montana residents using water for irrigation. Your local ditch company may provide additional assistance for understanding irrigation in your community and your local NRCS office will also provide resources for determining the most efficient irrigation system for your particular use.

Once you have confirmed how much water you have to use and where it can be applied you can determine if you have sufficient water to irrigate the size of orchard desired.  This can be done by dividing the water available in ac-ft by 1.7 ac-ft (the amount used by most apples) to determine the number of acres you can water.  For example if you have determined you have 8 ac-ft of water to use during the growing season you can water 4.7 acres of apples (8 ac-ft/1.7 ac-ft).  Be careful to also consider the flow rate available to you and the efficiency of your irrigation system when determining how much you can actually apply as this will influence how much water you need to irrigate your orchard.  Sprinkler and drip systems used for most orchard applications range in efficiency from 75-90% (Handley, 2018).  

Irrigation system design

The type of irrigation system you choose will depend on your water and soil resources, your orchard design and your budget.  As water resources become more scarce use of highly efficient orchard irrigation systems are on the rise.  The use of micro sprinklers in orchards is becoming more common as they are efficient, provide options for fertigation and if properly installed provide sufficient coverage to trees even in porous soils.  These systems also can reduce the spread of diseases by reducing canopy moisture.  Colorado State University's, Micro Sprinkler Irrigation for Orchards, provides guidance for growers interested in installing and using micro sprinklers in their orchard.  Your local NRCS office may also be able to provide guidance on sizing and designing a system.

Irrigation scheduling, how much and how often

WSU’s Determing Your Irrigation Schedule provides a simple explanation for determining how much water your trees need per week, how much water your soil can hold based on water holding capacity and determining your irrigation systems output.  The resource references typical evapotranspiration for apple trees in WA, however, real-time information can be found through your local Agrimet weather station which references data developed by WSU's AgWeathernet program.  Access to the tool requires registration through the AgWeathernet program which then uses Agrimet weather data to calculate apple crop specific evapostranspiration data. 

The Intermountain Tree Fruit Production Guide provides practical information for understanding the importance of irrigating properly, crop water use coefficients, basic irrigation terminology and special considerations for irrigating in the region's hot dry summers and often sandy soils.

This recorded Orchard Irrigation presentation from Utah State University's 2016 Fruit School provides information on understanding movement of water in soils and trees, tree water use, irrigation scheduling and evaluating water stress in trees.

Resources:

Handley, Kristine. (2018) Assessing your site for irrigation