Weather Considerations --- Soil Considerations --- Crop Growth Stage
Scheduling Irrigation --- Related Sites
Oregon potatoes are 100% irrigated in eastern Oregon and virtually so west of the Cascades. Round-white potatoes have been grown in the Willamette Valley on heavy soils without irrigation, but yields and tuber size have typically been commercially unacceptable.
Optimum irrigation rate and timing depend on a number of factors including weather, soil type, and crop growth stage (see sample Columbia Basin water use curve below). The total amount of water required during the growing season also varies with location and length of season. Varieties differ slightly in moisture needs. Because of these many interactions, irrigation scheduling recommendations must be tailored to specific farming operations.
Potatoes are relatively shallow-rooted with 90% or more of the effective root mass in the top foot. Water and nutrients below that level are only marginally available. Overwatering is both financially and environmentally wasteful because of potential loss of both water and nutrients to deeper soil layers, and eventually the soil water table.
As a rule of thumb, potato fields should be held above 60 or 65% available water in the rooting zone throughout the growing season. At the same time, prolonged waterlogging can be devastating due to bacterial soft rots and pink water rot (UC IPM Home Page; Online Guide to Plant Disease Control (Oregon) ). Soil moisture levels between 75 and 90% available would seem to be about ideal, but that level of consistency is obviously not feasible under real-world conditions. Of all varieties grown in Oregon, Russet Burbank is most susceptible to second growth, off shapes, and internal problems caused by nonuniform moisture regimes. Because of this sensitivity, Russet Burbank can not usually be grown with good success under rainfed conditions.
The following characteristics
and measurements are useful in determining irrigation regimes:
- 1 gallon of water = 83.4 lbs = 231 cubic inches; 1 lb = 453.6 grams
- 1 cubic foot of water = 7.48 gallons = 62.4 lbs
- 1 acre-inch = 3,630 cubic feet = 27,154 gallons; 1 inch = 25 millimeters
- 1 acre-foot = 43,560 cubic feet = 325,851 gallons; 1 cu. foot = 0.0283 cu. meter
- 1 acre inch/hour = 450 gallons/min = 646,317 gallons/day; 1 gallon/minute = 0.002228 cfs
- 1 lb/square inch = lift (water column) of 2.3 feet; 230 feet of lift = 100 psi
- 1 lb/square inch (psi) = 0.069 bar = 6.896 kilopascal; 1 atmosphere = 14.7 psi
Warm, windy weather removes more moisture from fields than any other set of conditions. At extremely high temperatures, plant stomates virtually close, greatly reducing transpiration, or water loss through plant leaves and stems. By contrast, they remain open and actively transpiring at moderate temperatures. Under such conditions, even a mild wind can remove the protective water vapor shell around the plants causing rapid water loss from both the plants and soil. Water losses from potato fields in the Columbia Basin can range from almost zero up to half an inch per day depending on the weather and crop growth stage (see sample water-use curve above).
Location affects watering regimes because of unique soil and climatic conditions. Total crop water losses seldom exceed 0.2 in/day in the Willamette Valley because of relatively high humidity especially during the night and early morning. By comparison, drier conditions may cause losses approaching half an inch at Hermiston. Crop Water Use Charts and Yearly Evapotranspiration (ET) Summary Data and many other essential inputs for successful irrigation are provided by the US Bureau of Reclamation through Agrimet, Pacific Northwest sites located in most production areas. See also CIMIS .
Soil texture, drainage, and organic matter content affect water-holding capacity. Texture is typically the most important of the three. The term available water is used to describe water available to the crop. Available water obviously varies slightly among crop species, but it is typically defined as soil moisture held between field capacity and the permanent wilting point. Field capacity is defined as the percentage of soil moisture held at 1/3 bar suction; it approximates the water in a soil after it has been fully wetted and all gravitational water has drained away, usually after a day or two. The permanent wilting point is described as the percentage of soil water held at 15 bars of suction.
On a hot, dry day potatoes may transpire heavily and temporarily wilt when soil moisture is at only 1 or 2 bars of suction, or readily available, but will quickly recover at night. At the permanent wilting point, plants will not recover even at night unless water is added to the soil.
Fine-textured soils hold much more water than sands, but they also hold it much more tightly. For example, while 75% of the water held at field capacity by a medium sand is available, only about 35% of the water held in a clay under the same conditions is available to the crop. The following table illustrates the relationship between soil texture and moisture availbility.