Efficient Irrigation Scheduling

Dr. Clint Shock, Oregon State University Malheur Experiment Station

Introduction

Irrigation scheduling consists of applying the right amount of water at the right time. With potatoes, growers have incentives to make irrigation scheduling work well.

Potatoes have a low tolerance for water stress. Tuber market grade, tuber specific gravity, and tuber processing quality for trench fries are all closely related to even low levels of water stress during tuber bulking. All these potato characteristics are closely related to the maintenance of soil water potential within a narrow range of values. Incentives to growers for precise irrigation scheduling include the following:

1. Under-irrigation leads to a loss in market grade, tuber quality, and contract price.

2. Over-irrigation leads to a loss in water, electricity for pumping, leaching of nitrogen, and wastes time. Over-irrigation increases crop N needs, fertilizer costs, and nitrogen losses to groundwater. Soil losses can be aggravated.

3. Under-irrigation and over-irrigation can occur during the same season in a given field.

Scheduling Methods

Growers irrigate using one of several criteria:
a. intuition,

b. calendar days since the last rainfall or irrigation,

c. crop evapotranspiration,

d. soil water.

Measurements of soil water or crop evapotranspiration provide objective criteria for irrigation management.

The Check Book Method: Crop Evapotranspiration

Crop evapotranspiration is a fancy word for the consumptive use of water. Consumptive water use is composed of evaporation of water off of the soil surface and transpiration of water through plant tissue to the air. Crop evapotranspiration is calculated using a weather station or an atmometer.

The grower needs to be concerned with keeping a checking account balance of the estimated evapotranspiration and the measured rainfall in the potato fields. The daily evapotranspiration is provided by a crop weather station or extension service. Here is how it works:

Table 1. Check book example of irrigation by crop evapotranspiration, ETc for a soil with allowable depletion of 1.4 inches of water. The soil used here is an Owyhee silt loam, Ontario, Oregon. The daily potato evapotranspiration amounts are the actual 1992 amounts at this arid location but the rainfall events are only hypothetical for instructional purposes.

Action _| August date _|ETc _ |Rainfall _|Accumulated ETc

- - - - - - - - - - - - - - inches - - - - - - - - - - - - - - - -

Irrigate _| 1 _ _ _ _ _ _|0.29 _ |_ _ _ _ _| 0.29
_ _ _ _ _| 2 _ _ _ _ _ _|0 37 _ |_ _ _ _ _| 0.66
_ _ _ _ _| 3 _ _ _ _ _ _|0.38 _ |0.08 _ _ | 0.96
_ _ _ _ _| 4 _ _ _ _ _ _|0.34 _ |1.45 _ _ | -----
_ _ _ _ _| 5 _ _ _ _ _ _|0.37 _ | _ _ _ _ | 0.37
_ _ _ _ _| 6 _ _ _ _ _ _|0.26 _ | _ _ _ _ | 0.63
_ _ _ _ _| 7 _ _ _ _ _ _|0.31 _ | _ _ _ _ | 0.94
_ _ _ _ _| 8 _ _ _ _ _ _|0.28 _ | _ _ _ _ | 1.22

The check book method is operated by keeping a daily record of rainfall, estimated ETc, and accumulated net ETc . Estimated crop evapotranspiration at Ontario, OR, is readily available at the AgriMet web page.  Rainfall is subtracted from the accumulated ETc . If there are rainfall events that make the accumulated ETc account negative, the negative balance is dropped. The reason that the negative balance is dropped is the it represents water applied in excess of the soil water holding capacity that will have been lost to leaching. The grower decides to irrigate by not allowing the accumulated ETc to exceed the allowable depletion. The grower decides how much to irrigate by not replacing more than the accumulated ETc. Note that we have come to clear decisions as to when to irrigate and how much to apply; the requirements for successful irrigation scheduling.

The use of the check book method is pretty straight forward, but the grower has to have access to the following information:

1. A local weather station estimating potato crop ETc based on the crop's coefficients and correct crop development data,

2. A rain gauge placed in each production field or group of closer adjacent fields,

3. A good estimate for the allowable depletion of water for each soil.

Acquiring all three of these needed pieces of information is feasible. The potato plant's water use is well known given weather data and the stage of development. But someone manually or automatically must calculate the daily potato ETc at each important weather station location. The allowable soil water depletion for potatoes can be calculated by extension agents, crop consultants, and growers. The calculation requires knowledge of the potato plants' effective rooting depth in a given soil and that soil's water retentive characteristics in the range where the potato plant does not suffer water stress. Potatoes are very sensitive to water stress, and caution is needed to avoid over estimation of a soil's allowable depletion.

Soil Moisture Measuring Techniques

Soil water can be measured by the methods that determine the soil water content or the soil water potential. Soil water content is the amount of water per volume of soil or weight of dry soil. Soil water potential is the force necessary to remove the next increment of water from the soil.

Different measurement methods have particular strengths and weaknesses. For example the gravimetric method is very accurate, but it is very slow and many samples are needed for each field and site specific interpretations are necessary. Strengths and weaknesses of several methods will be discussed.

Water Content

1. Feel method
2. Gravimetric
3. Neutron probe
4. Time domain reflectrometry (TDR)
5. Others

Soil Water Potential or Soil Water Tension

1. Tensiometers
2. Gypsum blocks
3. Granular matrix sensors (GMS)
4. Others

The use of soil water potential measurements with tensiometers or granular matrix sensors is provides a measurement analogous to the force (suction) necessary to extract water from the soil. The force is transmitted from the atmosphere through the plant to the roots.

Until recently growers had only tensiometers to accurately measure soil water potential. Growers have often been unwilling to properly manage tensiometers. Granular matrix sensors (Watermark Soil Moisture Sensor Model 200SS, Irrometer Co., Riverside, CA), a relatively new product on the market, could provide potato growers with an accurate and stable means to determine soil water potential for Malheur County soils in eastern Oregon. At the Malheur Experiment Station, we have successfully automated GMS to control drip irrigation.

Tensiometer and GMS are used in the following way. Starting in 1988, after a successful research program, GMS soil water potential readings made in growers fields were used to schedule irrigations. In the beginning the potato extension specialist, Lynn Jensen lead the program. The program has evolved to the point where 87 Malheur County potato fields were monitored in 1995 by the Soil Water Conservation District under the management of Ron Jones. The cost is paid for by the growers. Actual readings are made by student summer labor using a Model 30KTCD digital meter (Irrometer Co., Riverside, CA).

Twelve to fourteen GMS are used to characterize the soil water potential in each field. Two areas of the field are chosen by the grower based on irrigation experience. Often both a typical area and a difficult (usually drier) area are chosen. Six or seven GMS are distributed widely across each area and each GMS is connected by up to 150 ft of 18 gauge speaker wire to terminal strips. All sensors in a given area are wired to a single location for rapid reading. For each area, all but one of the sensors are installed at 8 inch depth (depth to the top of the sensor) in the shoulder of the potato hill and a single sensor is installed at the 16 inch depth. Responsive GMS placement has been determined.

Sensors are read daily and the soil water potential data is plotted daily. Copies of the data plotted stay in a news paper box at the site and with the person making the readings. The data is plotted for immediate interpretation and use by the grower. The average readings at 8 inch m depth and the single reading at 16 inch depth in each area is plotted. Also the soil water potential of the driest sensor at 8 inch depth is plotted. The graphs are designed to help the grower to irrigate at -50 kPa avoiding to let the soil dry beyond -60 kPa. In sprinkler-irrigated fields, information from the 16 inch depth helps avoid over irrigation which would keep the deeper part of the soil profile saturated.

Irrigating at the correct time is achieved by not allowing the soil in Malheur county, Oregon to become drier than -60 kPa. Irrigation with the right amount of water is possible using sprinkler irrigation, by only applying the amount of water necessary to refill the soil's water holding capacity in the root zone.

Relation of GMS readings to potato quality

Potato yields and quality can improve with control of soil water. Moderate water stress causes little damage to potatoes before tuber initiation but during tuber development, small amounts of water stress result in decreased tuber grade, decreased specific gravity, or increased incidence of dark-end fry colors. Research supported by the Oregon Potato Commission and completed by Eric Eldredge in his October 1991 Ph.D. thesis proved that a single, short duration water stress can lead to a reduction in tuber grade and dark fry colors. While these results are of critical importance to potato growers and processors, growers cannot easily implement these experimental results without quick and reliable field determination of soil water potential.

Work by Lynn Jensen and others has proven that GMS sensors are useful in managing soil water for potato production in Malheur County. When the soil was maintained moist the rest of the growing season, Eric Eldredge proved that a single episode of water stress as measured by GMS did not reduce Russet Burbank tuber yield, but tuber grade and specific gravity were reduced and dark-ends were increased . A single episode of water stress where GMS readings reached -75 kPa or drier was associated with a progressive loss in U.S. No. 1 tubers, increases in U.S. No. 2 tubers, and losses in tuber solids. A single episode of GMS readings of -100 kPa or drier was associated with increased incidence of USDA #3 and #4 dark-ends. These guidelines for quality tuber production need to be extended and interpreted so that they are useful in other soil types and climatic situations. For example, potatoes may need to be irrigated at lower readings (less negative readings) on lighter texture soils. Guideline for irrigation criteria may vary by region, soil, and variety, but potatoes must be considered a water stress sensitive crop.