Table 1
Economic Importance of Irrigation Management in Potato Production Systems of Idaho
B.A. King and J.C. Stark
Increasing production efficiency is becoming necessary for producers to maintain or increase their net return in an increasingly global market. In the case of irrigated agriculture, producers must also address increasing public concern about water conservation, water quality, and environmental protection.
Two irrigation management issues are important when striving to maximize production efficiency. These are irrigation scheduling and irrigation uniformity. Irrigation scheduling involves determining the proper amount and timing of water applications throughout the growing season. Irrigation uniformity describes how evenly an irrigation system distributes water over the field area. The economic importance of these irrigation management issues in potato production is highlighted below.
Irrigation Scheduling
The importance of scheduling irrigations to closely match water use (ET) of the potato crop is highlighted in Figure 1. These results were obtained from a 1995 study of water management practices on 45 commercial potato fields in southeast Idaho. Potato yield is reduced by both over- and under-irrigation. A mere 10 percent deviation from optimum water application for the growing season may begin to decrease yield. This sensitivity to water management is attributable to the sensitivity of potato plants to water stress, coupled with limited soil water storage to provide buffering of the soil-plant system. Yield reductions due to over-irrigation result from (i) reduced soil aeration which reduces water and nutrient uptake, (ii) increased nitrogen leaching and increased denitrification which reduce plant nitrogen availability, and (iii) increased disease problems caused by excessively wet soils which adversely impact plant growth. Proper irrigation scheduling can increase marketable yield and quality while reducing production costs by conserving water, energy, and nitrogen fertilizer, as well as reducing environmental impact.
Table 1
The economic importance of proper irrigation scheduling for potatoes is highlighted in Table 1. A difference of ±3 inches or more from actual seasonal crop water requirements reduces net return by nearly $300/acre. The yield and quality data shown in Table 1 are based on the 1995 study of irrigation management practices in southeastern Idaho. Production of U.S. No. 1 grade tubers is maximized by matching water application with crop water requirements.
Figure 2
The economic impact of over-irrigation on net return is summarized in Figure 2. These results are based on two years of research where optimum and excess irrigation was applied throughout the growing season. Regardless of nitrogen fertilizer applied, over-irrigation reduced yield, resulting in a decrease in net return. Additional nitrogen application will not overcome the effect of excess water application.
Irrigation Timing
The timing of water applications in relation to soil moisture levels is also a necessary part of irrigation scheduling. Matching seasonal water application with season crop water requirements is not sufficient to guarantee maximum yield. Soil moisture levels must also remain within the optimum range for crop production throughout the growing season. The optimum soil moisture range for potatoes is 70 to 90% available soil moisture. Thus, only 30 to 35 percent of the available soil moisture storage can be utilized between irrigations. This makes irrigation systems capable of light frequent irrigations well suited for potato production. Center pivots fulfill this requirement very well. However, irrigation timing is a potential problem with set-move sprinkler systems. Maximizing the number of days between irrigation minimizes the number of lines needed to irrigate a field, and thus minimizes capital cost of the irrigation system. The maximum irrigation interval for potatoes based on maintaining the optimal soil moisture range between irrigations is shown in Table 2 for different soil types and root zone depths based on a peak daily crop water use of 0.33 in/day. The maximum interval is less than five days in all cases. In practice, irrigation intervals exceeding five days are not uncommon.
Table 2
Field studies were carried out at Aberdeen in 1997 and 1999 to evaluate the effect of irrigation frequency on potato yield and quality. In 1997, irrigation intervals of 4, 5, 6, and 7 days during tuber development were included in the study, while in 1999, irrigation intervals of 4, 6, and 8 days were evaluated. The soil type was a Declo silt loam having a water holding capacity of approximately 2.2 in/ft. Water application amounts were determined based on replacement of crop water use as published by the AgriMet system. Total yield for both study years is shown in Figure 3.
Figure 3
An irrigation interval of 6 days resulted in the highest total yield for both years. Yield of U.S. No. 1 grade tubers in both years of the study decreased as irrigation interval increased beyond 5 to 6 days (Figure 4). This trend was more consistent and pronounced in the 1999 study. Yield of tubers over 10 oz also decreased as irrigation interval increased beyond 5 to 6 days (Figure 5). The overall results of the studies show that there is an optimum irrigation interval for maximizing total yield and quality based on the water holding capacity of the effective root zone and the rate of crop water use. This optimum interval is strongly dependent on soil texture and will be shorter for coarse textured soils than heavier textured soils. The percentage of large tubers is also strongly influenced by irrigation interval. Thus, irrigation management can be a useful tool is achieving tuber size goals for Russet Burbank potatoes.
Figure 4
Figure 5
Irrigation Uniformity
Uniform water application is not physically or economically feasible on a large scale. Thus, some degree of variability in water application exists for all irrigation systems. This variability is commonly quantified by a measure called the Christiansen Uniformity Coefficient (CU). The degree of water application uniformity is influenced by irrigation system type, design, and operating condition. Regular maintenance of all irrigation systems is necessary to achieve the highest degree of uniformity throughout the life of the irrigation system. Irrigation uniformity is economically important because variations in water application with each irrigation cumulate over the growing season.
Figure 6
This variation in seasonal water application over the field results in a corresponding variation in yield over the field. Field scale water distribution for two levels of irrigation uniformity is shown in Figure 6. An irrigation system with a CU of 70% applies ±1 inch of the target amount on a mere 10 percent of the field area. In comparison, an irrigation system with a CU of 90% applies ±1 inch of the target amount on 34 percent of the field area. This difference in irrigation uniformity can have a significant economic impact as shown in Table 3.
Table 3
The results shown in Table 3 are based on combining the data shown in Figures 1 and 6. Due to the sensitivity of potatoes to under- and over-irrigation, increasing CU from 70 to 90% results in a $144/acre increase in gross return. Replacement of sprinklers and regulators on a ¼ mile center pivot costs approximately $16/acre which is much less than the potential increase in gross return for the given example. Replacement of standard straight bore nozzles on a common set-move irrigation system costs less than $2/acre. The potential return in potato production system from periodic replacement of sprinklers and regulators on center pivot systems and nozzle replacement and routine sprinkler head maintenance on set-move systems is likely money well spent. Consider another scenario to highlight this point. A malfunctioning regulator would result in excess application of water. Two sprinklers centered at 1300 ft from the pivot point with a 10 ft spacing would cover 3.75 acres. An excess seasonal application of 3 inches or more would reduce yield by 42 cwt/acre (Figure 1). Using a cost of $4.83/cwt, the loss in gross return would be 42 cwt/acre x $4.83/cwt x 3.75 acre or $760. Thus, approximately $20 in malfunctioning equipment can cost $760 in lost gross return. Regular equipment maintenance and replacement is time and money well spent.
Figure 1
Additional Sources of Information
Potato Irrigation Management. Bulletin 789, University of Idaho, College of Agriculture, Moscow, Idaho.
Irrigation Uniformity. Bulletin 824, University of Idaho College of Agriculture, Moscow, Idaho.
Optional Performance from Center Pivot Sprinkler Systems. Bulletin 797, University of Idaho, College of Agriculture, Moscow, Idaho.
Irrigation Scheduling Using Water-Use Tables. CIS 1039, University of Idaho, College of Agriculture, Moscow, Idaho.
About the Authors
Bradley A. King is an irrigation research engineer in the Dept. of Biological and Agricultural Engineering a the University of Idaho, Aberdeen Research and Extension Center.
Jeffrey C. Stark is chair of the Division of Plant Science and professor of crop management at the University of Idaho, Aberdeen Research and Extension Center.