Water resources in the state of Nevada are in high demand among many competing users. Urban areas of the state with large populations are constantly looking to the rural areas for water resources that can be transported to the cities for their use. Competition for Nevada's water resources will only increase in the future. Consequently, a subsurface drip irrigation (SDI) project was undertaken at Lovelock, Nevada, to research and demonstrate the latest in irrigation technology and determine its applicability to alfalfa production in Nevada. After two years of operation, results indicate that SDI may be a technology that can increase yields and use water more efficiently in Nevada alfalfa fields.
Key words: alfalfa, subsurface drip irrigation, water use efficiency, Nevada
Agriculture consumed about 83 percent of the water used in Nevada in 1990, amounting to approximately 3.3 million acre-feet. Approximately 1.8 million acre-feet of the water withdrawn for irrigation purposes was used consumptively. The remaining 1.5 million acre-feet (45 percent) was lost to inefficiencies in the irrigation process (Nevada Water Facts, 1992). As Nevada's population grows, particularly in the urban areas, precious agricultural water supplies and use are coming under increased scrutiny. Alternative irrigation methods are needed that address the inefficiencies of traditional irrigation practices, as well as help Nevada's producers realize increased crop yields with unpredictable water supplies. Subsurface drip irrigation is one of several alternatives for addressing these issues.
This paper describes a project in which alfalfa is being irrigated with a subsurface drip irrigation system. The project is located at Lovelock, Nevada, and made possible by the combined efforts of several agencies, individuals, and businesses. Two full years of alfalfa production with the SDI system have been completed. This paper reports a description of the project and progress to date. Some of the data contained herein has been through a statistical analysis. On other data, such an analysis hasn't yet been completed.
The objectives of this project are to:
Installation of the SDI system and planting of the field took place in the summer of 1995. The project site is on a farm within the boundaries of the Pershing County Water Conservation District (PCWCD). The PCWCD collects surface water in Rye Patch Reservoir and distributes it to farmers through a series of irrigation canals. Individual farmers then use a level border irrigation system to water their fields. The border irrigation system was used to germinate the crop in this project. In years with adequate water, each farmer in the PCWCD is allocated 3 feet of water per acre. The major crop in the area is alfalfa; used either for forage or seed production.
The soil type at the project site is Sonoma Silt Loam. However, based upon CEC values from soil testing and the experience of project participants, it is clear there is a substantial amount of clay in the soil. At field capacity, Sonoma Silt Loam holds approximately 4.1 inches of water per foot of soil, and 2.9 inches of water at 50 percent available water depletion.
The SDI field is 180 feet wide by 108 feet long (approximately 3 acres) and is located within a larger field consisting of six level borders. There are eighteen plots in this project with individual plots being 48 feet wide and 102 feet long (Figure 1). Nine plots on the west end of the field are irrigated with Netafim Typhoon dripperline buried at 18 inches in depth and spaced 36 inches apart. Individual emitters are 24 inches apart along the dripperline with each emitter having a flow rate of 0.62 gallons per hour. Outside diameter of the dripperline is 0.625 inches and is 15 mils thick. Three of the plots at the 18-inch depth are irrigated at 75 percent of evapotranspiration (ET), three are irrigated at 100 percent of ET, and three are irrigated at 125 percent of ET. In the nine plots on the east half of the field, the dripperline is again spaced 36 inches apart, but is buried 12 inches deep. The 12-inch plots are irrigated at the same percentages of ET. Each plot contains an aluminum neutron probe access tube and eight plots contain an eight-foot deep observation well. An adjoining border is used as a control plot for partial comparison purposes.
The water for this project is surface water delivered from Rye Patch Reservoir through Union Canal. At the project site, water is pumped into a pipeline and then moves underneath a road to the field. At the field, the water pipeline emerges and sulfuric acid is injected to lower the water's pH from about 8.8 to 6.5. Nest, the water flows into two 24 inch Yardney sand media filters containing #20 silica sand. Filtered water then flows through a pressure-reducing valve to plots irrigated at 75 percent of ET, one of the six plots irrigated at 100 percent of ET, and one of the six plots irrigated at 125 percent of ET. In addition, there is a McCrometer flowmeter corresponding to each set of six plots. The flow rate to each set of plots is approximately 70 gallons per minute at 10-12 psi.
Irrigation with the SDI system is scheduled weekly based on readings taken at the field from an ET Gage Atmometer. Readings are taken every few days and averaged over the time period since the last reading to estimate the daily ET rate. Once daily ET is known, that value is multiplied by 1.25, 1.00, and 0.75. Nest, flow rates are calculated for each set of six plots and then a Rainbird irrigation clock is programmed to apply the correct amount of water to each set of plots. The plots are irrigated at 6:00 a.m. and 6:00 p.m. daily.
Maintenance operations on the system are performed after startup in the spring, after each cutting, and at shutdown in the fall. During each maintenance operation, the pH of the water is lowered to approximately 5.0 with sulfuric acid. Then 12 percent chlorine is injected downstream of the acid injection point at a concentration of 50 parts per million total chlorine (ppm). This solution sits in the system overnight and is flushed out the following morning. The irrigation clock is then reprogrammed and the system begins irrigating once again. In late October, when the system is shut off for the winter, all above ground equipment is drained and the laterals are blown out with compressed air.
A variety of monitoring activities are performed on a regular basis. System flow rates and pressures are monitored each week. Flows can be monitored through instantaneous readouts on the flowmeters, as well as through calculations based on system run time and flowmeter totals. Pressure in above ground lines as well as in the dripperlines is monitored with several pressure gauges. Any deviations from normal values indicate a possible problem requiring further investigation.
Various data are collected at different times throughout the year. Soil samples are taken at the one, two, three, and four foot depths in each plot at the beginning and end of the growing season. Each week soil moisture readings are taken at the one, two, three, and four foot depths with a Troxler soil moisture gauge. Readings are taken from observation wells on a weekly basis to determine if there is groundwater within eight feet of the soil surface. Flowmeter readings are also taken regularly to determine the total volume of water applied to each alfalfa cutting.
The SDI system is turned off several days before yield samples are taken and remains off until the producer finishes harvest and removes the bales from the plot area. Yield samples are taken with a Carter plot-harvesting machine three times a year at each cutting. Samples are also taken from each plot to determine forage quality and nutrient content of the alfalfa. After the yield and forage samples are taken, the producer begins his regular harvesting operations. When he is finished, the system is turned back on as soon as possible. Usually, the system if off for 10 to 14 days at harvest time.
Soils
The primary soil constituents of concern are the levels of sodium and soluble salts (EC) in the soil profile. These constituents are being monitored through the life of the project, and if levels build to harmful concentrations, a leaching irrigation with the flood system will be done. Table 1 shows an upward trend in the sodium and salinity levels in the 18-inch plots in 1997. Table 2 doesn't indicate obvious trends one way or the other in the 12-inch plots. Table 3, which contains data on the control plot, shows that generally sodium and salinity levels were lower in the fall than in the spring. Since the control field is flood irrigated, there is most likely some leaching of salts taking place in the soil profile. As can be seen in all plots, salinity levels are low while sodium levels are high to very high. High sodium levels are common in Nevada soils. Sodium and salinity values for 1998 were not available when this paper was prepared.
Soil Moisture
One goal of SDI systems is to reduce plant stress by maintaining soil moisture between field capacity and a specified available water depletion percentage. In this project, we attempt to maintain available water depletion above 50 percent in the 100 percent ET plots. This was generally accomplished in 1998, and happened in the other SDI plots as well. Initially, the authors expected to see more variability in soil moisture content between irrigation regimes throughout the season. In early 1998, we expected soil moisture trends similar to 1997's trends which showed plenty of plant available water, because of the substantial rainfall the plots received. In general, it appears the soil moisture levels are higher in 1998 than 1997, especially in the early part of the year. Much of this increase can be attributed to the fact that the SDI plots received 4.92 inches of rain between May 5 and June 16, nearly the average annual precipitation for this area.
Water applications through the SDI system were very small in April 198. In May, water applications through the SDI system were substantially larger. During this time period, the plots also received a large amount of rain. On May 8 alone, the plots received 1.05 inches of rain. This rainfall event resulted in a sharp increase in soil moisture occurring between April 30 and May 12 at nearly all levels. In hindsight, it appears the May water applications were much larger than needed. To the authors, this means two things. First, rainfall events can have a large impact on the soil moisture levels when combined with an SDI system. Second, great care must be taken to schedule irrigation, especially when there is abundant rainfall. In this trial, the large water applications during April and May hurt the quality of the crop and substantially reduced the yield. The control plots were not irrigated until after the first cutting in 1998, and had nearly the highest yields for the first cutting. As in 1997, it is easy to determine when the control plots were irrigated.
Yield
Producers in the Lovelock area get three cuttings of alfalfa in a normal year. In years when weather conditions are favorable, they may get a fourth cutting. The SDI plots were harvested three times in 1997 and three times in 1998. When properly managed, SDI may enable producers in Nevada to reduce the down time between harvests, allowing for an extra cutting each year. This hypothesis is not currently being tested on this SDI system. However, it is being tried on another SDI system at Lovelock. Time will tell whether or not four cuttings are realistic with SDI at Lovelock on an annual basis.
The average alfalfa yield for Nevada in 1996 was 4.5 tons per acre (Nevada Agricultural Statistics, 1995). Average yields in the SDI plots for 1997's three cuttings ranged from 7.45 to 8.06 tons per acre. Average yields for 1998's cuttings ranged from 5.83 to 6.71 tons per acre. Nearly the entire difference in the average yields between 1997 and 1998 is attributable to the first crop of 1998 and the excessive rainfall received during that time. Yield totals for the first cutting of 1998 ranged from 37 (control plots) to 63 percent less than the first cutting of 1997. The second and third cuttings were nearly equal in tonnage between the two years. There are no significant differences in average yield totals among the treatments in both 1997 and 1998. There are also no significant differences due to placement depth of the laterals; however, there are statistically significant differences in yield attributable to the number of the cutting.
Water Use Efficiency
Water Use Efficiency (WUE) in alfalfa can be defined as the amount of dry forage produced per unit of applied water. In this project, we are using the unit "inches of applied water per ton of dry forage" to quantify water use efficiency. Previous studies have investigated the amount of water needed to produce a ton of alfalfa. Jensen and Miller (1988) conducted a study near Wadsworth, Nevada, during the 1984 and 1985 growing seasons. Their work indicated it took from 6.1 to 8.4 inches of water to produce one ton of alfalfa. WUE values on the SDI plots range from 1.94 to 6.65 inches per ton (rainfall included) over the 1997 growing season. In 1998, WUE values range from 2.33 to 6.08. There are statistically significant differences in the amount of water used to produce a ton of alfalfa in both 1997 and 1998. There are also statistically significant differences in WUE attributable to placement depth of the laterals. This WUE trend is consistent with other SDI research conducted on various crops in the western U.S. The control plot was not included in the WUE comparisons, because it is flood irrigated and we can't accurately quantify the amount of applied water.
A publication entitled "Evaluation of Empirical Methods for Estimating Crop Water Consumptive Use for Selected Sites in Nevada" shows the amount of pan evaporation from April to the end of August is approximately 34.1 inches at Lovelock. In the same publication, methods such as the penman and Blaney Criddle show greater ET values during the same time period. During the period of April to the end of August 1997, 27.9 inches of water was applied to the 125 percent ET plots, 22.5 inches was applied to the 100 percent ET plots, and 18.5 inches was applied to the 75 percent ET plots. In 1998, these values are 26.9 inches, 22.8 inches, and 18.6 inches respectively. These values include rainfall. In most cases, the amount of water from the ET data is larger than the amount of water actually applied to the plots.
Forage Quality
Forage samples are taken at the time each plot is harvested and are sent to an analytical laboratory to be tested. Two forms of sampling are used; feed sampling and tissue sampling. Feed samples are taken from windrows containing the entire cut portion of the plant. They are tested for total digestible nutrients, acid detergent fiber, and percent protein. Only the upper one-third of the plant is used for the tissue samples, which are tested for nutrient content. Values for 1998 were not available at the time this paper was prepared, but there doesn't appear to be any significant differences between the SDI and control plots. The authors believe the lack of quality response is due to the way this particular SDI system is being managed.
In 1999, the SDI system will be operated once again, but with major changes. Irrigation regimes will be 50, 75, and 100 percent of ET. In addition, fertilizers will be injected into the system on a daily basis. If funding is secured, there will also be a groundwater monitoring component added to the project. Lastly, the economic evaluation of using SDI for alfalfa production is beginning in early 1999.
Water resources in the state of Nevada are in high demand among many competing users. Competition for Nevada's water resources will only increase in the future as our population grows. This project is intended to investigate the latest in irrigation technology and determine whether or not it is applicable for use in Nevada. Results to date indicate that SDI may be a technology that can increase yields and water use efficiency in Nevada alfalfa fields.
Jensen, E.H. and Miller, W.W. 1988. Effect of Irrigation on Alfalfa Performance. University of Nevada Cooperative Extension. Fact Sheet 88-20.
Nevada Agricultural Statistics Service. 1997. Nevada Agricultural Statistics, 1996-97. Reno, Nevada.
State of Nevada, Department of Conservation and Natural Resources, Division of Water Planning. 1980. Evaluation of Empirical Methods for Estimating Crop Water Consumptive Use for Selected Sites in Nevada. Carson City, Nevada.
State of Nevada, Department of Conservation and Natural Resources, Division of Water Planning. 1992. Nevada Water Facts. Carson City, Nevada.
Use of product names within this paper does not imply endorsement of any kind by the University of Nevada, Reno, or the authors.
Figure 1. Plot diagram for subsurface drip irrigation of alfalfa at Lovelock, Nevada.
|
Depth |
100 | 125 | 75 | A | ||
| 125 | 75 | 125 | B | |||
| 100 | 100 | 75 | C | |||
|
Depth |
75 | 100 | 125 | D | ||
| 125 | 75 | 75 | E | |||
| 100 | 125 | 100 | F | |||
| 1 | 2 | 3 |
75 = 75 percent of ET
100 = 100 percent of ET
125 = 125 percent of ET
----------North--------->
Table 1. Average sodium (ppm) and salinity (EC) levels in plots with laterals at 18 inches between the spring and fall 1997.
| Depth (in) % ET | Na - Spring | Na - Fall | EC - Spring | EC - Fall |
| 12 - 75 | 557 | 635 | 0.57 | 1.20 |
| 24 - 75 | 810 | 834 | 0.53 | 0.63 |
| 36 - 75 | 1128 | 1181 | 0.47 | 0.53 |
| 48 - 75 | 1136 | 1252 | 0.57 | 0.80 |
| 12 - 100 | 649 | 689 | 0.63 | 1.53 |
| 24 - 100 | 644 | 732 | 0.50 | 1.07 |
| 36 - 100 | 1013 | 1040 | 0.43 | 0.60 |
| 48 - 100 | 999 | 1154 | 0.53 | 0.93 |
| 12 - 125 | 637 | 816 | 0.53 | 1.37 |
| 24 - 125 | 747 | 802 | 0.53 | 0.87 |
| 36 - 125 | 1128 | 975 | 0.40 | 0.53 |
| 48 - 125 | 1188 | 1190 | 0.53 | 0.87 |
Table 2. Average sodium (ppm) and salinity (EC) levels in plots with laterals at 12 inches between the spring and fall 1997.
| Depth (in) % ET | Na - Spring | Na - Fall | EC - Spring | EC - Fall |
| 12 - 75 | 557 | 601 | 0.40 | 0.53 |
| 24 - 75 | 741 | 730 | 0.47 | 0.47 |
| 36 - 75 | 1053 | 922 | 0.47 | 0.33 |
| 48 - 75 | 1135 | 1139 | 0.47 | 0.67 |
| 12 - 100 | 458 | 572 | 0.43 | 0.47 |
| 24 - 100 | 615 | 793 | 0.53 | 0.67 |
| 36 - 100 | 936 | 915 | 0.60 | 0.43 |
| 48 - 100 | 1232 | 1192 | 0.80 | 0.47 |
| 12 - 125 | 541 | 563 | 0.60 | 0.63 |
| 24 - 125 | 777 | 860 | 0.73 | 0.73 |
| 36 - 125 | 1063 | 1055 | 0.73 | 0.50 |
| 48 - 125 | 2264 | 1413 | 0.83 | 0.50 |
Table 3. Average sodium (ppm) and salinity (EC) levels in control plots between the spring and fall 1997.
| Depth (in) % ET | Na - Spring | Na - Fall | EC - Spring | EC - Fall |
| 12 - 75 | 422 | 347 | 0.35 | 0.50 |
| 24 - 75 | 448 | 403 | 0.50 | 0.60 |
| 36 - 75 | 1213 | 770 | 0.65 | 0.35 |
| 48 - 75 | 1653 | 1313 | 0.85 | 0.65 |