Increasing Water-Use Efficiency on the Hammond Conservancy District through Education and Irrigation System Evaluation
This project was implemented to accomplish some of the objectives outlined in the Hammond Conservancy District’s (HCD) Water Management Plan prepared in 2002. The purpose of the plan was to help the District develop and implement a cost-effective, comprehensive water management strategy to achieve more efficient and effective water use over the next several years. Objectives outlined in the plan to help accomplish this goal included:
- An assessment of existing on-farm irrigation technologies being used in the HCD on both small and large parcels, including…
- The type of technology being used (sprinkler, flood, drip, etc.).
- The age and operating characteristics of the system (flow rate, operating pressures, nozzle size, state of repair, distribution uniformities, and overall efficiency, etc.).
- The crops and land area served by the system.
- An education program that:
- Informs irrigators of the best irrigation improvements available, and how to obtain technical and financial assistance for adopting them.
- Informs and trains HCD irrigators on the latest in irrigation water management strategies and best irrigated crop management practices.
- Provides introductory training in HCD procedures and efficient irrigation practices for irrigators new to the HCD, and…
- Institutes an orientation program for new irrigators as they arrive.
Activities related to Objective 1:
To accomplish objectives 1a and 1c, a questionnaire was mailed out to HCD irrigators in May 2006 to survey irrigation system types and methods currently being used to water crops on the project. Land owners were also asked to include crop types and irrigated acres of each.
Survey Results Summary
Approximately 30 irrigators (representing about 480 acres of irrigated land) completed and returned the survey. Of the total acres included in the survey results, about 460 acres (95%) were planted to alfalfa, pasture grass, or an alfalfa/grass mix. Of this area, about 75% (350 acres) was irrigated by side-roll (or wheel-move) sprinkler systems while more than 60% of the remaining acres were sprinkler-irrigated using hand move pipe. Only a small proportion of the total hay or pasture land (about 10%) was flood irrigated using gated pipe.
The small proportion of irrigated land (20 acres or 5%) not planted to alfalfa or pasture included vegetable or herb gardens, fruit trees, sod, trees for windbreaks, and turfgrass or other landscape plants. According to the survey, about 60% of this area was irrigated with sprinklers while 40% received flood irrigation. Less than 1% of the total irrigated land represented in the survey utilized drip irrigation.
Irrigation System Assessments (Audits)
To accomplish the objectives of 1b, audits were conducted on several sprinkler systems during the 2006 irrigation season. Initially, pumps, pipes, and sprinklers were inventoried, measured, and inspected for wear and/or damage. A drill bit set was used to measure nozzle sizes. To evaluate system water application uniformities, low-evaporation rain gauges (catch-cans) were set up systematically to collect applied water while the system was operating (Figure 1). Battery-powered, tipping bucket, recording rain gauges were also set up near the first and last catch-cans to monitor start and stop times of the system. All measurements were then used to determine system precipitation rates and water distribution uniformities using equations 1 through 3. With side-roll systems, measurements were made after completion of 2 sets (Fig. 1).
The equations below were used to calculate the average measured precipitation rate and the water distribution uniformity of each system.
Equation 1: Mean measured precipitation rate - PR(m).
PR(m) = (∑x) ÷ n ÷ t
PR(m) = mean measured precipitation rate (inches per hour)
∑x = sum of all measurements (inches)
n = number of measurements
t = time of set (hours)
Equation 2: Christiansen’s coefficient of uniformity (CU).
CU = 100 (1.0 - ∑x ÷ mn)
CU = uniformity coefficient (expressed as a percent)
x = the deviation of each measurement from the mean (inches)
m = the average or mean measurement (inches)
n = total number of measurements
Equation 3: Distribution uniformity (DU)
DU = 100 x (MQ1 ÷ M)
DU = Distribution uniformity (%)
MQ1 = mean of the lowest ¼ of measurements
M = mean of all measurements
During system operation, a pitot gauge was used to measure water discharge pressure at selected sprinkler nozzles along the sprinkler lateral. Equation 4 was used to calculate the theoretical precipitation rate of the system based on measured nozzle sizes, pressure measurements, and sprinkler spacings.
Equation 4: Calculated precipitation rate (PR)
PR = FR(sp) x 96.3 ÷ (ss x ls)
PR = calculated precipitation rate (inches/hour)
FR(sp) = the flow rate of each sprinkler in gpm (see Equation 5)
ss = sprinkler spacing (feet)
ls = lateral (set) spacing (feet)
Equation 5: Sprinkler flow rate in gallons per minute (gpm) (FRsp)
FR(sp) = 29 x √P x d2
FR(sp) = nozzle flow rate in gpm
√P = square root of the water discharge pressure measured at the nozzle (psi)
d2 = diameter of the nozzle in inches (measured with a drill bit) squared
Irrigation (or water application efficiency) was then defined by two terms; the uniformity coefficient (or DU) and the perceived application efficiency (mean measured precipitation rate divided by the mean calculated precipitation rate) or PR(m) ÷ PR.
Audit results summary
All but one system evaluated were side-roll (or wheel-move) sprinkler systems irrigating alfalfa or grass/alfalfa fields. The single exception consisted of small plastic impact sprinklers mounted on portable, tri-pod sprinkler stands fed with garden hoses from a 1.25 inch poly-pipe mainline. This system, when evaluated, was irrigating a blue grama grass pasture. No flood systems were evaluated during the 2006 irrigation season.
A typical side-roll system consisted of brass impact sprinklers (i.e. Rainbird 30) spaced 40 feet apart along a 4-inch diameter aluminum lateral. Set (side-roll move) spacings ranged from 50 feet to 62 feet. Main nozzle sizes ranged from 5/32 in. to 1/4 in. and spreader nozzle sizes, when present, ranged from 5/64 in. to 1/8 in. The most common combination was 3/16 x 1/8 inch. A Rainbird Model 30H sprinkler with these two nozzles is designed to operate between 25 and 80 psi and have a flow rate of 6.3 and 11.3 gpm at these two pressures, respectively. At a 40 foot by 60 foot spacing, the respective calculated (theoretical) precipitation rates are 0.25 and 0.45 inch per hour.
Measured nozzle pressures of the audited side-roll systems ranged from 8 psi (high sprinklers on a gravity system) to 37 psi (sprinklers close to a booster pump powered by an internal-combustion engine) and average measured precipitation rates ranged from 0.2 inch per hour in systems operated at low pressures (less than 25 psi) to 0.35 inch per hour in systems operated at pressures above 35 psi.
Most systems were set for either 12 or 24 hours and, in either case, (between the 2 lateral sets) more than 4 inches of water was applied per irrigation. Coefficients of uniformity (CU) ranged from 61% to 91% with the higher CUs generally being associated with higher operating pressures. However, one system operated at very low pressure (8 psi) provided a very acceptable CU (86%) at a lateral spacing of 50 feet. The side-roll system exhibiting the lowest CU (61%) was operated for only 6 hours (per set) at a low mean pressure (18 psi). The only non-linear system (plastic-impact sprinklers on moveable stands) was operated at the highest pressure (44 psi) and closest spacing (28 feet) of all the systems. This system exhibited a relatively high precipitation rate (0.32 inch per hour) but had a lower CU (77%) than the side-roll systems. This system ran for 8.5 hours and applied 2.7 inches of water to blue grama grass.
To determine soil type and fertility in the fields where system audits were performed, soil samples were taken in 1-foot increments to a depth of 3 feet and were sent to a soils lab for analyses of texture (sand, silt, and clay), pH, N, P, K, and micronutrients. The New Mexico State University Soil Test Interpretation Program (version 4.08) was used to classify soil nutrient levels (very low, low, moderate, high or very high) and provide fertilization recommendations based on the results of these analyses.
Top soil layer (0-12 inches)
The top foot of all soils was classified as sandy loam in texture, averaging 71% sand, 16% silt, and 12% clay. Soil solution pH ranged from 7.3 to 8.2 (mean = 7.7). Nitrogen (N) content was very low (less than 5 ppm or 20 lbs/acre) and soil phosphorus (P) content (as P2O5) was also very low (6 ppm [54 lbs/acre] or less) in all but 1 (low) sample. Soil K, Mg, Ca, and Fe contents were high to very high in most samples while Cu and Mn contents were moderate to high. Zinc (Zn) content tested low in about half of the samples.
Mid-level soil layer (12-24 inches)
This sandy loam soil layer had a slightly higher average pH (8.0) than the surface layer. As with the top layer, N and P fertility was classified as very low. Levels of K, Mg, Ca, Fe, and Cu ranged from moderate to very high while levels of Mn ranged from low to moderate. Zinc levels tested low in all samples.
Deep soil layer (24-36 inches)
In most cases, this soil layer had a higher proportion of sand (average of 77%) than the upper two layers and was classified as a loamy sand or sand in about half of the sites. Soil pH averaged 8.3 and soil N and P were very low. Magnesium (Mg) and Ca levels were high to very high while Fe content was moderate. Potassium (K) and Cu contents varied significantly between sites in this layer, ranging from low to very high. Once again, Zn levels were low in all samples and Mn levels were low in about 75% of the samples.
Discussion of Results and Preliminary Recommendations
The field assessments, irrigation system audits, and soil analyses identified a number of problems that contribute to irrigation inefficiencies and lower than potential crop production levels on the HCD.
- Poor crop stands: In alfalfa, it is recommended that stands be replaced (or over-seeded) when plant density falls below 5 plants per square foot http://www.sanjuanweeds.com/FactSheets/AlfalfaProdFS.pdf.
In many of the fields examined, plant density was much less than this.
- Weeds: As plant density decreases, weeds become more problematic and cause a reduction in hay quality. Numerous weed species (mustards, dandelion, sandbur, green foxtail, etc.) were identified in most of the audited fields. In lieu of complete stand replacement, weed control measures should be implemented: (http://www.sanjuanweeds.com/FactSheets/weedsinalfalfaFS.pdf )
- Gophers and prairie dogs: These rodents are plentiful in San Juan County and can severely limit alfalfa hay production and irrigation efficiencies. Their mounds interfere with harvesting and they can be hazardous to grazing livestock. Gopher mounds were noted on all fields evaluated during the assessments and on about half of the fields, infestation was severe. While sub-surface drip irrigation can be very water efficient, its use on the Hammond Conservancy District is not feasible without control of gophers and prairie dogs. http://cahe.nmsu.edu/pubs/_l/L-109.pdf
- Poor soil fertility: All soils tested low to very low for nitrogen (N) and phosphorus (P). Fertilization recommendations varied depending on the crop being grown. In pure alfalfa stands, the soil test interpretation program recommends that 200 lbs of P2O5 be applied per acre foot (or a total of 600 lbs P2O5 per acre if a 3-foot root zone is considered). In mixed grass/alfalfa stands, where grass is to be maintained, recommendations were for between 100 and 150 lbs/acre N and 80 and 120 lbs P2O5 per acre foot. For one of the soils testing low (less than 50 ppm) in K, an additional 160 lbs/acre of K2O was also recommended. On a few sites, small additions of Fe (2.5 lbs/acre) and Zn (15 lbs/acre) were also recommended.
Irrigation system evaluations
A number of factors that may contribute to low irrigation efficiencies were noted. These included:
- Operating pressures below system design range: As mentioned previously, some sprinklers were being operated at pressures below design specifications. This creates ‘donut’ water application patterns and results in low application uniformities. One suggestion is to reduce nozzle sizes to increase pressures. While this reduces flow rate, water application uniformities can be increased by decreasing lateral spacings.
- Mismatched sprinklers and/or nozzles: Some systems had a mixture of 1-nozzle and 2-nozzle sprinklers, and had several different sized nozzles. For optimum uniformity, all sprinklers should be of the same type and have the same nozzle sizes.
- Worn sprinkler bearings, bushings, seals, and nozzles: Recommendation repair and/or replace affected sprinklers http://www.rainbird.com/pdf/ag/imp.pdf.
- Leaks: Leaks did not appear to be a significant problem with the systems audited. However, leakage was observed on numerous other systems. Recommendation replace side-roll joint gaskets, sprinkler seals, punctured hoses, etc.
Irrigation scheduling recommendations
The total available water volume in a sandy loam soil ranges between 1.1 and 1.8 inches per foot. Generally, it is recommended that about 50% of soil available water (ASW) be extracted before the next irrigation. Assuming an effective 4-foot root depth for alfalfa, and starting with a full soil water profile (about 5.8 inches of water), approximately 3.0 inches of water (50% of ASW) could potentially be extracted between irrigations (assuming each irrigation refills the profile) on the audited fields. During the summer, water use (ET) of a good alfalfa stand averages about 0.3 inch per day. Considering this, it is recommended that for maximum water efficiency in summer, irrigation frequency be about every 10 days and that irrigation depth be sufficient to replace 3.0 inches of extracted soil water.
Actual system run time to satisfy this water replacement will depend on the system output and irrigation efficiency (including application uniformity). The audits indicate that water application uniformities of between 85 and 90% are common with systems having well-maintained sprinklers with uniformly-sized nozzles and being operated within design specifications. Theoretically then, assuming sprinkler systems are upgraded to satisfy design parameters and audits are performed to determine precipitation rates, sprinkler run durations should be adjusted to apply about 3.5 inches of water (3.0/0.85), plus a 10% leaching fraction (to prevent soil salt accumulation in the root zone) for a total of 3.85 inches, every 10 days between about mid-May and mid-August. Most irrigators using side-roll sprinklers on alfalfa used irrigation frequencies ranging from 8 to 12 days. Assuming a mean of 10 days, and a total measured irrigation depth of about 4.2 inches, about half of the audited fields may have been over-irrigated by about 9.1% ([4.2-3.85]/3.85x100).
The audits revealed that if sprinkler design specifications are used to calculate the system’s precipitation rate using equations 4 and 5, rather than using actual system audits (catch-can measurements), an additional irrigation efficiency adjustment of between 75 and 80% should be made to account for the discrepancy between calculated and measured rates. For example, in the case described, sprinkler run durations would be adjusted to apply a perceived 4.5 inches (3.0/0.77/0.85) every 10 days.
Some additional recommendations:
- While irrigation duration and/or frequency should be adjusted for rain and snow, single events of less than 0.20 inch in depth should be ignored. With events greater than 0.20 inch, only 75% of the amount greater than 0.20 inch is considered effective: http://www.farmwest.com/index.cfm?method=pages.showPage&pageid=234. During the past few years, effective precipitation in the HCD has been about 2 inches.
- With warming temperatures in February and March, perennial crops will begin to break dormancy and will mine much of the stored soil water before irrigation water becomes available (April 15). In alfalfa and permanent pasture then, the first irrigation of the season should be sufficient to fill the top 4 feet of the soil profile (about 5 inches of water in a sandy loam soil), particularly if the winter and early spring has been dry.
- For similar reasons, if possible, the last irrigation of the season should attempt to leave the soil with a filled profile. This stored water will help sustain the alfalfa roots through the winter and provide water for early green-up.
- Irrigation frequency and run times should be adjusted during the season to account for daily and seasonal variations in crop ET and irrigation efficiency. In spring for example, daily ET will be less than during summer because of smaller plant size and cooler temperatures. High winds common at this time of year, however, will cause irrigation efficiencies to be lower than during calmer summer days.
- Estimates of actual daily ET (available from the N.M.S.U. Agricultural Science Center web-site http://farmingtonsc.nmsu.edu) can be used to schedule irrigations based on crop coefficients developed at the center.
Activities related to Objective 2:
To educate and inform HCD irrigators of best management practices (BMPs) related to irrigation of crops in the district, a web-site was initiated in summer 2006: http://hammondcon.org. The site contains links to numerous agricultural, landscape, and irrigation web-sites that relate to these BMPs and to improved irrigation technologies. The site also provides water-management tip sheets (in progress) designed to assist HCD irrigators in crop and water management, including ET estimates for alfalfa and landscapes based on real-time weather data.
Links related to water-rights and other political issues that can potentially affect the HCD are also provided, as well as contact information, reservoir levels and river flows, and a list of related events. The web-site will be maintained and continuously updated.
To accomplish objectives 2c and 2d, a brochure is being prepared that explains HCD policies and procedures and contains other information that will be of value to new and established irrigators. Similar information will appear on the HCD web-site.
This project has set the foundation for a continuing effort by the HCD to achieve a more comprehensive, efficient and effective water management program in the upcoming years. Irrigation audits, followed up with on-farm crop and water management recommendations, will continue to be offered to irrigators and informational and educational resources will be provided through the established web-site and printed leaflets.