Managing Irrigation Water
Efficient water application is key under tight nitrogen management scenario.
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| Using atmometers to schedule irrigations is one cost-effective technique. |
Efficient water management is critical to achieving high nitrogen
(N) use efficiency. Nitrate-nitrogen is soluble and moves readily
downward with soil water. Applying more irrigation water than
can be stored in the crop root zone will increase nitrogen losses
through nitrate leaching and negate other positive N management
strategies. Adjusting irrigation management to increase both efficiency
and uniformity will result in more N available for crop uptake
throughout the growing season.
Producers should reevaluate their irrigation scheduling method
when growing crops under a tighter nitrogen budget. Decide when
to irrigate based upon an estimate of crop and soil water status,
as labor and water delivery permits. If currently irrigating on
a fixed day schedule, consider whether increasing the length of
time between irrigations is feasible, especially when the crop
is not in a sensitive growth stage.
Experienced producers know how long it takes them to get water across their fields and are proficient in avoiding crop stress during years of average rainfall. The difficulty lies in applying only enough water to fill the effective root zone without unnecessary deep percolation or runoff. Several devices, techniques, and computer aides are available to help producers in determining when they need water and how much is required.
Producers should choose the scheduling method which best suits their needs and management capabilities. Information necessary to improve scheduling includes:
• soil water-holding capacity
• current plant available soil moisture content
• crop water use or evapotranspiration (ET), daily ET rates available at www.coAgMet.com
• crop sensitivity to moisture stress at current growth stage
• irrigation and effective rainfall received
• availability of water supply
• length of time it takes to irrigate a particular field.
Consider using an irrigation scheduling service offered by crop consultants as a cost-effective method of scheduling irrigations to maximize return from N fertilizer inputs.
Improving the water distribution across a field (uniformity) is also important when focusing on keeping N in the root zone under surface irrigation. Producers should probe fields within 72 hours after irrigation to find depth of application along the irrigation gradient. Checking for visual signs of plant stress can also show areas of poor water penetration. Most commonly, the upper end of the field is over-watered and the lower end under-watered.
Surface irrigators should consider using surge irrigation or adjusting irrigation set size, stream size, set time, and length of run to improve both efficiency and uniformity. When properly used, surge valves can save labor and water with no loss of crop yield. Irrigators currently using conventional furrow irrigation on coarse-textured soils, fine soils with cracking problems, or slopes greater than 1% would benefit most from using surge valves
Using polyacrylamide (PAM) allows irrigators to use a higher stream flow rate (2x or higher), which also can improve application uniformity. However, using PAM without increasing flow rate is not recommended. Driving all rows can reduce excessive water intake on coarse soils early in the crop season before the first irrigation. Another irrigation strategy is to irrigate every other row and band apply nitrogen fertilizer on the ridge of the nonirrigated row. Researchers in Idaho (Lehrsch et al., 2000) found that this practice maintained or increased yields and increased N uptake on a silt loam soil. However, this practice may not be as beneficial on finer textured soils (clay loams).
Upgrading to an improved irrigation system usually results in improved water and nitrogen efficiency. Among these are systems such as low-pressure center pivot, LEPA (Low-Energy Precision Application), surge, and drip-irrigation. However, these improvements require capital, energy, or increased management costs, that may not pencil out in the short term. However, in some cases the additional labor savings will justify installation of improved systems over a period of several years.
