# Is Preseason Irrigation a Good Idea in 2014?

Figure 1. Percent of normal precipitation from Jan. 8 to April 7, 2014. (Source: High Plains Regional Climate Center)

Figure 2. April 8, 2014 U.S. Drought Monitor Map for Nebraska. (Source: US Drought Monitor) (Links to larger version.)

Many areas of Nebraska have received below average precipitation this winter and spring (Figure 1). This has resulted in the re-emergence of drought conditions in eastern Nebraska and worsening conditions in parts of central and western Nebraska (Figure 2).  With planting just around the corner and rains still sporadic, we face the increasing probability that we will not begin the growing season with a full soil moisture profile.  Some irrigators, especially those with limited irrigation well capacity, may be evaluating the use of preseason irrigation.

Research conducted on preseason irrigation in the Great Plains has shown that the crop water use efficiency is typically lower than can be achieved with in-season irrigation applications. (For more information and research data on preseason irrigation, see the March 22, 2013 CropWatch article.)

Based on previous research, the best practice if dry conditions continue would be to delay irrigation until May or early June, especially for irrigators with system capacities above 4 gallons per minute per acre. (See below for how to calculate system capacity and NebGuide 1851, Minimum Center Pivot Design Capacities in Nebraska, for information on recommended system capacity.)  By delaying irrigation until after crop emergence, we leave room in the soil profile to store rainfall that may occur through April or in early May.  In addition, the low crop ET in the early part of the season allows more water to be applied than the crop will use, resulting in stored soil water deeper in the profile.

## How to Calculate Gross System Capacity of an Irrigation System

Use the following formula:

Gross system capacity, gpm/ac =

System flow rate (gpm) divided by acres irrigated

For example, for a pivot that covers 125 acres and has a well that delivers 500 gpm:

Gross system capacity = 500 gpm/125 ac = 4.0 gpm/ac

Growers in southwest Nebraska with system capacities below 4 gallons per minute per acre should evaluate their stored soil moisture and may benefit from watering sooner than normal if dry conditions continue.  It's unlikely that growers in central or eastern Nebraska will see an economic return to preseason irrigation given our historic precipitation trends.

Irrigators with water allocations should avoid preseason irrigation as it could use up a large portion of their water allocation and may be against NRD groundwater management rules. Given that in season irrigation has a higher water use efficiency, it would be more profitable for those on water allocations to wait until later in the season to start applying water.  The use of deficit irrigation principles of delaying irrigation, even if it causes some crop stress, until as close to the reproductive growth phase as possible will likely result in the highest water use efficiency with limited water.

The use of monitoring equipment to measure soil water content (see March 21 CW)  is recommended for all parts of the state to get a better idea of how much water is present in the soil profile.  Another recommendation is to use ET data from nearby weather stations or an atmometer to estimate weekly crop water use as opposed to average crop ET values.  For more information about tools offered through the Nebraska Ag Water Management Network (NAWMN), please visit water.unl.edu/nawmn.

Aaron Nygren, Extension Educator
Derrel Martin, Extension Irrigation Specialist
Gary Zoubek, Extension Educator
Chuck Burr, UNL Water/Cropping Systems Extension Educator
Bill Kranz, Extension Irrigation Specialist

4/11/2014

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