June 15, 2012

Base irrigation on crop stage and actual crop water use, not the calendar.

As producers and agribusiness industry professionals we know that no two years are the same in Nebraska. This has probably never been more evident than this year. We had an early, warmer-than-normal spring that looked much like summer and accelerated early crop growth and loss of soil moisture.

Crop evapotranspiration has been almost twice what it usually is for this date, as seen in the ET data from York (Table 1). These higher cropwater ET rates depleted soil moisture much earlier in the season than normal. For example, in 2011 the crop ET for May 13-20 was 0.07 inch while in 2012 it was 0.29 inches.  For June 3-10, 2011, Crop ET was 0.42 inches and for June 4-11, 2012 it was 1.30 inches. With an early start to the irrigation season, especially in areas with below normal soil moisture profiles, effective irrigation management is essential to keep input costs in check.

Checking ET

To create a picture of crop water use at sites across the state, producers are posting their weekly ETgage readings at Water.unl.edu on the Nebraska Agricultural Water Management Network (NAWMN).

Several changes have been made to the NAWMN reporting site to make it even easier to use. Locations that have not been updated in the past week are grey and those that have been updated are red. This should make it easier to quickly identify sites in your area with current reference ET data. In addition, automatic weather station sites are shown in blue and a few sites measuring pasture or grass ET are shown in green.

The changes in crop water use from last season to this season reinforce the point that each year is different and needs to be monitored accordingly. For more effective in-season water management, producers should use equipment to estimate crop ET and monitor the soil water status in a given year  Since ET_ref is not limited by water availability or plant function, ET_ref depends on the ability of the environment to extract water from the surface. As such ET_ref varies significantly throughout the year and growing season as a function of climate as described in the UNL Extension publication Magnitude and Trends of Reference Evapotranspiration Rates in South Central Nebraska (EC765). Furthermore, ET_ref varies significantly across the state of Nebraska as climate varies significantly from the eastern to western edge of the state.

Resources

More information about irrigation management and crop water use for Nebraska can be found at http://water.unl.edu/cropswater/nawmdn and CropWatch website. Further information about reference ET and the variability of climate and ET from one year to another across Nebraska can be found at:

• Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients (G1994)
  
• Variability of Reference Evapotranspiration Across Nebraska (EC733)
• Magnitude and Trends of Reference Evapotranspiration Rates in South Central Nebraska (EC765)

Gary Zoubek
Extension Educator, York County
Suat Irmak
Extension Water Resources Engineer