A review of soil water data logs from farmers in the Upper Big Blue Natural Resources District indicates irrigators tend to overwater more in wetter years than dry ones. On very dry years like 2022, many farmers apply about the right amount of water. However, some still over irrigate, and some tend to under-irrigate.
The tendency should not be a surprise, because humans are creatures of habit. We get out of bed every day on the same side, we sit at the same place at the table for breakfast, etc. Farmers are no different — they tend to plant about the same number of seeds per acre each year, they apply about the same amounts of fertilizer each year, they plant the rows the same direction, etc. Well, you get the point, and without any compelling reasons, why make changes, right? However, irrigation scheduling should be an exception to this approach because the rainfall amounts and their timings are different each year.
The amount of irrigation applied last year or the year before may have very little to do with the amount needed this year. The long-term average of irrigation application depth and timing is relatively meaningless for the decisions producers need to make on any given day in the current growing season. However, without any additional data, all an irrigator can do is put on about the same amount of water as in the past and make slight adjustments if the weather is dry or wet.
To illustrate this, let’s look at some data from the Upper Big Blue NRD. The NRD requires growers in six areas that have an increased median nitrate-nitrogen level in the groundwater, to use soil water monitoring equipment in one irrigated field and report the data collected annually. Most of the producers impacted by this rule use Watermark sensors that measure soil matric potential; thus, this study has largely focused on Watermark data for easy comparison. The soils in the NRD are mostly a silt loam or similar soil texture, making the field-to-field data comparable as well. The assumption was made that the farmers using Watermark sensors would represent all irrigators in the area.
To protect privacy, the NRD removes names from the information prior to group analysis; therefore, the location for each field is an unspecified area in the Upper Big Blue NRD.
The Analysis
The analysis focused on dividing the fields into one of six categories based on the soil water levels in the heart of the season (July and August) and on Sept. 15. The six categories were developed based on the readings from the Watermark sensors. The sensors generate data reported in centibars and have a range from 0 (wet) to 240 (dryer) centibars (cb). Centibar is a unit of measure that refers to the force required by the plant to pull the water out of the soil and into the plant. Typically, installations use a set of three sensors, with one sensor installed at six to 12 inches, one at 18-24 inches, and one at 30-36 inches. The categories are as follows:
Good — At least one sensor out of the three depths drier than 70 cb early and one sensor drier than 70 cb on 9/15.
Fair — At least one sensor drier than 70 cb early and one sensor drier than 30 cb on 9/15.
Wet Late — At least one sensor drier than 70 cb early but no sensor drier than 30 cb on 9/15.
Wet Early — No sensor drier than 70 cb early, but one sensor drier than 30 cb on 9/15.
Wet All Season — Both sensors measuring the second and third foot not drier than 70 cb all year, but one sensor between 30 to 70 cb on Sept. 15. Very Wet All Season — Both sensors measuring second and third foot not drier than 70 cb all year and no sensor drier than 30 cb on Sept. 15.
These six categories were determined because the soil water level where deep percolation losses of water and nutrients slows to a low rate in a silt loam soil is about 70 cb; thus, it is recommended to keep at least one of the sensors in the second or third foot in this range or dryer. Yield losses would not be expected until all the sensors were approaching 140 cb, making the 70 cb level very achievable without any chance of causing yield loss. Field capacity in a silt loam is about 30 cb, so the goal is not to refill the soil above this level with irrigation in the second or third foot.
Source: unl.edu
Photo Credit: gettyimages-songbird839
Categories: Nebraska, Crops, Weather