According to the U.S. Geological Survey, irrigation accounted for 62% of the total U.S. freshwater consumption in 2005. This water is used to irrigate approximately 17% of U.S. cropland acres; yet, these acres account for 55% of total U.S. crop sales, including animal forage and feed crops (USDA Economic Research Service). Thus, irrigation plays a major role in both food production and the U.S. economy.
Demand for freshwater supplies has rapidly increased over the past few decades, and the necessity for more efficient irrigation systems is becoming critically important. Half of irrigated croplands across the U.S. are irrigated with more efficient center pivot systems, yet it has become evident through variable rate technologies that there is still room to improve the water-use efficiency with these machines.
Variable rate irrigation (VRI) seeks to apply water site-specifically to the field, depending on soil water holding capacities, crop type, and topography. Water savings with use of VRI has been contrasted in past research, with very few actual field studies being conducted to demonstrate differences in seasonal water application between site-specific and uniform treatments.
This past growing season, Valley conducted a field study with a VRI machine in central Illinois. The purpose of the research was to evaluate traditional uniform water application vs. site-specific irrigation treatments. Four main goals of this research were to: 1) understand how several factors, including soil type and slope, affect the variation of recommended watering rates in irrigation prescriptions; 2) identify how often irrigation prescriptions need to be updated throughout the growing season; 3) analyze water use under uniform and site-specific irrigation treatments; and 4) evaluate yield differences under uniform and site-specific irrigation treatments.
In this blog post, I’ll discuss some of the study results obtained from water-use efficiency between uniform and VRI treatments in a corn crop.
In general, the field where this study was conducted had significantly varying soils, ranging from sand on the hills (with 0-5% clay and 90-95% sand) and loam in the lower lying areas (0-5% sand). Thus, the available water holding capacity between these soils in the 3-foot root zone ranged from 3.75 inches in the sand to 7.2 inches in the loam. Twelve plots (three plots in sand and three plots loam soils for both VRI and uniform irrigation treatments) were identified in the field, each being approximately 1.5 acres in size. Three soil moisture probes (at 6-, 18-, and 30-inch depths) were placed in each of the plots.
Water application in the uniform treatment plots were based on irrigation scheduling recommended by the farmer, which was typically twice a week throughout the growing season. Application amounts were based on a “checkbook” method, determined by the soil water holding capacity and daily evapotranspiration rates. Thus, once it was determined that 50% of available moisture was present in the sand (which reached this level much sooner than the loam), a uniform treatment of water was applied to both the sand and clay soils to bring the available moisture back up to 85% of field capacity (which left additional room for rainfall).
Scheduling under VRI was based on reports from the soil moisture sensors. For example, in the sandy soils, water was applied once the moisture level reached 50% of field capacity. However, in the loamy soils, 50% of field capacity was a considerable 3.6 inches, which was too much to put back on at one time to bring the soil back up to near field capacity. Thus, the loamy soils were typically watered on days when the sandy soils were irrigated, but at nearly 50%-80% of the application depth.
The irrigation treatments between uniform and VRI were conducted from late June until early September. Throughout this time, 24 pivot passes were made over the study area. At the end of the season, irrigation treatments between the sand under VRI and the clay/sand under uniform treatment had similar total water applications of 14.4 and 14.5 inches respectively. However, the loam soil under the VRI treatment remained above 50% field capacity throughout the growing season even as it received a total of 8.9 inches.
Therefore, if this entire field were managed under VRI, there would be an estimated 31% reduction in water use. Although this study was conducted with two contrastingly different soils, it demonstrates how water saving can be achieved using variable rate irrigation.