Potential Pumping Reductions by Using VRI to Mine Undepleted Soil Water
Variable rate irrigation (VRI) may lead to a variety of benefits, which include but are not limited to decreasing pumping energy expenses, lowering the frequency/severity of yield loss and stuck pivots due to over irrigation, avoiding chemigation over noncropped areas, reducing irrigation runoff on hillslopes, and decreasing nitrate leaching. The map below shows estimates of potential pumping reduction from using VRI to account for spatial variability in root zone available water capacity (R), assuming a managed root zone of four feet and an ideal end-of-season soil water depletion of 50%. Conventional irrigation schedules are typically based on portions of the field where root zones hold the least available soil water, which leaves undepleted available water in areas with larger R. VRI allows a producer to irrigate low R soils while crops continue to use the stored water from precipitation in high R soils. The gridded Soil Survey Geographic database (gSSURGO; NRCS, 2014) was used to analyze 49,224 fields in Nebraska that used center pivot irrigation in 2005. Details of the procedure were presented in Lo et al. (2016). The analysis quantified potential pumping reductions resulting from zone control VRI; sector control (speed control) was not analyzed. The effects of topography were not considered in this analysis. Producers are encouraged to consider all potential benefits during the VRI investment decision process, as well as data collected from the field site under consideration. A brief economic analysis revealed that the use of VRI to improve yield may have the greatest potential to economically justify an investment in zone control VRI (Chapter 1, Lo, 2015). For fields in the Central Great Plains with adequate irrigation water supply to meet crop water requirements, this may be possible by reducing yield losses associated with excessive water. For fields under limited or deficit irrigation, VRI is unlikely to be used to reduce pumping; however, there may be applications of VRI for maximizing yield with a given water allocation.
1. Click on your county, zoom in, and click on your field of interest.
2. The map tool will display the potential pumping reduction from mining undepleted soil water both as a volume (ac-ft) and as a depth averaged across the field (in.). The field area (ac) and the distribution of R in the field are also shown.
3. Enter your annual cost ($) of pumping and your annual depth of irrigation (in), which are used to determine the cost of pumping per ac-ft of water.
4. Enter the irrigated area (ac) and the annual pumping reduction (in.) which were reported in the map tool.
5. Choose an annual interest rate (%) and an amortization period (yr), which is the payback period if you were to invest in a zone control VRI system.
6. Press “Calculate” to determine the present value ($) of pumping cost savings, which is how much you could afford to spend on a VRI system for this particular benefit of VRI. To estimate annual savings ($) instead of present value, enter an amortization period of 1 yr and a very low interest rate (0.01%).
Map Tool for Pumping Reduction and R
VRI Pumping Cost Savings Calculator
Results and Discussion
Pumping reductions exceeded two inches per year for 2% of the fields and exceeded one inch per year for 13% of the fields in Nebraska. Detailed results are presented in Lo et al. (2016). These data may be a conservative estimate of pumping reduction if undepleted water is mined early in the season and the soil water profile is refilled by precipitation, allowing undepleted water to be mined again. Following the Nebraska Extension recommendation of 60% depletion at the end of the season (NebGuide G1871) would result in a 20% increase in pumping reductions. A more precise estimate of pumping reduction could be determined by field measurements of soil properties (Lo et al., 2017), which may show greater variability in soil properties than the NRCS Soil Survey. This analysis quantified pumping reduction from mining undepleted soil water. Other potential sources of pumping reductions include eliminating application over uncropped areas, reducing application on hillslopes where runoff is high, and reducing application in depressional areas with high infiltration due to runon. Adoption of zone control VRI for pumping reduction is most feasible for fields where the pumping reduction from VRI is large and the pumping cost per unit of water is large. Speed control (i.e. sector control) is much less expensive than zone control VRI, although pumping reductions from speed control are likely to be smaller than pumping reductions from zone control. A reduction in pumping in over-irrigated areas may also reduce deep percolation and leaching of water and nutrients (e.g., nitrate – nitrogen) below the crop root zone.
VRI Map Tool Team
Tsz Him (Himmy) Lo, Derek Heeren, Joe Luck, Derrel Martin, Luciano Mateos, and Dean Eisenhauer
Water, Energy and Agriculture Initiative, which was made possible with funding from the Nebraska Corn Board, the Nebraska Soybean Board, the Agricultural Research Division at the University of Nebraska–Lincoln (UNL) and Nebraska Public Power District through the Nebraska Center for Energy Sciences Research at UNL
Water for Food Global Institute
Department of Biological Systems Engineering
USDA National Institute of Food and Agriculture, Hatch project 1009760
Lo, T., D. M. Heeren, L. Mateos, J. D. Luck, D. L. Martin, K. A. Miller, J. B. Barker, and T. M. Shaver. 2017. Field characterization of field capacity and root zone available water capacity for variable rate irrigation. Applied Engineering in Agriculture 33(4): 559-572, doi: 10.13031/aea.11963.
Lo, T., D. M. Heeren, D. L. Martin, L. Mateos, J. D. Luck, and D. E. Eisenhauer. 2016. Pumpage reduction by using variable rate irrigation to mine undepleted soil water. Transactions of the ASABE 59(5): 1285-1298, doi: 10.13031/trans.59.11773.
Lo, T. 2015. Quantification of variable rate irrigation benefits and spatial variability in root zone water holding capacity. MS thesis. Lincoln, Neb.: University of Nebraska–Lincoln, Department of Biological Systems Engineering.
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