Twin Cities WRS Seminar

Improving Irrigation Scheduling – An Evaluation of Water Balance Calculation Methods for Potato Grown on Sandy Soils in Humid Climates


Brian Bohman
Ph.D. Student, Water Resources Science

September 15, 2017


Supplemental irrigation is essential to produce water sensitive crops, such as potato (Solanum tuberosum L.), grown on sandy soils in humid climates. Proper management of irrigation is challenging, but essential for sustaining profitable crop production as well as groundwater supplies and quality. Commonly used methods of irrigation scheduling, such as the Checkbook method, are based on simplified calculations of reference evapotranspiration [ETref], crop coefficients [Kc], and leaching [L].. However, developments in computer software and processing power, improved soil moisture monitoring and remote sensing technology, and public investment in weather stations have made improvements in irrigation scheduling possible. This presentation will discuss improved techniques to calculate ETref, Kc, and L and evaluate the accuracy of various water balance calculation methods for use in irrigation scheduling for potato.

Measurements of soil moisture deficit from time-domain reflectometry [TDR-300, Spectrum Technologies], electrical resistance granular matrix sensors [Watermark, Irrometer Company], and gravimetric water samples were collected from a field study conducted in 2016 and 2017 on sprinkler irrigated potatoes grown on Hubbard loamy sand at the Sand Plains Research Farm in Becker, MN. Of all water balance methods evaluated, the Checkbook method was found to have the least accurate (Root Mean Square Error [RMSE] = 0.30 in) and most biased (Mean Bias [MB] = + 0.21 in) estimate of soil moisture deficit. In contrast, the most accurate (RMSE = 0.17 mm) and least biased (MB = – 0.04 in) water balance calculation method was using the American Society of Civil Engineers [ASCE] Standardized Reference [ETrs] calculated on an hourly time step, with a dual crop coefficient based on remote sensing observations, and leaching estimated from observed in situ soil moisture drainage. Using the improved and validated water balance model, irrigation decisions based on the Checkbook method for sprinkler-irrigated potatoes grown on a Hubbard loamy sand at the Sand Plains Research Farm from 2010 to 2015 are estimated to have been on average 4.3 inches per year [56 %] greater than the optimal irrigation rate. Based on these findings, it is estimated that utilizing improved water balance methods will reduce the volume of leaching by 3.9 inches per year [23 %]. These water balance calculation methods could also be extended to other crops and well-drained soils in humid climates where supplemental irrigation is required, and represent a significant opportunity for producers to reduce input cost and prevent contamination of groundwater resources.

Handout - PDF iconhandout_for_brians_seminar.pdf