of the U.S. Geological Survey (USGS) and Joseph Kanney of the U.S. Nuclear Regulatory Commission (NRC) developed the project scope. The NRC also developed the Statement of Work and actively participated in the design of the study. Karen R. Ryberg (USGS) provided most of the draft text, including contributions to the literature review section, and completed the initial data analysis and flood-frequency analyses. Kelsey A. Kolars (USGS) completed most of the literature review on flood-frequency estimation in consideration of stationary and nonstationary systems. Julie E. Kiang (USGS) contributed to the introduction, report structure, and revisions. Harry Jenter assisted with project scoping and oversight. Ryberg, Kolars, Kiang, and Meredith L. Carr (NRC) compiled the report; all authors contributed to addressing review comments. Steven K. Sando (USGS) and William H. Asquith (USGS) provided technical reviews of all material, and Tara Williams-Sether (USGS) provided an additional technical review of methods for including regional information. The NRC also contributed technical comments. This work was funded by the U.S. Nuclear Regulatory Commission (NRC-HQ-60-15-I-0006) with Carr acting as the Commission project manager. The authors acknowledge the USGS Water Mission Area Nonstationarity Workgroup (of which the USGS authors are members) for their in compiling a database of citations related to floods under nonstationary conditions before the start of this project. Many of those citations were used for this report.

Abstract. Most cropland in the upper Midwest will experience periods of excess water and drought conditions during a growing season. When the objective is to produce high yields, effective use of a subsurface water management system can help provide optimal soil moisture conditions for growth. A subsurface water management system includes draining excess water from the soil profile through subsurface drainage (SSD), managing the water table through controlled drainage (CD), or adding water to the drainage system during dry conditions (Subirrigation – SI). Subsurface water management can become difficult when determining the time and amount needed for SSD and SI, and (or) the optimal water table (WT) depth when using CD due to water movement in both the upward and downward directions. In this study, a 21 ha field with CD, a 17-ha field with CD + SI, and a 16 ha control field (surface drained only) over clay loam and silty clay loam soils were used to evaluate subsurface water management scheduling for corn (2013) and soybean (2014). The Checkbook Irrigation Scheduling method (Lundstrom and Stegman, 1988) was modified to include an algorithm to estimate the daily water balance contribution due to upward flux (UF) from a shallow water table. For the 2013 growing season, the UF reduction of the daily soil moisture deficit (SMD) was minimal due to deeper WT over the growing season and there was little difference between the modified and original Checkbook methods. For the 2014 growing season, the SMD estimates from the Modified Checkbook method produced closer estimates to the in-field SMD compared to the original Checkbook method. Therefore, adding SSD and shallow WT contributions in the Checkbook method produces similar, if not more accurate, estimations of daily SMD that can be used for subsurface water management. Keywords: Checkbook irrigation scheduling method, Model development, Subirrigation, Subsurface drainage.

vii 24. Graphs showing simulated 10-day mean streamflow for 100-year simulation period for the Souris River below Rafferty Reservoir, Saskatchewan, Souris River above Minot, North Dakota, and Wintering River near Karlsruhe, North Dakota, streamflow-gaging stations as compared to the median, 90th percentile, and maximum recorded values of annual maximum 10-day streamflow .

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