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The projected near-future climate (2031–2059) of wetter springs and drier summers may negatively affect agricultural production in the US Midwest, mostly through reduced aeration of the root zone due to excess soil water and frequent loss of nutrients such as nitrate (NO₃-N) and total phosphorus. Several agricultural adaptations—such as adding tile drains and increasing fertilizer rates—may be deployed to mitigate potential reductions in crop yield. However, these adaptations (generally driven by economic benefits) may have a severe impact on water quality, which is already under stress due to excess nutrient runoff from agricultural fields causing hypoxia in inland and coastal waters. Here, we evaluate the crop yield and water quality consequences of such adaptations under future climate with the Soil and Water Assessment Tool in a testbed watershed located in central Illinois. We show that additional tile drains and increased fertilizers can help achieve baseline (2003–2018) corn yields but with a nearly two-fold increase in riverine NO₃-N yield affecting a major drinking water supply source. However, a shift to spring-only fertilizer application may not require additional fertilizer and reduces the increase in NO₃-N loss to 1.25 times above the baseline. We also show that water quality may improve (better than baseline) with conservation measures such as cover crops and switchgrass. Our findings highlight the need to develop efficient climate change adaptation and conservation strategies for sustainable agriculture and water quality.

Data limitations often challenge the reliability of water quality models, especially in intensively managed watersheds. While numerous studies report successful hydrological model setup and calibration, few have addressed in detail the data challenges for multisite and multivariable model calibration to an intensively managed watershed. In this study, we address some of these challenges based on our reflective experience calibrating the Soil and Water Assessment Tool (SWAT) to the Upper Sangamon River Watershed in central Illinois based on daily flow, annual crop yield, and monthly sediment, nitrate, and total phosphorus loads. We highlight some challenges in SWAT calibration processes due to data errors and inconsistencies, and insufficient precipitation and water quality observations. Following, we demonstrate the merits of additional weather and water quality observations that could help reduce input uncertainties, and we provide suggestions for selecting appropriate observations for the model calibration. After dealing with the data issues, we show that the SWAT model could be calibrated with acceptable results for the case study watershed.

Aquifer depletion due to extensive and intensive irrigation in the High Plains has threatened the environmental sustainability of the region. The change of crop evapotranspiration (ET), the major form of agriculture water consumption, presents a critical signature of hydrologic cycle change. This study evaluates the relative contributions of climate and groundwater‐fed irrigation toETtemporal and spatial pattern change over the High Plains Aquifer, one of the most severely depleted aquifers in the US. We developed a framework to extend the Budyko hypothesis to assess the impact of catchment storage change on long‐termET. It is found that irrigation from groundwater pumping contributes more than half of the increase inET(74.1 mm) from the period of 1940–1975 to 1976–2010, despite an increase of precipitation (35.0 mm) in the region.ETseasonal variance is decreased by the decline in precipitation variability (at −20.7 mm) and increase in irrigation (at +14.2 mm). As expected, irrigation decreasesETcoefficient of variability (i.e., the ratio of standard deviation to mean). Spatially, we find that the human‐inducedETheterogeneity post‐1975 superimposes over the natural east‐to‐west gradient (in precipitation andET) of this region. A correlation between the statistics (mean vs. coefficient of variation) onETand crop yield provides promising signatures for understanding the coupled natural and human system of High Plains agriculture. Guides are discussed regarding how to handle the tradeoffs between agricultural development and natural resource sustainability under climate variability in the High Plains and other regions with similar conditions.

Efforts to reduce riverine phosphorus (P) loads have not been as fruitful as expected or hoped. One reason for the failure of these efforts appears to be that models used for watershed P management have understated and misrepresented the role of in‐stream processes in shaping watershed P export. Here, we update the latest release of the Soil and Water Assessment Tool (SWAT+), a widely used watershed management model, to better represent in‐stream P retention and remobilization (SWAT+P.R&R). We add new streambed pools where P is stored and tracked, and we incorporate three new processes driving in‐stream P dynamics: (a) deposition and resuspension of sediment‐associated P, (b) diffusion of dissolved P between the water column and streambed, and (c) adsorption and desorption of mineral P. The objective of this modeling work is to provide a diagnostic tool that enables researchers to challenge existing assumptions regarding how watersheds store, transform, and transport P. Here, in a first diagnostic analysis, SWAT+P.R&R helps reconcile in‐stream P retention theory (that P is retained at low flows and remobilized at high flows) and a discordant data set in our validation watershed. SWAT+P.R&R results (a) clarify that the theorized relationship between P retention and flow is only valid (for this point‐source affected testbed, at least) at the temporal scale of a single rising‐or‐falling hydrograph limb and (b) illustrate that hysteresis obscures the relationship at longer temporal scales. Future work using SWAT+P.R&R could further challenge assumptions regarding timescales of in‐stream P legacies and sources of P load variability.