Groundwater subsidies and penalties to corn yield

Proper water management is critical to closing yield gaps (observed yield below potential yield) as global populations continue to expand. However, the impacts of shallow groundwater on crop production and surface processes are poorly understood. The presence of groundwater within or just below the root zone has the potential to cause (via oxygen stress in poorly drained soils) or eliminate (via water supply in dry regions) yield gaps. The additional water use by a plant in the presence of shallow groundwater, compared to free drainage conditions, is called the groundwater subsidy; the depth at which the groundwater subsidy is greatest is the optimal depth to groundwater (DTGW). In wet years or under very shallow water table conditions, the groundwater subsidy is likely to be negative due to increased oxygen stress, and can be thought of as a groundwater penalty. Understanding the spatial dynamics of groundwater subsidies/penalties and how they interact with weather is critical to making sustainable agricultural and land-use decisions under a range of potential climates. Here, we examine patterns of groundwater subsidies and penalties in two commercial cornfields in the Yahara River Watershed, an urbanizing agricultural watershed in south-central Wisconsin. Water table levels are generally rising in the region due to a long-term trend of increasing precipitation over the last several decades. Biophysical indicators tracked throughout both the 2012 and 2013 growing seasons show a strong response to variable groundwater levels on a field scale. Sections of the field with optimal DTGW exhibit consistently higher stomatal conductance rates, taller canopies and higher leaf area index, higher ET rates, and higher pollination success rates. Patterns in these biophysical lines of evidence allow us to pinpoint specific periods within the growing season that plants were experiencing either oxygen or water stress. Most importantly, groundwater subsidies and penalties are directly related to year-end yield. During 2012 (a drier-than-normal growing season) corn in parts of the field with shallow groundwater had significantly higher yields than the rest of the field, indicating that groundwater can provide significant yield benefits during drought. In contrast, during 2013 (a wetter-than-normal growing season) areas with the shallowest groundwater experienced total yield losses due to early-season groundwater flooding and oxygen stress. This demonstrates that the optimal DTGW for agricultural production is variable and depends on growing season weather conditions. The presence or absence of shallow groundwater is an important and dynamic feature of many agroecosystems, and should be considered when making both field- and watershed-scale management decisions.