- Agronomic: corn, millet, potatoes
- Vegetables: peas (culinary), sweet corn
- Crop Production: cover crops, crop rotation, irrigation
- Education and Training: on-farm/ranch research
- Production Systems: agroecosystems
- Soil Management: soil physics
- Sustainable Communities: sustainability measures
The Wisconsin Central Sands region irrigates 80,000 hectares of potato, maize, pea, and bean crops by pumping groundwater from a coarse, shallow aquifer. The expansion and intensification of irrigated agriculture and recent surface water stresses have led to a community conflict over groundwater in this region. Effective regional groundwater management requires an increased understanding of groundwater recharge and evapotranspiration from irrigated cropping systems.
The overarching goal of this project was to quantify on-farm crop ET and potential groundwater recharge for dominant crop rotations in the Wisconsin Central Sands with greater spatial and temporal resolution than previous endeavors to test causal relationships and collect physiological and biophysical data required to parameterize and calibrate a process-based agroecosystem model of ET and recharge for the Wisconsin Central Sands. We implemented a semi-permanent network of 25 vadose zone lysimeters on Isherwood Farms, a 600-hectare, sixth generation family farm in Plover, WI. Between 2013-2015, we collected drainage, soil moisture and temperature, micrometeorological, crop physiological, and crop phenological data from irrigated potato, maize (field and sweet), and pea cropping systems under real agronomic management practices on Isherwood Farms. We observed significant intrafield variability in potential recharge and evapotranspiration from cropping systems on Isherwood Farms. This variability may the result of interfield variability in crop type (our initial hypothesis) or intrafield variability in soil texture, topography, and phenology. We have furthered our understanding of several biophysical mechanisms unique to irrigated agroecosystems in the WCS. We plan to use this increased mechanistic understanding, our robust field dataset, and site-specific physiological parameters to build process-based models of potential recharge and ET (Agro-IBIS) for potato, sweet corn, and pea functional types. By linking hydrology and carbon assimilation in these models, it will be possible to mechanistically explore the relationship between crop water use and crop productivity in the WCS. A future goal of this work will be to model regional solutions (i.e. precision irrigation, irrigation scheduling, deferred irrigation) over the WCS and assess their resilience to changes in climate.
Irrigated agriculture profoundly changes the coupled water and energy cycle from field to global scales. Pumping groundwater to the soil surface increases crop evapotranspiration (ET), which is considered consumptive groundwater use because water is depleted from a specific time and place in an aquifer (Winter, 1999). Globally, 4500 km3 of groundwater was depleted in the 20th century with agricultural demand for freshwater predicted to increase during the 21st century (Konikow, 2011). Regional stakeholders share groundwater as a common resource and the expansion of irrigated agriculture inevitably leads to community conflicts about water scarcity and equity.
The Wisconsin Central Sands (WCS, Fig. 1) irrigates 80,000 hectares of potato, maize, pea, and bean crops, which require over 335 billion liters of groundwater and comprise a $450 million agricultural and processing industry in the state of Wisconsin (NASS, 2013; Kraft, 2013; Keene and Mitchell, 2010). There are approximately140 family farms in the WCS, many of which survived Wisconsin’s Dust Bowl to build profitable agribusinesses contingent on unrestricted groundwater access (Goc, 1990). Though irrigation is still considered supplemental in the Midwest, it has been compulsory in the WCS since it tripled yields from fast-draining, sandy soils in the 1950s (French and Lynch, 1957).
Growers pump groundwater via high-capacity wells from an unconfined aquifer that replenishes 1000 km of headwater trout streams, 80 lakes, and extensive wetlands (Kraft et al. 2012; WI-DNR 2014). WCS residents and tourists prize these surface waters as ecosystems that support fishing, swimming, biodiversity, and spirituality. In the WCS, surface and groundwater are inextricably linked to one another, which directly ties the expansion of irrigated agriculture to the loss of aquatic ecosystem services. Prophetic hydrological studies in the late 1960s warned of future impacts to WCS surface waters if aquifer development for agriculture persisted, but these warnings went unheeded and the number of high-capacity wells in Wisconsin increased exponentially from less than 50 in 1960 to over 3800 in 2013 with the majority located in the WCS (Weeks et al. 1965; Weeks and Stangland, 1971; Smail, 2013).
In the late 2000s, hydrological predictions were actualized and many WCS surface waters began to exhibit severe stress and trout death (Kraft and Mechenich, 2010). These stresses include the drying of the Little Plover River in 2005-2009, which was named one of America’s most endangered rivers in 2013 (American Rivers, 2013). The hydrologic stress, trout death, and continued irrigation expansion in the WCS fuel a 60-year old, litigious community conflict between aquatic and agricultural stakeholders over consumptive groundwater use and equity.
Generally and in the WCS, consumptive groundwater use via crop evapotranspiration (ET) constitutes 70-85% of groundwater withdrawals for irrigation, while 15-30% of pumped groundwater may be recharged back into the aquifer (Weeks and Stangland, 1971; Winter, 1999). For the purpose of maintaining aquatic ecosystems, contemporary evidence suggests that consumptive groundwater use via crop ET is not recoverable. Pumping with high capacity wells shifts recharge patterns and exacerbates depletion in surface waters that depend on a critical zone of groundwater supplied from the first few meters of saturated aquifer thickness (Kraft et al. 2012; Condon and Maxwell, 2014). Irrigated crop ET is inferred to be of sufficient magnitude at 480-550 mm (WCS precipitation is 790-810 mm) to cause predicted and observed surface water depletion in the WCS (Weeks et al. 1965; Weeks and Stangland 1971; Tanner et al. 1974; Naber, 2011; Kraft et al. 2012; Kniffin et al. 2014) and there is enough existing information to begin equitable management of water resources. However, developing better management and policy requires an improved understanding of the spatiotemporal distribution of ET and “net” groundwater recharge (precipitation minus ET) from different irrigated cropping systems and how these relationships may be impacted by climate change (WCS Listening Sessions, 2011).
Though it is generally understood that irrigation increases cumulative annual crop ET and therefore decreases groundwater recharge by at least 50 mm, the spatiotemporal variability of ET and recharge from irrigated agroecosystems in the WCS remains uncertain. Spatially, differences in ET and recharge within a single irrigated field and crop type may be related to portions intrafield differences in topography, crop growth, and soil texture. Differences in daily ET between crop types on different fields within a close proximity to one another occurs because of crop growth, phenology, and agroecosystem management (e.g. residue and cover crop application). Characterizing these intrafield and intercropping system differences in ET and recharge from real agroecosystems in the WCS is critical to parameterize, calibrate, and validate process-based models of ET and groundwater recharge.
Process-based agroecosystem models of ET and net groundwater recharge such as Agro-IBIS (Kucharik, 2003) explicitly incorporate physiological and biophysical mechanisms taking place in the soil-plant-atmospheric based on causal relationships that may be continually tested in the field. Empirical models of potential or reference ET extrapolate correlative relationships between meteorological variables and crop physiology developed and validated for a particular site without explicit representation of soil-plant-atmospheric mechanisms. In the absence of site-specific crop or biophysical data, potential or reference ET models (e.g. Allen et al. 1998, Priestley-Taylor 1972, Hargreaves-Samani 1982) often serve as useful indices of overall evaporative demand and irrigation scheduling tools. Though approximating ET and net recharge using potential or reference ET and crop coefficient approaches may provide initial estimates for the WCS, these empirical models are unsuitable for analyzing differences between irrigated cropping systems or predicting how ET and recharge from these systems may respond to multidecadal climate change, interannual climate variability, or the implementation of water sustainability measures. Though the data requirements, field-tested causal relationships, and parameters are robust; a process-based agroecosystem model of ET and net groundwater recharge is required for resilient, comprehensive management of water resources in the WCS in the face of a changing climate.
The overarching goal of this project was to quantify on-farm crop ET and net recharge for dominant crop rotations in the WCS with greater spatial and temporal resolution than previous endeavors to test causal relationships and collect physiological and biophysical parameters required for building a process-based agroecosystem model of ET and net recharge for the WCS (Agro-IBIS, Kucharik, 2003).
The specific objectives were to (1) Utilize new vadose zone instrumentation and other biophysical field measurements in potato and maize cropping systems to quantify groundwater recharge under these crop types, and understand hydrogeological responses associated with crop type, irrigation, tillage, and cover crops; (2) Utilize field measurements to develop, parameterize, and validate potato and maize crop functional types in the Agro-IBIS agroecosystem model to link groundwater recharge to aboveground processes by capturing coupled carbon-water energy exchange; (3) Drive Agro-IBIS using a new, high resolution (8km x 8km) historical daily climate dataset (1948-2010) and varied land management scenarios (e.g. irrigation, crop/vegetation type, tillage) to understand how cumulative changes in climate and land management have impacted groundwater recharge and evapotranspiration in Wisconsin Central Sands over the past 60 years.