Global temperatures are rising, primarily due to greenhouse gas emissions from anthropogenic activities (Stocker et al. 2014). Future temperature increases are exceedingly likely (IPCC 2007), as is an increase in precipitation extremes in extratropical zones (O’Gorman and Schneider 2009). Embedded within this global context are changes to regional temperatures and precipitation (Christensen et al. 2007) that often result from the impacts of land management and land cover change on regional energy balances (Mahmood et al. 2014; Luyssaert et al. 2014). Some of these regional climate changes may be beneficial to agricultural and ecosystem management objectives, such as dampening extreme temperatures (e.g. (Juang et al. 2007; Mueller et al. 2015); others are not (e.g. Marshall et al. 2003; Mande et al. 2015). It is important to understand how land management impacts climate processes to develop strategies to minimize the deleterious impacts of climate change and become effective stewards of the earth system.
A unique interaction between land management and climate may have emerged across the northern Great Plains of North America (hereafter the NNAGP). Beginning in the 1960s and 1970s, concerns over soil health and profitability lead to widespread changes in agricultural management away from wheat-fallow rotation agriculture and toward a more diverse agricultural system that avoids bare ground by rotating wheat with pulses, cover crops, and other crops (Miller et al. 2002, 2003; Long et al. 2014). These changes appear to have unintentionally benefitted regional climate (Gameda et al. 2007). Agricultural areas of the Canadian Prairie Provinces have experienced a 6 W m− 2 cooling during summer across parts of this time period (Betts et al. 2013a). Summer maximum temperatures in the Canadian Prairies have decreased by nearly 2°C and extreme temperature events have become less frequent (Betts et al. 2013a,b). These regional climate effects have been attributed to the widespread decline of summer fallow from ca. 110,000 km2 (25% of Canada’s cultivated lands) to some 35,000 km2 (8%) (Gameda et al. 2007; Vick et al. 2016). In the U.S. portion of the NNAGP and across a similar time period, near-surface air temperatures have cooled by nearly 0.2°C decade− 1 during late spring and early summer, and near-surface atmospheric vapor pressure deficit (VPD) – which strongly depletes crop yields (López et al. 2021) – has decreased by − 0.04 kPa decade− 1 on average (Bromley et al. 2020). These changes to the regional climate are concurrent with a period of summer fallow decline on the order of 50,000 km2 in the United States from a peak in 1987 until 2012 (Fig. 1).
The observed climate changes are consistent with a transition of large expanses of land away from bare ground and toward crops which actively transport water from soil to atmosphere and increase latent heat flux and evaporative cooling (Vick et al. 2016). Changes to land management have likewise decreased the surface-atmosphere flux of sensible heat to help create a moister, shallower atmospheric boundary layer during summer (Gameda et al., 2007; Vick et al., 2016) through decreases in the Bowen ratio. Combined, these changes in surface fluxes have enhanced cloud formation (Betts et al. 2013b) and increased the probability of convective precipitation (Gerken et al., 2018). Monthly mean precipitation has increased in the Canadian Prairies by 10 mm decade− 1 (Betts et al. 2013b; Gameda et al. 2007) and the U.S. northern Great Plains by 8 mm decade− 1 (Bromley et al. 2020). Given the multitude of factors that drive precipitation change, the full suite of mechanisms that underlie these observed increases in precipitation and the potential role of land cover change remain uncertain.
Empirical observations and localized modeling studies to date have made critical inroads into our understanding of land-atmosphere-precipitation connections in the NNAGP. Planting crops at the expense of summer fallow decreases planetary boundary layer (PBL) height (Gameda et al. 2007; Vick et al. 2016; Gerken et al. 2018) and, coupled with increases in humidity, lowers the lifted condensation level (LCL) (Betts et al. 2013b; Betts and Desjardins 2018). Shallow cumulus clouds can result when the PBL crosses the LCL, a ‘necessary but not sufficient condition’ for the formation of convective precipitation (Juang et al. 2007). Calculations of PBL and LCL height based on eddy-covariance data and one-dimensional mixed-layer atmospheric models show that the likelihood of PBL-LCL crossings are maximized in May and June when ‘wet coupling’ (Roundy et al. 2013) prevails in the NNAGP such that increased moisture increases the likelihood of convective events (Gerken et al., 2018). This contrasts the prevailing ‘dry coupled’ conditions later in summer when convective precipitation is unlikely (Gerken et al. 2018). Mixed layer models of atmospheric boundary layer development and land-atmosphere coupling have demonstrated an increase in PBL-LCL crossings and an increase in the likelihood of convective rain events across parts of the NNAGP (Gerken et al. 2018), but the mechanisms underlying potential changes in convective precipitation across larger regions have not been explored.
At the same time, changes to surface-atmosphere fluxes due to land cover change may play a minor role in key aspects of the hydroclimate of the NNAGP. Convective precipitation in the NNAGP is dominated by mesoscale convective systems that are responsible for as much as 60% of the warm season precipitation (Carbone and Tuttle 2008). These systems form in the west and propagate eastward, often overnight, and mixed layer models are generally unable to account for these dynamics (Carbone and Tuttle 2008; Gerken et al. 2018). The buildup of convective available potential energy (CAPE) that supports the development of MCSs comes primarily from the advection of warm, moist air into the region in addition to the diabatic heating of the boundary layer by way of sensible and latent heat fluxes from the surface (Agard and Emanuel 2017). The reduction of summer fallow has the potential to influence the flux of heat and moisture into the boundary layer, leading to the buildup of CAPE. Several studies have posited that increased evapotranspiration from more continuous cropping – and less summer fallow – has led to more growing season convection and potentially stronger storms (Raddatz 1998; Shrestha et al. 2012). However, since the boundary layer has also become cooler and moister, the cooling may act to reduce CAPE and perhaps balance the tendency of added moisture to increase CAPE. Increases in temperature and moisture aloft will also increase convective inhibition (CIN) which may balance or even dominate the effects of any increase in CAPE. The exact response of convective processes to such changes in near-surface conditions is unclear and requires a mechanistic modeling environment that can explicitly account for the dynamics of convective precipitation across regional scales.
How do changes to agricultural management impact regional atmospheric and climate processes? We seek to understand how land management impacts the regional climate and hydrometeorology of the NNAGP, a critical global breadbasket for wheat production. To do so, we model the regional impacts of the reduction of summer fallow across the NNAGP using the Weather Research and Forecasting model (WRF) at a 4 km spatial grid to explicitly model convective precipitation processes across multiple year periods. After validating the model using reanalysis data products and describing the major results of the modeling analysis, we explore reanalysis datasets for a more comprehensive view of the changing hydroclimate of the NNAGP. We then discuss results in the context of the local and large-scale patterns that are consistent with observed climate trends across the region.