The Decline In Summer Fallow In The Northern Plains Cooled Near-Surface Climate But Had Minimal Impacts On Precipitation

Land management strategies can moderate or intensify the impacts of a warming atmosphere. Since the early 1980s, nearly 116,000 km 2 of crop land that was once held in fallow during the summer is now planted in the northern North American Great Plains. To simulate the impacts of this substantial land cover change on regional climate processes, convection-permitting model experiments using the Weather Research and Forecasting (WRF) model were performed to simulate modern and historical amounts of summer fallow, and were extensively validated using multiple observational data products as well as eddy covariance tower observations. Results of these simulations show that the transition from summer fallow to modern land cover lead to ~1.5 °C cooler temperatures and decreased vapor pressure decit by ~0.15 kPa during the growing season, which is consistent with observed cooling trends. The cooler and wetter land surface with vegetation leads to a shallower planetary boundary layer and lower lifted condensation level, creating conditions more conducive to convective cloud formation and precipitation. Our model simulations however show little widespread evidence of land surface changes effects on precipitation. The observed precipitation increase in this region is more likely related to increased moisture transport by way of the Great Plains Low Level Jet as suggested by the ERA5 reanalysis. Our results demonstrate that land cover change is consistent with observed regional cooling in the northern North American Great Plains but changes in precipitation cannot be explained by land management alone.


Introduction
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 bene cial 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 pro tability lead to widespread changes in agricultural management away from wheatfallow 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(Miller et al. , 2003Long et al. 2014). These changes appear to have unintentionally bene tted 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 km 2 (25% of Canada's cultivated lands) to some 35,000 km 2 (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 nearsurface atmospheric vapor pressure de cit (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 km 2 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 ux and evaporative cooling (Vick et al. 2016). Changes to land management have likewise decreased the surface-atmosphere ux 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. the LCL, a 'necessary but not su cient 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 uxes 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 uxes from the surface (Agard and Emanuel 2017). The reduction of summer fallow has the potential to in uence the ux 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 nearsurface 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.

Study Area
We de ne the semi-arid NNAGP following Bromley et al. (2020) as the combination of the Canadian Prairie Ecozone and the U.S. National Ecological Observation Network (NEON) Domain 9. Brie y, the NNAGP are dominated by grasslands, shrublands, and agriculture with minimal urban development and forests in isolated mountain ranges and river valleys ). The NNAGP is a critical region for the global production of wheat, pulses, and oilseeds, and corn-soy cropping is becoming increasingly common in its eastern portion (Maaz et   . From this perspective, our analysis represents a conservative interpretation of fallow change from the 1980s until the 2010s. We focus our analyses on the C11 and F11 simulations to isolate the role of land cover change apart from decadal global climate change on determining regional climate changes in the NNAGP. Statistical differences between simulations were assessed using the Mann-Whitney U test on daily data. The data processing work ow relied on the Climate Data Operators package (

Model Validation
The control simulations are extensively validated against observational datasets in the Appendix and validation will only brie y be discussed for completeness. Control simulations were compared to the Investigating warm season climate trends is the focus of this work, and changes to other seasons are intermittently discussed for completeness. Seasonal changes are taken to be the average change across the three-year simulation.

Changes to summer fallow extent and representation in WRF
The extent of summer fallow in Provinces and States that intersect the study area decreased from 151,900 km 2 in 1982 to 35,100 km 2 in 2012, the years closest to the study periods when data were available from the United States (data from Canada were available every year) (Fig. 1). Some 45% of the total decline in fallow of 116,800 km 2 was attributable to Saskatchewan alone (53,000 km 2 ). These published agricultural statistics were used to nudge the Landsat fallow attribution analysis on a per-Province and per-State basis (Fig. 2) to ensure that it simulated total fallow area, which was then used to adjust the bare ground fraction in Noah-MP as noted.

Changes to near-surface temperature, energy uxes, and humidity
Two-meter air temperature (T2) shows a domain-averaged increase of about 0.18°C during the growing season in F11 relative to C11 noting that the study period includes areas that experienced both increases and decreases in summer fallow (Figs. 1-3). The strongest simulated warming is limited to June, July, August (JJA) (Fig. 4). T2 cooled on average by − 0.5°C in areas where fallow decreased in C11 compared to F11. The northern and western part of the NNAGP in Alberta experienced a T2 increase on the order of + 1.5°C. T2 warmed on average by about 1.0°C in areas where fallow increased in F11 compared with C11 across the entire study domain. There is a linear relationship between fveg and T2 (Fig. 3b); T2 increases by 0.69°C for every 10% increase in summer fallow. There is a modest cooling signal during winter indicating the more vegetated C11 simulation is warmer by 0.25°C (Fig. 4) that follows the same spatial pattern as growing season temperature changes, but opposite in sign and of a lesser magnitude. Changes in VPD between F11 and C11 follow the same spatial pattern as T2; the near-surface atmosphere in areas that increased in fveg became moister and the areas that decreased in fveg became drier (Fig. 5b). The domain median change is − 0.045 kPa and with a wide distribution that encompasses both increases and decreases in VPD (Fig. 5a). The most widespread increase in VPD is in Alberta with a 0.15 kPa increase from F11 to C11 over most of the Province within the study area. The strongest increase in VPD is nearly 0. Study area-averaged sensible heat ux (H) is higher by 10 W m − 2 in F11 compared to C11 in areas where fveg is lower compared to C11 (Fig. 6a). The areas of largest change have magnitudes on the order of 20 W m − 2 . Changes to latent heat ux (LE) follow the same pattern as H, but the magnitudes are larger (Fig.   6b). Study area-averaged LE is − 16 W m − 2 lower in the F11 simulation compared to the C11 simulation.

Changes to convective environments
We separate our analyses of convective environments between the early warm season (May and June) and late warm season (July and August) given differences in surface-atmosphere coupling in the NNAGP during these periods (Gerken et al., 2018).
During May and June, changes to CAPE and CIN are not signi cantly different from background noise (Fig. 7a,b). The height of the LCL is higher in F11 compared to C11 for most of the study area. The mean change to LCL height is 13 m while some areas are more than 30 m (Fig. 7c). Manitoba is the only area where the LCL heights have decreased. Mean LCL heights in Manitoba decreased by 20-30 m. Differences in planetary boundary layer (PBL) heights closely follow the spatial pattern of differences in LCL heights (Fig. 7d) and, as a consequence, the areas that have a positive change fveg from F11 to C11 have higher PBL heights, while the opposite holds for areas that have a negative change in fveg such as Manitoba. The mean change in PBL heights within the study area is 8 m but some areas change up to 30 m.
During July and August, CAPE is lower across most of the study area in the F11 simulation by − 10 J kg − 1 with minima in Alberta and the Dakotas (Fig. 7a) where CAPE decreases by 20-40 J kg − 1 . Changes to CIN in July and August weaker than the changes to CAPE (Fig. 8b). CIN is lower in the F11 simulation than the C11 simulation in Alberta and parts of Saskatchewan by over − 5 J kg − 1 on average. Manitoba and north-eastern North Dakota exhibit higher CIN in F11 than C11 by 20-30 J kg − 1 . Differences in LCL heights show a similar pattern to May and June but much stronger with F11 simulating LCL heights that are 50 m higher than C11 across most of the study area (Fig. 8c). The largest differences are located along the North Dakota-South Dakota border, northern Saskatchewan, and Alberta. LCL heights in these areas are over 100 m higher. PBL heights in July and August follow a similar spatial pattern as in May and June but with greater magnitude (Fig. 8d). PBL heights in Alberta and northern Saskatchewan are higher in the

Changes to precipitation
Changes to precipitation were not appreciably different from background noise from May through August (Fig. 9). When aggregated to 100 km a slight drying becomes apparent during July and August, but any effects are not statistically signi cant.

Discussion
We demonstrate using convection-permitting WRF model simulations that land management change toward continuous cropping and away from summer fallow decreased near-surface air temperature and VPD but had muted impacts on precipitation in part because increases in instability through increased boundary layer moisture have been balanced by an increase in stability through a cooler boundary layer.
Precipitation did not appreciably change between the WRF simulations, indicating a possibility that precipitation processes in the NNAGP are not very sensitive to the land surface changes of the magnitude experienced in recent decades. Below, we elaborate on each of these ndings to describe how land cover change has modi ed important aspects of the regional climate of the NNAGP, especially near the land surface. We then add to emerging evidence that observed changes in precipitation are likely due to moisture advection into our study domain rather than regional surface-atmosphere interactions.

Temperature
To summarize ndings on the impact of summer fallow on near-surface climate: areas that underwent a fveg increase from F11 to C11 were cooler with lower VPD (Figs. 3 & 6). Near-surface warming and drying occurred in areas where fveg decreased. These results lend evidence to the notion that a reduction in summer fallow is largely responsible for the cooling and moistening trend that is observed across the NNAGP. The changes to temperature in the simulations are stronger than the trends calculated by Bromley et al., (2020), noting that the trends in the latter are calculated from a 1970 starting point, whereas these experiments simulate fallow reduction from the 1980s to 2010s. Temperature trends are stronger at nearly − 0.5°C decade − 1 (Bromley et al. 2020) when calculated using 1980 as a starting point, on the order of 1-1.5°C, similar to modeled changes in T2 associated with an fveg increase from F11 to C11. The temperature difference simulated here spans from May until September, but given wheat is often harvested in August (if not sooner for the case of winter wheat), the September T2 difference is likely due to the prescribed seasonal cycle for each land use category.
The winter warming in the C11 simulation relative to the F11 simulation is likely due to the decrease in albedo from increased fveg in the model. Since the fveg does not change based on a seasonal cycle, areas with greater fveg are assumed to be lower in albedo since the vegetation is not covered in snow. The bare ground areas are covered in snow and thus are higher in albedo. This is similar to year-round cover cropping and the winter warming effect has been noted in global climate models (Lombardozzi et al. 2018 Vapor Pressure De cit Plant stomata respond strongly to VPD. If VPD is too high, stomata will close to avoid evaporative water losses, effectively shutting off carbon uptake by plants (Eamus et  Novick 2017). The VPD change for an increase in fveg from F11 to C11 is on the order of − 0.45 kPa which corresponds to a rst order with the observed changes to VPD in the NNAGP.
Our modeling analysis suggests that the impacts of simulated fallow reduction on near surface climate has acted to create more favorable conditions for crop growth by reducing growing season temperatures and VPD (Hsiao et al. 2019). Wheat yields differ in their sensitivity at different crop growth stages, and early-season days with mean temperature > 28°C are especially detrimental (Asseng et al. 2015). It is interesting to note that the midwestern United States has experienced largely bene cial changes in near surface growing season climate as a result of agricultural intensi cation (Mueller et al. 2015), leading one to question if they can be sustained in the future as global climate change continues to stress water resources and production systems.

Boundary layer changes
The PBL by de nition is the near surface layer of the atmosphere that is strongly in uenced by surface uxes of water and energy, so it is not surprising that the systematic shift away from summer fallow affects PBL processes. The monthly mean boundary layer heights were 100 m higher in late summer in the F11 simulation where the fallow amounts were larger; the lowering of the PBL as fallow declines was proposed to be a consequence of the changes in energy partitioning from a fallow (bare) surface and a vegetated surface (Gameda et al. 2007) which was the case in these simulations (Fig. 5). The change in PBL height was previously assessed using a simple slab model with inputs from eddy covariance observations of turbulent uxes from wheat and fallow elds (Vick et  with the notion that summer fallow changes PBL and LCL heights but its realized impact on precipitation was relatively small and spatially variable (Fig. 9).

Precipitation
There is little evidence that precipitation changed appreciably between F11 and C11 (Fig. 9), suggesting that a reduction in summer fallow might not have had much of an impact on observed precipitation trends. July and August are 10-15 mm drier in the F11 simulation when the precipitation change is aggregated to 100 km × 100 km boxes but these changes are not signi cant at the 95% level. recipitation in the NNAGP increased by 8 mm decade − 1 in May and June, but July and August precipitation also increased, primarily on the eastern side of the NNAGP (Bromley et al. 2020). If the land surface is not appreciably changing mean precipitation, what is the source for the observed warm season precipitation increase (Bromley et al. 2020)?
Global mean precipitation has been increasing due to anthropogenic warming of the atmosphere at about a rate of 2% K − 1 (Held and Soden 2006;Pendergrass and Hartmann 2014). This rate comes from the thermodynamic change to precipitation, but does not account for changes to the dynamic components such as changes to circulation. Precipitation in the NNAGP is largest in the early warm To investigate the possibility that the observed increase in precipitation in the NNAGP is consistent with additional moisture sources from the south, we investigated meridional wind and speci c humidity trends in ERA5. Figure 10a shows a vertical cross-section along the 42º latitude line of 1979-2020 trends in monthly mean meridional wind and speci c humidity. Due to the lack of strong trends in meridional wind, meridional moisture transport trends are only slightly positive (Fig. 10b). There is not a clear signature of a strengthening GPLLJ, but the increase in surface speci c humidity corroborates Bromley et al., (2020); near-surface conditions are moistening during May and June. Trends in the North American Regional Reanalysis (NARR) dataset shows that moisture transport northward has increased during AMJ, particularly during days with MCS initiation (Feng et al. 2019; Barandiaran et al. 2013).
Most of the MCSs in the NNAGP occur during July and August, and the northward extension of the GPLLJ over the past four decades is clear in monthly mean trends (Fig. 11). Speci c humidity has increased at 0.3 g kg − 1 decade − 1 while meridional wind has increased at 0.35 m s − 1 decade − 1 . These trends add moisture to the NNAGP and likely contribute to the observed increase in precipitation on the eastern and southern boundaries of the NNAGP during summer as well as the lower VPD during JJA (Bromley et al., 2020). The magnitude of CAPE change was on average larger in July and August, which could mean that the increase in convective environments conducive to strong storms has been aided by the reduction of fallow (Brimelow et al. 2011). An analysis that tracks MCSs and looks at changes to convective environments, e.g. Feng et al, (2016), could perhaps show how much the land surface impacts these processes and resolve how changes in land cover and regional circulation processes have impacted the unique climate trends of the NNAGP.

Summary And Conclusions
Summer fallow in the NNAGP has declined from an estimated 151,900 km 2 in the 1980s to 35,100 km 2 in the 2010s, a decline of 116,800 km 2 which is approximately the land area of Pennsylvania. To investigate the climate impacts of this reduction in summer fallow, two three-year convection-permitting WRF simulations were performed, using ERA5 as the initial and lateral boundary conditions. The vegetation fraction of each simulation was adjusted using Landsat-estimated summer fallow and nudged to match published agricultural statistics for 2011 and 1984. The intention of these simulations is to understand how the near surface climate and precipitation processes have been impacted by these substantial changes in land cover. The summary of the results are: Two-meter air temperatures were 1-1.5ºC cooler and VPD was 0.15 kPa lower in areas where fveg increased between the fallow simulation and the control simulation.
The PBL and LCL were lower by 60 m, due to the cooler and more humid land surface.
CAPE increased by 20-30 J kg − 1 but there were minimal changes to CIN.
Precipitation did not change appreciably between the simulations, but the fallow simulation was 10-15 mm drier during July and August.
The results of these simulations suggest that observed near-surface cooling and moistening trends in the NNAGP are largely a result of the reduction in summer fallow. The lack of evidence for a land-surface induced change to precipitation stands in contrast to other observational studies focused on the same region; however, this is the rst modeling study looking at summer fallow reduction. Further work is needed to better understand the precipitation processes, perhaps tracking the evolution of precipitating storm systems as they move over the heterogeneous and changing landscape of the NNAGP.     Monthly differences in T2 between the F11 and C11 WRF simulations (Table 1). Positive values indicate the F11 simulation was warmer.
Page 24/30  The difference in sensible (a) and latent (b) heat ux for MJJA between the F11 simulation and the C11 simulation. Stippling indicates signi cant differences at the 95% level.    Changes to precipitation for May and June ("Early Warm", Row 1) and July and August ("Late Warm", Row 2) for the F11 and C11 WRF simulations (Table 1), their absolute difference (Abs Diff), and percent difference. Precipitation was aggregated to 100 km × 100 km boxes to to display regional trends Figure 10 Vertical cross-section of 1979-2020 May and June meridional wind trends (black contours) and speci c humidity trends ( lled contours) for the levels between 925 hPa to 800 hPa from the ERA5 reanalysis.
Inset axes show trends in meridional moisture transport (qv) for 1979-2020 and the location of the crosssection. Brown contour shows the pressure where topography is located along the cross section.