Observation of irrigation‐induced climate change in the Midwest United States

Abstract

Abbreviations: DTR, diurnal temperature range; e a , vapor pressure; ET, evapotranspiration; RET, reference evapotranspiration; T max , daily maximum temperature; T min , daily minimum temperature; VPD, vapor pressure deficit; WCS, Wisconsin Central Sands. AbstractIrrigated agriculture alters near-surface temperature and humidity, which may mask global climate change at the regional scale. However, observational studies of irrigation-induced climate change are lacking in temperate, humid regions throughout North America and Europe. Despite unknown climate impacts, irrigated agriculture is expanding in the Midwest United States, where unconfined aquifers provide groundwater to support crop production on coarse soils. This is the first study in the Midwest United States to observe and quantify differences in regional climate associated with irrigated agricultural conversion from forests and rainfed agriculture. To this end, we established a 60 km transect consisting of 28 stations across varying land uses and monitored surface air temperature and relative humidity for 31 months in the Wisconsin Central Sands region. We used a novel approach to quantify irrigated land use in both space and time with a database containing monthly groundwater withdrawal estimates by parcel for the state of Wisconsin. Irrigated agriculture decreased maximum temperatures and increased minimum temperatures, thus shrinking the diurnal temperature range (DTR) by an average of 3°C. Irrigated agriculture also decreased the vapor pressure deficit (VPD) by an average of 0.10 kPa. Irrigated agriculture significantly decreased evaporative demand for 25% and 66% of study days compared to rainfed agriculture and forest, respectively. Differences in VPD across the land-use gradient were highest (0.21 kPa) during the peak of the growing season, while differences in DTR were comparable year-round. Interannual variability in temperature had greater impacts on differences in DTR and VPD across the land-use gradient than interannual variability in precipitation. These regional climate changes must be considered together with increased greenhouse gas emissions, changes to groundwater quality, and surface water degradation when evaluating the costs and benefits of groundwater-sourced irrigation expansion in the Midwest United States and similar regions around the world. K E Y W O R D S agriculture, climate, groundwater, irrigation, land use, Midwest United States, temperature, vapor pressure deficit | 3473 NOCCO et al.

Contrasting long-term temperature trends reveal minor changes in projected potential evapotranspiration in the US Midwest

“…A body of evidence has emerged in support of the latter explanation [39][40][41][42] , with the hypothesized link being an evaporative cooling effect (i.e., latent heat) as a consequence of elevated rates of ET due to denser canopies 11,40 and increased irrigation [43][44][45][46] . Agricultural management changes throughout the second half of the 20th century, such as increased plant populations, earlier planting, and fertilizer additions, have indeed resulted in prolonged periods with full canopy closure (i.e., leaf area index >3) at which ET would be expected to be near to PET if sufficient soil moisture is available 11 .…”

Global assessment of the impact of irrigation on land surface temperature

“…S7-S11 online). Our findings highlight the broad range of the impact of irrigation on local climate across the globe, especially in arid regions, where the irrigationinduced cooling benefits may surpass the warming pressure caused by urbanization [10][11][12] and/or greenhouse gas emissions [5,6,8]. However, besides local climate, irrigation may also have an impact on remote regions through land-atmosphere interactions [13,14], which needs to be further investigated by combining multi-source datasets and coupling observational and modeling methods.…”

“…Modeling results are effective in presenting mechanistic understandings of the effects of irrigation on climate, but show high uncertainties in the sign, magnitude, and spatial distribution of the predicted effects, due to their heavy dependence on the model's structure and parameterization [3,4]. Observations from in-situ measurements (e.g., weather stations and field experiments) can provide local reliable evidence to verify the model results [5]. However, the previous observational studies have been mostly restricted to local regions [6][7][8], because of the insufficient coverage of in-situ measurements, and it is still unclear as to what extent this observed local evidence can be extrapolated to larger areas.…”

Quantifying irrigation cooling benefits to maize yield in the US Midwest

“…The cooling effect is greater in reducing maximum temperature (Bonfils & Lobell, ) and becomes more pronounced during hot days, where irrigation decreases annual maximum temperature by −0.78 K, a four times larger effect than on mean temperature owing to increased irrigation application (Thiery et al, ). Since crop yield is highly sensitive to high temperature and vapor pressure deficit (Lobell et al, ), this cooling effect benefits crops through reducing heat stress (Siebert, Ewert, Rezaei, Kage, & Graß, ; Siebert et al, ) and evaporative demand (Nocco, Smail, & Kucharik, ), and therefore mitigates the impacts of extreme heat (Vogel et al, ). In particular, irrigation cooling can shift the high temperature thresholds of crops beyond which yield declines so that crops become less susceptible to extreme weather (Carter, Melkonian, Riha, & Shaw, ; Lobell et al, ; Schlenker & Roberts, ; Troy et al, ).…”