Abstract
Long-term tendencies in annual, seasonal, and monthly (March) precipitation, evapotranspiration, and air- and dew-point temperature values were correlated with county-level changes in irrigated area across Nebraska over the 1979–2015 period. A statistically significant linear relationship (slope of − 1.65 ± 0.33 mm decade−1 per % decadal change in irrigated area, with a correlation coefficient of − 0.47) was found between the trends in annual precipitation and irrigated land area. Precipitation dropped by 1 mm annually if the level in irrigation expansion reached about 8% per decade, while the rest of the state enjoyed about a half-millimeter overall increase in annual precipitation rates. The drop was not evenly distributed within the year: the largest decrease took place in the spring, followed by autumn, while the winter and summer months experienced a slight precipitation increase independent of land use. In contrast to what was reported for Eastern Africa by Alter et al. (Nat Geosci 8:763–767. https://doi.org/10.1038/ngeo2514, 2015), the evaporation-enhanced colder land surface of the irrigated fields stabilizes the overlying atmosphere most effectively not in the summer months when precipitation is most abundant in Nebraska, but rather in early spring and fall. The observed precipitation suppression of irrigation only works at the regional scale; it does not apply to irrigated land that is not an integral part of the more-or-less continuous irrigated land region of east-central Nebraska.
Highlights
Irrigated agriculture represents 20% of the global cultivated land area, but contributes 40% of the total worldwide food production (FAO 2016)
The aim of the present study is to provide further support for the large-scale irrigation expansion versus precipitation suppression feedback mechanism, as was first proposed by Szilagyi (2018a) for Nebraska, unaware of the above studies with similar findings
Decadal changes in irrigated land area were derived on a county basis from linear tendencies of the countywide irrigated areas over the available 1978–2012 period (Fig. 2) and assumed valid for 1979–2015
Summary
Irrigated agriculture represents 20% of the global cultivated land area, but contributes 40% of the total worldwide food production (FAO 2016). Large-scale longsustained irrigation has led to several environmental problems, such as salinization (0.37 million km worldwide), groundwater-level and streamflow reductions, seawater intrusion in coastal regions, and the dramatic shrinking and breaking up of the Aral Sea into several disconnected water bodies in Central Asia (FAO 2016) with its entailing public health and environmental disaster (Waehler and Dietrichs 2017). Due to the ongoing climate change and the resulting increased variability in weather extremes (such as flooding and droughts, often in the same area and year), a still growing worldwide population will depend even more strongly on large-scale irrigation developments which raises the issue of long-term sustainability. From all the different potential dimensions (energy and water-use efficiency, productivity, and water quality), this study is about to focus on the most unexpected and least trivial one: the suppression of local precipitation by large-scale irrigation
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