Agricultural systems are being challenged to decrease water use and increase production while climate becomes more variable and the world's population grows. Low water use efficiency is traditionally characterized by high water use relative to low grain production and usually occurs under dry conditions. However, when a cropping system fails to take advantage of available water during wet conditions, this is also an inefficiency and is often detrimental to the environment. Here, we provide a systems-level definition of water use efficiency (sWUE) that addresses both production and environmental quality goals through incorporating all major system water losses (evapotranspiration, drainage, and runoff). We extensively calibrated and tested the Agricultural Production Systems sIMulator (APSIM) using 6 years of continuous crop and soil measurements in corn- and soybean-based cropping systems in central Iowa, USA. We then used the model to determine water use, loss, and grain production in each system and calculated sWUE in years that experienced drought, flood, or historically average precipitation. Systems water use efficiency was found to be greatest during years with average precipitation. Simulation analysis using 28 years of historical precipitation data, plus the same dataset with ± 15% variation in daily precipitation, showed that in this region, 430 mm of seasonal (planting to harvesting) rainfall resulted in the optimum sWUE for corn, and 317 mm for soybean. Above these precipitation levels, the corn and soybean yields did not increase further, but the water loss from the system via runoff and drainage increased substantially, leading to a high likelihood of soil, nutrient, and pesticide movement from the field to waterways. As the Midwestern United States is predicted to experience more frequent drought and flood, inefficiency of cropping systems water use will also increase. This work provides a framework to concurrently evaluate production and environmental performance of cropping systems.
7 Figures and Tables
Fig. 1 Cumulative annual precipitation at the experimental site for each year (solid line) and the 30-year average cumulative annual precipitation (dashed line).
Table 1 Planting dates and N fertilization inputs. CC, continuous corn; CCW, continuous corn with cover crop; CS, corn–soybean rotation; and SC, soybean- corn rotation
Table 2 Model performance statistics for biomass and yield at harvest. CC is continuous corn, CCW is continuous corn with rye, CS is corn–soybean rotation, and SC is soybean–corn rotation
Fig. 4 Simulated (line) and observed (points) soil water content (mm mm 1) in continuous corn (a–e) and continuous corn with winter rye cover crop (f–j) across five soil depths 5 cm (a, f), 10 cm (b, g), 15 cm (c, h), 35 cm (d, i), and 50 cm (e, j).
Fig. 5 Simulated (line) and observed (points) soil water content (mm mm 1) in corn–soybean (a–e) and soybean–corn (f–j) across five soil depths 5 cm (a, f), 10 cm (b, g), 15 cm (c, h), 35 cm (d, i), and 50 cm (e, j).
Fig. 6 Observed (black) and simulated (gray) subsurface drainage during the growing season. Treatments labeled as corn and soybean are the respective crops from the corn–soybean rotation.
Fig. 7 Physiological (WUE) and systems water use efficiency (sWUE) in corn averaged over all corn treatments and soybean. Error bars represent one standard error of the mean.
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