Climate Change, Global Warming, and Agricultural Droughts - Monitoring and Predicting Agricultural Drought: A Global Study - page 435

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Figure 34.1 The historical variability in wheat yields in Colorado, North Dakota, and
Ka nsas, USA. (a) Annual variability in yield (metric tons per ha) and its trend (solid line), (b)
yie ld anomalies from five-year means (solid line); two horizontal dashed lines show the 15%
yield anomalies; (c) S -aridity index and its five-year means (solid line), and (d) hydrothermal
coefficient (HTC) and its five-year means (solid line); dashed line shows the HTC normal.

Europe

Figure 34.2 shows wheat yield anomalies for Bulgaria for 1960-2001 after
removing the linear (assumed) technological trend (i.e, the yield increase
on account of fertilizers, mechanization, and the introduction of high-
yield varieties). Such trends were first developed for the United States and
the former Soviet Union (Menzhulin and Nikolayev, 1987; Menzhulin
et al., 1987). For this purpose fertilizer used (kg/ha), combine harvesters