*3.3. Potato Harvest Variability*

The evaluation of the yield, made on the basis of the actual results obtained from WUSP [2000–2019], indicates a systematic decrease in potato yields in the analyzed period. Large differences in the value of this feature between consecutive seasons resulted from changes in the cultivation area and the size of the potato yield. The regression analysis of the value of this feature showed a third-degree relationship with the years of research (Figure 4). The highest value characterizing the potato harvest was recorded in 2001, and the lowest was recorded in 2016. This indicates a clear downward trend in potato harvest over the last 20 years. The coefficient of determination of this equation (R2 = 91.56%) indicates its very high reliability [32].

**Figure 4.** Changes in the total potato harvest in the southeastern part of Poland in 2000–2019.
