*3.7. Influence of Hydrothermal Conditions on the Formation of Tuber Yield*

The impact of these conditions was similar to that of the rainfall in April–May. From the regression equation described by the formula in Figure 9 results, it can be observed that thermal and precipitation conditions negatively influenced the amount of potato yield. With the increase in the value of the hydrothermal index, the value of the yield decreased systematically. Of all the meteorological elements described by the coefficient of determination, the hydrothermal index turned out to be the most reliable (R<sup>2</sup> = 87.72%).

**Figure 9.** Partial dependence of the potato tuber yield on the value of the hydrothermal coefficient in April–May.

In Figure 10, the values of changes in precipitation are correlated with the average air temperature. Regression analysis showed that these data accounted for 38–49% of the variability in rainfall levels. The diagram also shows the correlation between the yield and meteorological elements in individual localities. The diagram presents selected meteorological parameters, and Student's *t*-test showed that rainfall during the growing season was the most statistically significantly (*p* ≤0.01) correlated with the air temperature in June–July.

**Figure 10.** The relationship between fluctuations in rainfall level and various meteorological parameters. Y—yield; X1—temperature of the April–may; X2—temperature of June–July; X3—temperature of August– September; X4—rainfalls of April–September; X5—indicators of hydrothermal of April–September.
