**2. Materials and Methods**

#### *2.1. Study Area*

The study area is located in the temperate continental climate zone, and is a forested ravine–gully system surrounding the valley of a small river (Bytytsia river, right tributary of the Psel river, Dnieper basin) [38,39]. The studied valley of a branched ravine is situated in the woodland area—Vakalivschyna tract (wet oak forest, 150 m a.s.l., coordinates of the section of the ravine—51◦02 353 N, 34◦55 266 E and 51◦02 249 N, 34◦55 591 E), 22 km north of Sumy city (northeastern Ukraine).

In the early 90s of the 20th century, the valley of the ravine was swampy. During this period, the high humidity determined the microclimatic conditions in this ravine. The stream was maintained in spring during snowmelt and during rainy periods. The stream did not dry up during the year as it filled up due to infiltration of water from the forested watersheds along both slopes of the gully. Beginning in the 2000s, an increase in average temperatures in the region was accompanied by a decrease in the water content in the study area due to earlier melting of snow and a reduction of rainy

periods. As a result of the drought period, in summer, the stream dried up, resuming in spring during the snowmelt and in autumn during the rainy season.

This paper analyzed the data for two periods differing in climatic conditions: wet (1993, 1994) and drought (2009, 2010); the interval between which is 14 years. During this period in Ukraine, average temperatures steadily increased. The mean annual temperatures in the wet period were −4 ◦C in January and 25 ◦C in July; in the dry period, they were −3 ◦C in January and 31 ◦C in July [40]. The wet two-year period was characterized by a large amount of annual precipitation—from 1392 to 1452 mm, while the second period (also a two-year period) was dry, and compared to the first one, was characterized by half the amount of annual precipitation—from 610 to 584 mm [41] (Figure 1).

**Figure 1.** Average monthly precipitation during the two-year wet (1993–1994) and two-year drought (2009–2010) periods in the study area.
