Dynamics of the Natural Afforestation Process of a Small Lowland Catchment and Its Possible Impact on Runoff Changes

Abstract

Changes in land use have an impact on changes in renewable water resources. Land use especially in the context of urbanization has been and continues to be widely studied. The role of the forests in ecosystems is well known but there is not much research investigating the impact of natural afforestation on water resources. This paper shows the results of the quantitative changes in the use of the lowland catchment in the last 50 years on the example of the Zagożdżonka river located in central Poland. The results show that the impact of climatic factors on the reduction of water resources in the Zagożdżonka catchment is compounded by socio-economic changes in agriculture, and the positive role of forest areas in the catchment depends on the water demand of the stand. The interactions between forest ecosystems and water retention are strong and, depending on the habitat conditions, they may reduce ground runoff and deepen the water deficit in the catchment area in dry periods.

1. Introduction

Accelerated changes in land use, population, and climate are causing spatial and temporal changes in renewable water resources. For a better understanding of these changes and effective managing of water resources, hydro-meteorological investigations in river watersheds have been carried out around the world [1,2]. In Poland, annual sums of precipitation have been already slightly growing; however, these changes are not statistically significant in the entire area [3,4,5,6]. Additionally, there is a clear change in the precipitation structure reported in Poland, with an increase in the number and intensity of torrential rainfall mostly in spring and summer [7]. There are torrential rains, the intensity of which exceeds the design values determined based on multi-annual data [8]. An increasing duration of sunshine and consequent evapotranspiration, especially in early summer, have been observed over the few last decades [4]. Further, studies presented in the literature indicate a higher than ever before temperature rise in the last 30 years, manifested by a significantly increased frequency of the warmest months, seasons, and years [9]. The increase in temperature triggers a series of feedback loops, consisting of an increase in evaporation, condensation of water vapor, and the formation of convective clouds [9].

The analysis of the impact of observed changes on the runoff from catchments requires long series of hydrometric measurements. Multi-annual observations from a small catchment of natural character are of special worth [1,10]. Due to their small spatial scale, factors possibly affecting the hydrological cycle are usually more transparent than in larger watersheds [2]. It is rather difficult to find, in Poland, small catchments with long-lasting observation series. The Zagożdżonka river catchment is one of a few in Poland, with long-term records on rainfall and runoff [11]. In addition to the observation of hydrological factors, information on changes in the use of the catchment was collected. In the 1980s, agricultural land (arable land, pastures) covered about 60% of the catchment area, and forests 40%. Initial observations indicate that over the years, as a result of social and economic changes, the area of grasslands has significantly decreased, while the number of forested areas has increased, accounting for over 60% of the catchment area in 2018 [12]. Therefore, arable lands have been partially left as fallow and thus the ecological succession has started. As a result, a rapid increase in the ratio of the forest areas can be observed during the period 2001–2006 [13]. The reasons can be found in the situation of small farms, which in Europe looks unfavorable. Most of them, in recent years, have either collapsed or lost their production function and now produce only for their own needs. This is because the support system in the European Union is focused primarily on large farms [14]. Studies of hydrological characteristics conducted for many years in this research catchment show a clear process of decreasing the runoff [2,12,15].

Changing the runoff is accompanied by the process of land-use change and social changes. The role of afforestation in the process of changes in the runoff deserves special attention. Due to its high retention capacity, forests regulate the outflow of water from the catchment area by diverting a large part of it to the underground. Afforestation, thus, increases the underground runoff and reduction of surface runoff, and therefore has a significant impact on the formation of high and low flows. The reducing effect of forests on high floods is quite well known, and its beneficial effect occurs mainly in the upper parts of rivers. The importance of the positive role of forests in reducing the risk of floods is important, especially as the frequency and social and economic “costs” of floods are expected to increase in the future, not only due to changes in land use but also due to climate change [16,17]. Although the system of hydro-meteorological hazard forecasts is getting better, catastrophic, deadly floods happen even in very well-developed countries, such as those that took place in Germany and Belgium in July 2021 [18]. In the literature [19], one can often find the opinion that areas with high forest cover, in comparison with the catchments used for agriculture, are characterized by lower runoff, mainly due to the use of large amounts of water by the forest for evaporation. However, the impact of the forest on the amount of runoff depends on many variables, especially on the current hydrological and meteorological conditions, the season of the year, and, above all, on the type of forest habitat, i.e., stand composition and age. The experience presented so far in the literature, on the modeling of hydrological effects caused by climate change on the example of selected large catchments in Poland (Narew, Pilica, Reda, Vistula, and Odra catchments), show that the strongest effects on runoff changes are due to the climatic factor, and land-use changes have a local effect, smaller than that caused by climatic factors [20,21].

The previous study in the Zagożdżonka catchment shows that the estimated contributions of climate and land-use changes to the decrease in mean annual runoff in the studied catchment amount to between 60% and 80% and between 40% and 20%, respectively [13]. However, the impact may be different depending on local conditions. The latest model investigations on African catchments with limited current and historical climate data change [22] suggest that the land-use influence on the surface runoff response is more significant than that of climate. Research in Montana, U.S., could also be an example of the technique to remove the effect of climate change from the total change in the seasonal hydrological drought [23]. Results from the application of Contemporary Watershed Management show that urban/suburban development and agriculture are primary drivers of alterations to watershed hydrology, streamflow regimes, transport of multiple water quality constituents, and stream physical habitat [24].

In terms of forest area, Poland is at the forefront of Europe, where forests currently cover 29.6% of the country’s territory and growing in an area of 9.3 million ha [25]. The majority of them are state forests, managed by the “State Forests” holding, and, as an additional tool in sustainable water management in the country, they provide the potential in supplying groundwater [21]. In Poland, the dominant species is pine (56.5%), and in general, conifers constitute 69%, and deciduous species 31% [26]. It should be emphasized that the positive role of coniferous forests is, unfortunately, smaller than the deciduous ones. Coniferous forests can transpire water into the atmosphere much longer than deciduous ones (the process stops after the leaves fall) and rather use up groundwater reserves than rebuild them. The litter under deciduous trees also has a greater retention capacity than in coniferous forests, which has a better influence on the process of groundwater recharge. The species composition of Polish forests is not diverse enough to be properly prepared for climate change. For example, Scots pine, European larch, and silver birch, covering a total of 75% of the forest area in Poland, are the most sensitive to the global temperature increase and its indirect consequences [27]. Currently, a common phenomenon occurring in many European countries is also the dieback of oaks, probably as a result of changes in water conditions [28], as a consequence of long-term droughts, heatwaves, and the related fluctuations in groundwater levels or sudden and severe frosts [29]. In 2019, the International Union for Conservation of Nature (IUCN), for the first time in the history of this organization (existing since 1948), published the Red Book of European Trees at Risk of Extinction, in which 42% of 454 tree species are “highly endangered” [30]. An important direction of the forest adaptation strategy to environmental (climate) changes, apart from increasing species diversity of stands, age structure, and increasing intra-species genetic variability, is preventing the excessive growth of forest stands [30].

This research aims to show the quantitative changes in the use of the lowland catchment in the last 50 years, taking into account the socio-economic conditions of the country, with the use of available aviation archives, using the example of the Zagożdżonka river located in central Poland. The second aim of the study is an attempt to present the problem of the natural afforestation process of a small lowland catchment against the hydrometric conditions of the period 1963–2018 in the aspect of the observed decrease in a runoff.

2. Materials

2.1. Study Area and Data

Zagożdżonka is the left tributary of the Vistula and flows into it above the Radomka River. The Zagożdżonka river catchment area is located in the Masovian Lowland, and its upper part is the research catchment area of the Department of Water Engineering and Applied Geology at the Warsaw University of Life Sciences. Systematic hydrological measurements began in July 1962 in the Płachty Stare water gauge profile (Figure 1a), which closes the catchment area with an area of 82.4 km². Since 1980, research has also been carried out in the Czarna profile (A = 23.4 km²). In 1991, the modernization of the devastated mill weir was started at the measuring station in Czarna (Figure 1b), which was equipped with devices for automatic hydrometeorological measurements. The system includes sensors installed in the measurement cross-section of the river directly above the overflow and in a test stand for measuring sediment dragging, as well as sensors and measuring devices located in a nearby meteorological garden [12].

2.2. Characteristics of Precipitation, Temperature, and Runoff

Masovian Lowland is one of the most arid regions of the country, with relatively long periods without precipitation and negative values of the climatic water balance. Especially in recent years, in spring and early summer, the values of the climatic water balance (CWB) have been getting lower, which means that the droughts that occur are more and more severe [12]. The analysis of the average annual air temperature at the Puławy meteorological station (Figure 2), located close to the Zagożdżonka river catchment (about 40 km), clearly shows its increase, which corresponds to the results presented for Poland [2].

There is also an increase in the annual evaporation value for the catchment area [13]. However, the evaluation of the long-term series of measurement data (1963–2018) concerning the total annual rainfall in the Zagożdżonka river catchment indicates that, despite the downward tendency (Figure 2), there are no statistically significant changes in their trend [12,31]. Similar conclusions can be found in the publications of other researchers, especially in the work by Czarnecka and Nidzgorska-Lencewicz [3], which included 38 meteorological stations of the Institute of Meteorology and Water Management–National Research Institute, from the area of Poland (excluding the mountainous are), where none of the analyzed precipitation characteristics showed a statistically significant linear trend, even at the level of α = 0.10 in the period 1951–2010. However, in the prevailing area of the country, a slight tendency of an increase in the precipitation in spring and autumn was found, as well as a decreasing share of summer precipitation in the annual total precipitation. The change in precipitation pattern is more pronounced if we consider semi-annual and monthly precipitation totals and relate them to the long-term norm. Years are more common, where extremely dry and very dry months are accompanied by extremely humid and very humid months in terms of the sum of precipitation to the long-term norm [12]. In the Zagożdżonka river catchment area, precipitation is unevenly distributed throughout the hydrological year. The average monthly sums of precipitation in the summer half-year significantly exceed the amount of precipitation in the winter half-year [32], when snow or snowfall occurs. Research on the deposition of snow cover in the Zagożdżonka catchment area conducted since 2003 and shows a visible decrease in the total number of days with snow cover in the winter half-year and a decrease in the number of days with thick cover (>20 cm), wherein during the hydrological years 2004, 2008, 2009, 2012, 2015, 2016, 2017, and 2018, thick snow cover did not appear at all [12].

In the Zagożdżonka river catchment, despite the lack of a statistically significant change in the average total annual rainfall in the years 1963–2018, there is a clear process of decreasing the runoff from the catchment (Figure 2) at Płachty Stare gauge. Both the low flows and average flows are decreasing, and these changes are statistically significant. The analysis of the number of days with low and high flows in the multi-year period 1963–2018 in Płachty Stare clearly shows two opposite, statistically significant trends: a decreasing number of days with high flows and an increase in the number of days with low flows. Answers to the question about the causes of the observed decrease in the value of the runoff from the Zagożdżonka catchment area, which is disturbing for the environment and agriculture, should be sought in the changes resulting from the increase in temperature, i.e., in the change in the structure of rainfall and the increased amount of evaporation, as well as in warm and little snowy winters in recent years and the influence of human pressure. The studies conducted so far in the catchment area indicate that the decreasing trend of renewable resources observed in the Zagożdżonka river catchment, manifested by a decrease in the runoff by 0.924 mm per year, which corresponds to a decrease in the average unit runoff by 0.029 l/s/km²/year, as well as lower than average runoff, is mainly attributed to climatic factors. However, the change in the land use in the catchment during the last 30 years, where it increased in the number of forested areas of the catchment at the abandoned agricultural land, may also be significant [20]. Research on the precipitation and extreme flows occurring in the period 1963–2018 indicate that in the Zagożdżonka catchment, as well as in a large area of the country, three periods can be distinguished with the measures. The first period is 1963–1969, when dry and average years prevailed over wet years, and long-term low-flow periods appeared in the river. The second period 1970–1981, when wet and very wet years prevailed over the average, and the river had the most numerous flood flows in the whole multiannual period. The last, third period is 1982–2018, in which half of the years were characterized by average annual rainfall, and another 40% were dry and very dry years, especially at the turn of the 1980s and 1990s and the first decade of the 21st century, while wet flows did not reach the values observed in the 1970s. In the last, third analyzed period, 1982–2018, low flows occured in almost every summer period, except for three years: 1997, 1998, and 2014 [32]. The average annual size of the runoff determined for the last period 1982–2018, at 87 mm, is lower than the long-term average (1963–2018) of 107 mm. Summing up, from the beginning of the observation period in the hydrological years 1963–2018, in the Zagożdżonka river catchment up to the Płachty Stare gauge, a statistically significant increase was observed in the number of days with low flows and a decrease in the number of days with flood flows, accompanied by an increase in air temperature and evaporation, as well as a change in the structure of rainfall.

3. Methods

3.1. Changes in the Land Use of the Catchment

The analysis of land-use changes in the Zagożdżonka river catchment was carried out for the period 1970–2018. The land use of the catchment was established for four years: 1970, 1990, 2000, and 2018. To continue the long-term research carried out in the catchment, its boundaries were maintained in accordance with those originally designated, when the first research in the Zagożdżonka river basin began, in the early 1960s, based on the available topographic maps and field reconnaissance. For the catchment area, the oldest aerial imagery, which covered the catchment in its entirety and was of satisfactory resolution, comes mostly from 1970 (a small part of the catchment area is included in the photo from 1973). Nine black-and-white aerial photographs were used for the Zagożdżonka river catchment to Płachty Stare gauge. The boundaries of the 2018 map divisions were imposed on the analyzed aerial photos from 1970. The analysis of aerial photos was performed using the Arc-Gis 10.5.1 software. Previously, the photos were rectified based on topographic maps and vectorized using polygons developed for 2018, which were modified according to the content visible in the aerial photos.

For 1990, 2000, and 2018, information from the CORINE Land Cover 1990 (CLC1990), 2000 (CLC2000), and 2018 (CLC2018) databases were used [33]. The base map was a map created based on separations from the CLC2018 geodatabase detailed by a photo of information about changes from the current orthophotomap of the area of Poland available through the WMS service of the national geoportal (https://www.geoportal.gov.pl/ (accessed on 10 May 2021)) [34]. The 2018 land-use map was saved in the spatial reference system PL_EUREF89/1992 (EUREF-POL) with the following parameters: ellipsoid: GRS80, longer axis of the ellipsoid: 6378137, projection: Transverse Mercator, units: meters, central meridian: 19.000000, coordinates of the origin of the system: x = 500,000; y = −5,300,000, scale variation on the axial meridian: 0.9993.

There are five main categories of land-use/cover, mostly consistent with the five main types of terrestrial cover (the first level of land cover classes distinguished in the CORINE Land Cover program): anthropogenic areas (in the case of this catchment, limited to farm buildings); land cover agricultural (additionally divided into two categories: arable land and permanent grassland); forests and semi-natural ecosystems (in the case of the described catchment, the category limited to forests); water areas (limited to surface hydrographic objects such as ponds). Hence, ultimately, changes in the land use of the Zagożdżonka river catchment are presented in 5 categories: forests, ponds, grasslands, farm building, and arable lands.

3.2. The Age Structure of Stands

On the 2018 map, two subcategories were separated from the “forests” category. These are: young tree stands as a succession in unused agricultural areas and young trees as renewal of the existing forest (Figure 3). Information on the age structure of the forest and tree species was obtained from the Geoportal of the State Forests, as of 2016 (https://www.bdl.lasy.gov.pl/portal/mapy (accessed on 10 May 2021)) [35], for the catchment to Płachty Stare gauge.

3.3. Changes in the Type of Buildings

Information about the number of residential buildings in the catchment was also collected. Three periods were taken into consideration: 1970, 2000, and 2018. Information about buildings in 2018 was downloaded from the BDOT10K topographic database in the form of a WMS service provided by the national infrastructure geoportal (geoportal.gov.pl (accessed on 8 May 2021)) and updated with information from the current orthophotomap of the national geoportal [34] and the Google Earth service (https://earth.google.com/ (accessed on 8 May 2021)) [36]. A map layer of buildings from 1970 was created based on recorded archival aerial photos.

3.4. Changes in Farms

GUS (Statistics Poland) data obtained from the Local Data Bank (stat.gov.pl (accessed on 11 May 2021)) [37,38,39] were used to analyze socio-economic changes in the catchment area. The information for the Pionki commune was used because it covers the largest area of the catchment. The available information was taken from the national censuses of 2002, 2011, and the agricultural census of 1996.

4. Results and Discussion

4.1. Changes of the Catchment Land Use in the Context of Socio-Economic Changes in the Country

In 1970, the basic form of land use of the Zagożdżonka river catchment was agriculture—arable land and meadows accounted for over 50% of the catchment area to Płachty Stare gauge, and over 60% in the Czarna profile catchment area (

Table 1. Structure of land use in the Zagożdżonka catchment in years: 1970, 1990, 2000, and 2018.

Land-Use Type	Surface [%]
	1970	1990	2000	2018
Up to Płachty Stare gauge
Arable lands	42	32	28	25
Grasslands	10	16	20	11
Forests and afforestations	45	49	49	61
Farm building	2	2	2	2
Ponds	1	1	1	1
Up to Czarna gauge
Arable lands	58		37	30
Grasslands	10		27	31
Forests and afforestations	29		32	35
Farm building	2		3	3
Ponds	1		1	1

, Figure 4).

At that time, meadows appeared along the valley of the watercourses and sporadically in in-field depressions. A significant part of the grasslands had been drained a few years earlier in the period 1962–1965 [40]. Forest and wooded areas covered 45%. Some of them are the complex of Kozienice Landscape Park (area under protection).

In 1970, the catchment area was typically agricultural. The water users included agricultural irrigation and municipal management (water supply to farms and public utilities. In the following years 1990 and 2000, the share of arable land in the catchment to Płachty Stare gauge, compared to 1970, decreased from 42% to 32% in 1990 and 28% in 2000, respectively, while the forest cover of the catchment increased by 10% (Table 1). There were also more grasslands and built-up areas. This period coincided with the systemic changes that took place in Poland after 1989 and contributed significantly to ownership transformations and changes in the structure of land use [41]. Similar directions of changes in the structure of land use to those observed in the Zagożdżonka river catchment area were also visible in other parts of the country [42], where a change from the dominance of field crops to meadow use with a large share of sodded land not used for agricultural purposes was found. In the Zagożdżonka catchment, the area of arable land in 1970 decreased by 1/3 compared to 2000, while the area of meadows increased almost threefold from 9.8% in 1970 to 29.9% in 2000 (Table 1).

After almost two decades, in 2018 there was another significant change in the land use of the Zagożdżonka river catchment (Table 1). In the period 2000–2018, the catchment area occupied by grasslands decreased significantly from 20 to 11%, which means a decrease in their area by almost half in 2018, compared to 2000. Area of arable land also decreased, but on a smaller scale, within 10%. In the Zagożdżonka river catchment to Czarna gauge, the observed changes were not of such intensity. In 2018 (Figure 5), it was observed that the area of arable land and forests accounted for a similar size of about 30% of the catchment area, resulting from the process of decreasing the area of arable land and the increase in forest and grassland areas. Comparing the use of the catchment in 1970 with the available archival topographic map of the Kingdom of Poland from 1839 [43], it can be assumed that the dynamics of changes in forest areas were small, and the changes concerned the deforestation of forests for new grasslands. The location of forest areas within the catchment area was stable and concerned private forests typical of this region.

The changes observed in the period 2000–2018 reflect the socio-economic situation in Poland, resulting, inter alia, from production limitations related to Poland’s accession to the European Union (1 May 2014), the necessity of fierce competition on the market of agricultural products, and resulting from the poor economic condition agricultural holdings [44]. The observed reduction in the area of arable land can be explained by the consequence of the poor economic profitability of agricultural production from traditional plants. On the other hand, a significant decrease in the area of grasslands can be explained by the process of extensification of meadows on a national scale, caused mainly by the concentration and specialization of farms in milk production, where hay is no longer the basic fodder, but only one of the components [44,45]. At the same time, the agri-environmental programs implemented in the country aimed at the protection of nature on grasslands, are, in the economic sense, area payments that promote further extensification of meadow production [44].

The research on the structure of farms in Poland, carried out for many years by the Central Statistical Office, shows the directions of changes taking place in Polish agriculture, caused by the implementation of the Common Agricultural Policy, as well as changes in the economic situation in global and domestic agriculture, which were influenced, among others, by the biological nature of agricultural production and agrometeorological conditions, changes in the eating habits of the population, the transition to modern ways of feeding livestock, as well as the search for new sources of energy. The processes of modernization and specialization of farms, as well as concentration of production, are becoming more and more visible, especially in regions with good production conditions. At the same time, unprofitable small farms are being liquidated, with extensification of production, most often conducted only for household needs, or even its complete abandonment [38]. Adaptation of farms to the EU requirements often requires expenditure and it is usually easier to do in large farms, which already achieve quite good financial and economic results than in small farms that are in fact on the verge of survival and try to survive on the market rather than crash and go out of business [14]. The observed constant tendency to decrease the number of farms is also related to the decrease in the number of farmers and spouses working on individual farms. There is also a significant decrease in the number of other family members by 1/3 contributing work to the family farm, mainly due to the abandonment of agriculture. In 2006, there were 1.8 million farms in Poland, in 2016 their number was 1.4 million, of which 99.7% were individual farms. The highest dynamics of decline were recorded in the group of the smallest and least numerous farms (i.e., with an area of up to 1 ha of agricultural land inclusive) [38]. The situation of small farms looks bad not only in Poland, but also in Europe in general. Most of them have been either bankrupt in recent years or have lost their production function and they produce only for their own needs. This is because the support system in the European Union is focused mainly on large farms, although small farms, apart from area payments, can also participate in agri-environmental programs and thus increase their income using European payments. Still, these payments are treated rather as social rent, and not as the creation of certain production and development potential by these farms. Basic European support is horizontal, i.e., all farmers in the European Union, depending on their cultivation area and the area in which they produce, receive area support. On the other hand, what different countries do is that, under other programs, they may grant larger or smaller funds, e.g., for supporting the modernization of agricultural production or for adapting agricultural production to environmental needs under agri-environmental and climate programs. In Poland, however, area support is still of key importance, depending on the size of the farm, and not how the farmer manages and provides ecological services, protecting water resources, climate, and biodiversity [14].

The effects of the processes presented above are visible in the Zagożdżonka river catchment, where small-scale farms, typical for Poland, occur (Figure 6). In the catchment to Płachty Stare gauge, the area occupied by arable land and grasslands has decreased from 50% to about 36% over 50 years. When analyzing the information on the number of farms available in the Central Statistical Office, for the Pionki commune, which covers the administrative part of the catchment area up to the Płachty Stare gauge, a 30% decrease can be observed, from the number of 2006 in 1996 to 1416 in 2010. It should be noted that in the census in 2002, the number of farms amounted to 2,139. The liquidation of small farms in the Zagożdżonka river catchment led to a spontaneous forest succession of abandoned agricultural areas. According to data from the Central Statistical Office of Poland, from 2000 to 2015, in the area of the Pionki commune, the area of private forested land approximately doubled.

Another observed change in the use of the Zagożdżonka river catchment area is the development of dispersed, suburban buildings. Public interest in the issue of land as land intended for housing construction is observed throughout the country along with economic development [41]. In the Zagożdżonka river catchment, between the extreme years included in the analysis, i.e., in the period of 50 years, more than 200 residential buildings, which are not farms, were added (

Table 2. Change in the number of residential buildings in the Zagożdżonka river catchment in 1970–2018 [own study based on: Database of Topographic Objects updated with information obtained from archival aerial photos and the current orthophoto map].

Year	To Płachty Stare Gauge	To Czarna Gauge
	Number of Households
2018	1304	586
2000	1176	524
1970	1074	463

).

In general, in Poland, the difference between the inflow and outflow of people from the countryside is positive, migration of people to the countryside for permanent residence is observed. In the rural commune of Pionki in the years 2002–2019, the number of inhabitants increased by 1.6% [39].

4.2. The Dynamics of Changes in the Forested Areas in the Zagożdżonka River Catchment

The biggest change in the land use of the Zagożdżonka river catchment to Płachty Stare gauge in the last 50 years, is the decline in agricultural land in favor of forest areas. In 1970, forests and trees accounted for 45% of the catchment area up to the Płachty Stare gauge and 30% of the catchment area up to the Czarna gauge. In the next analyzed year, 1990, the increase in forest areas in the catchment to Płachty Stare gauge in relation to 1970 was small—about 8%; in 2000 the forest area did not change compared to 1990 and covered about 50% of the catchment area. On the other hand, a clear change in the size of forest areas in the catchment area took place over the next 20 years. In 2018, the area of forests and trees in the catchment area to Płachty Stare gauge was 60% (Figure 7). There was a dynamic increase in the area under consideration of over 20% in the period 2000–2018. Comparing the extreme observation years of 1970 and 2018, the area of forests and trees stands with 45% covered 60% of the catchment area (Figure 7). In the case of the Zagożdżonka river catchment to Czarna gauge, this change was not significant and between 2000 and 2018 the increase in forest areas and trees was several percent, while from 1970 to 2018 the area of forests and trees increased from 29% to 35%.

When examining the nature of the growing forest areas, two subcategories on the maps from 1970 and 2018 were distinguished from the “forests” category. These are: young tree stands as a succession on unused agricultural land and young tree stands as renewal of the existing forest (Figure 5). In the catchment up to the Płachty Stare gauge, 3.4% of agricultural land transformed into forests, either on purpose or in the process of natural succession after the cessation of agriculture, appeared on agricultural land in 2018 (Figure 5—red color), as a result of changing economic conditions in agriculture during the period considered. The renewal of forest areas covered with the young forest was also observed in the amount of 2.6% (Figure 5—dark green), which, together with the areas covered by succession or restoration of forest areas after logging, constitutes 6% of the forest area with young, very water-consuming stands (Figure 7—dark green).

The commentary should explain the area in the northern part of the catchment area on the maps of changes in management covers the forest (as of 1990), although on the maps of the forest species structure this place seems to be devoid of a tree stand. This state of affairs is such since these are not areas belonging to the state forests or private forests. These are agricultural areas where their use was discontinued very early (forest was marked here in the 1990s) and they were not separated in the category of spontaneous succession, which was the result of socio-economic changes at the turn of the century.

4.3. Changes in the Structure of the Stand

The dominant tree species in the Zagożdżonka river catchment area in forest areas belonging to the Holding “State Forests” and private forests is pine, with the remaining species accounting for about 13% (Figure 8). Considering the age of the pine stand, in 2016, based on the available data from the State Forests [35] for the area in question, it appears that about 33% of them are aged 1 to 50 years, while the rest are older (the most numerous pine age range is 51–75 years—35%), and only a few (4%) exceed the age of 100.

It was also found that new forest areas in unused agricultural areas in the catchment area of the Zagożdżonka river are overgrown with conifers, and it can be assumed that self-sowing pine also dominates there. In Poland, for several years now, pine (Pinus sylvestris) has been a more and more frequently naturally renewed species in forest practice, and in self-seeding of this species, the possibility of increasing the area of natural regeneration is seen [46]. The map of the structure of tree species in the area of the State and private forests from 2016 (Figure 8) does not show the forest areas in the northern part of the catchment area, although this area has already been recognized as a forest on the 1990 map (Figure 7). This state of affairs is due to the fact that these are not areas belonging to the State Forests or private forests. These are agricultural areas where agricultural use was abandoned earlier, before the period of socio-economic changes resulting from the country’s political transformation and accession to the EU. These areas were not separated in the “spontaneous succession” category on the 2018 map, since it occurred earlier than in the last decades.

4.4. Forests and Runoff from the Catchment Area

In the analyzed period of 1963–2018, an intensive afforestation process was found in the catchment after the year 2000. There are two issues related to the observed phenomenon: a young stand is a significant water consumer, which needs more water to grow than a mature stand, and, due to the larger vegetation area, 15–25% more rainwater evaporates from the forest than from the agricultural area [47]. The forest uses more water than other crops with a shorter growing season, which results from greater interception of forest areas and higher transpiration, made possible by deeper rooting [48]. Water requirements depend not only on the age but also on the species composition of the stand. Coniferous species, mainly pine, dominate in the forests of the Zagożdżonka river catchment area. According to the research of the Forest Research Institute carried out in the 1960s, it appears that the greatest water needs of pine stands, growing on sandy soils commonly found in the catchment area, occur in the phase of height gain culminating in a 30-year-old stand [47]. Air temperature should also be taken into account, as climate warming increases the productivity of forest habitats, which affects the intensity of evapotranspiration [49]. However, the factors influencing the outflow in periods of drought are complex and, depending on local conditions, forests may mitigate or aggravate the effects of drought [48,50]. Increased water consumption by trees may reduce low flows, but much depends on soil type and geology. Permeable areas are the most sensitive, while flow reduction will be small in impermeable areas with poorly structured soils. In this case, afforestation improves soil infiltration, leading to the discharge of more rainfall to the deeper layers of the soil profile and the replenishment of low flows [48]. Forest soils have a higher water content, retaining water during drought, which slows down and reduces ground runoff [51]. It can therefore be assumed that the observed significant increase in pine forest areas along with increasing air temperature affects the increase in water needs of stands in the Zagożdżonka river catchment, which use groundwater resources, and this, in turn, reduces the runoff, which is more intensive than rainfall (no statistically significant downward trend). The degree of the assumed impact, however, requires separate studies, taking into account the state of the groundwater table, analysis of groundwater abstraction for municipal and agricultural purposes (irrigation, filling of ponds) in the Zagożdżonka catchment area, and soil and geological conditions.

5. Conclusions

Long-term research conducted in a small catchment area of central Poland presents, on a local scale, a picture of changes taking place in water resources as a result of the cumulative effects of climate change and human pressure. In the period 1963–2018, in the Zagożdżonka river catchment, several changes related to meteorological characteristics and socioeconomic changes were noted, which influenced the state of renewable water resources in the study area. The conducted analyzes show the role of the processes taking place in the society of the agricultural environment on the shaping of the decrease in the amount of outflow from the catchment area. In most of Poland’s area, the atmospheric drought of 1982 and the hydrological drought of 1983 began a permanent dry period, interrupted only by short periods with amounts of rainfall greater than normal (average amounts). In the Zagożdżonka river catchment, numerous dry periods are accompanied by: an increase in air temperature, evaporation, and visible change in the annual precipitation structure in recent years, with the annual rainfall total being close to the norm, as well as warm and little snow winters. After 2000, a dynamic increase in forest areas in the catchment was observed, covered mainly with young pine trees, which needed much more water than mature stands (valuable for the climate due to the possibility of CO₂ storage), caused by a spontaneous succession of agricultural land unsuitable for agricultural production, as a result of changes taking place in Polish agriculture. The generally observed increase in evaporation caused by the temperature increase and the increased biomass production of the young stand in the former grasslands negatively affects the outflow from the catchment area. The presented results show that the negative impact of climatic factors on the reduction of water resources in the Zagożdżonka catchment is compounded by socio-economic changes in agriculture, and the positive role of forest areas in the catchment depends on the water demand of the stand. The interactions between forest ecosystems and water resources are strong and, depending on the habitat conditions, they may reduce ground runoff and deepen the water deficit in the catchment area in dry periods. It has to be underlined also that the presented results were obtained at the particular area and should be treated as a case study. Future analyses should also consider groundwater resources.