The knowledge of runoff conditions is of great importance for decision-making in river basins. In the context of global climate change, this knowledge is essential to support and substantiate the formulation and implementation of public policies with an emphasis on the management and conservation of ecosystems (Avanzi; Borges; Carvalho, 2009; Santos; Vajapeyam; Srinivasan, 2007). From this perspective, it can be seen that in both national and international literature, the Curve Number method is one of the most widely used to estimate potential runoff conditions from precipitation events, given the different land cover and land use scenarios, as well as the different types and conditions of land use and cover that characterize landscapes in the regional context (Lal; Mishra; Kumar, 2019).

The Curve Number method, also known in the literature as the Curve Number (CN) Method, was developed and presented by Soil Conservation Service (SCS) of the United States of America (USA) in the 1930s, based on a great need for knowledge of hydrological processes, especially those associated with surface runoff and water storage capacity in small and medium-sized watersheds. In this context, the purpose was precisely to support managers and decision-makers in the conservation of soil and water resources. This method, characterized as being conceptual and empirical, is relatively simple to implement with just one CN parameter, making it the most widely used method for calculating runoff potential based on rainfall events (Araújo Neto et al., 2012; Carvalho; Rodrigues, 2021; Hawkins, 1993; Oliveira et al., 2022).

According to Anjinho et al. (2018), the dimensionless parameter CN ranges from 0 to 100, based on the assumption that values close to one hundred (100) represent the boundary conditions of a completely impermeable watershed, in other words, a watershed with an equal and/or practically zero water retention rate. On the other hand, the assumption for values close to zero (0) is that these areas indicate the opposite conditions: a high rate of water retention in the respective watershed, thus representing very permeable watersheds, understood that surface runoff is significantly low, regardless of the amount of accumulated rainfall recorded. Specifically, the methodology it is used to convert the volume of precipitation into the volume of surface runoff by applying the Equation 1 (Rezende; Ribeiro; Mendes, 2018; Soil Conservation Service, 1972):

Where:

Q – Volume of rainwater runoff (mm)

P – Volume of precipitation (mm)

CN – Curve Number (dimensionless).

Given these aspects, the main objective of this study is to estimate and spatialize the values inherent to the CN parameter in different land uses and land cover conditions in a watershed located in the Semiarid Region of Brazil. Specifically, the area defined for the study comprises the Riacho da Consulta watershed (Figure 1). The justification for choosing this area is based on the fact that it is a watershed that has still been studied very little, and that it is a watershed where, although a significant part of it (83%) has natural vegetation cover (Caatinga vegetation), there is a strong occurrence of erosion processes, causing many of its continuous landscapes with natural vegetation cover, gradually, degraded and abandoned (Silva, 2023).

In the Semiarid Region of Brazil, especially in the Caatinga Biogeographic System, scientific studies about the dynamics of water resources, landscapes and the processes that lead to their degradation are extremely relevant to society as a whole, given that environmental degradation in this region of Brazil is one of the main challenges to be overcome. In this perspective, it is important to consider the challenges posed by current and future global climate change scenarios, with a significant increase in areas at high risk of desertification, significant changes in the dynamics of ecosystems and ecosystem services, the impacts on the maintenance of life and biodiversity, on the economy and, consequently, on the way of life and culture, and quality of life of the human beings who inhabit this region that, historically, has lived with various socio-economic challenges.

On Figure 2 it is presented the maps for altitude, slope, land use and land cover, and the different soil classes that occur in the context of the study area, Riacho da Consulta watershed. With specific regard to pedology, five (5) soil classes can be observed throughout the Riacho da Consulta watershed: Latossolos Vermelhos and Neossolos Flúvicos, both classified in hydrological group B, totaling approximately 380 hectares, 85% of the watershed’s total area; Organossolos Háplicos, classified in hydrological group C, comprising around 1.84 hectares, 0.4% of the study area; Neossolos Regolíticos and Neossolos Quartzarênicos, falling into hydrological group D, occupying around 62.73 hectares, 14.6% of the total area of the Riacho da Consulta watershed.

The most part (78.6%) of the study area is characterized, pedologically, by the occurrence of: Latossolos Vermelhos, well-developed, deep and drained soils; Neossolos Regolíticos (8.6%), especially in the north and west, these soils are poorly developed, sandy in texture and highly erodible; followed by the Neossolos Flúvicos (6.8%), associated mainly with valley bottoms and intermittent streams, areas that have moderate and/or high availability of water in the soil profile for a large part of the year, contributing to groundwater levels, not by chance, since the presence of typical and/or common species in the dense phytophysiognomies of the Caatinga is common; Neossolos Quartizarênicos (5.5%), these soils are also poorly developed and sandy; and finally the Organossolos Háplicos (0.4%), occurring in the form of a discontinuous patch in the extreme north of the study area, associated with the floodplain area.

The mapping of land use and land cover (Figure 2) shows the different aspects inherent to natural and anthropogenic factors on a landscape scale in the Riacho da Consulta watershed. Land cover conditions are evidenced by the occurrence of the different phytophysiognomies characteristic of the Caatinga’s natural vegetation cover. Specifically considering the Dense Caatinga phytophysiognomies, which comprise only 5.1% of the watershed’s total area, it can be seen that these are mostly associated with valley bottom areas, occurring mainly in the central portion of the watershed, areas whose hypsometric classes vary between 518 and 609 meters in altitude, where one can also observe the fragmentation of continuous landscapes with natural vegetation for land use by pastureland.

Areas covered by Caatinga Herbaceous and Caatinga Arbustive phytophysiognomies occupy around 84.0% and 4.3%, respectively, of the watershed, with the latter (Caatinga Arbustive phytophysiognomies) contrasting most with areas of exposed soil, which comprise 3.4% of the study area, mainly associated with Latossolos on gently undulating reliefs. From the terrain slope map in the context of the study area, it can be seen that the average slope observed is 16.1%, characterizing undulating reliefs. On Figure 3 it is presented the spatialization of CN in the study area. As can be seen, areas with high CN values predominate, showing a watershed with high runoff potential and low infiltration capacity.

In agreement with another studies (Silva, 2023), who points out that the Riacho da Consulta watershed is highly susceptible to erosion and that the areas with a predominance of undulating (8-20%), strongly undulating (20-45%) and mountainous (45-75%) terrain comprise areas with restricted use by human activities and need to be effectively earmarked for the conservation of fauna and flora in the context of the study area; Figure 3 shows that the areas used for human activities are slightly associated with areas with high runoff potential, demonstrating the potential impacts of a possible increase in these areas on water resources in the Riacho da Consulta watershed.

As can be seen in Figure 3, the average NC observed for the entire study area was 79, showing that the Riacho da Consulta watershed has high runoff conditions. Specifically, it can be seen that the predominance of areas with high CV values occurs spatially throughout the watershed, mainly in its northern and northeastern and southwestern portions, respectively. As shown in Figure 3, from fieldwork carried out on September 22nd (09) of the year 2023, Figure 4 shows a record of the landscape in an area with high runoff potential, located in the northeastern portion of the study area.

As can be seen (Figure 4), this is an area in an advanced stage of soil degradation (Latossolo Vermelho), located in the interfluve to the northeast of the watershed. Although this area has these conditions, it is important to note that, during the dry season (July to November), it is often used for grazing by livestock farmers, due to the fact that pasture areas in the region are practically exhausted by this time. It is important to understand that, this practice further accelerates the process of soil degradation in these areas and, as Silva (2023) proposes, these areas are more suitable, from a land use perspective, for biodiversity conservation.

Figure 5, which is also the result of fieldwork carried out on September 22nd (09) 2023, shows a second record of the landscape in another area that also showed high runoff potential, located in the northern portion of the Riacho da Consulta watershed. Similarly to what was observed in Figure 4, the landscape in question (Figure 5) shows that land use is geared towards farming activities, with degraded pastures and a predominant presence of species commonly found in environments covered with Arbustive Caatinga.

The preferential corridors commonly seen in cattle (Figure 5), goat and sheep grazing areas can be seen, highlighting the process of soil compaction, a process which directly limits the infiltration of rainwater and increases the potential for surface runoff. Soil compaction is an easily identified process in cultivated pasturelands and is quickly related to the practice of overgrazing, highlighting the lack of good management practices, which in the short, medium and long term can lead to the degradation of these areas. Thus, in both landscapes visited through fieldwork, comprising areas with high runoff potential, human interventions in ecosystems were identified. As mentioned above, neglecting good management practices where these activities take place can jeopardize the ecosystem balance of the watershed.

By estimating and spatializing the values inherent in the CN parameter, an analysis was carried out of the potential runoff conditions in the Riacho da Consulta watershed, considering different land cover and land use conditions. The average CN value observed for the entire watershed was 79, showing that the potential for surface runoff in the Riacho da Consulta watershed is high.

On the other hand, it shows that the Riacho da Consulta watershed has significant restrictions in terms of water infiltration into the soil, which can trigger other processes that impact the soil, water resources, biodiversity, ecosystem services and the culture of the inhabitants who have historically had links with the area. Furthermore, it is understood that the absence of conservation practices in the watershed can accentuate the challenges related to erosion processes, for example, with the consequent loss of soil and silting up of its water resources.

About the potential for surface runoff in relation to the dynamics of land use and land cover conditions, this study showed that land use areas have higher CN values than land cover areas, thus demonstrating a high potential for surface runoff in the former and a low potential for surface runoff in the latter, even though the conditions and properties of the soils in the areas where the latter occur are not favorable to infiltration. In this sense, it is important to carry out fieldwork to validate the results and, above all, the processes that help to understand and explain the high runoff potential in the study area.

It also highlights the importance of implementing conservation practices in the context of the Riacho da Consulta watershed and, above all, practices aimed at proper soil management, especially in areas used for grazing activities. It is hoped that future studies will contribute to soil loss in the Riacho da Consulta watershed. Similarly, other studies should investigate the importance of forest fragments for the implementation of an ecological corridor in the context of the watershed in question.

The results found, presented and discussed within the scope of this study can support the formulation and implementation of public policies with an emphasis on conservation practices in the context of the area under study, for example, supporting the definition of areas of restricted use by human activities in the Riacho da Consulta watershed.