Impacts of erosion and sedimentation on reservoirs in the Seridó river basin: a hydrosedimentological assessment in the brazilian semiarid region

Impactos da erosão e do assoreamento nos reservatórios da bacia do rio Seridó: avaliação hidrossedimentológica no semiárido brasileiro

ABSTRACT

Soil erosion and sedimentation challenge water resources management in semiarid regions, where extreme climatic conditions and inadequate land use amplify these issues. These processes compromise reservoir storage, water quality, and regional sustainability. This study evaluates their impacts on reservoirs (Mulungu, Caldeirão de Parelhas, Várzea Grande, Esguicho, Itans, Cruzeta, São João do Sabugi, and Boqueirão de Parelhas) in the Seridó River Basin, Brazil, using hydrosedimentological modeling based on the Universal Soil Loss Equation (USLE) and sedimentation data. Sedimentation rates ranged from 0.12% to 8.12% per decade, causing notable storage losses, such as 11.45 hm³ in Cruzeta over 81 years and 17.80 hm³ in Itans over 82 years. Reservoir lifespans were projected between 31 years and over 3,800 years, averaging 96 years. Sediment accumulation averaged 6.39 million tons, increasing evaporation losses and degrading water quality. The dense dam network (0.55 km²/reservoir) disrupts sediment connectivity, emphasizing the importance of conservation to mitigate soil degradation and maintain reservoir functionality. This study advances understanding of hydrosedimentological dynamics in arid and semiarid regions, underscoring the need for sustainable water resource management strategies.

Keywords:
Soil erosion; Sedimentation; Semiarid; Water resource

RESUMO

A erosão do solo e a sedimentação representam desafios para a gestão de recursos hídricos em regiões semiáridas, onde condições climáticas extremas e práticas inadequadas de uso da terra agravam esses problemas. Esses processos comprometem a capacidade de armazenamento dos reservatórios, a qualidade da água e a sustentabilidade regional. Este estudo avalia seus impactos nos reservatórios (Mulungu, Caldeirão de Parelhas, Várzea Grande, Esguicho, Itans, Cruzeta, São João do Sabugi e Boqueirão de Parelhas) na Bacia do Rio Seridó, Brasil, utilizando modelagem hidrossedimentológica baseada na Equação Universal de Perda de Solo (USLE) e dados de sedimentação. As taxas de sedimentação variaram de 0,12% a 8,12% por década, causando perdas significativas de armazenamento, como 11,45 hm³ no reservatório Cruzeta ao longo de 81 anos e 17,80 hm³ no reservatório Itans ao longo de 82 anos. A vida útil dos reservatórios foi projetada entre 31 anos e mais de 3.800 anos, com uma média de 96 anos. O acúmulo médio de sedimentos foi de 6,39 milhões de toneladas, aumentando as perdas por evaporação e degradando a qualidade da água. A densa rede de barragens (0,55 km²/reservatório) interrompe a conectividade dos sedimentos, destacando a importância da conservação para mitigar a degradação do solo e manter a funcionalidade dos reservatórios. Este estudo aprimora a compreensão das dinâmicas hidrossedimentológicas em regiões áridas e semiáridas, ressaltando a necessidade de estratégias sustentáveis de gestão de recursos hídricos.

Palavras-chave:
Erosão do solo; Assoreamento; Semiárido; Recursos Hídricos

INTRODUCTION

Soil erosion, a mechanical process occurring at the surface and resulting in the removal of soil profiles and sediment transport, is a natural phenomenon often exacerbated by improper land use associated with human activities. Soil erosion leads to organic matter losses, reduced fertility, and diminished water storage capacity, while contributing to siltation and pollution of water bodies (Rabelo & Araújo, 2019; Frascareli et al., 2023). These processes negatively impact the environment and generate social problems such as land abandonment, rural community decline, and rural exodus, further intensifying urban pressures and socio-economic challenges such as unemployment and marginalization (Rabelo, 2017).

The environmental and economic damages caused by sediment accumulation in reservoirs can be challenging to mitigate, particularly in arid and semi-arid regions (International Commission on Large Dams, 1989). A World Bank study revealed that the average lifespan of reservoirs worldwide decreased from 100 years to 22 years due to siltation, estimating an annual economic loss of 6 billion USD for sediment removal (Mahmood, 1987). Wisser et al. (2013) employed a more recent approach based on hydrological modeling and observational data to estimate storage loss in large reservoirs on a global scale. The study considered sedimentation rates, reservoir lifespan, and trends in storage capacity, suggesting that many systems may have already reached or exceeded their peak storage capacity. Furthermore, the study indicated that the average annual volume loss of reservoirs due to sediment deposition was approximately 1%, depending on the region. In Brazil, studies conducted by Eletrobrás/IPH concluded that the annual storage capacity loss of reservoirs is approximately 0.5% (Carvalho, 1994).

In Brazil’s semi-arid region, siltation reduces reservoir storage capacity by an average of 0.2% per year (Araújo, 2003). Araújo et al. (2006) highlighted that siltation decreases water availability in reservoirs through two main mechanisms: increased exposed surface area, leading to evaporation, and reduced usable storage volume, thereby intensifying “spillway losses.” Consequently, investigating the processes affecting reservoir water availability, with particular emphasis on erosion and sediment production, becomes imperative. Several researchers have undertaken significant sedimentological monitoring studies in semi-arid regions, including Cavalcante (2012), Araújo (2003), Araújo et al. (2006), Melo et al. (2008), Galvão (2008), Silva Junior et al. (2011), Piscoya (2012), Morais (2015), Araújo et al. (2017), Xavier et al. (2017), Almeida (2019), Bento et al. (2019), Gil et al. (2020), Barra et al. (2022), and Rabelo et al. (2023).

To estimate the volume of eroded soil, the Universal Soil Loss Equation (USLE) is a widely employed empirical model developed based on runoff and soil loss data collected from over 10,000 experimental plots under both natural and simulated rainfall conditions (Wischmeier & Smith, 1965). The USLE model is grounded in historical precipitation records, soil typology, topography, cropping systems, and soil conservation practices. It was developed during the 1960s and 1970s by the United States Department of Agriculture Agricultural Research Service to support soil planning and conservation efforts in rural watersheds (Wischmeier & Smith, 1965). Since its inception, the model has undergone multiple revisions, research advancements, and extensions, such as the MUSLE (Williams, 1975), USLE-M (Kinnell, 1999), and dUSLE (Flacke et al., 1990). These advancements culminated in a significant update, resulting in the Revised Universal Soil Loss Equation (RUSLE) (Renard et al., 1998), aiming to enhance the model's applicability. Despite these modifications, the USLE remains widely applied (Barretto et al., 2008). In addition to the USLE, other hydro-sedimentological models, such as SWAT (Arnold et al., 1998), WASA-SED (Mueller et al., 2010), WEPP (Laflen et al., 1991), SWIM (Krysanova et al., 1996), SedNET (Prosser, 2001), and LASCAM (Sivapalan et al., 1996), are derived from the USLE and often integrate sediment transport or sediment delivery equations (SDR) (Araújo, 2003; Rabelo et al., 2023).

This study aimed to analyze soil erosion, siltation, and sediment production in monitored reservoirs within the Seridó River Basin, located in the Brazilian semi-arid region, using modeled and secondary data through a four-step approach: (i) estimation of gross erosion in the drainage basin of each monitored reservoir, (ii) assessment of the number of dams in the Seridó River Basin, (iii) evaluation of the operational lifespan of each reservoir based on siltation processes, and (iv) identification of correlations between gross basin erosion and sediment production within monitored reservoirs. Applying this approach enables the association of degraded and eroded soils with the respective monitored reservoirs. In this study, the proposed method was implemented through area-based modeling using the Universal Soil Loss Equation (USLE) combined with sediment data consolidated in the scientific literature.

METHODOLOGY

Study area

The monitored reservoirs are located within the Seridó River Basin, which spans an area of 9,931 km² and constitutes the main sub-basin of the Piranhas-Assu River system (Figure 1). In the Seridó region, coping with drought through the construction of reservoirs is a widely adopted strategy, characterized by the presence of a large number of dams. Among the principal monitored reservoirs in the region are the Várzea Grande Reservoir, Mulungu Reservoir, Esguicho Reservoir, Boqueirão Reservoir, Caldeirão de Parelhas Reservoir, São João do Sabugi Reservoir, Cruzeta Reservoir, and Itans Reservoir. Of these, only the Várzea Grande Reservoir is located in the state of Paraíba, while the others are situated in the state of Rio Grande do Norte.

Figure 1
Location of the study área.

The geological structure of the study area is dominated by Upper Precambrian metamorphic rocks, which directly influence the relief, such as the Sertaneja Depression (Souza, 2000). The Borborema structural province, located in the central portion, is characterized by the Seridó fold belt, featuring shear zones, fractures, and folds as primary evidence of the tectonic forces that shaped the Borborema Plateau (Pires, 2006). Most of the Seridó River Basin is within the Seridó Group—comprising the Seridó, Equador, and Jucurutu Formations—situated in tectono-stratigraphic terrains whose ages and evolution remain subjects of research (Almeida, 1977; Van Schmus et al., 2000; Nascimento & Sial, 2001). Residual massifs of granitoid plutons are notable features in the region (Nascimento, 2002).

The relief also includes flat-topped areas of Cenozoic sediments from the Serra dos Martins Formation, consisting of siliciclastic sediments with variable grain sizes and thicknesses ranging from 5 to 60 meters, with an average of 25 to 30 meters ( Morais Neto & Alkmim, 2001). According to the IBGE, the region experiences a tropical equatorial semi-arid climate, with a dry season lasting 7 to 10 months. The average annual precipitation is 500 mm, concentrated in the northern region due to the Intertropical Convergence Zone (ITCZ), which causes rainfall to be temporally and spatially irregular (Rabelo et al., 2022).

The geomorphological features reflect the interplay between climate and geology. Differential erosion in magmatic and metamorphic rocks has created varied landscapes, including residual massifs and dissected valleys. Processes of relief inversion, driven by tectonic uplift or differential erosion, are prominent on the Borborema Plateau (Maia et al., 2016). Two major morphostructures dominate: the Borborema Plateau and the Sertaneja flattened surface (Rabelo, 2016). The basin exhibits a pedological mosaic, with soils such as Neosols, Planosols, Luvisols, Argisols, Cambisols, and Latosols (Rabelo et al., 2024). Latosols in plateau areas are deep, exceeding 100 cm, composed of mineral material with smooth horizon transitions (Embrapa, 2009). Luvisols are associated with flat areas over metamorphic rocks, reflecting the region's mineralogical structure.

The predominant vegetation is the Caatinga, typical of the semi-arid region, consisting of small to medium-sized species ranging from shrub to tree forms. This vegetation supports economic activities such as agriculture, livestock farming, ceramics, and mineral processing industries (Rabelo et al., 2022). Areas with dense Caatinga vegetation exhibit reduced land use and greater conservation. Open areas are marked by extensive livestock farming and mining activities, especially near rivers. The reservoirs, used for public water supply, aquaculture, and irrigation, reflect a critical strategy for coping with drought conditions.

Data collection

Several institutional agencies were consulted to obtain data for the present research area, including the Executive Water Management Agency of the State of Paraíba (AESA) and the Agricultural Research Corporation of Rio Grande do Norte (EMPARN) for rainfall data; the Brazilian Agricultural Research Corporation (EMBRAPA/Solos) for soil information; and the National Institute for Space Research (INPE), from which raster data from the Shuttle Radar Topography Mission (SRTM) of scenes 06S375SA and 07S375SA were acquired. Additionally, satellite images from Landsat 8 with the Operational Land Imager (OLI) sensor were used.

Bathymetric data were sourced from various references: studies by Miranda (2007) regarding the Várzea Grande Reservoir, located in the municipality of Santa Luzia, Paraíba; the Water Management Institute of Rio Grande do Norte (IGARN) for reservoirs located in the municipalities of Cruzeta, Currais Novos (Mulungu Reservoir), Ouro Branco (Esguicho Reservoir), and Parelhas (Boqueirão Reservoir and Caldeirão de Parelhas Reservoir); and the National Water and Basic Sanitation Agency (ANA) for reservoirs located in the municipalities of São João do Sabugi and Caicó (Itans Reservoir).

The analyzed reservoirs exhibit a wide range in both contributing area, which varies between 44 km² and 1,500 km², and water storage capacity, which ranges from 0.17 hm³ to 17.80 hm³, as detailed in Table 1. Data on sediment production, the bulk density of the soil, and the accumulated sediment mass in the mentioned reservoirs were extracted from the study conducted by Rabelo et al. (2023) in the region. These datasets were essential for the quantitative analysis of erosive processes and the evaluation of sedimentation in the studied reservoirs, as methodologically suggested for reservoirs by Carvalho et al. (2000), Araújo (2003), Gaiser et al. (2003), and Ferreira et al. (2014).

Table 1
Reservoirs Analyzed in the Seridó River Basin.

Reservoir	Year Built	Year of Bathymetry	Reservoir Age (year)	Volume (hm3)		apparent specific mass (m3)	Drainage basin area (km2)	Geographic coordinates
				Built	Bathymetric
Mulungu	1980	2008	28	1.790	1.618	1.23	44.2	6°14’14.91”S / 36°17’42.90”W
Caldeirão de Parelhas	1967	2009	42	10.196	9.321	1.28	195.1	6°42’22.77”S / 36°41’21.39”W
Várzea Grande	1933	2006	73	11.960	8.650	1.30	217.5	6°52’30.92”S / 36°55’26.60”W
Esguicho	2001	2010	9	30.149	27.937	1.25	448.0	6°44’0.52”S / 36°57’15.09”W
Itans	1935	2017	82	81.750	63.944	1.22	1268.0	6°29’28.59”S / 37° 3’48.89”W
Cruzeta	1929	2010	81	35.000	23.546	1.31	1400.0	6°24’34.44”S / 36°47’45.78”W
Sabugi	1965	2017	52	65.335	61.849	1.36	1428.0	6°38’52.46”S / 37°11’49.56”W
Boqueirão de Parelhas	1988	2009	21	85.013	84.792	1.28	1519.0	6°41’42.57”S / 36°37’36.08”W

• Source: Prepared by the authors.

Thus, the results were presented separately for each reservoir following the methodology of Araújo (2003), which proposes an individualized analysis based on specific factors such as topography, rainfall regime, soil characteristics, and land cover. This approach allows for a more precise assessment of erosion processes and sediment transport, taking into account the particularities of each system.

Gross erosion

According to Rabelo & Araújo (2019), gross erosion refers to the displacement of eroded material along slopes, without necessarily resulting in the effective loss of soil at the watershed scale. In contrast, sediment yield corresponds to the fraction of soil lost within the watershed due to erosive processes, with this material subsequently being exported beyond the watershed boundaries.

At the watershed scale, the dynamics of gross erosion were estimated using the factors included in the Universal Soil Loss Equation (USLE), as proposed and revised by Wischmeier & Smith (1978). This model provides an estimation of the quantity of sediment eroded locally over a given period. The model is represented by Equation 1:

$\epsilon =R.K.L.S.C.P$ (1)

where:

ε = soil loss due to erosion (ton.ha־¹.y־¹); R = rainfall erosivity (MJ.mm. ha־¹. h־¹. y־¹); K = soil erodibility (ton.ha.MJ־¹.mm־¹); LS = topographic factor (dimensionless); e CP = vegetation cover factor multiplied by the land use and management factor (dimensionless). Each variable in this equation was processed using geoprocessing software, producing results corresponding to the factors of rainfall erosivity (R), slope steepness (S), slope length (L), soil erodibility (K), vegetation cover and land use (C), and conservation practices (P). Ultimately, the synthesis of these variables represents the gross erosion of each sub-basin associated with the respective reservoir.

The data for each USLE parameter were obtained from the study by Rabelo & Araújo (2019). Monthly and annual rainfall data from thirty rain gauges in the Seridó region, covering a 23-year historical series (1992–2015), were used to estimate rainfall erosivity following the methodology of Lombardi Neto & Moldenhauer (1992). Soil erodibility data were sourced from Rabelo et al. (2024).

The topographic factor was calculated based on slope length and steepness derived from SRTM processing, using the methodology proposed by Carvalho (1994). The vegetation cover and land use factor were determined using supervised classification through the MAXVER algorithm and reclassified with values ranging from 0 to 1, following relevant literature (Carvalho, 1994). The conservation practices factor was not included, as no such practices were implemented in the region at the time of the study.

Statistical correlation of erosion

The Pearson correlation coefficient (r) was used to assess the relationship between soil erosion data (USLE) and sediment production identified by Rabelo et al. (2023). The r value ranges from -1 to 1, where values close to 0 indicate little to no linear correlation, while values approaching -1 or 1 represent a stronger negative or positive linear correlation, respectively. This coefficient quantifies the strength and direction of the linear association between two variables, providing a robust statistical measure for evaluating their relationship (Bachiller et al., 2019).

Projection of the useful time of reservoirs

The projection of the useful lifespan of the reservoirs was carried out based on the siltation data obtained for the main reservoirs in the Seridó River Basin. To estimate the time required until the reservoirs' capacity is fully compromised by sediment accumulation, Equation 2, as proposed by Mahmood (1987), was used:

$T_u=\frac{C_i}{A}$ (2)

where:

Tu is the estimated useful lifespan of the reservoir in years, Ci​ is the initial capacity of the reservoir (hm³), and A is the average siltation rate (hm³/year). The values for initial capacity (Ci) and siltation rate (A) were derived from bathymetric data and literature, using decadal rates. The initial storage capacity of each reservoir was divided by this rate to estimate the useful lifespan until complete infill by sediments. The analysis was applied to eight reservoirs: Mulungu, Caldeirão de Parelhas, Várzea Grande, Esguicho, Itans, Cruzeta, São João do Sabugi, and Boqueirão de Parelhas.

Quantity mapping

For mapping the models, the software ArcGIS 10, QGIS, and AutoCAD were used. The estimated erosion production was mapped according to the natural flow propagation order, using a model developed by the Hydro-Sedimentological Research Group of the Semi-Arid Region (HIDROSED-UFC), which is based on the concept of sedimentological connectivity (Lira et al., 2014).

Satellite images from SENTINEL-2A, provided by the United States Geological Survey (USGS), were acquired. These images, captured by the MSI sensor, cover zones T24MXT, T24MYT, and T24MXU, corresponding to the Seridó basin area, and are dated May 25, 2018. The images were used to quantify the reservoirs in the basin at a 1:30,000 scale. The satellite features a spatial resolution of 10 meters, with radiometric and geometric corrections applied in the UTM/WGS84 projection system.

In reservoir mapping, Sentinel-2 images were processed using bands 3 and 8. Based on this, the Normalized Difference Water Index (NDWI), proposed by McFeeters (1996), was calculated. This index enhances water reflectance in the green band (B3) while reducing it in the near-infrared band (B8), as applied for Sentinel-2 data in Equation 11:

$NDWI=\left(B3-B8\right)/\left(B3+B8\right)$ (3)

For the spatial distribution of point data, the Kriging method was used as an interpolator for these points. For model processing, map algebra was employed, an approach that utilizes the properties of Geographic Information Systems (GIS) to perform primitive operations while respecting the fundamental properties of the processes involved (Tomlin, 1990).

All modeling, spatial analysis, and mapping were based on the SIRGAS2000 UTM 24S cartographic projection. The scales used varied from 1:30,000 to 1:200,000 at different levels—ranging from macro to micro scales concerning the watersheds—with the final map scale set at 1:500,000.

RESULTS

Figure 2 presents the distribution of variables associated with gross erosion in the reservoirs of Mulungu, Caldeirão de Parelhas, Várzea Grande, Esguicho, Itans, Cruzeta, São João do Sabugi, and Boqueirão de Parelhas, located in the Seridó Basin. The analyzed parameters include erosivity, erodibility, topographic factor, and soil and vegetation cover, represented by boxplots that indicate data dispersion and variability among the reservoirs.

Figura 2
The box-plot charts of the USLE reservoir parameters.

Erosivity, calculated in MJ·mm·ha⁻¹·h⁻¹·year⁻¹, exhibits a relatively concentrated distribution, with a median around 4000 and values ranging from approximately 3000 to 4500, suggesting moderate differences in the intensity of rainfall kinetic energy in the region. Erodibility, expressed in ton·ha·MJ⁻¹·mm⁻¹, shows greater dispersion, with values ranging from about 0.005 to 0.03 and a median close to 0.02, indicating heterogeneity in soil susceptibility to erosion. The topographic factor varies between 0.5 and 3.5, with a median around 1.5, reflecting differences in slope and slope length, which influence surface runoff dynamics. Finally, soil and vegetation cover exhibits an asymmetric distribution, with values ranging from 0.1 to 0.9 and a median around 0.4, highlighting significant variations in soil protection against erosion across the analyzed reservoirs. These results demonstrate considerable spatial variability in the factors controlling gross erosion in the Seridó Basin, which may directly influence sedimentation processes in the studied reservoirs.

The estimated gross erosion values for the reservoirs ranged from 2.13 to 139.59 ton·ha⁻¹·year⁻¹. This variation is represented by a color scale (Figures below), where areas with lower erosion (2.13 to 19.38 ton·ha⁻¹·year⁻¹) are shown in green, indicating regions with lower erosive impact, while areas with the highest erosion values (107.79 to 139.59 ton·ha⁻¹·year⁻¹) are highlighted in red, suggesting critical zones with greater susceptibility to sediment transport. Intermediate regions, ranging from 19.38 to 107.79 ton·ha⁻¹·year⁻¹, are represented in shades of yellow and orange, reflecting areas of moderate erosion.

Mulungu Reservoir

The Mulungu Reservoir is located in the municipality of Currais Novos, Rio Grande do Norte, upstream in the Seridó River Basin. This reservoir has a low storage capacity of approximately 1.790 hm³ (Table 2). The contributing watershed covers an area of 44.2 km². The dam was completed in 1980, with its primary purposes being human water supply and agriculture. The watershed of the Mulungu Reservoir is predominantly covered by shrub-tree Caatinga vegetation, with few exposed soil areas where evidence of vegetation extraction is observed. The gross erosion (USLE) in the reservoir's watershed (Figure 3) showed an average value of 25.04 Ton.ha־¹.year־¹. Siltation data obtained from IGARN revealed a reduction in the reservoir's capacity of approximately 0.172 hm³ over 28 years, corresponding to a 3.55% loss per decade due to siltation. The observed siltation equated to 212,000 tons of sediments, given that the bulk density of the deposited material is 1.23 t.m־³, with a specific sediment yield of 1.77 Ton.ah־¹.y־¹ (Rabelo et al., 2023).

Table 2
Physical characteristics and measurement results in the Mulungu Reservoir.

Mulungu Reservoir - Currais Novos/RN
Measured Variables	Value
Initial Storage Capacity	1.790 hm3
Drainage Basin Area	44.2 km2
Construction Year	1980
Monitoring Year	2008
Temporal Interval	28 years
Storage Capacity at Monitoring Year	1.618 hm3
Apparent Mass Density	1.23 t.m-3
Brune Sediment Retention Index	97%
Sedimentation (volume)	0.172 hm3
Sedimentation Rate (volume)	3.55%.decade-1
Sedimentation (Mass)	2,12E+05 t
Average USLE-Based Erosion (Mass)	25.04 Ton.ha-1.y-1
Sediment Yield (Mass)	1.77 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 3
Gross erosion map (USLE) of the Mulungu Reservoir basin.

Caldeirão de Parelhas Reservoir

The Caldeirão de Parelhas Reservoir is located in the municipality of Parelhas, Rio Grande do Norte, upstream in the Seridó River Basin. The reservoir has a medium storage capacity of approximately 10.196 hm³ (Table 3). The contributing watershed covers an area of 195.1 km². The dam was completed in 1967, with its primary purposes being human and industrial water supply, as well as agriculture. The watershed of the Caldeirão de Parelhas Reservoir has dense vegetation in its upper course and some exposed soil areas. Shrub-tree Caatinga vegetation predominates across the watershed, though evidence of vegetation extraction practices is also present. The gross erosion (USLE) in the reservoir's watershed (Figure 4) showed an average value of 21.69 Ton.ha־¹.year־¹. Siltation data, obtained from a private company and later validated by IGARN, revealed a reduction in the reservoir's capacity of approximately 0.874 hm³ over 42 years. This corresponds to a 2.11% loss per decade due to sedimentation. The observed siltation equaled 1.12 million tons of sediments, with the bulk density of the deposited material being 1.28 t.m־³ and a specific sediment yield of 1.41 Ton.h־a¹.y־¹ (Rabelo et al., 2023).

Table 3
Physical characteristics and measurement results at the Caldeirão de Parelhas Reservoir.

Caldeirão de Parelhas Reservoir - Parelhas/RN
Measured Variables	Value
Initial Storage Capacity	10.196 hm3
Drainage Basin Area	195.1 km2
Construction Year	1967
Monitoring Year	2009
Temporal Interval	42 years
Storage Capacity at Monitoring Year	9.321 hm3
Apparent Mass Density	1.28 t.m-3
Brune Sediment Retention Index	97%
Sedimentation (volume)	0.874 hm3
Sedimentation Rate (volume)	2.11%.década-1
Sedimentation (Mass)	1.12E+06 t
Average USLE-Based Erosion (Mass)	21.69 Ton.ha-1.y-1
Sediment Yield (Mass)	1.41 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 4
Gross erosion map (USLE) of the Caldeirão de Parelhas Reservoir basin.

Várzea Grande Reservoir

The Várzea Grande Reservoir is located in the municipality of Santa Luzia, Paraíba, upstream in the Seridó River Basin. The reservoir has a medium storage capacity of approximately 11.960 hm³ (Table 4). The contributing watershed spans an area of 217.5 km². The dam was completed in 1933, with its primary purpose being human water supply for the city of Santa Luzia. The watershed of the Santa Luzia Public Reservoir is characterized by anthropized areas near its flooded zone, as it is located within the city. In non-urban areas, open shrub Caatinga vegetation predominates. The gross erosion (USLE) in the reservoir’s watershed (Figure 5) showed an average value of 42.16 Ton.ha־¹.year־¹. Bathymetric data from the study by Miranda (2007) revealed a reduction in the reservoir’s capacity of approximately 3.310 hm³ over 73 years, indicating a loss of 4.34% per decade due to sedimentation. The observed siltation amounted to 4.29 million tons of sediments, with the bulk density of the deposited material being 1.30 t.m־³ and a specific sediment yield of 2.78 Ton.ha־¹.y־¹ (Rabelo et al., 2023).

Table 4
Physical characteristics and measurement results at the Várzea Grande Reservoir.

Várzea Grande Reservoir - Santa Luzia/PB
Measured Variables	Value
Initial Storage Capacity	11.960 hm3
Drainage Basin Area	217.5 km2
Construction Year	1933
Monitoring Year	2006
Temporal Interval	73 years
Storage Capacity at Monitoring Year	8.650 hm3
Apparent Mass Density	1.30 t.m-3
Brune Sediment Retention Index	98%
Sedimentation (volume)	3.310 hm3
Sedimentation Rate (volume)	4.34%.decade-1
Sedimentation (Mass)	4.29E+06 t
Average USLE-Based Erosion (Mass)	42.16 Ton.ha-1.y-1
Sediment Yield (Mass)	2.78 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 5
Gross erosion map (USLE) of the Várzea Grande Reservoir basin.

Esguicho Reservoir

The Esguicho Reservoir is located in the municipality of Ouro Branco, Rio Grande do Norte, downstream in the Seridó River Basin. The reservoir has a medium storage capacity of approximately 30.14 hm³ (Table 5). The contributing watershed covers an area of 448 km². The dam was completed in 2001, with its main purposes being human water supply for local communities, floodplain agriculture, and fishing and recreational activities. The watershed of the Esguicho Reservoir is characterized by anthropized areas with diverse agricultural practices and exposed soils near the reservoir. Open shrub Caatinga vegetation dominates other areas but offers limited protection against rainfall erosivity. The gross erosion (USLE) in the reservoir’s watershed (Figure 6) showed an average value of 42.85 Ton.ha־¹.year־¹. Bathymetric data collected by IGARN revealed a reduction in the reservoir's capacity of approximately 2.21 hm³ over 9 years, indicating a loss of 8.12% per decade due to sedimentation. The observed siltation amounted to 2.77 million tons of sediments, with the bulk density of the deposited material being 1.25 t.m־³ and a specific sediment yield of 7.00 Ton.ha־¹.y־¹ (Rabelo et al., 2023).

Table 5
Physical characteristics and measurement results at the Esguicho Reservoir.

Reservatório Esguicho - Ouro Branco/RN
Measured Variables	Value
Initial Storage Capacity	30.14 hm3
Drainage Basin Area	448 km2
Construction Year	2001
Monitoring Year	2010
Temporal Interval	9 years
Storage Capacity at Monitoring Year	27.93 hm3
Apparent Mass Density	1.25 t.m-3
Brune Sediment Retention Index	98%
Sedimentation (volume)	2.21 hm3
Sedimentation Rate (volume)	8.12%.decade-1
Sedimentation (Mass)	2.77E+06 t
Average USLE-Based Erosion (Mass)	42.85 Ton.ha-1.y-1
Sediment Yield (Mass)	7.00 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 6
Gross erosion map (USLE) of the Esguicho Reservoir basin.

Itans Reservoir

The Itans Reservoir is located in the municipality of Caicó, Rio Grande do Norte, downstream in the Seridó River Basin. The reservoir has a large storage capacity of approximately 81.750 hm³ (Table 6). The contributing watershed covers an area of 1,268 km². The dam was completed in 1935, with its primary purposes being human and industrial water supply, irrigation, aquaculture, and livestock watering. The watershed of the Itans Reservoir is predominantly covered by open shrub Caatinga vegetation, with many degraded areas. The gross erosion (USLE) in the reservoir's watershed (Figure 7) showed an average value of 40.99 Ton.ha־¹.year־¹. Bathymetric data collected by ANA revealed a reduction in the reservoir's capacity of approximately 17.806 hm³ over 82 years. The results indicated a loss of 2.95% per decade due to sedimentation. The observed siltation totaled 22.7 million tons of sediments, with the bulk density of the deposited material being 1.22 t.m־³, which is relatively low due to lesser compaction and the deposition of silt and clay. The specific sediment yield was estimated at 2.27 Ton.ha־¹.y־¹ (Rabelo et al., 2023).

Table 6
Physical characteristics and measurement results at the Itans Reservoir.

Reservatório Itans - Caicó/RN
Measured Variables	Value
Initial Storage Capacity	81.750 hm3
Drainage Basin Area	1268 km2
Construction Year	1935
Monitoring Year	2017
Temporal Interval	82 years
Storage Capacity at Monitoring Year	63.944 hm3
Apparent Mass Density	1.22 t.m-3
Brune Sediment Retention Index	96%
Sedimentation (volume)	17.806 hm3
Sedimentation Rate (volume)	2.95%.decade-1
Sedimentation (Mass)	2.27E+07 t
Average USLE-Based Erosion (Mass)	40.99 Ton.ha-1.y-1
Sediment Yield (Mass)	2.27 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 7
Gross erosion map (USLE) of the Itans Reservoir basin.

Cruzeta Reservoir

The Cruzeta Reservoir is located in the municipality of Cruzeta, Rio Grande do Norte, upstream in the Seridó River Basin. The reservoir has a large storage capacity of approximately 35.000 hm³ (Table 7). The contributing watershed covers an area of 1,400 km². The dam was completed in 1929, with its primary purposes being human and industrial water supply, irrigation, and livestock watering. The watershed of the Cruzeta Reservoir is predominantly covered by open shrub Caatinga vegetation, with some areas of exposed soil. The gross erosion (USLE) in the reservoir's watershed (Figure 8) showed an average value of 32.90 Ton.ha־¹.year־¹. Bathymetric data collected by IGARN revealed a reduction in the reservoir's capacity of approximately 11.454 hm³ over 81 years. The results indicated a loss of 4.78% per decade due to sedimentation. The observed siltation amounted to 15 million tons of sediments, with the bulk density of the deposited material being 1.31 t.m־³ and a specific sediment yield of 1.36 Ton.ha־¹.y־¹ (Rabelo et al., 2023).

Table 7
Physical characteristics and measurement results at the Cruzeta Reservoir.

Cruzeta Reservoir - Cruzeta/RN
Measured Variables	Value
Initial Storage Capacity	35.000 hm3
Drainage Basin Area	1400 km2
Construction Year	1929
Monitoring Year	2010
Temporal Interval	81 years
Storage Capacity at Monitoring Year	23.546 hm3
Apparent Mass Density	1.31 t.m-3
Brune Sediment Retention Index	97%
Sedimentation (volume)	11.454 hm3
Sedimentation Rate (volume)	4.78%.decade-1
Sedimentation (Mass)	1.50E+07 t
Average USLE-Based Erosion (Mass)	32.90 Ton.ha-1.y-1
Sediment Yield (Mass)	1.36 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 8
Gross erosion map (USLE) of the Cruzeta Reservoir basin.

São João do Sabugi Reservoir

The São João do Sabugi Reservoir is located in the municipality of São João do Sabugi, Rio Grande do Norte, downstream in the Seridó River Basin. The reservoir has a large storage capacity of approximately 65.335 hm³ (Table 8). The contributing watershed spans an area of 1,428 km². The dam was completed in 1965, with its primary purposes being human water supply, irrigation, and livestock watering. The watershed of the São João do Sabugi Reservoir is predominantly covered by open shrub Caatinga vegetation, with some areas of exposed soil. The gross erosion (USLE) in the reservoir’s watershed (Figure 9) showed an average value of 37.44 Ton.ha־¹.year־¹. Bathymetric data collected by ANA revealed a reduction in the reservoir's capacity of approximately 3.486 hm³ over 52 years. The results indicated a loss of 1.05% per decade due to sedimentation. The observed siltation amounted to 4.73 million tons of sediments, with the bulk density of the deposited material being 1.36 t.m־³. This higher value is attributed to the coarser, sandier granulometry during the deposition process, with a specific sediment yield of 0.65 Ton.ha־¹.y־¹ (Rabelo et al., 2023).

Table 8
Physical characteristics and temperature results in the São João do Sabugi Reservoir.

Sabugi Reservoir - São João do Sabugi/RN
Measured Variables	Value
Initial Storage Capacity	65.335 hm3
Drainage Basin Area	1428 km2
Construction Year	1965
Monitoring Year	2017
Temporal Interval	52 years
Storage Capacity at Monitoring Year	61.849 hm3
Apparent Mass Density	1.36 t.m-3
Brune Sediment Retention Index	97%
Sedimentation (volume)	3.486 hm3
Sedimentation Rate (volume)	1.05%.decade-1
Sedimentation (Mass)	4.73E+06 t
Average USLE-Based Erosion (Mass)	37.44 Tona.h-1.y-1
Sediment Yield (Mass)	0.65 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 9
Gross erosion map (USLE) of the São João do Sabugi Reservoir basin.

Boqueirão de Parelhas Reservoir

The Boqueirão de Parelhas Reservoir is located in the municipality of Parelhas, Rio Grande do Norte, upstream in the Seridó River Basin. The reservoir has a large storage capacity of approximately 85.013 hm³ (Table 9). The contributing watershed spans an area of 1,519 km². The dam was completed in 1988, with its primary purposes being human and industrial water supply, as well as livestock watering. The watershed of the Boqueirão de Parelhas Reservoir is predominantly covered by open shrub Caatinga vegetation, with some areas of exposed soil. The gross erosion (USLE) in the reservoir’s watershed (Figure 10) showed an average value of 17.49 Ton.ha־¹.year־¹. Siltation data, obtained from a private company and later validated by IGARN, revealed a reduction in the reservoir’s capacity of approximately 0.221 hm³ over 21 years. The results indicated a loss of 0.12% per decade due to sedimentation. The observed siltation amounted to 282,000 tons of sediments, with the bulk density of the deposited material being 1.28 t.m־³ and a specific sediment yield of 0.09 Ton.ha־¹.y־¹ (Rabelo et al., 2023).

Table 9
Physical characteristics and measurement results at the Boqueirão de Parelhas Reservoir.

Boqueirão de Parelhas Reservoir - Parelhas/RN
Measured Variables	Value
Initial Storage Capacity	85.013 hm3
Drainage Basin Area	1519 km2
Construction Year	1988
Monitoring Year	2009
Temporal Interval	21 years
Storage Capacity at Monitoring Year	84.792 hm3
Apparent Mass Density	1.28 t.m-3
Brune Sediment Retention Index	98%
Sedimentation (volume)	0.221 hm3
Sedimentation Rate (volume)	0.12%.decade-1
Sedimentation (Mass)	2.82E+05 t
Average USLE-Based Erosion (Mass)	17.49 Ton.ha-1.y-1
Sediment Yield (Mass)	0.09 Ton.ha-1.y-1

• Source: Prepared by the authors.

Figure 10
Gross erosion map (USLE) of the Boqueirão Reservoir basin.

Projection of the useful time of reservoirs in the Seridó River Basin

The analysis of the estimated useful lifespan of the reservoirs in the Seridó River Basin revealed significant variations, reflecting geomorphological characteristics, land use, and dam density (Table 10). The results underscore the importance of conservation practices in the Brazilian semi-arid region, aligning with recent studies on reservoir management in arid and semi-arid regions (Panagos et al., 2015; Alatorre et al., 2021).

Table 10
Projection of the useful life of each Seridó reservoir.

Reservoir	Initial Capacity (hm3)	Siltation Rate (hm3/year)	Estimated Useful Time (years)
Mulungu	1.79	0.0172	104
Caldeirão de Parelhas	10.196	0.0874	117
Várzea Grande	11.96	0.331	36
Esguicho	30.149	0.221	136
Itans	81.75	1.78	46
Cruzeta	35	1.145	31
São João do Sabugi	65.335	0.348	188
Boqueirão de Parelhas	85.013	0.0221	3.846

• Source: Prepared by the authors.

The estimated useful lifespans varied widely among the analyzed reservoirs. The lowest values were observed in the Cruzeta Reservoir (31 years) and the Várzea Grande Reservoir (36 years), indicating high vulnerability due to elevated siltation rates relative to their initial volumes. Conversely, the highest lifespans were recorded in the São João do Sabugi Reservoir (188 years), the Esguicho Reservoir (136 years), and the Boqueirão de Parelhas Reservoir (3,846 years), reflecting greater resilience to siltation due to proportionally lower rates. The overall average lifespan was estimated at 96 years, corroborating previous studies that highlight variability in the useful life of reservoirs in semi-arid regions (Galharte & Leal, 2014; Silva & Corrêa, 2022).

The impacts of siltation include reduced storage capacity, increased evaporation due to larger exposed surface areas, and degradation of water quality. These factors directly affect the region’s water sustainability, as emphasized by Fryirs (2013a), who highlighted the role of dams in disrupting sedimentological connectivity and increasing sediment retention in densely dammed watersheds.

DISCUSSION

The average silted mass in the reservoirs was 6.39 × 10⁶ tons of sediments. These values align with those reported for the semi-arid region (Araújo, 2003; Lima, 2010; Taveira et al., 2019; Lima et al., 2024) and demonstrate the substantial amount of sediments retained in the reservoirs. As the residence time of these reservoirs is higher compared to other semi-arid reservoirs, sediments can only be expelled during years of above-average rainfall.

Based on the historical analysis of climatic events, it was estimated that, on average, the reservoirs overflowed approximately 1.7 times per decade. For example, the Itans Reservoir had an average of 1.59 overflow events every 10 years, while the Cruzeta Reservoir exhibited the highest frequency, with 1.98 overflows per decade.

During this period, the Boqueirão de Parelhas Reservoir overflowed only three times; the Caldeirão de Parelhas Reservoir, seven times; the São João do Sabugi Reservoir, nine times; the Várzea Grande Reservoir, thirteen times; the Itans Reservoir, thirteen times; and the Cruzeta Reservoir, sixteen times. Additionally, silted areas have a significant impact on water quality, increase the reservoir's surface area, and consequently amplify water losses due to evaporation, negatively affecting the population's interaction with water resources.

It is worth noting that the low sediment production in some reservoirs may be attributed to the high density of reservoirs per square kilometer in the watershed. Molle (1994) estimated the number of reservoirs in the Seridó region, showing a density of 1.5 reservoir/km² in 1985, compared to 2.5 reservoir/km² in 1965. Using remote sensing data, the NDWI index was employed to estimate the reservoirs in 2018, revealing a concentration of 0.55 km² per reservoir in the Seridó River Basin. This high density of reservoirs can interfere with hydro-sedimentological processes due to damming. As Fryirs (2013b) noted, the position of connectivity-blocking elements within the watershed and sediment residence time can trap sediments and disrupt sediment dynamics.

The sediment yield data from the reservoirs showed a correlation with USLE data. Figure 11 indicates a positive Pearson correlation (𝑟=0.61), where the modeled average gross erosion data (USLE) for each reservoir watershed aligns with the sediment production data for each reservoir and its drainage area.

Figure 11
Correlation graph of USLE data and sediment yield in reservoirs.

The Esguicho Reservoir, positioned at the far right, exhibits the highest sediment yield (7 ton·ha⁻¹·yr⁻¹) relative to its gross erosion (42.85 ton·ha⁻¹·yr⁻¹), indicating greater efficiency in sediment transport to the reservoir. This behavior may be influenced by sub-basin characteristics such as lower upstream reservoir density, reduced sediment retention capacity along the drainage network, or higher hydro-sedimentological connectivity. On the other hand, reservoirs such as Boqueirão de Parelhas and Caldeirão de Parelhas, which exhibit low sediment yield relative to gross erosion, suggest greater sediment retention along the flow path. This may be associated with factors such as a higher density of reservoirs in the contributing basin, which act as barriers to sediment transport, reducing the sediment load reaching the final reservoir.

The Itans, Cruzeta, and Várzea Grande reservoirs, which are located closer to the trend line, exhibit sediment yield values that align more consistently with the expected relationship between gross erosion and sediment yield, reinforcing the general estimate that sediment deposition in reservoirs corresponds to approximately 10% to 20% of total basin erosion, as reported by Araújo (2003). However, the variability observed among the reservoirs suggests that site-specific factors—such as slope gradient, vegetation cover, and land use and land cover dynamics—may significantly influence sediment retention and transport processes.

This relationship reinforces the applicability of the USLE in watershed studies, even in semi-arid conditions, as highlighted by authors such as Panagos et al. (2015), who emphasize the relevance of the USLE in global and regional erosion studies. Similarly, studies such as those by Silva & Corrêa (2022) discuss the importance of validating erosion models with field data, particularly in regions where sediment transport dynamics are influenced by intense precipitation events.

The errors associated with measurements (represented by error bars) indicate variations in observed data, which can be attributed to factors such as differences in the apparent bulk density of the soil and variability in sediment deposition in the reservoirs, as discussed by Rabelo et al. (2023). These factors should be considered to calibrate models and improve the accuracy of estimates.

Reservoirs that control their respective basins represent specific units within the drainage basin. The dynamics of hydro-sedimentological processes demonstrate that these reservoirs act as primary sediment sinks. The quantity of sediment accumulated in reservoirs reflects the integration of internal processes as well as all interactive processes of erosion and sediment transport on the slopes of the drainage basin (Schiefer et al., 2001).

CONCLUSION

This study analyzed the impacts of erosion and siltation on reservoirs in the Seridó River Basin using the Universal Soil Loss Equation (USLE) and empirical sedimentation data to quantify hydro-sedimentological dynamics in a semi-arid region. Siltation rates ranged from 0.12% to 8.12% per decade, with significant capacity reductions, such as in the Cruzeta Reservoir, which lost 11.45 hm³ over 81 years (4.78% per decade), and the Itans Reservoir, with a reduction of 17.80 hm³ over 82 years (2.95% per decade). These values resulted in projected lifespans ranging from 31 years (Cruzeta) to 3,846 years (Boqueirão de Parelhas), with an overall average of 96 years. These data reflect the cumulative impacts of improper land use and climatic irregularity in the region.

The average silted mass in the reservoirs was 6.39 million tons, representing a considerable impact on storage capacity and reservoir functionality. This sediment accumulation also contributed to an increase in the exposed surface area of the reservoirs, intensifying evaporation losses and compromising water sustainability in a region that faces extreme climatic variability and high demand. Additionally, siltation reduces the reservoirs' ability to regulate water availability over time, leading to a progressive decline in their storage efficiency and operational lifespan.

In addition to storage limitations, the high density of dams in the basin (an average of 0.55 km² per reservoir) disrupted sedimentological connectivity, transforming reservoirs into significant sediment sinks. However, erosive processes remain strongly influenced by local conditions, such as the predominance of exposed soils and inadequate management practices, which intensify sediment transport to the reservoirs. These results highlight the urgent need for conservation practices to mitigate soil degradation and prevent the loss of storage capacity and regulatory efficiency, ensuring greater hydrological and environmental resilience for the region.

DATA AVAIABILITY:

Research data is only available upon request.

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Edited by

• Editor in-Chief:
  Adilson Pinheiro

• Associated Editor:
  Fernando Mainardi Fan

Publication Dates

• Publication in this collection
  07 July 2025
• Date of issue
  2025

History

• Received
  01 Jan 2025
• Reviewed
  31 Mar 2025
• Accepted
  29 Apr 2025