Assessment of the impact of land cover changes on sediment yield: a case study

Soil erosion is one of the most serious future threats to agriculture and the environment (Montoya et al. 2021). Human activities on rivers may have a significant impact on water and sediment regimes (Frangipane & Paris 1994). In addition to the climatic change, hydrological and environmental circumstances, human-induced factors, such as land use changes, influence sediment load trends. Human-induced activities impact erosion risk, which is directly tied to land cover, vegetation, and forest area. Dinçsoy (2013) pointed out that forests control stream flows and filter both surface and groundwater. It was stated that afforestation and terrace construction are the principal methods for preventing erosion in basins in Türkiye. Furthermore, it was reported that afforestation has previously been utilized to reduce erosion and regulate sediments in the basins, with positive outcomes (Dinçsoy 2013).

Soil erosion influences agricultural regions and silt accumulation in dam reservoirs (Patil et al. 2021). Because river morphology changes over time because of variables such as climate change (Li et al. 2022), continual monitoring of flow and sediment transport in rivers is required to manage the river basin system and the functioning of water infrastructure in the basin network.

Land use significantly affects surface runoff, soil erosion, and sediment delivery (van Rompaey et al. 2002). Changes in land use/land cover (LULC) effect soil erosion and sediment deposition in watersheds, hence they should be included in water and land resource planning and management studies for environmental protection. Water and land resources are critical for mankind and the long-term management and preservation of the environment. Increasing demands and human activities have made changes in the features and surface topography of land resources. The differences in LULC over time, climate change impacts, and sediment transport dynamics all impact the design stages of water infrastructure.

In terms of assuring intended performance, sedimentation plays an important role in the planning and project design of water structures in water resource development and management (Güvel & Yurtal 2020). The features of river basin water and land resources, as well as hydrological circumstances, are concerns connected to sediment movement in rivers, and observation and monitoring studies are required for long-term management.

Problems concerning the erosion process, sediment transport, and sediment production in catchments are critical for assessing existing and future water and land management research. Soil erosion formation and consequences must be examined to analyze the impacts of soil erosion on water infrastructure and determine the negative effects in operation-maintenance studies. In the past years, studies were limited due to the lack of land cover data, but today, ever-evolving technologies such as remote sensing have made it possible to provide continuous data in remote and hard-to-access areas. For many years, studies on sediment yield inquiry and estimate in basins have been conducted (Evans & Seamon 1997; Kothyari & Jain 1997; Guyot et al. 2007; Jain et al. 2010; Wang et al. 2012; Fang 2017; Colman et al. 2018; Abdul Razad et al. 2020; Güvel et al. 2021; Kolli et al. 2021).

The Seyhan River basin, which is one of Türkiye's 25 hydrological basins, is located in the south of the country. The Seyhan River basin stretches from the Taurus Mountains' 3,500 m above sea level in the north to the Mediterranean coastlines in the south. The river basin has both high mountains and vast plains. The Göksu and Zamantı rivers merge to form the Seyhan River. The Seyhan River stretches from the Taurus Mountains and the Eğlence, Körkün, Üçürge, and Çakıt streams merge with the Seyhan River. The Seyhan River passes through Adana City, then meanders in the Çukurova Plain, and arrives in the Mediterranean Sea.

For many years, water and land resource development and management initiatives for the Seyhan River basin have been in place. Multipurpose dams and hydropower projects, as well as irrigation, flood and sediment control, and drinking water supply projects, were built in the river basin's mainstream and tributaries in recent years. Many research on LULC changes (Zadbagher et al. 2018), climate change impacts (Tanaka et al. 2006; Fujihara et al. 2008a, 2008b), sediment yield estimations (Kokpinar et al. 2015), and sediment deposition in reservoirs (İlter & Ağıralioğlu 1987; Göğüş & Adıgüzel 1991; Kırkgöz et al. 1992; Fakıoğlu 2005; Güvel 2007; Güvel & Yurtal 2020) were undertaken in the Seyhan River basin.

The Seyhan Dam, built on the Seyhan River and put into operation in 1956, is in the province of Adana, in the Seyhan River Basin. Çakıt and Körkün Streams are tributaries of the Seyhan River, which drains into the reservoir of the Seyhan Dam.

The goal of this study is to examine land use and land cover changes in the Çakıt and Körkün Stream sub-basins using CORINE data in order to contribute to the evaluation of the impacts of sediment transport in the streams and on the operation and maintenance activities of water infrastructure. Sediment management approaches and strategies were also evaluated at the Çakıt and Körkün Stream sub-basins by examining the changes induced by LULC changes. Moreover, in this study, it was examined how to provide practical answers to difficulties that may arise, to contribute to long-term sediment management. In comparison with previous studies on the topic, within the scope of this study, the impact of land cover changes on sediment yield in the examined sub-basins was assessed using both suspended sediment observation data from sediment observation stations, bathymetric maps, and CORINE data.

The Çakıt Stream originates in the highlands surrounding Ulukışla. The Çakıt stream gets its name from where the Şekerpınarı water meets the stream. The Çakıt stream, which runs for 162 km through the valleys up to Arapali village, is joined into the Seyhan Dam reservoir (DSİ 2014). The Çakıt sub-basin has previously been reported to have a major landslide problem. The causes of landslides and erosion in the Çakıt sub-basin have been investigated, and many structural solutions have been offered (DSİ 2014).

The Körkün Stream is also a tributary of the Seyhan River. The branches that originate on the western slopes of Aladağlar merge and flow south, where they meet the Ecemiş Stream and continue to flow and mix with the streams that originate on the side branches. It follows the Koca Stream, joins with the Kadıoğlu Stream, and eventually enters the Seyhan Dam reservoir. Dams for energy production, drinking water supply, and agricultural irrigation purposes have been proposed for the Körkün sub-basin in earlier studies (DSİ 2014).

Sediment monitoring data in Çakıt sub-basin were limited in the past. Sediment flow from the Çakıt Stream to the Seyhan Reservoir has occurred because of landslides and erosion events in the higher elevations of the Çakıt Stream, and a sediment observation station was established on Çakıt Stream in 2005 to improve the sediment monitoring system. The observation station at Çakıt Stream Salbaş (E18A028) is located 300 m upstream of the bridge over Çakıt stream at the entrance to Salbaş Village, 20 km from the Adana Karaisalı Highway. Monthly suspended sediment observation data at this station between May 2005 and December 2018 were used in the study (DSİ 2018).

The water resources of the Seyhan Dam reservoir are the Seyhan River, Körkün, and Çakıt streams. In 1997, Çatalan Dam was built upstream of Seyhan Dam on the Seyhan River, preventing silt movement into the Seyhan Dam reservoir. As mentioned in previous research, the silt from the Körkün stream that reached the dam reservoir had little impact on its operation. The silt from the Çakıt Stream basin, on the other hand, fills the dam reservoir and shortens the dam's life. In past years, afforestation was undertaken in the Çakıt stream sub-basin. Planning studies for bed enhancement measures to control problems such as slope collapses and landslides were conducted (Fakıoğlu 2005).

The time period for producing hydrographic maps is determined by official regulations (DSİ 2007). Bathymetric surveys in the Seyhan Dam reservoir were carried out in 1966, 1971, 1976, 1980, 1986, 1991, 2005, and 2019. In this study, the bathymetric survey data were used to assess and validate the impact of land use changes on reservoir sediment buildup.

Coordination of Information on the Environment (CORINE) land use cover data were used in this study to detect changes in LULC, particularly in forest areas, in the Çakıt and the Körkün sub-basins.

As a consequence of the CORINE land use cover (CLC) investigations, which began in 1985, the initial map was developed in 1990 and was revised in 2000, 2006, 2012, and 2018. The standard CORINE land cover nomenclature has three levels. The CLC classes are defined as artificial surfaces, agricultural areas, forest and semi-natural areas, wetlands, and water bodies at Level 1 in the CORINE Land Cover nomenclature (Büttner et al. 2021). The CLC data contain 44 distinct land cover classifications and were taken from https://land.copernicus.eu/pan-european/corineland-cover.

The spatial distribution of LULC changes has been studied in several studies using CLC data (Falcucci et al. 2007; Ruiz Benito et al. 2010; Diaz-Pacheco & Gutiérrez 2014; Cole et al. 2018, 2022; Waltner et al. 2020; Aune-Lundberg & Strand 2021; Güvel et al. 2021).

Temporal changes and spatial distribution of LULC were examined, and areal values in km² on the specified dates were calculated using CLC data. The impact of LULC changes on sediment yield was analyzed using suspended sediment observation data and CLC data.

In data analysis and assessments of spatial changes in the selected study area, geographic information system (GIS) technique was used. GIS is a widely used technique in many fields of science around the world, such as water resource planning and management (McKinney & Cai 2002; Chowdary et al. 2003; Rao et al. 2004; Naidu & Giridhar 2011; Suresh Babu et al. 2012), disaster management (Sajjad et al. 2020), risk analysis (Samarasinghe et al. 2010; Guoqing et al. 2011), forestry (Soubry et al. 2021), construction management (Palve 2013; Al-Saggaf & Jrade 2015; Patel et al. 2017; Chowdary et al. 2019; Han et al. 2020), environment (Mahé et al. 2013), and/or natural resource management (Kingra et al. 2016).

Suspended sediment observation data from the Körkün Stream Hacılı Bridge station between 1977 and 2018 were investigated, and it was discovered that suspended sediment concentration (C, ppm) in Körkün Stream declined with time while forest area decreased (Figure 13). The correlation coefficient between forest area and suspended sediment concentration in the Körkün sub-basin was determined to be 0.4431. It was determined that the drop in suspended sediment content was caused by the installation of water structures on the Körkün Stream.

Analyzing changes in forest and agricultural areas in the Çakıt sub-basin using CLC data, it was determined that the increase in forest areas between 1990 and 2018 was one of the primary causes for the decrease in suspended sediment transport in the Çakıt Stream (Figure 8). When the suspended sediment observation years in the Körkün sub-basin are considered, it is apparent that the sediment content in the stream decreases over time (Figure 13). Future monitoring is advised to better quantify sediment transport in streams once the planned upstream facilities are operational.

Previous studies looked at probable future events involving sediment transport. According to Güvel & Yurtal (2020), the water and sediment structures constructed upstream of the river branches that connect to the Seyhan Dam reservoir would limit silt transfer to the reservoir.

In the research region, small dams were put into operation in 2016. Small dams, Demirçit and Yağlıtaş, were built in the Çakıt and Körkün sub-basins, respectively. The Karakuz hydroelectric power plant was completed in 2015 and is located on the Körkün stream. Technical studies for the construction of some other minor dams are also ongoing.

Bathymetric data taken in the Seyhan Dam reservoir in previous years were utilized to confirm that variations in the LULC in the Çakıt stream upper basin are the major source of silt deposition with water storage capacity losses when Çakıt stream enters the reservoir. As previously stated, the rate of capacity loss in the reservoir was estimated as sediment buildup (Güvel & Yurtal 2020). Sediment movement on the Çakıt and Körkün Streams to the Seyhan Dam reservoir continues and must be considered in future water structure operations and maintenance.

The study has some limitations in terms of spatial resolutions and temporal data collection. One of the limitations of this study is the lack of particular plant-specific information on land use maps. It would be advantageous to do research using more precise land use maps to establish which plant kinds are included in these detailed land use maps, such as in agricultural regions, as well as to assess the root length of the plants and their influence on sediment transport. Furthermore, as technology advances, using data that has high spatial and temporal accuracy will positively impact assessments on sediment yield.

This research was conducted to explore the effect of LULC changes on sediment transport in the Seyhan River Basin's Çakıt and Körkün Streams. Land use trends in the Çakıt and Körkün Stream sub-basins were assessed using CORINE data. The findings of the study are expected to be useful to decision-makers and technical staff in charge of sediment control in water structure operations.

The following conclusions were derived after evaluating CLC data between 1990 and 2018 in the Çakıt and Körkün sub-basins.

The forest area in the Çakıt sub-basin drainage area was determined to be approximately 15 and 19% of the drainage area in 1990 and 2018, respectively. It was discovered that increased forest area in the Çakıt sub-basin drainage region until 2018 helped to reduce sediment movement. The agricultural area (Level 1) in the Çakıt sub-basin drainage area was determined to be approximately 25 and 22% of the drainage area in 1990 and 2018, respectively. The arable land in the Çakıt sub-basin drainage area was determined to be approximately 7 and 11% of the drainage area in 1990 and 2018, respectively. Permanent crops were determined as 0.67 and 0.69% of the drainage area in 1990 and 2018, respectively. The heterogeneous agricultural area in the Çakıt sub-basin drainage area was determined to be approximately 16 and 10% of the drainage area in 1990 and 2018, respectively. Pastures in the Çakıt sub-basin drainage area were determined to be approximately 1.5 and 0.7% of the drainage area in 1990 and 2018, respectively. The results demonstrate that the amount of forest areas, and water bodies expanded in the Çakıt sub-basin between 1990 and 2018, and this result was identified to assist minimize sediment flow until 2018.

The forest areas in the Körkün sub-basin drainage area were assessed to be approximately 8 and 7% of the drainage area in 1990 and 2018, respectively. Between 1990 and 2018, water bodies and mineral extraction sites increased while forest areas decreased in the Körkün sub-basin. It was established that the agricultural area (Level 1) in the Körkün sub-basin drainage area remained approximately 21% between 1990 and 2018. The arable land in the Körkün sub-basin drainage area was determined to be approximately 2 and 3% of the drainage area in 1990 and 2018, respectively. Permanent crops were determined as 3.16 and 3.26% of the drainage area in 1990 and 2018, respectively. The heterogeneous agricultural area in the Körkün sub-basin drainage area was determined to be approximately 16 and 14% of the drainage area in 1990 and 2018, respectively. Pastures in the Körkün sub-basin drainage area were determined to be less than 0.1% of the drainage area between 1990 and 2018.

The water structures built in the two sub-basins decreased sediment transport to the Seyhan Dam reservoir by trapping sediment in the artificial reservoirs in the upper reaches of the streams. The correlation coefficient values between forest area and suspended sediment concentration in Çakıt and Körkün sub-basins were found to be 0.9521 and 0.4431, respectively.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.