**1. Introduction**

Soil is a precious natural resource that covers Earth's land surfaces, and it contributes to basic human needs like food, clean water, and clean air, as well as being a major carrier for biodiversity. There have been antecedents (from 3500 B.C to 17th century) of soil knowledge and its relationship with human practices before soil scientific studies. Soil is an integrated discipline within soil sciences, geography, and land management, and it was developed in parallel with agriculture [1,2]. In the globalized world of the 21st century, soil sustainability depends not only on management choices by farmers, foresters, and land planners but also on political decisions on rules and regulations; it also requires a large effort of awareness raising and the communication of issues related to the degradation of soils and land by scientists, civil society organizations, and policy makers [3]. Estimations have shown that worldwide, 75% of land is degraded due to physical, chemical, and biological processes [4]. Soil erosion has a severe impact on the degradation of quality fertile topsoil. Worldwide, soil erosion losses are the highest in the agro-ecosystems of Asia, Africa, and South America, averaging 30–40 t ha−<sup>1</sup> year<sup>−</sup>1, and it is the lowest in the United States, Europe, and Australia, averaging 5–20 t ha−<sup>1</sup> year−<sup>1</sup> [5,6]. The multifunctional use of land is needed within the boundaries of the soil–water system to achieve land degradation neutrality, avoid further land degradation, and promote land restoration [7]. Keesstra et al. [7] introduced four concepts (systems thinking, connectivity, nature-based solutions, and regenerative economics) in a more integrated way to accomplish land degradation neutrality in an effort to achieve the soil-related Sustainable Development Goals (SDGs). A robust soil–water system is essential to achieve interlinked SDGs through smart planning based on a socio–economical–ecological systems analysis [3,7].

Agricultural land degradation in rainfed mountainous areas is a major onsite problem (the removal of top soil) that also causes offsite effects, such as downstream sediment deposition in fields, floodplains, and water bodies. The costliest offsite damages occur when the soil particles enter lakes or river systems [8,9]. Annual soil loss in the middle Yellow River basin of China amounts to 3700 t km<sup>−</sup>2, the largest sediment-carrying river in the world [10]. The world's 13 large rivers carry 5.8 billion tons of sediments to reservoirs every year [11]. The Indus River in Pakistan ranks third in the world, with an annual sediment load of 435 million tons in the Tarbela dam, which has lost about 35% of initial reservoir capacity (11,600 Mm3) [12]. Water and soil are the most crucial natural resources for agriculture and livestock production. Globally, water resource deterioration caused by soil erosion is a growing concern. An estimated productivity loss of US\$13–28 billion annually in drylands can be attributed to soil erosion as well [13].

In Pakistan, dryland farming is practiced on 12 Mha of the Pothwar Plateau, the northern mountains, and the northeastern plains. Soil erosion is a severe problem due to erratic rainfall, varied soil slopes, and land use. A lot of land has been converted into gullies that are difficult to restore. Different studies related to soil erosion severity have been conducted in the Pothwar region. Hussain et al. evaluated the soil erosion parameters and estimated the annual sediment loss in small watersheds of the Dhrabi River using the Soil and Water Assessment Tool (SWAT) model. The annual sediment yield ranged from 2.6 to 31.1 t hm−<sup>2</sup> for the non-terraced catchments, while it ranged from 0.52 to 10.10 t hm−<sup>2</sup> for the terraced catchments [14]. Iqbal et al. studied runoff plots in the Dhrabi watershed in Chakwal Pakistan; cultivated slopes produced the highest soil loss (8.96 Mg ha<sup>−</sup>1) annually compared to both undisturbed gentle and steep slopes at approximately 2.08 and 4.66 Mg ha−<sup>1</sup> [15].

Nasir et al. applied the Revised Universal Soil Loss Equation (RUSLE) and a Geographic Information System (GIS) at the small mountainous watershed of Rawal Lake near Islamabad. The predicted annual soil loss ranged from 0.1 to 28 t ha−<sup>1</sup> [11]. Similarly, Ahmad et al. reported annual soil loss rates of 17–41 t ha−<sup>1</sup> under fallow conditions, as well as an annual rate of 9–26 t ha−<sup>1</sup> under vegetative cover in the Fateh Jang watershed with a slope of 1–10% [16]. Saleem et al. assessed the annual soil erosion (70–208 t ha<sup>−</sup>1) of the Pothwar region using the RUSLE model integrated with a GIS [17]. Bashir et al. estimated the soil erosion risk using the Coordination of Information on the Environment (CORINE) model in the Rawal watershed. The annual soil loss ranged between 24 and 28 t ha<sup>−</sup>1, with a high erosion risk (26%) in areas with steep slopes and low vegetative cover [18].

The highest estimated record of soil erosion was 150–165 t ha−<sup>1</sup> year−<sup>1</sup> in the Dhrabi watershed of the Pothwar region [12]. Nabi et al. reported that in the Soan watershed of Pothwar, the soil loss rates in barren and shrub land were 63.41 and 53.41 t ha−<sup>1</sup> year−1, respectively, whereas those in low and high cropping intensity land were 34.91 and 25.89 t ha−<sup>1</sup> year<sup>−</sup>1, respectively [19]. Vegetation cover on sloped ground helps to reduce soil loss; however, during field preparation and cultivation, surface soil becomes pulverized and easily eroded, causing acute topsoil erosion due to the removal of vegetation cover. Therefore, during the cultivation of sloping land, measures should be adopted to stop fertile surface soil loss caused by substantial rainfall–runoff. If such measures are not applied, agricultural land may turn barren in only a few years. Vegetation cover is a key measure for soil protection against water erosion; it reduces the flow velocity of surface runoff by increasing surface roughness, in addition to increasing the infiltration rate of soil [20–22].

Considerable increase in sediment yield at the expense of soil development poses a major threat to soil and water resource development. Though water erosion is a function of many environmental factors, its assessment and mitigation at the watershed level are complex phenomena; this is due to the unpredictable nature of rainfall and topographic heterogeneities, climate, and land use–land cover variability, as well as other watershed features for the specified areas under study. In addition, inappropriate land management practices and human activities increase the dynamics of these factors.

At present, many models with a broad spectrum of concepts—which are classified as spatially lumped, spatially distributed, empirical, regression, semi-distributed eco-hydrological models, and factorial scoring models—are in use for modelling the rainfall–runoff–soil erosion and sediment transport processes at different scales [23]. The empirical models are generally the simplest, limited to the conditions and parameter inputs for which they have been developed. For example, the Universal Soil Loss Equation (USLE) [24], the Agricultural Non-Point Source Pollution Model (AGNPS) [25], and the Sediment Delivery Distributed (SEDD) model [26]. In conceptual models, a watershed is represented by a storage system, such as the SWAT [27], the Large Scale Catchment Model (LASCAM) [28], or the European Modeling and Simulation Symposium (EMSS) [29]. Physics-based models rely on the solution of fundamental physical equations and are used for the quantification of physical processes. Areal Nonpoint Source Watershed Environment Response Simulation (ANSWERS) [30]; Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS) [31]; the Watershed Erosion Simulation Program (WESP) [32]; Système Hydrologique Européen Sediment (SHESED) [33]; and the European Soil Erosion Model (EUROSEM) [34] are some examples of physically based erosion and sediment transport models.

This research was conducted in ungauged micro-watersheds of the Chakwal and Attock districts of the Pothwar region. Soil erosion and water loss are extreme hazards in this area due to cultivated highland slopes where timely soil and water conservation strategies and remedial measures are required for sustainable crop productivity. A large number of loose stone structures have been built by public departments and farmers themselves to reduce the soil erosion and moisture conservation upside of these structures. There are few measurement points for rainfall and runoff, and most of the watersheds are ungauged; both of these issues hamper model calibration and validations. The purpose of the study was to evaluate the effectiveness of soil and water conservation structure for soil erosion control using the SWAT model. The calibrated and validated model related to soil erosion was adopted from Hussain et al. [14], where an experimental setup was used to monitor the soil and water loss from agricultural catchment. The collected data were used to calibrate and validate soil erosion parameters using the SWAT model.

This validated model was further modified for the application of soil and water conservation structures, eventually to be recommended by this study as a strategy to counteract the soil erosion with soil and conservation structures at a broader scale. Several studies related to soil and water conservation intervention were carried out to control soil erosion at the field and sub-watershed scale within the Gumara-Maksegnit watershed in the northern Highlands of Ethiopia [35–37], while Melaku et al. predicted the impact of soil and water conservation structures on runoff and erosion processes using the SWAT model [38]. However, studies on the impacts of soil and water structures on the erosion process at the watershed scale that have used the SWAT model have been limited.

Our study was localized to the micro-watersheds with soil and water conservation structures installed through the cooperative project coordinated by the International Centre for Agricultural Research in the Dry Areas (ICARDA), the Centre of Excellence in Water Resources Engineering, (CEWRE), and the Soil and Water Conservation Research Institute (SAWCRI). To the best of our knowledge, no study has been conducted to evaluate the effectiveness of these structures to control

soil erosion in the Pothwar region. The study results may encourage the stakeholders to extend this practice to a larger scale by knowing the quantitative benefits of soil conservation structures. Therefore, the SWAT model was adopted due to the availability of a comprehensive agricultural management database, as well as a reduced time and cost [39–41]. In this context, the objective of this work was to evaluate the effectiveness of soil and water conservation structures for soil erosion control using the SWAT model in the micro-watersheds of the Chakwal and Attock districts.
