**1. Introduction**

The amount and occurrence time of sediment export from the watersheds are mainly caused by different types of soil erosion, such as channel bed and bank erosion, overland surface erosion, and glacier erosion [1]. As soil erosion causes shear stress by rainfall, surface runoff brings most sediment yields from the overland eroded soil and then is usually transported to the downstream [2]. Moreover, extreme natural disturbances (i.e., typhoons, earthquakes, landslides, and floods) also play as the trigger for abnormal sediment yields in some regions [3], and further change the characteristics of sediment yields and sediment transports in watersheds [4]. Many studies have indicated that climate change has caused higher rainfall intensity and annual precipitation, leading to increases in sediment yields and soil erosion rates [5].

In order to comprehensively quantify the impacts of rainfall, soil erodibility, land use cover, topography, and support practice to sediment yields, the universal soil loss equation (USLE) [6] and the modified universal soil loss equation (MUSLE) [7] have been developed. These two empirical equations have been used to estimate the soil loss in watersheds worldwide [8]. Some other physically-based erosion models were developed in the past decades, such as those used in ANSWERS (Areal Nonpoint Source Watershed Environmental Response Simulation) [9], EPIC (Environmental Policy Integrated Climate) model [10], WEPP (Water Erosion Prediction Project) [11], and EUROSEM (European Soil

Erosion Model) [12]. The main difference between MUSLE and USLE is that USLE uses rainfall as an indicator of erosive energy, while MULSE uses the amount of runoff to simulate erosion and sediment yield. Sadeghi et al. [8] reviewed 49 papers of MUSLE application worldwide, and presented that most of MUSLE studies were to estimate the sediment yield on a storm basis (73.91%), monthly basis (2.17%), and annual basis (17.39%), while other applications of MUSLE (6.53%) were studied in soil erosion in storm-wise scale, pollutant estimation, and return periods of annual sediment yield. Therefore, MUSLE can better simulate the sediment yields during single storms with a low level of estimation error [13]. Since USLE and MUSLE were developed based on experiment watersheds in the U.S., [8] indicated that MUSLE would have huge errors without the calibration but can present reliable results for sediment yield on a storm basis after calibration, especially when it is applied under appropriate conditions (i.e., rangeland watersheds, similar climatic conditions as the USA) similar to where the original model was developed.

The process of erosion is usually described in three stages, which are detachment, transport and deposition; and four main types, which are sheet, rill, gully and in-stream erosion [14]. Sheet erosion and rill erosion are often classified as overland flow erosion caused by raindrop or overland flow [2,15]. Both types of erosions are usually considered together in erosion modelling. Gully erosion is the removal of soil along the channels of concentrated flow and is controlled by the thresholds related to slope and catchment area [2]. In-stream erosion involves the direct removal of sediment from stream beds and stream banks, especially during high flow periods huge amount of sediment transported through the stream network originates from the stream channel [14]. Erosion/sediment transport models can be categorized into three types: empirical model, conceptual model, and physics-based model [14]. The empirical models, the simplest ones, are based on the statistical observations of experiment areas in response to the characteristics of sediment transport [16], while conceptual models usually represent the catchment as a series of internal storages with parameters values determined through calibration against observed data [17]. Physics-based models integrate some fundamental physical equations, such as the equations of conservation of mass and momentum for flow, and those equations for sediment [14]. Compared to physics-based models and more complex conceptual models, which usually lack of sufficient spatially distributed input data, empirical and conceptual models are suggested to be combined for presenting the event responsiveness and sensitivity to climate variability [14].

The soil and water assessment Tool (SWAT) is a semi-distributed hydrological watershed model, which can simulate water balance, plant growth, and transport of sediment, nutrients, heavy metals and pesticides. The SWAT model has been used worldwide for simulating the impact of climate change and land use change on streamflow, sediment and nutrients exports, and best management practices (BMPs) on watershed responses in different countries [18–20]. Generally, the SWAT model performed well for simulating streamflow, sediment and nutrient exports at various spatio-temporal scales. MUSLE plays an important role of the simulation of continuous sediment loads [21]. Sediment transport is often called total sediment load, which is the sum of bed load and suspended load. The sediment transport is generally modeled through the sediment flow caused by overland flow and the channel erosion [22]. Some studies indicated unsatisfactory sediment simulation of the SWAT model. Addis et al. [23] used the SWAT model to simulate discharge and sediment transport at a small mountainous catchment in Ethiopian plateau, and showed that SWAT performed well for discharge but overestimated daily sediment transport with NSE = 0.07 and −1.76 for calibration and validation, respectively, mainly due to insufficient observed data and the heavy rainfall during simulation period. Bressiani et al. [24] indicated that the SWAT model was suitable for sediment simulation in most areas in Brazil and suggested the change in transmission of sediment should be reflected in the current SWAT version.

As the sediment transport and sediment concentration can affect the nutrient and turbidity in water, the estimation of sediment transport or sediment concentration need to be more reasonable. Arnold et al. [25] advised that SWAT users need to calibrate the discharge and sediment transport or

concentration sequentially before simulating the nutrients. However, extreme rainfalls in Taiwan result in serious debris flows and landslides, making it more difficult to simulate the sediment transport in the river. As the runoff and sediment hazards in Taiwan are increasing, Lee et al. [26] indicated that the runoff and sediment yield would increase with the storm events and become more frequently occurred, especially in the small mountainous catchments. Chiu et al. [27] simulated the sediment yield in the Shihmen reservoir in Taiwan, and revealed that natural distributions (i.e., typhoon, storm, and earthquake) have become an important potential source of sediment yield. Therefore, Chang et al. [28] suggested that landslide should be considered in the model to simulate more accurate and reasonable sediment exports. In order to more accurately simulate the sediment transport and sediment yields in a small mountainous watershed in Taiwan, we aim to: (1) integrate Taiwan universal soil loss equation (TUSLE) and the landslide area-volume estimation equation into the SWAT model (version 664) as SWAT-Twn model; (2) examine five sediment transport methods in the SWAT model; (3) provide the calibration and validation experience of sediment transport simulation for the SWAT users.
