Estimation of Potential Impact of Land Use Change on Sediment Yield From a Small Tropical Watershed Using the TREX Erosion Model

Abstract

Analysis of the influence of changes in land use on the sediment yield from watersheds provides crucial inputs that aid the development of appropriate strategies for the sustainable management of consequent land degradation. Event‐based hydrologic models can be used for performing such analysis, as long‐term hydrological datasets are not available for most of the rivers in developing countries like India. In this study, an event‐based Two‐Dimensional Runoff, Erosion, and Export (TREX) model was used to simulate the soil erosion process in Moozhy, an ungauged watershed in the Karamana River basin in Thiruvananthapuram District, Kerala State, India. Rainfall, streamflow, and sediment concentration data corresponding to three isolated storm events were used to calibrate and validate the model. The performance of the model was assessed using four statistical measures, namely, Percent bias (PBIAS), Nash‐Sutcliffe Efficiency Coefficient (NSEC), Coefficient of determination (R2) and RMSE‐observations standard deviation ratio (RSR). The values of PBIAS varied between 46% and 49%, indicating satisfactory performance of the model. Also, from the values of NSEC and RSR, it can be concluded that the TREX model yields acceptable results. For a rainfall event that occurred on the 17 September 2017, the simulated values showed that about 1054 t of suspended sediments are transported from the watershed to the stream channels, and about 1020 t is carried out of the Moozhy watershed. Only a very small amount of sediment is left in the channel at the end of the simulation. About 34 t of sediment is deposited in the channel bed, about 0.04 t remains in suspension, and the balance 0.04 t remains in suspension as a suspended load. About 3% of the total sediment entering the reach of the stream considered in this study is deposited on the riverbed. The major component of the settled sediments is silt; about 12% of the total silt size fraction entering the stream settles on the river bed. Future scenarios of land use in the watershed were modelled using a hybrid Artificial Neural Network (ANN) and Cellular Automata (CA) model. These were used for simulating sediment discharge using the TREX erosion model. The predicted land use scenarios were used to investigate both short‐term (2025 and 2029) and long‐term (2037, 2045, and 2053) variations in sediment discharge. The sediment yield for the storm event that occurred on 17 September 2017 was used to benchmark the variation in sediment yield under the predicted land use scenarios. Results indicate that the peak runoff and the runoff volume are expected to increase by about 29% and 22% respectively in the period from 2005 to 2053. The expected increase in the volume of sediments in this period is about 50%; the peak sediment concentration is likely to increase by about 56%. The study highlights the threats of likely increase in soil erosion and consequent land degradation posed by unscientific changes in land use caused by urbanisation and calls for proper management interventions to address the problem.