Abstract. The Soil and Water Assessment Tool (SWAT) is a globally applied river basin ecohydrological model used in a wide spectrum of studies, ranging from land use change and climate change impacts studies to research for the development of the best water management practices. However, SWAT has limitations in simulating the seasonal growth cycles for trees and perennial vegetation in the tropics, where rainfall rather than temperature is the dominant plant growth controlling factor. Our goal is to improve the vegetation growth module of SWAT for simulating the vegetation variables -such as the leaf area index (LAI) -for tropical ecosystems. Therefore, we present a modified SWAT version for the tropics (SWAT-T) that uses a straightforward but robust soil moisture index (SMI) -a quotient of rainfall (P ) and reference evapotranspiration (ET r ) -to dynamically initiate a new growth cycle within a predefined period. Our results for the Mara Basin (Kenya/Tanzania) show that the SWAT-Tsimulated LAI corresponds well with the Moderate Resolution Imaging Spectroradiometer (MODIS) LAI for evergreen forest, savanna grassland and shrubland. This indicates that the SMI is reliable for triggering a new annual growth cycle. The water balance components (evapotranspiration and streamflow) simulated by the SWAT-T exhibit a good agreement with remote-sensing-based evapotranspiration (ET-RS) and observed streamflow. The SWAT-T model, with the proposed vegetation growth module for tropical ecosystems, can be a robust tool for simulating the vegetation growth dynamics in hydrologic models in tropical regions.

Most common numerical solutions used in CSTR-based in-stream water quality simulators are susceptible to instabilities and/or solution inconsistencies. Usually, they cope with instability problems by adopting computationally expensive small time steps. However, some simulators use fixed computation time steps and hence do not have the flexibility to do so. This paper presents a novel quasi-analytical solution for CSTR-based water quality simulators of an unsteady system. The robustness of the new method is compared with the commonly used fourth-order Runge-Kutta methods, the Euler method and three versions of the SWAT model (SWAT2012, SWAT-TCEQ, and ESWAT). The performance of each method is tested for different hypothetical experiments. Besides the hypothetical data, a real case study is used for comparison. The growth factors we derived as stability measures for the different methods and the R-factor-considered as a consistency measure-turned out to be very useful for determining the most robust method. The new method outperformed all the numerical methods used in the hypothetical comparisons. The application for the Zenne River (Belgium) shows that the new method provides stable and consistent BOD simulations whereas the SWAT2012 model is shown to be unstable for the standard daily computation time step. The new method unconditionally simulates robust solutions. Therefore, it is a reliable scheme for CSTR-based water quality simulators that use first-order reaction formulations.

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Existing dimensionless expressions that represent the incipient motion of sediments are based on studies of non-cohesive sediments. Because of the complex behaviour of cohesive sediments, many simulators also assume non-cohesiveness when simulating the erosion of cohesive sediments. However, studies show that the critical shear force needed for entrainment is much higher for consolidated cohesive sediments than for similarly sized non-cohesive sediments. Treating cohesive sediments as non-cohesive sediments thus will introduce a significant error with regard to quantifying the eroded sediment mass. On the other hand, the existing expressions of non-cohesive sediments require relatively detailed hydraulic calculations to estimate the shear velocity or the bed shear stress and thus cannot be used with simplified simulators. Therefore, it is essential to have a versatile simple explicit method that estimates the incipient motion condition of both the consolidated cohesive and non-cohesive sediments whenever needed. In this paper, explicit analytical expressions are proposed that simulate the incipient motion of consolidated cohesive and non-cohesive sediments, based on the critical erosion curves of the Hjulstr€ om-Sundborg-Miedema diagram. The new method reproduces the latter diagram with high precision. It also mimics the critical incipient condition of non-cohesive sediments determined by a well-known analytical method for other experimental data sets and for the East Fork River without the need of an iterative solution. The new approach provides essential information for estimating the entrainment condition of pebbles or finer sediments. Besides, the use of the mean flow velocity and the flow depth as predictors of incipient condition allows for its easy and efficient implementation in conceptual simulators that do not perform detailed hydraulic calculations and for use by modelers that are not familiar with the hydrotechnical literature. It also reduces the computation time required for simulation.