Abstract
Understanding the dynamics of pesticide transport in the Malewa River and Lake Naivasha, a major fresh water resource, is critical to safeguard water quality in the basin. In this study, the soil and water assessment tool (SWAT) model was used to simulate the discharge of sediment and pesticides (notably the organochlorine residues of lindane, methoxychlor and endosulfan) into the Malewa River Basin. Model sensitivity analysis, calibration and validation were performed for both daily and monthly time steps using the sequential uncertainty fitting version 2 (SUFI-2) algorithm of the SWAT-CUP tool. Water level gauge data as well as a digital turbidity sensor (DTS-12) for suspended sediment transport were used for the SWAT calibration. Pesticide residues were measured at Upper and Down Malewa locations using a passive sampling technique and their quantity was determined using laboratory gas chromatography. The sensitivity analysis results showed that curve number (CN2), universal soil loss equation erodibility factor (USLE-K) and pesticide application efficiency (AP_EF) formed the most sensitive parameters for discharge, sediment and pesticide simulations, respectively. In addition, SWAT model calibration and validation showed better results for monthly discharge simulations than for daily discharge simulations. Similarly, the results obtained for the monthly sediment calibration demonstrated more match between measured and simulated data as compared to the simulation at daily steps. Comparison between the simulated and measured pesticide concentrations at upper Malewa and down Malewa locations demonstrated that although the model mostly overestimated pesticide loadings, there was a positive association between the pesticide measurements and the simulations. Higher concentrations of pesticides were found between May and mid-July. The similarity between measured and simulated pesticides shows the potential of the SWAT model as initial evaluation modelling tool for upstream to downstream suspended sediment and pesticide transport in catchments.
Highlights
Lake Naivasha in Kenya serves as a source of fresh water for most of the farms in the area.It provides fresh water for domestic consumption and supports a variety of wildlife around theLake [1]
This study focuses on hydrological modeling of runoff, sediment and pesticides residues in the Malewa River Basin located in the Lake Naivasha catchment, Kenya
The results showed that the discharge peaks during the simulations were most sensitive to curve number (CN2) values and the SOL_K parameter, which both vary spatially in the basin, and other parameters were in lower level of sensitivity
Summary
Lake Naivasha in Kenya serves as a source of fresh water for most of the farms in the area.It provides fresh water for domestic consumption and supports a variety of wildlife around theLake [1]. Lake Naivasha in Kenya serves as a source of fresh water for most of the farms in the area. It provides fresh water for domestic consumption and supports a variety of wildlife around the. The long-term use of agrochemicals and the continuous upstream to downstream transport of suspended sediment into the lake could endanger the ecosystem as well as the livelihood of the local people. Intensive and extensive agricultural activities have been identified as the main contributors to sediment generation in the Lake Naivasha catchment [1]. Pollution by agrochemicals causes the death of the aquatic life that provides a livelihood for many fishermen in the area.
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