Abstract
ABSTRACT Riparian vegetation plays an important role in sediment retention, thus reduces sediment yield in watersheds. The Brazilian Forest Law (Law 12,651/2012) requires maintenance of fixed-width buffers of riparian vegetation but allows the continuity of agriculture, livestock, and forestry farming activities in some parts of the Areas of Permanent Preservation (APP). This paper aimed to evaluate sediment reduction by recovering the APPs with vegetation strips of permitted widths (5, 8, 15, and 30 m), as per the Forest Law. We considered three land use scenarios that present distinct erosion rates - predominance of areas with forest cover, pasture, and agriculture. The Soil and Water Assessment Tool (SWAT) model was used to simulate sediment yield in these scenarios at the Jundiaí-Mirim Watershed in São Paulo, Brazil. The SWAT was calibrated and validated for monthly streamflow. We obtained statistical indices for the processes of calibration and validation, respectively, as: NS = 0.77 and 0.70, PBIAS = -10.2 and -12.5, and RSR = 0.48 and 0.55. The highest reduction in sediment yield (30%) was observed with the total recovery of the APPs (vegetation strips of 30 m) in the current land use scenario. The recovery of the APPs with vegetation strips of 5, 8, and 15 m yielded sediment reduction below 10% in the alternative land use scenarios. The APP strips with reduced recovery maintained high rates of sediment yield. Additionally, even with a total recovery of the APP it is necessary to adopt soil conservation practices throughout the basin’s agricultural area to minimize the impacts on water resources.
Highlights
Land use affects the components of the hydrological cycle and, sediment flows in watersheds (Ghaffari et al, 2010; Alvarez-Garreton et al, 2019; Kang et al, 2020; Zhang et al, 2020)
The study was performed at the Jundiaí-Mirim Watershed (JMW), which is a part of the “Water Resources Management Unit 5” (UGRHI 5; acronym in Portuguese) in São Paulo, Brazil
We found that the most sensitive parameters for the JMW streamflow, in the descending order of importance, were: soil evaporation compensation factor (ESCO), curve number for moisture condition II (CN2), depth to the bottom of the soil layer (SOL_Z), and available water capacity of soil layer (SOL_AWC)
Summary
Land use affects the components of the hydrological cycle and, sediment flows in watersheds (Ghaffari et al, 2010; Alvarez-Garreton et al, 2019; Kang et al, 2020; Zhang et al, 2020). Since the removal of natural vegetation induces accelerated soil erosion, replacing forests with agricultural land significantly increases erosion rates (Germer et al, 2009). It is estimated that the replacement of forests with cropland increases soil erosion by 52% worldwide (Borrelli et al, 2017). The sediments generated due to erosion are carried to watercourses, lakes, ponds, and artificial dams, taking nutrients and pesticides adsorbed on their surfaces. This results in silting up of river channels and contamination of water bodies, putting them at risk (Hajigholizadeh; Melesse; Fuentes, 2018; Himanshu et al, 2019). There are costs resulting from the repair of damages caused by sediment deposition in rivers, lakes, and dams (Batista et al, 2017; Food and Agriculture Organization of the United Nations FAO, 2019). Alternative uses of the strips surrounding the water bodies result in less ground cover and tend to increase sediment yield since the strips do not retain enough sediments
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