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Impacts hydro-sédimentaires des feux et de la faune envahissante sur le bassin versant de la Thiem : Touho, Nouvelle-Calédonie

Résumé : In New Caledonia wildfires and invasive fauna (deer and wild pigs) constitute major threats to forests ecosystems. In the northeast coast characterized by a humid tropical climate (2400 mm y-1 of rainfall) wildfire pressure constrains progressively the forest to thalwegs at low altitude, creating a mosaic-like landscape composed of savannas resistant to fire and humid forests. This study aims at characterizing the impact of the pressures induced by such effects on the hydrological regime of the Thiem watershed, where a reduction of the low flows has been observed for the past few decades. Runoff from five drainage areas ranging from 100 m2 plots to a 3.1 km2 watershed and erosion from three of them were studied to quantify the hydrological and sedimentary response of different and characteristic land surfaces. Pathologically high runoff and erosion were found at the 100 m2 forest plot highly impacted by ungulates and at the 3.1 km2 mixed watershed with a 76 % forest cover. We showed that the forest fragmentation as well as the invasion of ungulates create discontinuities in the soil profile along the flow pathways, which disrupt the continuity of subsurface flow, major component of the hydrological regime in the study area characterized by highly anisotropic soils. Subsurface flow exfiltration in critical areas favors erosion and reinforce the surface runoff, which exacerbates the floods. The flooding water never gets the chance to infiltrate and supply the deep-water resources that feed the rivers during the dry season, which explain the low flows decrease in the study area. The topsoil is lost through a positive feedback mechanism which leads to the conclusion that a healthy hydrological regime cannot be regained without an intervention. Special attention should be given to the watersheds providing drinking water, where wildfires must be completely excluded. Tree plantations located in strategic areas, including the forest edges, as well as ungulates regulation should be implemented.
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https://hal-unc.archives-ouvertes.fr/tel-03725625
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Soumis le : lundi 18 juillet 2022 - 02:57:58
Dernière modification le : vendredi 22 juillet 2022 - 04:11:27

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  • HAL Id : tel-03725625, version 1

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Caroline Marie Clémence Tramier. Impacts hydro-sédimentaires des feux et de la faune envahissante sur le bassin versant de la Thiem : Touho, Nouvelle-Calédonie. Hydrologie. Université de la Nouvelle-Calédonie, 2021. Français. ⟨NNT : 2021NCAL0011⟩. ⟨tel-03725625⟩

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