Impacts of forest conversion and agriculture practices on water pathways in Southern Brazil

Abstract

Land-use/cover change (LUCC), and more specifically deforestation and multidecadal agriculture, is one of the various controlling factors of water fluxes at the hillslope or catchment scale. We investigated the impact of LUCC on water pathways and stream stormflow generation processes in a subtropical region in southern Brazil. We monitored, sampled and analysed stream water, pore water, subsurface water, and rainwater for dissolved silicon concentration (DSi) and 18 O/ 16 O (δ 18 O) signature to identify contributing sources to the streamflow under forest and under agriculture. Both forested and agricultural catchments were highly responsive to rainfall events in terms of discharge and shallow groundwater level. DSi versus δ 18 O scatter plots indicated that for both land-use types, two run-off components contributed to the stream discharge. The presence of a dense macropore network, combined with the presence of a compact and impeding B-horizon, led to rapid subsurface flow in the forested catchment. In the agricultural catchment, the rapid response to rainfall was mostly due to surface run-off. A 2-component isotopic hydrograph separation indicated a larger contribution of rainfall water to run-off during rainfall event in the agricultural catchments. We attributed this higher contribution to a decrease in topsoil hydraulic conductivity associated with agricultural practices. The chemical signature of the old water component in the forested catchment was very similar to that of the shallow groundwater and the pore soil water: It is therefore likely that the shallow groundwater was the main source of old water. This is not the case in the agricultural catchments where the old water component had a much higher DSi concentration than the shallow groundwater and the soil pore water. As the agricultural catchments were larger, this may to some extent simply be a scale effect. However, the higher water yields under agriculture and the high DSi concentration observed in the old water under agriculture suggest a significant contribution of deep groundwater to catchment run-off under agriculture, suggesting that LUCC may have significant effects on weathering rates and patterns.

Effects of Reforestation of a Degraded Imperata Grassland on Dominant Flow Pathways and Streamflow Responses in Leyte, the Philippines

“…Zhou et al () and Krishnaswamy et al () similarly showed major reductions in the storm runoff response after reforesting bare land in South China and overgrazed degraded forest land in India, respectively. The results are also in agreement with previous hydrochemical studies (Chaves et al, ; Robinet et al, ) and runoff plot studies (Chandler & Walter, ) that suggested that overland flow was more widespread in grazed pastures or pasture fallows than in (re)forested sites. In addition to the much higher infiltration rates in the reforested Manobo catchment than the degraded Basper catchment and resulting decreases in overland flow, the soil storage capacity of the Basper grassland has likely been diminished by the continued soil erosion and landsliding (cf.…”

“…Groundwater levels rose to 30 cm below the surface at the footslope in Manobo, thereby likely activating lateral macropores flow (cf. Noguchi et al, ; Robinet et al, ; Schellekens et al, ). Chandler and Bisogni () found that pipe flow contributed most of the interflow at a forest site on epikarst in the Philippines.…”

“…For instance, a hydrochemical study of two ephemeral streams in southwestern Amazonia on Precambrian basement rocks (granite and gneiss) showed that the runoff ratio was very small for an old growth forest microcatchment (0.8%) and streamflow during rainfall events consisted mainly of throughfall (57%), groundwater (24%), and shallow soil water (19%), but runoff ratios were much larger for a pasture (17%), where the dominant streamflow components were overland flow (60%), groundwater (35%), and soil water (5%; Chaves et al, ). Isotope and silica data from catchments with Acrisols on predominantly rhyolitic and rhyodacitic rocks in southern Brazil suggested that in a forested catchment stormflow consisted mainly of rapid subsurface flow and macropore flow, while in an agricultural catchment stormflow was mostly due to overland flow (Robinet et al, ).…”

Land use impacts on soil erosion and rejuvenation in Southern Brazil

“…We investigated the combined effect of geomorphology and land use on soil redistribution and rejuvenation in a subtropical region where the majority of Atlantic rain forest was converted to agricultural land in the 1960s (Minella, 2003;Minella et al, 2008). Our study is part of a larger project on soil systems on global change, where a forested and agricultural catchment in the Guapore river basin in Southern Brazil were studied for short-and long-term soil-landscape dynamics (Ameijeiras-Mariño et al, 2018;Robinet et al, 2018aRobinet et al, , 2018bUnzué-Belmonte et al, 2017;Schoonejans et al, 2017).…”

Water within, moving through, and shaping the Earth's surface: Introducing a special issue on water in the critical zone

“…The impact of vegetation on the CZ was also examined by Robinet et al (), who explored difference in the structure, hydrology, and geochemistry between natural forests and croplands in subtropical southern Brazil, where agriculture is fast expanding. The authors monitored stream, subsurface, and rain water for dissolved silicon concentration and O and H isotopes, to identify contributing sources to streamflow under forest versus agriculture landcover.…”