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The expansion and intensification of soya bean agriculture in southeastern Amazonia can alter watershed hydrology and biogeochemistry by changing the land cover, water balance and nutrient inputs. Several new insights on the responses of watershed hydrology and biogeochemistry to deforestation in Mato Grosso have emerged from recent intensive field campaigns in this region. Because of reduced evapotranspiration, total water export increases threefold to fourfold in soya beanwatersheds compared with forest. However, the deep and highly permeable soils on the broad plateaus on which much of the soya bean cultivation has expanded buffer small soya bean watersheds against increased stormflows. Concentrations of nitrate and phosphate do not differ between forest or soya bean watersheds because fixation of phosphorus fertilizer by iron and aluminium oxides and anion exchange of nitrate in deep soils restrict nutrient movement. Despite resistance to biogeochemical change, streams in soya bean watersheds have higher temperatures caused by impoundments and reduction of bordering riparian forest. In larger rivers, increased water flow, current velocities and sediment flux following deforestation can reshape stream morphology, suggesting that cumulative impacts of deforestation in small watersheds will occur at larger scales. [ABSTRACT FROM AUTHOR]

Abstract: Landscapes in the humid tropics are undergoing a continuous change in land use. Deforestation is still taking its toll on forested areas, but at the same time more and more secondary forests emerge where formerly agricultural lands and pastures are being abandoned. Regarding soil hydrology, the extent to which secondary succession can recover soil hydrological properties disturbed by antecedent deforestation and pasture use is yet poorly understood. We investigated the effect of secondary succession on saturated hydraulic conductivity (Ks) at two soil depths (0–6 and 6–12cm) using a space-for-time approach in a landscape mosaic in central Panama. The following four land-use classes were studied: pasture (P), secondary forest of 5–8 years of age (SF5), secondary forest of 12–15 years of age (SF12) and secondary forest of more than 100 years of age (SF100), each replicated altogether four times in different micro-catchments across the study region. The hydrological implications of differences in Ks in response to land-use change with land use, especially regarding overland flow generation, were assessed via comparisons with rainfall intensities. Recovery of Ks could be detected in the 0–6cm depth after 12 years of secondary succession: P and SF5 held similar Ks values, but differed significantly (α =0.05) from SF12 and SF100 which in turn were indistinguishable. Variability within the land cover classes was large but, due to sufficient replication in the study, Ks recovery could be detected nonetheless. Ks in the 6–12cm depth did not show any differences between the land cover classes; only Ks of the uppermost soil layer was affected by land-use changes. Overland flow – as inferred from comparisons of Ks with rainfall intensities – is more likely on P and SF5 sites compared to SF12 and SF100 for the upper sample depth; however, generally low values at the 6–12cm depth are likely to impede vertical percolation during high rainfall intensities regardless of land use. We conclude that Ks can recover from pasture use under secondary succession up to pre-pasture levels, but the process may take more than 8 years. In order to gain comprehensive understanding of Ks change with land use and its hydrological implications, more studies with detailed land-use histories and combined measurements of Ks, overland flow, precipitation and throughfall are essential. [Copyright &y& Elsevier]

The article presents the study which determines the impact of land management on runoff sources in Amazon watersheds. It uses an end member mixing analysis approach (EMMA) to examine the contributing sources of streamflow during an entire rainy season in a forest and a pasture watersheds. The study shows differences in the proportions of total streamflow between the forest and pasture watershed. It suggests that there is great potential for alteration of the hydrological budgets of larger watersheds as the proportion of deforested land in Amazon increases. Moreover, the more rainfall diverted into fast flowpath to streams in established pastures, the more water with higher solute concentration generated by erosion or by bypassing sites of solute removal will be delivered.

Prolonged dry periods, and increasingly the generation of smoke and dust in partially-deforested regions, can influence the chemistry of rainfall and throughfall in moist tropical forests. We investigated rainfall and throughfall chemistry in a palm-rich open tropical rainforest in the southwestern Brazilian Amazon state of Rondônia, where precipitation averages 2300 mm year−1 with a marked seasonal pattern, and where the fragmentation of remaining forest is severe. Covering the transition from dry to wet season (TDWS) and the wet season (WS) of 2004–2005, we sampled 42 rainfall events on event basis as well as 35 events on a within-event basis, and measured concentrations of DOC, Na+, K+, Ca2+, Mg2+, NH, Cl−, SO, NO and pH in rainfall and throughfall. We found strong evidence of both seasonal and within-event solute rainfall concentration dynamics. Seasonal volume-weighted mean (VWMS) concentrations in rainfall of DOC, K+, Ca2+, Mg2+, NH, SO and NO were significantly higher in the TDWS than the WS, while VWMS concentrations in throughfall were significantly higher for all solutes except DOC. Patterns were generally similar within rain events, with solute concentrations declining sharply during the first few millimeters of rainfall. Rainfall and throughfall chemistry dynamics appeared to be strongly influenced by forest and pasture burning and a regional atmosphere rich in aerosols at the end of the dry season. These seasonal and within-event patterns of rainfall and throughfall chemistry were stronger than those recorded in central Amazônia, where the dry season is less pronounced and where regional deforestation is less severe. Fragmentation and fire in Rondônia now appear to be altering the patterns in which solutes are delivered to remaining moist tropical forests. [ABSTRACT FROM AUTHOR]

Abstract: Many modern landscapes in the humid tropics appear as a mosaic of primary forest, agricultural land and abandoned areas at various stages of regrowth. Whereas the hydrological functioning of primary forest and agricultural ecosystems is known to some degree, the impact of secondary forests on the hydrological cycle has yet to receive the same attention; this is particularly true for the effects of forest regrowth on soil hydrology. We investigated the effects of land-use and land-cover types representative of largely deforested areas of the Amazon basin on soil hydrological flowpaths by quantifying infiltrability and field-saturated hydraulic conductivity (Ksat) at two depths (12.5 and 20cm) under primary forest, recently cleared secondary forest (prepasture), teak (Tectona grandis), pasture and secondary forests originating from a former banana–cacao plantation (banana) and from pasture (capoeira). We further inferred potential changes in the hydrological flowpath regime by comparing our results with prevailing rainfall intensities. Infiltrability varied (α =0.05) among the cover types according to: {forest}>{banana, capoeira, teak}>{pasture}, with prepasture overlapping with forest, banana and capoeira. Although the infiltrability of pasture is lower by an order of magnitude in comparison to the other cover types, this decrease does not affect flowpath patterns as infiltrability still exceeds even the highest short-term, such as 5min, rainfall intensities. Ksat at the 12.5cm depth varies as follows: {forest, banana, capoeira}>{teak}>{pasture}, with prepasture being undistinguishable from the first group. Under teak and pasture, Ksat is lower than prevailing rainfall intensities, which implies the possible generation of a perched water table at a shallow depth and hence the generation of saturation overland flow. The impact of 13 years of cattle grazing prior to planting teak is still measurable even after 10 years of growth. At the 20cm depth, the differences in Ksat among the land uses are minor compared to prevailing rainfall intensities, i.e., the vertical flow of water is impeded regardless of land use. We conclude that there is a considerable memory effect; hence, any assessment of the hydrological functioning of secondary forests, quasi-natural or man-made, must take into account the kind, intensity and duration of land use prior to regrowth. [Copyright &y& Elsevier]

Abstract: In the humid tropics, continuing high deforestation rates are seen alongside an increasing expansion of secondary forests. In order to understand and model the consequences of these dynamic land-use changes for regional water cycles, the response of soil hydraulic properties to forest disturbance and recovery has to be quantified. At a site in the Brazilian Amazônia, we annually monitored soil infiltrability and saturated hydraulic conductivity (K s) at 12.5, 20cm, and 50cm soil depth after manual forest conversion to pasture (year zero to four after pasture establishment), and during secondary succession after pasture abandonment (year zero to seven after pasture abandonment). We evaluated the hydrological consequences of the detected changes by comparing the soil hydraulic properties with site-specific rainfall intensities and hydrometric observations. Within one year after grazing started, infiltrability and K s at 12.5 and 20cm depth decreased by up to one order of magnitude to levels which are typical for 20-year-old pasture. In the three subsequent monitoring years, infiltrability and K s remained stable. Land use did not impact on subsoil permeability. Whereas infiltrability values are large enough to allow all rainwater to infiltrate even after the conversion, the sudden decline of near-surface K s is of hydrological relevance as perched water tables and overland flow occur more often on pastures than in forests at our study site. After pasture abandonment and during secondary succession, seven years of recovery did not suffice to significantly increase infiltrability and K s at 12.5 depth although a slight recovery is obvious. At 20cm soil depth, we detected a positive linear increase within the seven-year time frame but annual means did not differ significantly. Although more than a doubling of infiltrability and K s is still required to achieve pre-disturbance levels, which will presumably take more than a decade, the observed slight increases of K s might already decrease the probability of perched water table generation and overland flow development well before complete recovery. [Copyright &y& Elsevier]

Summary: Soil compaction that follows the clearing of tropical forest for cattle pasture is associated with lower soil hydraulic conductivity and increased frequency and volume of overland flow. We investigated the frequency of perched water tables, overland flow and stormflow in an Amazon forest and in an adjacent 25-year-old pasture cleared from the same forest. We compared the results with the frequencies of these phenomena estimated from comparisons of rainfall intensity and soil hydraulic conductivity. The frequency of perched water tables based on rainfall intensity and soil hydraulic conductivity was expected to double in pasture compared with forest. This corresponded closely with an approximate doubling of the frequency of stormflow and overland flow in pasture. In contrast, the stormflow volume in pasture increased 17-fold. This disproportional increase of stormflow resulted from overland flow generation over large areas of pasture, while overland flow generation in the forest was spatially limited and was observed only very near the stream channel. In both catchments, stormflow was generated by saturation excess because of perched water tables and near-surface groundwater levels. Stormflow was occasionally generated in the forest by rapid return flow from macropores, while slow return flow from a continuous perched water table was more common in the pasture. These results suggest that deforestation for pasture alters fundamental mechanisms of stormflow generation and may increase runoff volumes over wide regions of Amazonia. [Copyright &y& Elsevier]