Studies that relate changes in land cover with changes in river discharge at the small scale (<1 km2) are abundant. These studies generally indicate that deforestation causes an increase in the annual mean discharge. However, previous studies that evaluated the effects of changes in land cover in larger river basins (>100 km2) usually have not found similar relationships. Here we analyse a 50-year long time series of discharge of a tropical river, the Tocantins River at Porto Nacional (175,360 km2), as well as precipitation over this drainage area, during a period where substantial changes in land cover occurred in the basin (1949–1998). Based on agricultural census data, we estimate that, in 1960, about 30% of the basin was used for agriculture. Previous work indicates that by 1995, agriculture had increased substantially, with about 49% of the basin land used as cropland and pastures. Initially, we compare one period with little changes in land cover (period 1-1949–1968) with another with more intense changes in land cover (period 2-1979–1998). Our analysis indicates that, while precipitation over the basin is not statistically different between period 1 and period 2 (α=0.05), annual mean discharge in period 2 is 24% greater than in period 1 (P<0.02), and the high-flow season discharge is greater by 28% (P<0.01). Further analyses present additional evidence that the change in vegetation cover altered the hydrological response of this region. As the pressure for changes in land cover in that region continue to increase, one can expect important further changes in the hydrological regime of the Tocantins River.

The El Niño–Southern Oscillation (ENSO) phenomenon is one of the dominant drivers of environmental variability in the tropics. In this study, we examine the connections between ENSO and the climate, ecosystem carbon balance, surface water balance, and river hydrology of the Amazon and Tocantins river basins in South America. First we examine the climatic variability associated with ENSO. We analyze long‐term historical climate records to document the “average” climatic signature of the El Niño and La Niña phases of the ENSO cycle. Generally speaking, the “average El Niño” is drier and warmer than normal in Amazonia, while the “average La Niña” is wetter and cooler. While temperature changes are mostly uniform through the whole year and are spatially homogeneous, precipitation changes are stronger during the wet season (January‐February‐March) and are concentrated in the northern and southeastern portions of the basin. Next we use a land surface/ecosystem model (IBIS), coupled to a hydrological routing algorithm (HYDRA), to examine how ENSO affects land surface water and carbon fluxes, as well as changes in river discharge and flooding. The model results suggest several responses to ENSO: (1) During the average El Niño, there is an anomalous source of CO2 from terrestrial ecosystems, mainly due to a decreased net primary production (NPP) in the north of the basin. There is also a decrease in river discharge along many of the rivers in the basin, which causes a decrease in flooded area along the main stem of the Amazon. (2) During the average La Niña, there is an anomalous sink of CO2 into terrestrial ecosystems, largely due to an increase in NPP in the northern portion of the basin. In addition, there is a large increase in river discharge in the Amazon basin, especially from the northern and western tributaries. There is a corresponding increase in flooded area, largely in the northern rivers. These results illustrate that changes in water and carbon balance associated with ENSO have complex, spatially heterogeneous features across the basin. This underscores the need for comprehensive analyses, using long‐term observational data and model simulations, of regional environmental systems and their response to climatic variability.

This article examines how climate variability and ecological disturbances affect ecosystem composition and functioning in the Amazon region. We use a terrestrial ecosystem model, IBIS, and vary the treatment of climate (using either long‐term average climate, or actual historical variations in climate) and disturbances (with uniform disturbance rates applied through the region). Interannual climate variability and frequent disturbances both favor grasses over trees, causing large increases in the geographic extent of savanna in the south and east of the region. A more constant climate and less frequent disturbances both favor trees over grasses, causing forest to dominate most of the study area. While climate variability and disturbances have a major impact on ecosystem structure, we find that compensating processes between herbaceous and woody plants damp the simulated response of carbon and water fluxes. Nevertheless, the cumulative impact of these changes in vegetation structure results in significant changes in soil and vegetation carbon stocks (up to 36%).

[1] The Amazon basin contains some of the most productive ecosystems on the planet, yet we have little understanding of their long-term behavior. By examining historical climate records over the Amazon, we identify several modes of climatic variability-including previously undocumented long-term modes. Furthermore, using a process-based ecosystem model, we show that these variations in climate generate variations in terrestrial carbon fluxes on short (3-4 year), intermediate (8-9 year), and long (24-28 year) time scales. The long-term cycles in terrestrial carbon balance have not been previously suggested. Finally, we find that time-lags between productivity and decomposition enhance the short-term variations in net carbon balance, while slightly dampening the long-term variations. Given the worldwide attention on terrestrial carbon cycling, and the potential for "carbon sinks", we suggest that an improved understanding of long-term climatic and ecosystem processes is crucial. Other regions should be examined for potential long-term carbon cycle variations.

A terrestrial ecosystem model (integrated biosphere simulator (IBIS)) and a hydrological routing algorithm (HYDRA) are used in conjunction with long time series climate data to simulate the river discharge and flooded area of the Amazon/Tocantins River basin over the last 60 years. Evaluating the results of this modeling exercise over the entire basin yields three major results: (1) Observations at 121 stations throughout the basin show that discharge is well simulated for most tributaries originating in Brazil. However, the discharge is consistently underestimated, by greater than 20%, for tributaries draining regions outside of Brazil and the main stem of the Amazon. The discharge underestimation is most likely a result of underestimated precipitation in the data set used as model input. (2) A new flooding algorithm within HYDRA captures the magnitude and timing of the river height and flooded area in relatively good agreement with observations, particularly downstream of the confluence of the Negro and Solimões Rivers. (3) Climatic variability strongly impacts the hydrology of the basin. Specifically, we find that short (∼3–4 years) and long (∼28 years) modes of precipitation variability drive spatial and temporal variability in river discharge and flooded area throughout the Amazon/Tocantins River basins.

[i] A terrestrial ecosystem model (integrated biosphere simulator (IBIS)) and a hydrological routing algorithm (HYDRA) are used in conjunction with long time series climate data to simulate the river discharge and flooded area of the Amazon/ Tocantins River basin over the last 60 years. Evaluating the results of this modeling exercise over the entire basin yields three major results: (1) Observations at 121 stations throughout the basin show that discharge is well simulated for most tributaries originating in Brazil. However, the discharge is consistently underestimated, by greater than 20%, for tributaries draining regions outside of Brazil and the main stem of the Amazon. The discharge underestimation is most likely a result of underestimated precipitation in the data set used as model input. (2) A new flooding algorithm within HYDRA captures the magnitude and timing of the river height and flooded area in relatively good agreement with observations, particularly downstream of the confluence of the Negro and Solimoes Rivers. (3) Climatic variability strongly impacts the hydrology of the basin. Specifically, we find that short (∼3-4 years) and long (∼28 years) modes of precipitation variability drive spatial and temporal variability in river discharge and flooded area throughout the Amazon/Tocantins River basins.

SummaryLeaf phenology describes the seasonal cycle of leaf functioning. Although it is essential for understanding the interactions between the biosphere, the climate, and biogeochemical cycles, it has received little attention in the modelling community at global scale. This article focuses on the prediction of spatial patterns of the climatological onset date of leaf growth for the decade 1983–93. It examines the possibility of extrapolating existing local models of leaf onset date to the global scale. Climate is the main variable that controls leaf phenology for a given biome at this scale, and satellite observations provide a unique means to study the seasonal cycle of canopies. We combine leaf onset dates retrieved from NOAA/AVHRR satellite NDVI with climate data and the DISCover land‐cover map to identify appropriate models, and determine their new parameters at a 0.5° spatial resolution. We define two main regions: at temperate and high latitudes leaf onset models are mainly dependent on temperature; at low latitudes they are controlled by water availability. Some local leaf onset models are no longer relevant at the global scale making their calibration impossible. Nevertheless, we define our unified model by retaining the model that best reproduced the spatial distribution of leaf onset dates for each biome. The main spatial patterns of leaf onset date are well simulated, such as the Sahelian gradient due to aridity and the high latitude gradient due to frost. At temperate and high latitudes, simulated onset dates are in good agreement with climatological observations; 62% of treated grid‐cells have a simulated leaf onset date within 10 days of the satellite observed onset date (which is also the temporal resolution of the NDVI data). In tropical areas, the subgrid heterogeneity of the phenology is larger and our model's predictive power is diminished. The difficulties encountered in the tropics are due to the ambiguity of the satellite signal interpretation and the low reliability of rainfall and soil moisture fields.

We coupled a global biome model iteratively with an atmospheric general circulation model to study the possible role of vegetation in the climate system, at the time of glacial inception 115,000 years ago. Orbital forcing alone was not sufficient to initiate glaciation when other components of the climate system were kept as present (atmospheric composition, oceans, biosphere and cryosphere). Summers were however cold enough to induce major vegetation shifts in high northern latitudes. Southward migration of the boreal forest/tundra limit helped to create favourable conditions for continental ice‐sheet growth, with increasing snow depth and duration in Labrador, Arctic Canada and northern/western Fennoscandia. These results support a role for biogeophysical feedback in initiating glaciations.