Abstract
This study is an exploratory, spatio-temporal analysis of river discharge in the upper Teles Pires contribution area for the period between 1993 and 2006, and its relationship with deforestation dynamics on a watershed scale. After an evaluation of temporal variability of monthly rainfall and runoff time series, generalized linear models (GLM) have been fitted to examine the relationships between discharge with indicators of deforestation in the contribution area of six sub-watersheds. Monthly discharges at the main outlet show a significant positive trend, whereas increase of precipitation has no statistical significance. Besides precipitation, principal determinant of specific discharge, deforestation percentages and the annual relative rates of deforestation are significant predictors in the three smaller sub-basins with areas up to about 14.000 km2. In the most altered sub-basin under intensive agricultural use, the ratio of residence and hydrological response have positive correlations with deforestation indicators, evidencing the removal of arborous formations diminish evapotranspiration and favor discharge. With increasing contribution areas up to about 54,000 km2, the interference of deforestation on discharge and hydrological parameters decrease and precipitation remains the unique significant predictor.
Highlights
Deforestation of savannahs and forests for crop and cattle farming is considered one of the major anthropogenic factors on the water balance, river discharge and water resource availability (Sahin & Hall, 1996; Bosch & Hewlett, 1982)
On the background of the scarcity of studies on Land Use and Cover Change (LUCC) on discharge in neotropical savannahs and adjacent transitional forests region (Oliveira et al, 2015) and contradicting results on the effects of deforestation on discharge (Krishnaswamy et al, 2012; Andreassian, 2004), we examine spatio-temporal variation of river runoff on the watershed scale in the upper Teles Pires river contribution area, which includes portions of one of the most productive crop farming region worldwide and major projects for the installation of new hydropower plants
For the delimitation procedure only, the original digital elevation model (DEM) was corrected by the TOPOGRID algorithm (Hutchinson, 1989), with imposition of a single line hydrographic network in a 1:100.000 scale, obtained from the Mato Grosso State Environmental Agency (SEMA)
Summary
Deforestation of savannahs and forests for crop and cattle farming is considered one of the major anthropogenic factors on the water balance, river discharge and water resource availability (Sahin & Hall, 1996; Bosch & Hewlett, 1982). Vegetation cover removal tends to reduce storage by interception, litter and in the root zone, changing groundwater recharge principally during strong precipitation events (Bronstert et al, 2002). Evapotranspiration is the main process causing changes in annual discharge after deforestation (Brown et al 2005). The reduction of evapotranspiration triggers an increase in annual discharge. This effect, varies in different climates, under different physiographic characteristics of the watersheds and along the intra- and interannual cycles (Hibbert, 1967; Andreássian, 2004; Zhang and Schilling, 2006). Discharge increases after deforestation are stronger during the rainy season, favoured by a reduced infiltration due to a degradation of soil structure (Bronstert et al, 2002; Zuazo and Pleguezuelo, 2008). During dry periods runoff may decrease, induced by channel sedimentation and reduced retention by soil loss (Harr et al.,1982; Cheng, 1989; Cornish, 1993; Gustard and Wesselink, 1993)
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