A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Modeling the effects of hydrogeology and land cover conversion on runoff processes andrates in Rondônia, Brazil.T Dunne, Donald Bren School of Environmental Science and Management, University ofCalifornia, Santa Barbara, CA 93106; ph. 805-893-7557; fax 805-893-7612; e-mail:tdunne@bren.ucsb.eduJ A Ballantine, Department of Geography, University of California, Santa Barbara, CA93106; ph. 805-893-8816; e-mail:andyb@bren.ucsb.eduJorge M. Moraes, Centro de Energia Nuclear na Agricultura/USP, Piracicaba, SP –Brazil13416-903; ph. 55 (-19) -429-4678; email moraes@cena.usp.br.Hydrologic records in the state of Rondônia are still too sparse and short, and the extentof deforestation too small and transient for recognition of deforestation signals in runofffrom mesoscale river basins (100s -1000s of km 2 ). Large changes in soil recharge areknown to result from deforestation at a point, but the response of river flow to suchchanges is complicated by subsurface transfer of this water, transient subsurface storage,and increases in the amount of overland flow. These influences depend in turn on thetopographic ruggedness and hydrogeologic properties of a basin. Using data fromtopographic and geologic maps, seasonal extreme water-table depths, and hydrogeologicproperties (both measured in situ and back-calculated from streamflow), we havemodeled runoff responses to deforestation that should be expected from typical hillslopesin gauged river basins of Rondônia. The hillslopes vary in length and gradient, and indegree of rockiness. The computations show that although complete deforestation shouldincrease total runoff by about 25%, the partitioning of the runoff into subsurface andsurface paths and the proportion of the flow reaching the river as quickflow should beaffected by the interaction of these increased volumes of runoff with the hydrogeologyand geometry of the basin. Steeper sloping basins, such as the Rio Massangana arepredicted to generate larger amounts of quickflow than low-gradient topography such asthat of the Rio Jacundá. However, when low-gradient hillslopes are deforested in themodel, the increased runoff is partitioned mainly into quickflow through an increase insaturation overland flow.