20 21Large-scale soy agriculture in the southern Brazilian Amazon now rivals 22 deforestation for pasture as the region's predominant form of land use change. Such 23 landscape level change can have substantial consequences for local and regional 24 hydrology, which remain relatively unstudied. We examined how the conversion to soy 25 agriculture influences water balances and stormflows using stream discharge (water 26 yields) and the timing of discharge (stream hydrographs) in small (2.5 to 13.5 km 2 ) 27 forested and soy headwater watersheds in the Upper Xingu Watershed in the state of 28Mato Grosso, Brazil. We monitored water yield for one year in three forested and four 29 soy watersheds. Mean daily water yields were approximately four times higher in soy 30 than forested watersheds, and soy watersheds showed greater seasonal variability in 31 discharge. The contribution of stormflows to annual streamflow in all streams was low (< 32 13% of annual streamflow), and the contribution of stormflow to streamflow did not 33 differ between land uses. If the increases in water yield observed in this study are typical, 34 landscape-scale conversion to soy substantially alters water-balance, potentially altering 35 the regional hydrology over large areas of the southern Amazon. 36 37 3 Introduction 38

20 21Large-scale soy agriculture in the southern Brazilian Amazon now rivals 22 deforestation for pasture as the region's predominant form of land use change. Such 23 landscape level change can have substantial consequences for local and regional 24 hydrology, which remain relatively unstudied. We examined how the conversion to soy 25 agriculture influences water balances and stormflows using stream discharge (water 26 yields) and the timing of discharge (stream hydrographs) in small (2.5 to 13.5 km 2 ) 27 forested and soy headwater watersheds in the Upper Xingu Watershed in the state of 28Mato Grosso, Brazil. We monitored water yield for one year in three forested and four 29 soy watersheds. Mean daily water yields were approximately four times higher in soy 30 than forested watersheds, and soy watersheds showed greater seasonal variability in 31 discharge. The contribution of stormflows to annual streamflow in all streams was low (< 32 13% of annual streamflow), and the contribution of stormflow to streamflow did not 33 differ between land uses. If the increases in water yield observed in this study are typical, 34 landscape-scale conversion to soy substantially alters water-balance, potentially altering 35 the regional hydrology over large areas of the southern Amazon. 36 37 3 Introduction 38