Conversion to soy on the Amazonian agricultural frontier increases streamflow without affecting stormflow dynamics

Abstract

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

Soil hydraulic response to land-use change associated with the recent soybean expansion at the Amazon agricultural frontier

“…Thus, our results reveal a soilscape wellbuffered with respect to near-surface hydrology in this part of Amazonia and underpin the findings of Hayhoe et al (2011) that the conversion to soybeans did not affect stormflow dynamics.…”

Solute and sediment export from Amazon forest and soybean headwater streams

“…Mean annual temperature is 27°C and mean annual precipitation is 1,800 mm/yr (1987Tanguro Ranch, unpublished data), almost all of which falls from September to April. The landscape at Tanguro Ranch is undulating, with wide interfluves that grade to streams with generally less than 65 m in elevation change and channel slopes between 0.3° and 1.9° (Hayhoe et al 2011). The region lies in the headwaters of the Xingu River, the fifth largest tributary of the Amazon River by watershed size (basin area, 446,203 km 2 ; length, 1,640 km; average discharge, 8,665 m 3 /s).…”

“…Tertiary and Quaternary fluvial deposits cover Precambrian gneisses of the Xingu Complex (Projeto Radambrasil 1981). Topographic differences between plateaus and stream channels indicate the depth to the water table ranges from 20 to more than 40 m (Hayhoe et al 2011). The landscape at Tanguro Ranch is undulating, with wide interfluves that grade to streams with generally less than 65 m in elevation change and channel slopes between 0.3° and 1.9° (Hayhoe et al 2011).…”

“…We calculated hourly discharge from rating curves extrapolated with power functions based on periodic measurements of stream cross-sectional area and water velocity across a range of discharges (Gore 2007, Hayhoe et al 2011; Appendix S1: Fig. Stream stage was measured hourly with HOBO pressure loggers (Onset Computer, Bourne, Massachusetts, USA).…”

“…They tend to be up to several meters deep (Negreiros et al 2009), and in some places display a pronounced microaggregation (pseudosand, Schaefer 2001). The increased resistance to surface compaction and high infiltrability of these soils has been observed to reduce the production of overland flow despite large increases in streamflow in cropland compared to forests (Hayhoe et al 2011, Scheffler et al 2011. The increased resistance to surface compaction and high infiltrability of these soils has been observed to reduce the production of overland flow despite large increases in streamflow in cropland compared to forests (Hayhoe et al 2011, Scheffler et al 2011.…”