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

The fate of phosphorus fertilizer in Amazon soya bean fields

“…There is little topographic variation. The landscape consists of plateaus 360-390 m above sea level grading to streams 320-350 m above sea level [36]. The soils along geographical plateaus are medium-textured, well-drained ustic Oxisols (Latossolo vermelho-amarelo distrófico).…”

“…We additionally hypothesized that the high P-binding capacity of soils would keep P surpluses in the upper soil, and that P transport deeper in the soil column would be relatively small [34]. Based on our previous measurement of low dissolved P losses from soya bean streams [35] and minimal occurrence of overland flow [36,37], we hypothesized that losses to streams would be low.…”

“…In fact, with no movement of P below 20 cm and with more than 20 m of soil and almost exclusively vertical flow paths [36,37], it does not appear that soya bean cultivation is increasing the risk of P losses to surface waters.…”

Environmental and spatial processes determining Ephemeroptera (Insecta) structures in tropical streams

“…The study region is under strong pressure of agricultural expansion, aggravated by poorly planned rural settlements, leading to direct and indirect effects on conservation areas. The main economic activities in the area are logging, monocultures of rice and soybean, and extensive cattle raising (Schwartzman and Zimmerman, 2005;Hayhoe et al, 2011). The Suia´-Missu region has a seasonally tropical climate, with a dry period from May to October and a rain period from November to April.…”

Influence of watershed‐climate interactions on stream temperature, sediment yield, and metabolism along a land use intensity gradient in Indonesian Borneo

“…Forest conversion to cropland often results in increased water yield due to associated changes in canopy interception, evapotranspiration (ET), and soil infiltration [Brauman et al, 2012;Coe et al, 2011;Costa et al, 2003;Giambelluca, 2002;Hayhoe et al, 2011;Nik, 1988]. Agricultural conversion also affects stream physical and biochemical properties.…”