More cultivation with lower intensity in the South American Chaco: A double hydrological challenge

GIMÉNEZ R; MERCAU JL; HOUSPANOSSIAN J; BALDI, G; KUPPEL, S; JOBBAGY, E

San Francisco

Congreso; American Geophysical Union Fall Meeting 2014; 2014

American Geophysical Union

As in other semi-arid plains of the world, long-scale deforestation to establish croplands in the South American Chaco may disrupt the regional w ater balance. As annual crops use less w ater than the native perennial system, w ater in ex cess usually translates into serious degradation processes such as run-off driven erosion, or the onset of groundw ater recharge w hich can develop flooding and dryland saliniz ation. Agriculturally, w ater ex cess could be reduced by using more intensive crop systems w hich consume w ater ex haustively. We used MODI S imagery (2000-present) from Bandera, Argentina (28.8S 62.2W), a major agricultural cluster in the region, to assess deforestation, to identify the main crop systems, and to analyz e the impact of crop ex pansion and phenological shifts on the regional w ater balance. Three cover classes (Dry Forest DF, Agriculture AG, and Pastures PA) and five AG crop types w ere distinguished (w inter W, spring Sp,summer S, and late-summer LS single crops, w inter/summer DC and spring/summer DC double crops). Each season, w ater use (annual evapotranspiration, ET) for each cover/crop type (10 pix els/class) w as computed w ith a daily w ater balance based on meteorological data and 2 remote sensing-derived indices: N ormaliz ed Difference Vegetation I ndex , to capture canopy conditions, and Dead Fuel I ndex to represent mulch cover conditions. Throughout 14 crop seasons AG ex panded from 20 to 50% of the study area (1M ha) mostly replacing DF. Also, AG gradually evolved from a more intensive and diversified pattern dominated by DC (45-50%), S (28%) and Sp (16%) systems, to a more w ater-conservative system dominated by LS (60-80% in the last 3 seasons). Crop type differences in ET (DC ≈DC ≈FG>S>Sp>LS≈W) w ere stronger in w et years (>1000mm) but nil in dry ones (250mm for the less intensive W and LS in w et years. Weighting each cover/crop class by their area, w e found that the current ex pansion and reduced intensity of cultivation has cut regional ET of w et years by 50-100mm (compared to early 2000s land cover/crop pattern in similar w eather conditions), generating a grow ing w ater ex cess that is likely contributing to drive the w ater table rises and more frequent floods observed in this region.