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

Abstract:

As in other semi-arid plains of the world, long-scale deforestation to establish croplands in the South American Chaco may disrupt the regional water balance. As annual crops use less water than the native perennial system, water in excess usually translates into serious degradation processes such as run-off driven erosion, or the onset of groundwater recharge which can develop flooding and dryland salinization. Agriculturally, water excess could be reduced by using more intensive crop systems which consume water exhaustively. We used MODIS 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 analyze the impact of crop expansion and phenological shifts on the regional water balance. Three cover classes (Dry Forest DF, Agriculture AG, and Pastures PA) and five AG crop types were distinguished (winter W, spring Sp,summer S, and late-summer LS single crops, winter/summer DC_WS and spring/summer DC_SpS double crops). Each season, water use (annual evapotranspiration, ET) for each cover/crop type (10 pixels/class) was computed with a daily water balance based on meteorological data and 2 remote sensing-derived indices: Normalized Difference Vegetation Index, to capture canopy conditions, and Dead Fuel Index to represent mulch cover conditions. Throughout 14 crop seasons AG expanded 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 water-conservative system dominated by LS (60-80% in the last 3 seasons). Crop type differences in ET (DC_WS≈DC_SpS≈FG>S>Sp>LS≈W) were stronger in wet years (>1000mm) but nil in dry ones (<550mm). As a result, water excess (precipitation-ET) ranged from 0mm for most crop types in the driest year to >250mm for the less intensive W and LS in wet years. Weighting each cover/crop class by their area, we found that the current expansion and reduced intensity of cultivation has cut regional ET of wet years by 50-100mm (compared to early 2000s land cover/crop pattern in similar weather conditions), generating a growing water excess that is likely contributing to drive the water table rises and more frequent floods observed in this region.