Higher water-table levels and flooding risk under grain vs. livestock production systems in the subhumid plains of the Pampas

Although the strong influence of vegetation shaping the hydrological cycle is increasingly recognized, the effects of land-use changes in very flat regions (i.e. hyperplains, regional slope < 0.1%) are less understood in spite of their potentially large magnitude. In hyperplains with sub-humid climates, long-lasting flooding episodes associated to water-table raises are a distinctive ecohydrological feature and a critical environmental concern. We evaluated the hydrological impacts caused by the replacement of livestock systems, dominated by perennial alfalfa pastures, by grain production systems, dominated by annual crops, that has been taking place in the Pampas (Argentina). For this purpose, we combined remote sensing estimates of vegetation transpiration and surface water coverage with long-term (1970-2009) hydrological modeling (HYDRUS 1D), and water-table depth and soil moisture measurements. The NDVI derived from MODIS imagery was 15% higher in dairy systems than in grain production ones, suggesting higher transpiration capacity in the former (852 vs. 724 mm y-1). Even higher contrasts were found among individual cover types, with perennial pastures having the highest NDVI and transpiration potential rates (0.66 and 1075 mm y-1), followed by double winter/summer crops (0.55 and 778 mm y-1) and single summer crop (0.45 and 679 mm y-1). Significantly deeper long-term average water-table levels in dairy system compared to single and double cropping (4 m, 2.1 m and 1.5 m, respectively) were suggested by the hydrological modeling and confirmed by field observations at nine paired sites (pasture vs. cropland, p<0.05) and two transects. At two additional paired sites, continuous water-table depth monitoring with pressure transducers, provided insights about the mechanisms behind these contrasts, which included enhanced groundwater recharge in the cropland and direct groundwater discharge by the pasture. Soil profiles, being notably drier under pastures (316 vs. 552 mm stored at 0-3 m depth, p<0.05), prevented the recharge episodes experienced by agricultural plots after an extraordinary rainy period. Our study highlights the key role of land-use on the hydrology of subhumid hyperplains, supporting the linkage of groundwater level raises and flood frequency and severity increases with the expansion of grain production systems in the Pampas. Given the spatial connectivity imposed by the hydrologic system and the strong association observed between the plot water balance and regional flooding, it is highly relevant to improve the quantification of the hydrological responsibility and interdependence of land use decision across plots and farms. This further step should support territorial policies that optimize the hydrological services of the region.