The imprint of land use on water and salt dynamics in the semiarid plains of Argentina

Abstract

As semiarid forests become increasingly replaced by rain-fed annual crops throughout the sedimentary plains of Argentina, changes in the water and salt balance of ecosystem and groundwater are likely to occur. This possibility is suggested by the recent formation of saline seeps and springs in the region and by a large body of observations under similar land cover transitions in other continents. Dry forests worldwide have shown extremely low deep drainage (groundwater recharge) and high vadose salt accumulation. Their cultivation initiated deep drainage, solute displacement, and rising water tables in many cases, affecting the long term viability of agriculture. To explore this possibility we characterized vadose moisture and chloride storage and transport in a sedimentary watershed that started to get cleared and cultivated ~100 years ago, and linked this observation to the sudden formation of salty streams through sediment liquefaction during the last 20 years. Within this watershed we selected 7 paired stands under dry forest and crops and sampled their sediments (n=3) down to 6 m of depth using hand augers. We characterized current surface water and salt export from the new streams at several points along their course and at three dates within a growing season (Jun/2008, Oct/2008 and Apr/2009). A consistently dry and salty zone was found below two meters of depth in dry forest stands, suggesting that they experienced very low to negligible deep drainage. Under cultivated dry forests chloride storage losses were >70% and moisture gains were of >30% more, suggesting a net chloride leaching of 150 to 1600 g m-2 following deforestation. New streams had a discharge of 10 mm yr-1 and salt exports of 6 g m-2 yr-1. Based on the cultivated area of the watershed and the net chloride lost from its soil profiles we estimate that at current water yield rates between 25 to 250 years of stream flow will be necessary to remove the leached salts out from the region. While the steady long term precipitation raise experienced by this region (30% more in in the last 100 years) has been assumed as the ultimate cause triggering the sudden formation of salty streams, dry forests have maintained their vadose salt stock and their typical low deep drainage rates until the present, suggesting that the combination of higher precipitation and extensive clearing of forest are two converging causes of hydrological change.