RECENT DRAINAGE SYSTEM EVOLUTION OF THE RIO NUEVO (SAN LUIS PROVINCE, ARGENTINA) DRIVEN BY THE INTERACTIVE EFFECTS OF LAND USE CHANGES AND INCREASES IN MONSOONAL PRECIPITATION

Central western Argentina is a semiarid region, at the southernmost reach of the South American Monsoon, which receives >70% of yearly rainfall during the austral monsoon season. Precipitation totals have increased in this area by ~20-30% from the 1970s to the present. This increase in available water leads to agricultural expansion and the conversion of forested rangeland to cropland since ~ 1975. On this eolian plain which lacked an integrated river network, new rivers formed and incised up to 25 m. Previous studies indicate that the land use changes were the dominant driver for drainage system formation of the Río Nuevo (“New River”), with> 75% of the catchment cultivated by 2001. The Río Nuevo formed in erodible, unconsolidated Holocene eolian sands associated with a regional rise in groundwater, accelerated percoline flow, and massive sapping. This study utilized a time series of Landsat TM imagery between 2001 and 2016 to quantify the rate of headward and downward extension of the Río Nuevo, with images from the summer months. A combination of image bands (e.g. near-IR, mid-IR, and red) revealed annual changes in river length and widening of channels. Precursor areas of stream extension were identified with increases in vegetation productivity (e.g. NDVI) which were areas with a rising water table, visible pools and inferred piping. The average channel extension rate was 2.3 km/y between 2001 and 2016 with peak rates exceeding 4 km/y. Extension rates and channel slope appear to be inversely proportional during the wettest interval between 2006 and 2009, with peak extension rates associated with slopes ≤1%. River incision was coincident with the greatest slope, > 1%. Río Nuevo extension rates appear also to correlate (r² >0.48) with bi-decadal-scale summer precipitation anomalies from 1903 to 2016. This analysis indicates that a sustained increase in precipitation precedes acceleration of extension rates for the Río Nuevo in the past 15 years. Thus, the increase in monsoonal rainfall appears to be an important driver for stream extension even for fully cultivated land surfaces. This study indicates that the initial rise of cultivated areas and the continued increase in precipitation resulted in a positive, sustained water balance and the continued formation of new drainage systems where none previously existed.