The ecohydrological imprint of deforestation in the semiarid Chaco: insights from the last forest remnants of a highly cultivated landscape

Abstract

The semiarid Chaco plains present one of the highest rates of forest clearing and agricultural expansion of the world. In other semiarid plains, such massive vegetation replacements initiated a groundwater recharge and salt mobilization process that, after decades, raised regional water tables and salts to the surface, degrading agricultural and natural ecosystems. Indirect evidence suggests that this process (known as dryland salinity) began in the Chaco plains. Multiple approaches (deep soil profiles, geoelectric surveys and monitoring of groundwater salinity, level and isotopic composition) were combined to assess the dryland salinity status in one of the oldest and most active agricultural hotspots of the region, where isolated forest remnants occupy an extremely flat cultivated matrix. Full vadose moisture and chloride profiles from paired agriculture-forest stands (17 profiles, six sites) revealed the following: a generalized onset of deep drainage with cultivation (32 to >87 mm year À1 ), full leaching of native chloride pools (13.7 ± 2.5 kg m À2 ) down to the water table after >40 years following clearing and differential groundwater table rises (0.7 to 2 m shallower water tables under agriculture than under neighbouring forests). Continuous level monitoring showed abrupt water table rises under annual crops (up to 2.6 m in 15 days) not seen under forests or pastures. Varying deep drainage rates and groundwater isotopic composition under agricultural plots suggest that these pulses are strongly modulated by crop choices and sequences. In contrast to other dryland salinity-affected areas of the world, forest remnants in the study area (10-20% of the area) are not only surviving the observed hydrological shifts but also sustaining active salty groundwater transpirative discharge, as evidenced by continuous water table records. The overall impact of these forest remnants on lowering neighbouring water tables would be limited by the low hydraulic conductivity of the sediments. As highly cultivated areas of the Chaco evolve to new hydrological conditions of shallower saline water tables, innovative crop rotations that minimize recharge, enhance transpirative discharge and tolerate salinity will be needed.

On the Fundamental Causes of High Environmental Alkalinity (pH ≥ 9): An Assessment of Its Drivers and Global Distribution

“…; George et al . ; Gimenez et al ., ). On the other hand, decreasing rainfall or the introduction of vegetation types with high water consumption rates (e.g., fast growing tree plantation on grasslands) in subhumid plains with stagnant hydrology can depress water table levels, create longer groundwater trajectories, and decrease the alkalinizing potential of the solutions that reach evaporative discharge areas (Jobbágy & Jackson ; Nosetto et al .…”

Reviewing our options: Managing water‐limited soils for conservation and restoration

“…Irrigation waters also introduce salts to the soil, which can accumulate in the upper soil profile due to lack of drainage from the soil profile (Pitman & Läuchli, 2002). Water-limited systems are especially vulnerable (Gimenez, Mercau, Nosetto, Paez, & Jobbagy, 2016) and may be further threatened by human regulation of waterways (Jolly, Walker, & Thorburn, 1993). Land clearing and deforestation promote salinization, as water-levels rise due to a lack of water-uptake by deeper roots, exacerbated by declining precipitation following clearing (Sheil & Murdiyarso, 2009).…”

Preface for the South American Hydrology Virtual Special Issue

“…Excitingly, three more manuscripts tackled broader hydrologic process science through the lens of ecohydrology. Two papers studied vegetation responses to agriculture expansion (Giménez, Mercau, Nosetto, Páez, & Jobbágy, ) and rainfall variability (Souza et al, ), and a third characterized runoff generation processes in a high‐elevation tropical ecosystem (Mosquera et al, ). Giménez et al () combined multiple approaches (deep soil profiles, geoelectric surveys and monitoring of groundwater salinity, and level and isotopic composition) to assess the effects of forest clearing and agricultural expansion in the semiarid Chaco plains.…”

“…Two papers studied vegetation responses to agriculture expansion (Giménez, Mercau, Nosetto, Páez, & Jobbágy, ) and rainfall variability (Souza et al, ), and a third characterized runoff generation processes in a high‐elevation tropical ecosystem (Mosquera et al, ). Giménez et al () combined multiple approaches (deep soil profiles, geoelectric surveys and monitoring of groundwater salinity, and level and isotopic composition) to assess the effects of forest clearing and agricultural expansion in the semiarid Chaco plains. This suggests that the Chaco plains are beginning to suffer from dryland salinity, that is, increasing groundwater recharge and salt mobilization that raise water tables and salts to the surface, which will potentially degrade agricultural and natural ecosystems.…”

“…Excitingly, three more manuscripts tackled broader hydrologic process science through the lens of ecohydrology. Two papers studied vegetation responses to agriculture expansion (Giménez, Mercau, Nosetto, Páez, & Jobbágy, 2016) and rainfall variability (Souza et al, 2016), and a third characterized runoff generation processes in a high-elevation tropical ecosystem (Mosquera et al, 2016) Five papers investigate groundwater-surface water interactions across a variety of South American landscapes. Boutt, Hynek, Munk, and Corenthal (2016) investigated recharge processes to the halitehosted brine aquifer of the economically important lithium brine of the Salar de Atacama in Nothern Chile.…”