The Effects of Salinity and Sodium Adsorption Ratio on the Water Retention and Hydraulic Conductivity Curves of Soils From The Pampa del Tamarugal, Chile

Abstract:

The Pampa del Tamarugal is located in the Atacama Desert, the most arid desert of the world. It has important reserves of groundwater, which are probably fed by infiltration coming from the Andes Mountain, with groundwater levels fluctuating between 3 and 10-70 m below the land surface. In zones where shallow groundwater exists, the capillary rise allows to have a permanently moist vadose zone, which sustain native vegetation such as the Tamarugos (Prosopis tamarugo Phil.) and Algarrobos (Prosopis alba Griseb.). The native vegetation relies on the soil moisture and on the evaporative fluxes, which are controlled by the hydrodynamic characteristics of the soils.

The soils associated to the salt flats of the Pampa del Tamarugal are a mixture of sands and clays, which have high levels of sulfates, chloride, carbonates, sodium, calcium, magnesium, and potassium, with high pH and electrical conductivity, and low organic matter and cationic exchange capacity. In this research, we are interested in evaluating the impact of salinity and sodium adsorption ratio (SAR) on the hydrodynamic characteristics of the soil, i.e., water retention and hydraulic conductivity curves.

Soils were collected from the Pampa del Tamarugal and brought to the laboratory for characterization. The evaporation method (HYPROP, UMS) was used to determine the water retention curve and the hydraulic conductivity curve was estimated combining the evaporation method with direct measurements using a variable head permeameter (K_SAT, UMS). It was found that higher sodium concentrations increase the water retention capacity and decrease the soil´s hydraulic conductivity. These changes occur in the moist range of the hydrodynamic characteristics. The soil’s hydraulic properties have significant impact on evaporation fluxes, which is the mayor component of the water balance. Thus, it is important to quantify them and incorporate salt precipitation/dissolution effect on the hydrodynamic properties to correctly simulate evaporation in saline soils.