EFFECT OF SODICITY ON SOIL HYDRAULIC PROPERTIES AND ROOT WATER UPTAKE

Abstract

Soil-water retention and hydraulic properties are important parameters that affect moisture movement in the vadose zone. The parameters play an important role in modelling moisture flow and solute transport dynamics and in the analysis of infiltration and drainage processes. These soil properties are dependent of the soil physical and textural properties, and are often considered to be invariant of the salt concentration in the soil-water. Due to increased agricultural and industrial demands, water from various sources such as sewage treated water and sometimes industrial treated water with mixed concentration of salts has been used with fresh water to meet the agricultural water demands in arid and semi-arid regions. It is known that, presence of salts in the soil-water affects the structural stability, water retention characteristics and hydraulic properties of soils. The effects of salinity and sodicity on soil hydraulic and water retention properties are complex in nature. Many studies have been done to analyse to better understand the effects of mixed concentration of various salts on the soil-water retention characteristics, soil hydraulic properties and water use efficiency of plants. The most of the studies conducted are focused on the qualitative aspects of effects of salt water concentrations only, very few studies have been carried out which have quantitatively analysed the effect of salt water concentrations on the soil water retention and hydraulic parameters. The change in water retention and hydraulic parameters consequent to the soil-mineral and salt-ion interactions affects the moisture flow dynamics, which in turn affects the irrigation management practices. In the present study, pressure plate experiments were conducted with four sodic treatments and varying matric suction on wide range of soil ranging from silt to sand. van Genuchten water retention parameters were estimated from the numerical inversion of experimentally obtained matric suction and moisture content data. Permeameter experiments were conducted with different sodic treatments for determining saturated hydraulic conductivity of soil samples. The estimated parameters were used for the simulation of hypothetical infiltration in initially dry soils and gravity drainage in initially saturated soils. The results of the water retention experiments indicates that with an increase in the sodic concentrations, for a given matric suction, the soil-water content decreases for all soil samples. The change in water retention is evaluated in terms of change in water retention parameters. van Genuchten water retention parameters (α, n, 𝜃𝑠 and 𝜃𝑟) were evaluated from the inversion of soil water retention curve. It was observed that, no observable change is found in the values of parameter 𝜃𝑠 and 𝜃𝑟 because these parameters play insignificant role in shaping the soil water retention curve. However a consistent increase in the value of α is observed with an increase in sodicity concentrations in fine-textured soils, and in coarse-textured soils, no significant trend in the changes in the value iii of parameter α is observed. The value of parameter n increased monotonically for all the soils as the concentration of sodic solutions increased. The results of permeameter experiments show that, saturated hydraulic conductivity 𝐾𝑠 decreased in fine textured soils and increased in coarse textured as the concentration of salts in the sodic solution increased. The results of infiltration experiment show that, wetting front movement decreases in fine textured soils and increases in coarse textured soils with the increase in sodicity. The gravity drainage simulations show that, as the sodicity concentration was increased gravity drainage slowed in fine-textured soils, and it was increased in coarse-textured soils. For studying the effect of salt water concentrations on the crop growth and root zone soil moisture dynamics, a root water uptake model is developed. The model is based on the numerical solution of Richards equation, governing unsteady vertical flow in the root zone, and the root water uptake is modelled as a sink term, which evaluates water uptake under varying water and salt stresses. The model is used for simulation of soil moisture and root water uptake for the irrigation field experiments, where paddy was grown with irrigation water with varying salinity concentrations. The growth of the crop was monitored regularly, crop parameters such as leaf area index and root depth along with the soil moisture profile were measured during the crop growth period. The non-linear parameter used for root water uptake model was estimated using an empirical relation in terms of observed crop variables for each saline irrigation condition. Root zone soil moisture and root water uptake for the prevailing hydro-meteorological conditions during the crop period and soil-crop parameters was simulated for analysing the effect of saline irrigation on the crop growth and moisture dynamics. The irrigation experiments carried out with saline water show that the growth of crop is significantly affected by the salt concentration in soil-water, and the crop canopy (LAI) and root depth decreases with increase in the salt concentration. The model simulation results show that increase in salt concentration in irrigation water results in reduced root water extraction, and reduced moisture content in soil profile.