Evaluating fluxes in Histosols for water management in lettuce: A comparison of mass balance, evapotranspiration and lysimeter methods

The evaluation of downward water fluxes in Histosols is critical for water budget and irrigation studies in natural and cultivated wetlands. By analyzing water fluxes in cultivated organic soils, our study aimed to (i) compare drainage water fluxes measured using in situ lysimeters to fluxes estimated by three indirect methods, specifically, a simplified water balance, a water balance including soil water storage changes, and a method based on the Buckingham–Darcy law; and (ii) investigate water flux mechanisms using a bromide tracer in the same lysimeters. In 2008–2009, three sites were each set up with four drainage lysimeters equipped with water content probes and tensiometers. Bromide was applied over each lysimeter at the beginning of the lettuce growing cycle. Daily outflow water volumes were measured and solute samples were collected. The results showed that measured cumulative drainage was very low (0–40.2 mm) compared to water input (30–128 mm). A rupture point in the unsaturated hydraulic conductivity at matric potentials around −4.5 to −6 kPa, combined with the stratified profile of the organic layers, may in part explain these observations. The measured water fluxes were also significantly lower (by 1.2 to 1255.3 times) than the estimations made with the three indirect methods. Finally, the analyses of soil and outflow bromide concentrations confirmed the low fluxes through the lysimeters and revealed an accumulation at the surface layer interface (at a depth of about 0.3 m), indicating that water perched in the profile and suggesting that preferential flow occurred, leading to episodic solute leaching. Direct evaluation of water and solute fluxes are needed in such soils, as all three methods presented important biases.