Water content (%) Dry density (kN/m3)
Chapter 6 Hydraulic characteristics of Vegetated soil-WH composite
6.2 Hydraulic characteristics of Vegetated soil-WH composite
6.2.3 Interpretation of monitoring results
Figure 6.3 shows the evolution of desiccation cracks at the end of each drying cycles in the form of average CIF for both bare (BS) and vegetated soil (SG, SGWH). The corresponding GD for SG and SGWH are also reported. In the current study, all parameters except SC and SL are measured at the end of each drying cycle-DC. Based on the comparison of CIF for both BS and SG, it can be seen that both exhibit similar cracking for the first two DCs corresponding to initial GD (10%). Beyond this, CIF for bare soil increases up to 2.65% ± 0.56% at the end of the 6th cycle and becomes constant thereafter. Peak CIF for the same soil compacted at 0.8 MDD and similar irrigation patterns previously reports a CIF of 3.15 (Gadi et al. 2017). SG showcases barely any increase in CIF after GD of 40% and 6th DC. The CIF was found to actually decrease in last two cycles for SG. This is noteworthy, as even though initial grass growth (40%) induces CIF up to 2.1 ±0.26 (not as much as BS) but thereafter, it reduces crack formation. This behaviour of CIF in SG for last 5 cycles may be attributed to two factors; (i) the root bridge effect which restricts crack growth (Zhou et al. 2009); and (ii) intercepted radiant energy due to growing grass cover reduces suction on the surface (Garg et al. 2015b). On the contrary to both BS and SG, WH fiber inclusions
in SGWH resulted in lower CIF throughout the monitoring period. As compared to maximum CIF in BS and SG, WH-fiber inclusion lowers the desiccation potential by 55.5% and 25% respectively.
This reduction in CIF for SGWH throughout monitoring period can be attributed to “bridge effect”
of WH-fibers.
Fig. 6.3 Variation of CIF (bare and vegetated) and GD measured during the monitoring period The vegetation growth for SG and SGWH is discussed by GD observed in the monitoring period. It is clearly seen that SGWH showcases a higher GD, especially after the 3rd cycle. The possible reasoning is discussed with the water retention behaviour discussed later. GD only gives lateral vegetation growth and thus the average SL was also measured manually (Fig. 6.4a) to take into account for longitudinal growth. It can be observed from figure that SGWH showcases higher SL than SG which indicates that WH-fiber inclusions aids in the initial establishment of vegetative layer.
Fig. 6.4 Variation of measured parameters (a) Stomatal Conductance; (b) Evaporation and evapotranspiration; and (c) Shoot length during the monitoring period
Stomatal conductance-SC of both species was further monitored after 35 days of transplantation in all vegetated columns to the effect on plant growth (i.e., photosynthesis) due to inclusion of WH-fibers. Fig. 6.4b shows that Cynadon Dactylon has higher SC as compared to Axonopus Compressus for all test series. However, there is no significant variation observed in SC values for either species with WH-fiber inclusions. Based on measured SC and atmospheric conditions, the potential Etr was estimated for both series (SG and SGWH). The measured Etr was noted to be same for both SG and SGWH. However, the potential Etr of SGWH is seen to be relatively higher than that of SG.
The measured ψ-θ data points for all three series are shown in Fig. 6.5 and is representative of the variation observed after 43rd day of monitoring when vegetation growth was adequate. It is clearly seen that SG showcases higher water retention for most of the suction range (25-1000 kPa) as compared to BS. Increase in water retention for SG can be related to the change in soil pore size and distribution which influence water retention (Ng and Leung 2011). The increase in maximum water retention for SG (35.7%) as compared to BS (32.1%) is attributed to three possible reasons.
They are (i) water retention in roots (Taleisnik et al. 1999); (ii) root reduces overall pore size (Scanlan and Hinz 2010, Scholl et al. 2014); and (iii) root exudation which secretes enzymes in the root zone (Traoré et al. 2000). Pore size decrease can thus increase retention capacity.
The highest water retention among the three series is shown by SGWH. Addition of WH- fibers increase the composites’ water retention capacity throughout the suction range. This hydrophilic nature of the composite can be addressed from the surface morphology of WH-fibers and its inherent bio-chemical composition. FE-SEM images shows the surface morphology of the fiber and it is stacked with fine pores that is conducive to absorb moisture (Methacanon et al., 2010). Furthermore, WH has a very high hemicellulose content which is a bio-polymer with
available hydroxyl groups that attracts water molecules (Rowell and Stout 2002). WH-fibers being highly hydrophilic, naturally improves the soil’s ability to retain more water and is conducive for the roots even at drought conditions. This can be a probable reason for high grass growth (GD and SL) in case of SGWH as compared to SG.
Fig. 6.5 Measured suction and water content relationship of the three series of soil