Farm Table says:
Water use, root activity and deep drainage within a perennial legume-grass pasture: A case study in southern inland Queensland, Australia.
What is the problem?
Irrigated systems, capable of using large volumes of water with minimal risk impact on natural aquifers, are needed.
The decision by CSG companies to irrigate leucaena combined with Rhodes grass (Chloris gayana) was based on the hypothesis that the roots of trees and grass occupy different soil strata when growing in association (Schroth 1999) and are capable of maximizing water use in the profile and minimizing deep drainage.
This study aimed to monitor soil water extraction under a Leucaena-Rhodes grass pasture using EnviroSCAN to provide background information on (a) the maximum depth of water extraction and by inference root activity, (b) the amount and pattern of water extraction, (c) the likelihood of deep drainage below 4 m depth.
What did the research involve?
- moisture usage was monitored at Santos’ Fairview gas field north-east of Injune, Queensland (25°44’40″S, 149°3’19″E), where 234 ha of Leucaena leucocephala ssp. glabrata and Chloris gayana were being irrigated using desalinated CSG water under 4 center-pivot irrigation system
- volumetric soil water content was monitored at 4 sites using 8 EnviroSCAN probes connected to dataloggers (RT6 logger, Sentek Pty. Ltd.) with a sampling interval of 15 minutes
- the depth of water extraction, assumed to be indicative of the maximum depth at which roots were actively taking up water, was estimated using the IrriMAX 9.1.1 software tools by measuring the depletion of water in the soil profile during days when no precipitation was recorded
- decreases in soil water content could be due to evapotranspiration, plant water uptake (WU), runoff (R) or drainage (D). The EnviroSCAN data were used to calculate WU and D for the top 4 m of soil profile from 1 August 2011 to 27 August 2013 at 15-minute intervals
- a total of 72,635 data points was logged for each probe, totaling 581,080 data points during the 757 days of study
What were the key findings?
- EnviroScan sensors were a useful tool for characterizing spatial and temporal patterns of water extraction, and by inference root activity of leucaena-Rhodes pasture
- a marked seasonal water extraction was observed which was greater during growing seasons and lower in cool dry seasons
- both leucaena and Rhodes grass extracted a greater amount of water in the upper layers, suggesting high levels of competition for water resources between species
- low rainfall, defoliation and low temperatures negatively affected depth of water extraction and therefore reduced total water extraction
- there was some evidence that leucaena roots were active slightly deeper in the soil profile than roots of Rhodes grass. The highest values of deep drainage below 4 m occurred when rainfall events coincided with soil moisture near to 100% PAW. Therefore, irrigation should be avoided at this time
- deep drainage below 4 m within leucaena twin rows differed little from that within the grassed inter-rows
Given that water usage by both leucaena and Rhodes grass was greatest in the upper layers of soil (<1.5 m), future research should focus on how the level of competitive interaction might be managed by choice of row spacing and frequency of irrigation.