Predicting the impact of perennial phases on average leakage from farming systems in south-western Australia

Abstract

Rising watertables and dryland salinity in southern Australia are due to excess groundwater recharge after the replacement of native vegetation by annual crops and pastures. The inclusion of perennial plants into agricultural systems has been proposed as a possible method of recharge reduction, through the creation of a buffer (extra water storage capacity generated by the perennial in comparison with an annual crop or pasture). However, the role of perennial phases under conditions of highly episodic leakage is not well understood. In this paper, a simple Leakage/Buffer Model (LeBuM) was developed to determine the effect of perennial phases on long-term average annual leakage, incorporating episodic events. Mechanistic modelling studies on contrasting soil types were used to demonstrate that leakage for any given May–December period was directly related to soil water storage at 1 May. From this finding, it follows that leakage from a phase rotation can be calculated if the size of the buffer, and the leakage quantity in the absence of a buffer, are known for each stage of the rotation. LeBuM uses a long-term sequence of leakage values in the absence of a buffer as input, and the maximum buffer size, its rate of development, and the length of perennial and annual phases are specified as parameters. LeBuM was applied to leakage data modelled for 5 contrasting soil types over 100 years at 24 sites in the Western Australian wheatbelt. Phase rotations on duplex, waterlogging duplex, or loamy sand soils reduced leakage by >90% for regions with <380 mm annual rainfall, but were less effective in wetter regions and on deep sands or acid loamy sands. Nevertheless, phase rotations if adopted widely could delay the onset of salinity by as much as several decades.

Farming carbon: an economic analysis of agroforestry for carbon sequestration and dryland salinity reduction in Western Australia

“…This causes salinity when the groundwater rises and brings to surface dissolved salts that hinder plant growth. Recharge values for annual crops or pastures and trees on each LMU were estimated using a model of water leakage for agricultural areas, LBUM (Ward 2006). Different soils have different properties which affect their recharge rate.…”

Perennial pastures for recharge control in temperate drought-prone environments. Part 2: soil drying capacity of key species

“…Much of the environmentally degrading deep drainage that occurs is attributable to episodic rainfall events occurring outside the normal growing season (Black et al 1981;Asseng et al 2001;Ward 2006). However, the experimental sites in this research received few such episodic rainfall events.…”

“…The superior ability of lucerne (Medicago sativa L.) to dry the soil profile in a range of environments is well documented (Christian 1977;Lolicato 2000;Latta et al 2001;Ridley et al 2001a;Brown et al 2005;Bell et al 2006;Fillery & Poulter 2006;Sandral et al 2006;Ward 2006). Rarely has any pasture species been reported to exceed lucerne in its capacity to dry the soil profile, although occasionally an alternative species is reported to have achieved the same level of drying as lucerne.…”

Optimal management of annual ryegrass (Lolium rigidum Gaud.) in phase rotations in the Western Australian Wheatbelt*

“…The length of the lucerne phases in the rotations containing perennial pasture in Sections 4.1-4.4 are sufficient to prevent the development of dryland salinisation on the field of interest. This is identified through the calculation of mean recharge over the last century for each rotation using the Leakage-Buffer model (Ward 2006) (data not shown). Furthermore, Doole (2007) identified that, if the value of lucerne phases for weed management is not considered and the only effect of salinisation is the loss of agricultural production, perennial pasture is only profitable to adopt for salinity prevention if a saline water table is less than 3.5 m from the soil surface.…”

“…In addition, crop yield declines if insufficient weed control is attained. Continued reliance on shallow-rooted annual crops and pastures in rotations has also encouraged the onset of dryland soil salinisation by allowing a higher proportion of rainfall to recharge saline water tables, relative to that occurring under native vegetation (Pannell and Ewing 2006;Ward 2006).…”