Thursday, February 09, 2006

Green water efficiencies help salinity on Australian drylands.

Drainage and change in soil water storage below the root-zone under long fallow and continuous cropping sequences in the Victorian Mallee
Mark G. O'Connell, Garry J. O'Leary and David J. Connor

A field study investigated drainage and changes in soil water storage below the root-zone of annual crops on a sandy loam soil in the Victorian Mallee for 8 years. It was designed to compare the effects of the common long (18-month) fallow in a 3-year rotation (fallow–wheat–pea, FWP) with a rotation in which the fallow was replaced with mustard (Brassica juncea), viz. mustard–wheat–pea (MWP). Drainage was measured over 2 periods (1993–98 and 1998–2001) using 9 in situ drainage lysimeters in each rotation. The first period of ~5 years was drier than average (mean annual rainfall 298 cf. 339 mm) and drainage was low and variable. Drainage was greater under the fallow rotation (average 0.24 mm/year) than under the non-fallow rotation (average <0.01>
Australian Journal of Agricultural Research 54, 663–675 (2003)

Production and Environmental Aspects of Cropping Intensification in a Semiarid Environment of Southeastern Australia
VĂ­ctor O. Sadras* and David K. Roget

Low and highly variable rainfall are major sources of risk for farms in semiarid environments, including the Mallee region of Australia where risk management is largely based on a conservative, low-input approach. This approach has substantial opportunity costs (missing the benefits of wetter seasons) and low yield per unit rainfall. We combined field and modeling experiments to evaluate an intensive, flexible cropping approach based on (i) an opportunistic combination of crops, including wheat (Triticum aestivum L), canola (Brassica napus L.), and grain legumes, and (ii) a close matching of N input to soil and seasonal conditions. In a 4-yr field trial established on a coarse-textured soil, an intensive cropping approach doubled gross margin and halved its coefficient of variation in relation to current practice. Modeling experiments revealed the underlying mechanisms of this response and estimated the effect of cropping intensification on N leaching and deep drainage. Simulated yield improvement under intensive cropping was related to increased water use efficiency [biomass per unit evapotranspiration (ET)] at the expense of N use efficiency (biomass per unit of N uptake); this is consistent with the theoretical expectation that plant growth is maximized when all resources are equally limiting. Simulations indicated no substantial increase in N leaching and moderate decrease in drainage beyond the root zone with the more intensive approach. The approach to intensification in this research provides a platform to improve production and profit and to reduce its seasonal variation with neutral or positive effects on environmentally relevant processes.
Abbreviations: ET, evapotranspiration • MAP, monoammonium phosphate • PAW, plant available water • T, transpiration

Published in Agron. J. 96:236-246 (2004).

Water balance changes in a crop sequence with lucerne
F. X. Dunin, C. J. Smith, S. J. Zegelin and R. Leuning

In a detailed study of soil water storage and transport in a sequence of 1 year wheat and 4 years of lucerne, we evaluated drainage under the crop and lucerne as well as additional soil water uptake achieved by the subsequent lucerne phase. The study was performed at Wagga Wagga on a gradational clay soil between 1993 and 1998, during which there was both drought and high amounts of drainage (>10% of annual rainfall) from the rotation. Lucerne removed an additional 125 mm from soil water storage compared with wheat (root-zone of ~1 m), leading to an estimated reduction in drainage to 30–50% of that of rotations comprising solely annual crops and/or pasture.

This additional soil water uptake by lucerne was achieved through apparent root extension of 2–2.5 m beyond that of annual crops. It was effective in generating a sink for soil water retention that was about double that of annual crops in this soil. Successful establishment of lucerne at 30 plants/m2 in the first growing season of the pasture phase was a requirement for this root extension. Seasonal water use by lucerne tended to be similar to that of crops in the growing season between May and September, because plant water uptake was confined to the top 1 m of soil. Uptake of water from the subsoil was intermittent over a 2-year period following its successful winter establishment. In each of 2 annual periods, uptake below 1 m soil depth began late in the growing season and terminated in the following autumn.

Above-ground dry matter production of lucerne was lower than that by crops grown in the region despite an off-season growth component that was absent under fallow conditions following cropping. This apparent lower productivity of lucerne could be traced in part to greater allocation of assimilate to roots and also to late peak growth rates at high temperatures, which incurred a penalty in terms of lower transpiration efficiency. The shortfall in herbage production by lucerne was offset with the provision of timely, high quality fodder during summer and autumn. Lucerne conferred indirect benefits through nitrogen supply and weed control. Benefits and penalties to the agronomy and hydrology of phase farming systems with lucerne are discussed.

Keywords: crop rotation, evapotranspiration, deep drainage, water use efficiency.

Australian Journal of Agricultural Research 52(2) 247 - 261
Full text doi:10.1071/AR00089

Estimating episodic recharge under different crop/pasture rotations in the Mallee region. Part 2. Recharge control by agronomic practices
L. Zhanga, W. R. Dawesa, T. J. Hattonb, I. H. Humec, M. G. O'Connelld, D. C. Mitchellc, P. L. Milthorpe and M. Yeee

Much environmental degradation, including salinity in the Mallee region of southeastern Australia, is associated with the loss of native vegetation and increased recharge. As a result, various agronomic practices have been proposed to reduce groundwater recharge. This study was conducted to evaluate the impact of these practices on recharge, in particular episodic recharge. A biophysically based model (WAVES) was used to estimate recharge rates under some typical crop and pasture rotations in the region using long-term meteorological data. Results show that: (1) recharge just below the root zone was episodic and that just 10% of annual recharge events contributed over 85% of long-term totals. Management options such as incorporating lucerne and deep-rooted non-fallow rotations can reduce both, mean annual recharge, and the number of episodic events, but not eliminate recharge completely; (2) winter fallows increased soil-water storage and some of the additional water was stored in the lower portion of the root zone or below it. This can increase the risk of recharge to groundwater system; (3) changes in land management may take a considerable period of time (>10 years) to have any noticeable impacts on recharge; and (4) recharge under lucerne was ≈30% of that under medic pasture.
Author Keywords: Agronomic practices; Episodic recharge; Fallowing; Root zone.

Agricultural Water Management
Volume 42, Issue 2 , November 1999, Pages 237-249

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