THE VIABILITY OF A PHASE ROTATION IN SALINITY CONTROL

Transcription

1 THE VIABILITY OF A PHASE ROTATION IN SALINITY CONTROL P.R. Ward CSIRO Plant Industry, Private Bag No 5, Wembley WA S. Asseng CSIRO Plant Industry Abstract Soil salinity is one of the most serious issues currently facing agriculture and the environment in Australia. It is due to an imbalance between groundwater recharge and discharge, caused by large-scale clearing of native perennial vegetation. The incorporation of perennial vegetation such as lucerne or trees into current farming systems has been proposed as a possible means to try to redress the imbalance. Lucerne and other perennials have been shown to create a zone of dry soil below the normal cropping root zone, which acts as a buffer against further leakage. In this paper, a series of 81 years of modelled annual leakage amounts from a current farming system was used to calculate the potential impact of incorporation of a perennial phase (either lucerne or blue gum) on average long-term leakage in two contrasting environments in Western Australia. The environments chosen were based on Merredin (average May-October rainfall 214 mm) and Moora (average May-October rainfall 369 mm), and correspond to the wetter and drier ends of the WA wheat-growing spectrum. Calculations suggest that at Merredin, if a lucerne phase created a buffer of 100 mm (as has been measured), long-term drainage would be reduced by 85% in a rotation of 3 years perennial, 5 years crop on a duplex soil, and by 73% on a deep sandy soil. The same rotation at Moora reduced drainage by 43% on the duplex soil and 26% on the sandy soil. For Blue gums with a 600 mm buffer in a 5 year blue gum/10 year crop rotation, leakage was reduced by more than 99% at Merredin regardless of soil type, and by 92 % on the duplex soil and 76% on the sandy soil at Moora. In theory, phase rotations can be designed to achieve just about any leakage target, although further research is required to define acceptable leakage targets for the various regions and soil types of southern Australia. Policy incentives may be necessary to encourage the rate and scale of adoption of phase rotations necessary to have an impact on the spread of salinity. Introduction Across southern Australia, the removal of native vegetation and its replacement with agricultural crops and pastures has caused an increase in groundwater recharge. Under the native ecosystem in the wheat belt, recharge was generally less than 1 mm/year, but after clearing, recharge increased to at least 6-10 mm/year (George 1992). In many regions of southern Australia, the very low slope of the land limits the groundwater discharge rate, and so the groundwater levels in these regions have risen in response to the increased recharge. When the water table rises to within about 2 m of the soil surface, evaporation from the capillary fringe causes the salt concentration to increase, and secondary salinity starts to develop. Once the salinisation process has reached this stage, reversal through biological options is difficult, because plants must cope with both waterlogging and salinity. Water use rates under these conditions tend to be much 150

2 Agriculture for the Australian Environment slower than would otherwise be expected (Barrett-Lennard 2002), and so a major impact on the water table height is unlikely. However, there may be options to slow the rate of water table rise, or perhaps even reverse it, if biological intervention occurs before the water table starts to interact with the soil surface (George et al. 1999). If perennials are established in the farming system while the water table is still deep enough, they have the opportunity to grow deep roots and dry the soil above the water table, creating a soil water storage zone (referred to as a buffer ) below the normal rooting zone of current agricultural crops and pastures. In theory, the storage zone is used when leakage from the root zone of agricultural crops and pastures occurs, preventing or reducing the amount of water reaching the water table. In order to be effective, this strategy must be employed over most, if not all, of the landscape. Although trees such as oil mallees (various Eucalyptus species) have a definite and important role in future landscapes, it is likely that they will be incorporated as distinct alleys, rather than at the scale necessary to control recharge at all points in the landscape. The rest of this paper will focus on leakage generated under broad-acre rotations including perennial plants as part of a phase rotation. Perennial plants proposed for broad-acre inclusion in phase farming systems of southern Australia include herbaceous perennial pastures such as lucerne (Medicago sativa) (Ward et al. 2001; Dunin et al. 2001), and short-rotation (about 5 years) trees such as blue gum (Eucalyptus globulus) (Harper et al. 2000), sometimes referred to as kamikaze forestry. In a recent review of lucerne performance in southern Australia, Ward and Dunin (2001) found that lucerne could create a buffer of up to 500 mm, but was more commonly in the mm range. For woody species, Knight et al. (2002) found buffers of up to 600 mm under belts of trees. In the wheatgrowing regions of southern Australia, where various estimates of average leakage range between 6 mm/year (George 1992) and 140 mm/year (Asseng et al. 2001), buffers of the magnitude measured under both trees and lucerne should have a substantial impact. However, actual leakage is very variable from year to year (Asseng et al. 2001; Zhang et al. 1999). According to Asseng et al. (2001), annual modelled leakage from a deep sandy soil varied from mm at Moora (average annual rainfall 461 mm), and from mm at Merredin (310 mm). With this degree of variation, the impact of a buffer of any particular size on longterm average annual leakage is much harder to predict. In this paper, a simple model (LeBuM Ward et al., in press) is used to calculate the impact of buffers of sizes typically created by lucerne or blue gums on leakage in high rainfall and low rainfall environments, on two contrasting soil types, within the WA wheat belt. Methods The Leakage/Buffer Model (LeBuM) has been described in detail by Ward et al. (in press). It works on an annual time step, using a long-term series of annual leakage amounts under conventional cropping systems, generated by models such as APSIM (McCown et al., 1996) or SWIM (Ross 1990). The user can specify the size of the buffer and its rate of development, and the length in years of the buffer-generation and cropping phases. LeBuM then calculates the average annual leakage expected under the new phase rotation, assuming that the buffer is completely filled during the cropping phase before leakage recommences. In other words, the 151

3 model assumes that preferential water flow through the buffer does not occur. For example, if the buffer was set at 100 mm at the end on the perennial phase, and leakage under a cropping rotation for the next three years was 60 mm, 30 mm and 50 mm, LeBuM would calculate the total leakage for these three years as 0 mm in the first year (buffer now reduced to =40 mm), 0 mm in the second year (buffer now reduced to 40-30=10 mm), and 40 mm in the third year (buffer absorbs the first 10 mm out of 50 mm, but the other 40 mm becomes leakage). LeBuM was applied to leakage data generated by APSIM for wheat crops grown in the period (81 years in total). Environments at Merredin (annual rainfall 310 mm) and Moora (461 mm), on soil types of acid loamy sand (90 mm available water) and shallow waterlogging duplex (81 mm available water) were selected for analysis. To simulate a blue gum phase rotation, the blue gum phase was set at 5 years, and the cropping phase was varied between 0 and 10 years. The buffer was set at 150 mm after the first year of blue gums, 300 mm after the second year, 450 mm after the third year, and 600 mm after the fourth and fifth years. For a lucerne phase rotation, the length of the lucerne phase was set at 3 years, and the cropping phase was again varied from 0-10 years. The buffer was set at 60 mm after the first year, 80 mm after the second year, and 100 mm after the third year. Ward et al. (2002) and Latta et al. (2002) showed that crops grown after lucerne may be able to extract water from deeper soil layers (if water is available there) than crops grown in conventional rotations, presumably by using deep root channels opened by the lucerne. A similar mechanism may operate under blue gums (Harper et al. 2000). To examine the possible impact on long-term leakage, extra water use by crops in LeBuM after either a blue gum or lucerne phase was set at either 0 or 10 mm. Results Leakage from the blue gum phase rotation was less than leakage from the lucerne phase rotation (Figure 1), for all soil types and locations. Similarly, leakage was greater at Moora (high rainfall) than at Merredin (low rainfall), and greater from the acid loamy sand than from the shallow duplex (waterlogging) soil. Allowing crops to use an extra 10 mm of soil water (if available) reduced leakage by a small amount for all lucerne phase rotations and for the blue gum phase rotations at Moora. In relative terms, a lucerne phase was much more effective at Merredin on either soil type than at Moora, particularly when combined with extra water use in the cropping phase (Figure 2). Blue gum phase rotations were more effective than lucerne phase rotations for all treatments, but were markedly less effective on the acid loamy sand at Moora than for other sites and soil types. 152

4 Agriculture for the Australian Environment Figure 1. Impact of the length of the cropping phase on leakage, in mm, from a 3-year lucerne phase rotation (a and b) and a 5-year blue gum phase rotation (c and d), with zero extra crop water use (a and c), or 10 mm extra crop water use (b and d). Note the different scale for the lucerne and blue gum graphs. als = loamy sand; sd = shallow duplex. 153

5 Figure 2. Impact of the length of the cropping phase on relative leakage, where a value of 1 is sequivalent to leakage from the current farming system, from a 3-year lucerne phase rotation (a and b) and a 5-year blue gum phase rotation (c and d), with zero extra crop water use (a and c), or 10 mm extra crop water use (b and d). Note the different scale for the lucerne and blue gum graphs. als = acid loamy sand; sd = shallow duplex. 154

6 Agriculture for the Australian Environment Discussion Lucerne and blue gum phase rotations A phase rotation involving 5 years of blue gums, creating a buffer of 600 mm, was far more effective in reducing long-term leakage than one involving 3 years of lucerne, creating a buffer of 100 mm. Under the blue gum phase rotation, leakage was reduced to virtually zero everywhere except the acid loamy sand in the high-rainfall district. However, as noted by Harper et al. (2000), there is considerable uncertainty regarding the soil water use characteristics of blue gums in the mm rainfall zone. The values used here of 150 mm in the first year, 300 mm in the second, 450 mm in the third, and 600 mm in the fourth and fifth years, represent an educated, but at this stage unverifiable, guess. There is also uncertainty associated with the economic analysis of blue gums when used in a phase rotation, particularly with respect to products and markets, and treatments (such as stump removal) necessary to return the land to a cropping phase. Inclusion of a phase of blue gums (or any other tree) into a farming system is a major change to the farming system, and will require different management strategies. For this reason, the rate of uptake by farming communities is likely to be slow. Nevertheless, the large buffers potentially created by trees in a phase rotation appear capable of controlling leakage in all but the most leaky soils at the wetter edge of the current wheat belt. In contrast, phase farming with lucerne represents a much less significant change to current farming systems. Economic analyses, such as those by Bathgate and Pannell (2002), are generally more accurate than those for blue gum phase rotations, and in this case, suggest that the inclusion of lucerne on 25% of a farm on the south coast of WA can increase the profitability of the farm as a whole. Recent research (eg Ward et al. 2001; Ridley et al. 2001) has clearly demonstrated that lucerne is capable of creating a buffer of around 100 mm in many different soil types. However, according to LeBuM calculations, the relatively small buffer size created by lucerne appears to restrict its effective area to the less leaky soil in the drier part of the wheat belt. The ability of perennials to reduce leakage to the groundwater is influenced by both the volume of the buffer (as investigated here), and by how readily the buffer is filled during periods of excess water. In some situations, excess water may flow preferentially through large pores through the buffer, bypassing the majority of the potential storage volume. Under conditions of preferential flow, the impact of the buffer on total leakage will be restricted. Preferential flow is known to occur in some soils, but at this stage, its impact on long-term average annual leakage is not known. Leakage targets It is both impractical and undesirable to eliminate leakage entirely: impractical, due to the episodic nature of leakage. As demonstrated here, even a buffer of 600 mm in the drier parts of the wheatbelt cannot reduce leakage to zero. Undesirable, because there must be a mechanism of salt export from the system. If leakage were eliminated, salt could accumulate in the root zone, eventually causing reduced plant growth and death. Therefore, agricultural rotations should be designed to achieve a pre-determined leakage target. An acceptable leakage target would be 155

7 related to the discharge capacity of the groundwater, and will vary considerably depending on the location within the catchment and the size of the groundwater system (Ferdowsian et al. 2002). In Western Australia, the low-rainfall parts of the wheatbelt tend to be of low relief, and the groundwater discharge capacity of these regions is probably of the order of a few millimetres per year (Clarke et al. 2002). Because of the low groundwater discharge rates, soils in these regions are generally at a high risk of developing secondary salinity. Both blue gum and lucerne in a phase rotation appear to be capable of achieving this leakage target, particularly if water use by conventional crops following the perennial phase can be increased slightly. In the high-rainfall parts of the wheatbelt, the topography tends to be more undulating, and the groundwater discharge capacity may be of the order of a few tens of millimetres. In this environment, blue gums may be a better option than lucerne. Of course, there will be many exceptions to the generalities discussed above, but it does appear that in many situations, phase rotations may be capable of restoring groundwater systems in the Western Australian wheatbelt to a state of balance. However, phase rotations will only be effective if they are established in regions before the water table starts to interact with the soil surface and secondary salinity becomes a problem. Phase rotations will also need to be implemented on the majority of land in a region, to reduce recharge over as much of the landscape as possible. In order to achieve the necessary level and rate of adoption, policy incentives may be required. Conclusions Assuming that the buffer created during the perennial phase was completely filled before drainage re-commenced, a perennial phase was more likely to have a significant impact, in terms of percentage reduction in drainage, in the drier environment. The drier environments of WA are also where the threat of lost production due to dryland salinity appears to be greatest. Reductions in drainage of more than 80% in the drier environment were possible with realistic assumptions of lucerne s performance, and with reasonable lengths of phase rotations. Incorporation of a phase of blue gums was even more beneficial, but the major changes required in the farming system may restrict the rate of adoption by landholders. From these calculations, a phase rotation involving a suitable perennial, if practised across an entire region in combination with other groundwater management strategies, could prevent further groundwater table rise. Input from groundwater hydrologists is necessary to confirm likely acceptable recharge targets. Policy incentives may be necessary to encourage land managers to adopt phase rotations at a suitable scale. References Asseng, S., Fillery, I. R. P., Dunin, F. X., Keating, B. A. and Meinke, H. (2001). Potential deep drainage under wheat crops in a Mediterranean climate. I. Temporal and spatial variability. Australian Journal of Agricultural Research 52: Barrett-Lennard, E. G. (2002). Restoration of saline land through revegetation. Agricultural Water Management 53:

8 Agriculture for the Australian Environment Bathgate, A. and Pannell, D. J. (2002). Economics of deep-rooted perennials in western Australia. Agricultural Water Management 53: Clarke, C. J., George, R. J., Bell, R. W. and Hatton, T. J. (2002). Dryland salinity in southwestern Australia: its origins, remedies, and future research directions. Australian Journal of Soil Research 40: Dunin, F. X., Smith, C. J., Zegelin, S. J., Leuning, R., Denmead, O. T. and Poss, R. (2001). Water balance changes in a crop sequence with lucerne. Australian Journal of Agricultural Research 52: Ferdowsian, R., Ryder, A., George, R., Bee, G. and Smart, R. (2002). Groundwater level reductions under lucerne depend on the landform and groundwater flow systems (local or intermediate). Australian Journal of Soil Research 40: George, R. J. (1992). Hydraulic properties of groundwater systems in the saprolite and sediments of the wheatbelt, Western Australia. Journal of Hydrology 130: George, R. J., Nulsen, R. A., Ferdowsian, R. and Raper, G. P. (1999). Interactions between trees and groundwaters in recharge and discharge areas - A survey of Western Australian sites. Agricultural Water Management 39: Harper, R. J., Hatton, T. J., Crombie, D. S., Dawes, W. R., Abbott, L. K., Challen, R. P. and House, C. (2000). Phase farming with trees. Rural Industries Research and Development Corporation Publication No 00/48. Knight, A., Blott, K., Portelli, M. and Hignett, C. (2002). Use of tree and shrub belts to control leakage in three dryland cropping environments. Australian Journal of Agricultural Research 53: Latta, R. A., Cocks, P. S. and Matthews, C. (2002). Lucerne pastures to sustain agricultural production in southwestern Australia. Agricultural Water Management 53: McCown, R. L., Hammer, G. L., Hargreaves, J. N. G., Holzworth, D. P. and Freebairn, D. M. (1996). APSIM: A novel software system for model development, model testing, and simulation in agricultural systems research. Agricultural Systems 50: Ridley, A. M., Christy, B., Dunin, F. X., Haines, P. J., Wilson, K. F. and Ellington, t. l. A. (2001). Lucerne in crop rotations on the Riverine Plains. 1. The soil water balance. Australian Journal of Agricultural Research 52: Ross, P. J. (1990). SWIM - a simulation model for soil water infiltration and movement: Reference Manual. Townsville, Australia, CSIRO Division of Soils 59pp. Ward, P.R. and Dunin, F.X. (2001) The influence of soil and climate on soil water use by lucerne. Grains Research and Development Corporation Preliminary Report,

9 Ward, P. R., Dunin, F. X. and Micin, S. F. (2001). Water balance of annual and perennial pastures on a duplex soil in a Mediterranean environment. Australian Journal of Agricultural Research 52: Ward, P. R., Dunin, F. X. and Micin, S. F. (2002). Water use and root growth by annual and perennial pastures and subsequent crops in a phase rotation. Agricultural Water Management 53: Ward, P. R., Dolling, P. J. and Dunin, F. X. (in press) The impact of a lucerne phase in a crop rotation on groundwater recharge in south-west Australia. Proceedings, 11 th Australian Agronomy Conference, Geelong, Zhang, L., Dawes, W. R., Hatton, T. J., Hume, I. H., O'Connell, M. G., Mitchell, D. C., Milthorp, P. L. and Yee, M. (1999). Estimating episodic recharge under different crop/pasture rotations in the Mallee region. Part 2. Recharge control by agronomic practices. Agricultural Water Management 42: