Lucerne in crop rotations on the Riverine Plains 1. The soil water balance

Dryland salinity, caused largely by insufficient water use of annual crops and pastures, is increasing in southern Australia. A field experiment in no rth-eastern Victoria (average annual rainfall 600 mm) assessed the potentia l for lucerne grown in rotation with crops to reduce the losses of deep dra inage compared with annual crops and pasture. Soil under lucerne could stor e 228 mm of water to 1.8 m depth. This compared with 84 mm under continuous crop (to 1.8 m depth), except in 1997-98 where crop dried soil by 162 mm. Between 1.8 and 3.25 m depth lucerne was able to create a soil water defici t of 78 mm. The extra water storage capacity was due to both the increased rooting depth and increased drying ability of lucerne within the root-zone of the annual species. Large drainage losses occurred under annuals in 1996 and small losses were calculated in 1997 and 1999, with no loss in 1998. Averaged over 1996-1999, drainage under annual crops was 49 mm/year (maximum 143 mm) and under annu al pastures 35 mm/year (maximum 108 mm). When the extra soil water storage under lucerne was accounted for, no drainage was measured under this treatm ent in any year. Following 2 years of lucerne, drainage under subsequent cr ops could occur in the second crop. However, with 3 or 4 years of lucerne, 3-4 crops were grown before drainage loss was likely. Our calculations sugg est that in this environment drainage losses are likely to occur under annu al species in 55% of years compared with 6% of years under lucerne. In wet years water use of lucerne was higher than for crops due to lucerne's abili ty to use summer rainfall and dry soil over the summer-autumn period. Durin g the autumn-winter period crop water use was generally higher than under l ucerne. The major period of increased soil water extraction under lucerne was from late spring to midsummer, with additional drying from deeper layers until a utumn. Under both lucerne and crops, soil dried progressively from upper to lower soil layers. Short rotations of crops and lucerne currently offer th e most practical promise for farmers in cropping areas in southern Australi a to restore the water balance to a level which reduces the risk of seconda ry salinity.