Water use and drainage under phalaris, cocksfoot, and annual ryegrass pastures

The water balance equation was used to calculate plant water use and drainage below a depth of 1.1 m for phalaris, cocksfoot; and annual ryegrass pastures and bare fallow at Rutherglen in north-eastern Victoria. Rainfall from 1990 to 1993 averaged 693 mm/year. Soil water use was greater under perennials over the summer-autumn period and the soil profile was approximately 50 mm drier at the beginning of each drainage season. Following autumn rains, soil water profiles under all treatments converged, usually reaching similar water contents within 4-6 weeks. Over 4 years, soil under phalaris became 33 mm drier, and cocksfoot 24 mm drier (P < 0.001), than under annual ryegrass or bare fallow. Phalaris had higher actual evapotranspiration (P < 0.05; average, 642 mm/year) than cocksfoot (619 mm/year) and annual ryegrass (606 mm/year), the latter two not differing significantly. Drainage occurred during winter and early spring, ranging from 2 to 12 mm in 1991 (515 mm rainfall) to >100 mm/year in 1990 and 1992 (671 mm and 901 mm rainfall, respectively). The variation between years was greater than the differences between pastures in any one year. Averaged over the 4 years, drainage losses below ! lm decreased in the order bare fallow > annual ryegrass > cocksfoot > phalaris, although differences between the 3 pasture types were not statistically significant. Drainage under phalaris and cocksfoot may have been overestimated relative to annual ryegrass and fallow because of subsurface flow, at the top of the B horizon, between the wetter and drier plots. The drainage under phalaris may also have been overestimated because this pasture extracted water below the depth of soil water measurement. Allowing for these effects, the estimated drainage under phalaris may have been 49-56 mm/year compared with 80-87 mm/year under annual ryegrass, an overall reduction of more than one-third. Although perennial pasture grasses are unlikely to stop all recharge to groundwater in high rainfall areas (>600 mm/year) of south-eastern Australia, they offer a practical way to combine profitable agriculture with reduced land degradation.