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

Abstract

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 1 .1 m 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.

Subsoil constraints to grain production in the cropping soils of the north-eastern region of Australia: an overview

“…Armstrong et al (1999) showed that perennials were able to extract more water from the soil than the annual legumes and sorghum. Similarly, Ridley et al (1997) demonstrated the potential of lucerne grown in rotation with crops to reduce water losses to deep drainage compared with annual crops and pastures. …”

Maximising the use of soil water by herbaceous species in the high rainfall zone of southern Australia: a review

“…Phalaris transpired 130 mm more water (33% increase) and produced 17% more dry matter than annual pasture under a medium cutting height (Table 5). Similarly, in the study of Ridley et al (1997), the yield of phalaris + N treatment increased from around 8 t to more than 16 t/ha between 1992 and 1993, despite similar pan evaporation in both years and higher April-October rainfall in 1992 (580 mm) than in 1993 (368 mm). The increased yield was more likely due to increased soil fertility, as these pastures received 400 kg N in 1992 and 500 kg N in 1993.…”

“…Therefore, a 50% improvement in the net pasture production, say from 8 t to 12 t, through improved agronomic practices and/or use of higher yielding varieties would use approximately 160 mm more water, irrespective of the water lost through soil evaporation or evaporative demand. Studies by Ridley et al (1997) and Clifton and Taylor (1995) reported that increased yields through better agronomic management practices resulted in higher soil water use. Clifton and Taylor (1995) reported that swards of both phalaris (Phalaris aquatica) and cocksfoot (Dactylis glomerata) that were allowed to grow to higher herbage mass were able to transpire more water than swards that were kept short.…”

“…Furthermore, the perception of substantial extraction of water from below 1 m depth by a 'negligible' amount of deep roots may prove to be confounded, if the relationship between intercepted loss and reduced wetting of soil under these conditions could be quantified. Unfortunately, intercepted loss has been ignored in most soil water balance studies due to the complexities involved with its measurement (Ridley et al 1997;Dunin et al 2001;Ward et al 2001Ward et al , 2002Dunin 2002). The wide variations in how well the reported data describe the relationship between water use and biomass production could be largely due to variation in the estimates of ET obtained from different indirect methods such as the Bowen ratio, soil water change, long-term weather data, and simulations, compared with more reliable direct measurements from weighing lysimeters.…”

“…Evaporative demand is a combined function of microclimatic parameters such as atmospheric temperature, leaf temperature, solar radiation, light intensity, and relative humidity. Measurement of actual ET under field conditions is highly complex; therefore, ET (actual) is mostly estimated as the potential ET or derived from the ET reference value using the modified Penman equation (Doorenbos and Pruitt 1984;Ridley et al 1997;White et al 2000). Large variations can occur in the estimates of ET from different methods such as the Bowen ratio method through continuous measurement of micrometeorological parameters, long-term weather data, and changes in soil water content.…”