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Potential crop water use is influenced by potential evapotranspiration (EP) which summarises complex relationships between environmental factors such as; global radiation, temperature, wind run, and was described by Penman (1948). EP consists of two parts; transpiration by the crop (EPT) and evaporation of water from the soil surface

(ES). The partitioning of EP between the two is a function of crop canopy cover, and ES

is dependent on soil surface wetness (Ritchie, 1972).

2.5.2.1 Crop water use

Potential crop water use (EPT) is normally measured when supply is non-limiting and

the ratio of EPT to EP peaks at 1.1 for lucerne (Carter and Sheaffer, 1983b; Brown,

2004) and is reduced in relation to the LAI and canopy cover (French and Legg, 1979). For example, Carter and Sheaffer (1983b) showed actual crop water use (ET) relative to

EP increased exponentially from 0.6 to 1.2 when the LAI of an irrigated lucerne crop increased from ~1.0 to 4.5. An increase in LAI increases the intercepted radiation which provides the latent energy that evaporates water from the stomata within the canopy. For example, Brown et al. (2012) showed strong (R2_{> 0.90) linear}

relationships between accumulated ET and intercepted radiation (Ri), however the slope

of the regression differed amongst regrowth cycles throughout the season from 0.10 to 0.27 mm/MJ/m2. When ET/Ri was regressed against mean vapour pressure deficit

(VPD) for individual regrowth cycles 75% of the variation was explained. VPD is the difference in the saturated vapour pressure at a given temperature and actual vapour pressure which creates the difference in water vapour concentration between stomata and the external air that drives ET (Tanner and Sinclair, 1983). When VPD increases,

ET increases but there is no associated increase in photosynthesis, therefore the water

use efficiency (WUE) decreases.

2.5.2.2 Soil evaporation, ES

Water loss from the plant-soil system can continue in the absence of a crop canopy due to soil evaporation (ES) (Ritchie, 1972). ES is the loss of water vapour from the soil

which largely depends on atmospheric demand and soil wetness. The accuracy of estimating ES is often disregarded in annual crops because full canopy cover is

maintained for a longer duration of the season than in grazed forages (Jamieson et al., 1995a). However, lucerne is repeatedly defoliated throughout the year and Brown (2004) showed lucerne experienced ~100 days per year with incomplete canopy cover. This incomplete canopy cover showed annual ES from lucerne grown under rainfed and

fully irrigated conditions contributed ~30% of total water use. When shelters were used to exclude rainfall, ES contributed 9% of total water as water loss ceased with the drying

of the topsoil. ES needs to be accounted for in the current study to compare ET of crops

grown on soils with different patterns of leaf area index.

2.5.2.3 Crop water extraction

Crop rooting characteristics depend on crop age and soil characteristics (Jamieson and Ewert, 1999). The pattern of water extraction from when the roots reach an individual soil layer can be described by an exponential rate of decline of SWC over time from DUL to LL, known as the ‘Monteith Framework’ (Passioura, 1983; Monteith, 1986). The rate of extraction is quantified by the extraction rate constant (-kl), where the k is the soil dependent diffusion constant (cm2_{/day) and l represents root length density}

12.5 and 15.6 mm/day for seedling and regrowth lucerne, respectively, and the rate of water extraction was 0.02 to 0.03/day. These parameters were combined to predict the water supply in relation to demand and the subsequent loss in yield when water limitation occurred (Brown et al., 2009). Because the ‘Monteith Framework’ can only be applied when water demand is greater than supply it is unknown how different water supplies, normally varied by irrigation, influence the above parameters. An alternative is to grow the crops in the same environment, however on soils with similar soil texture which differ in their PAWC, which is a key aspect of the current research.

Janson and Knight (1973) showed the application of irrigation to lucerne on a Lismore stony silt loam in Canterbury increased DM yields three fold. They also observed the fully irrigated crop exhibited water stress and suggested the root system may limit the extraction of water in this soil type. Fick (1984) also noted the potential growth rates of pasture across a wide range of soils in New Zealand were not realised in soils with less than 115 mm of PAWC in the root zone at field capacity. Again, root characteristics were suggested to be the limiting factor in these crops. The root system of lucerne has been described as non-branching relative to annual crops (Sheaffer et al., 1988) which may result in less exploitation of soil by roots searching for water. Dardanelli et al.

(1997) showed the rate of extraction for lucerne was consistent with that found by Brown et al. (2009), however this was a third of that of annual crops such as maize. Rather than an inefficient root system the authors suggested lucerne implemented the strategy to persist by using water conservatively and extract water from depth. Lucerne conservative daily water use of ~3% day (Dardanelli et al., 1997; Brown et al., 2009) of PAWC means 3.6 mm/day can be extracted from soils with 120 mm PAWC in the rooting zone. In Canterbury, summer daily EP often exceeds 5 mm (Jamieson et al., 1995a; Brown et al., 2012) which means crops grown on these soil must display greater EFV and –kl, or supply will not meet demand and water stress will occur. Thus, the pattern of water extraction for crops in the present study will be quantified to determine the water supply from these soils.