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35 ,
<br />30
<br />2 5
<br />Average W LT6 = 0.16 Mg ha enr
<br />r , .= 0.651
<br />• 20 •
<br />te
<br />•:
<br />-z 15 o •
<br />• so... • p
<br />ft i,
<br />1 0 i _• . • ., : •• •
<br />5 ; s • • g•• • a
<br />1 . ; •
<br />• •
<br />o.
<br />0 25 50 75 100 125
<br />ET (cm)
<br />• •• •-•
<br />• • •
<br />• ••
<br />150 175
<br />200
<br />Fig. I. Season long alfalfa biomass yield and evapotranspiration
<br />(ET) from studies with variable irrigation in the Great Plains
<br />and Intermountain West of the United States. Each study (not
<br />each point) was weighted equally to create the regression line.
<br />The slope of the line represents a regional average water -use
<br />efficiency (WUE) of 0.16 Mgha 1 cm'I.
<br />irrigation treatments, and 39 cm for dryland treatments (Table
<br />1). Average annual biomass yield was 16.6 Mgha 1 under
<br />full irrigation, 11.1 Mg hat under a variety of deficit irriga-
<br />tion treatments, and 6.0 Mgha 1 under dryland conditions.
<br />Although the irrigation treatments are pooled over a wide
<br />range of climatic conditions, the comparison illustrates the
<br />magnitude of water use by irrigated alfalfa, the potential water
<br />savings from converting full irrigation alfalfa to deficit irriga-
<br />tion or dryland, and the extent of biomass yield reduction that
<br />would accompany the reduced irrigation levels.
<br />A regional average WUE for alfalfa was estimated from the
<br />slope of a water production function created by aggregating the
<br />results of the reviewed studies. To illustrate the spatial and tem-
<br />poral variability in the water production function, results from
<br />individual study years were used (Fig. 1) rather than multi -year
<br />treatment averages (Table 1). Alfalfa biomass yield responds in
<br />apositive, linear relationship to increasing ET, with an aver-
<br />age WUE of 0.16 Mgha 1 cm -1 . This WUE is consistent with
<br />values reported for this region by others (Hanson and Putnam,
<br />2000). There is wide variation in reported WUE, with total
<br />seasonal WUE values ranging from 0.08 to 0.231vig ha cm
<br />(Table 1 and Fig. 1). To normalize ET and biomass yield for
<br />different climatic and environmental conditions, a relative water
<br />production function with multi -year average data was created
<br />(Fig. 2). Relative biomass yield declines at a greater rate than rela-
<br />tive ET (slope = 1.28), indicating that WUE will decrease with
<br />decreasing ET under either deficit irrigation or dryland condi-
<br />tions. The relative water production function (Fig. 2) can be used
<br />with localized estimates of maximum yield and ET to predict
<br />alfalfa production under planned reductions in irrigation.
<br />Several individual studies confirm decreasing WUE with defi-
<br />cit irrigation over a wide range of environmental conditions. Retta
<br />and Hanks (1980) used line source irrigation in an alfalfa study
<br />in Utah and found that WUE ranged from 0.19 Mgha 1 cm -1
<br />under deficit irrigation to 0.21 Mg ha cm -1 for full irriga-
<br />tion (Table 1). Sammis (1981) found that WUE ranged from
<br />0.09 Mgha 1 cm in the driest conditions to 0.15 Mgha cm
<br />in the wettest conditions under line source irrigation in New
<br />Mexico (Table 1). Another New Mexico study under line
<br />source irrigation (Smeal et al., 1991) supports these findings;
<br />WUE ranged from 0.07 Mgha 1 cm in the driest conditions
<br />to 0.14 Mg ha an in the wettest conditions (Fig 3) A similar
<br />observation of declining WUE with ET was observed in a more
<br />humid environment in Minnesota, where average WUE values
<br />were 0.23, 0.23, 0.21, and 0.12 Mg hat cm for high, medium
<br />high, medium low, and raided irrigation treatments (Carter
<br />and Sheaffer, 1983). Thus, over a wide range of environmental
<br />conditions, there is a consistent trend of decreased \XTUE with
<br />decreasing ET under deficit irrigation.
<br />Seasonal Differences in Water -Use Efficiency
<br />Water -use efficiency varies with harvest interval due to dif-
<br />ferences in environmental conditions and plant carbohydrate
<br />partitioning. Smeal et al. (1991) compared biomass yield and
<br />transpiration over harvest interval. Greatest WUE was measured
<br />during the first harvest of each season, and the remaining har-
<br />vests had lesser WUE (Table 2). A study conducted by Under -
<br />sander (1987) in Texas also found that the greatest WUE was
<br />observed for the first harvest (Table 2), and the trend is echoed in
<br />the results of both Daigger et al. (1970) and Wright (1988).
<br />Variation in solar irradiance helps explain seasonal differences
<br />in WUE. Smeal et al. (1991) compared biomass yield per unit of
<br />transpiration with level of solar irradiance over time. The results of
<br />this experiment showed that biomass yield increased with increas-
<br />ing average daily solar irradiance (S during the growing period
<br />from a low of 16.1 MJ m-2 d-1 to a maximum of 28.0 MJ m d
<br />Values of S greater than 28 MJ m dl resulted in biomass yields
<br />that decreased slowly or plateaued. Based on a plant growth simu-
<br />lation model, the increase in biomass yield per unit of transpiration
<br />was explained by increasing S due to increased light penetration
<br />into the canopy rather than an increase in heat energy (Holt et al.,
<br />1975). Both conditions, high light and relatively low temperatures
<br />only occur in the spring, whereas, light levels are limiting in the
<br />fall. Thus, harvest intervals correspond i g to the greatest WUE
<br />occur when solar irradiance is high enough to induce high levels of
<br />photosynthesis and temperatures are low enough to keep evapora-
<br />tion at a minimum, such as the first harvest in the spring (Delaney
<br />et al, 1974; Leavitt et al., 1979).
<br />The seasonalvariarion in WUE also relates to carbohydrate
<br />reserve Aux in the alfalfa plant. Biomass development early in the
<br />season depends on carbohydrate reserves accumulated during the
<br />fall of the previous season and results in the highest WUE of the •
<br />season (Smith, 1962; Robinson and Massengale, 1968). After the
<br />first harvest, carbohydrates for growth and restoration of root
<br />reserves come from photosynthesis in new leaves, and transpira-
<br />tion is correlated with leaf surface area (MacAdam and Barta,
<br />2007). As day - length shortens and temperatures decline in the
<br />late summer and early fall, greater amounts of photosynthate are
<br />partitioned into root reserves, resulting in lower levels of aboveg-
<br />round biomass, yield, and WUE (Hanson et aL, 1988).
<br />These observations suggest an approach to saving water from
<br />irrigated alfalfa may be to concentrate irrigation during periods
<br />of the growing season that have the greatest WUE, such as the
<br />first harvest interval, followed by deficit or no irrigation during
<br />periods of the growing season with the least WUE, such as mid-
<br />summer harvest intervals. This approach, referred to as irriga-
<br />tion termination or partial season irrigation, has the potential
<br />to improve WUE relative to a controlled deficit irrigation that
<br />is applied uniformly through the growing season. None of the
<br />studies reported in Table 1 managed irrigation in this manner,
<br />46 Agronomy Journal • Volume 103, Issue 1 • 2011
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