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Evapotranspiration from agricultural plant communities in the high rainfall zone of the southwest of Western Australia
Journal of Hydrology, 146 (1993) 301-319 Elsevier Science Publishers B.V,, A m s t e r d a m
301
[3]
Evapotranspiration from agricultural plant communities in the high rainfall zone of the southwest of Western Australia P.R. Scott and R.A. Sudmeyer
Western Australian Department ~f Agriculture, Division of Plant Industries, Baron-Hay Court, South Perth, W.A. 6151, Australia (Received 27 March 1992; accepted 4 November 1992)
ABSTRACT Scott, P.R. and Sudmeyer, R.A., 1993. Evapotranspiration from agricultural plant communities in the high rainfall zone of the southwest of Western Australia. J. Hydrol., 146: 301-319. The clearing of native vegetation and its replacement with shallow rooted, annual crops and pastures has resulted in rising groundwater levels and concentration of salts in the surface soils of resulting groundwater discharge areas in the southwest of Western Australia. The potential to manipulate the recharge rates to groundwaters by using agronomic techniques to change catchment evapotranspiration (Et), has been the subject of much discussion. From 1986 to 1989, annual Et was estimated from daytime measurements of Et from annual pasture (existing pasture, subterranean clover, Medicago murex), crops (lupins, oats, rape, barley and wheat) and two perennial pastures (lucerne and phalaris) at a site near Collie in the southwest of Western Australia. The ventilated chamber technique was used to measure Et rates, together with ancillary measurements of above ground biomass and rooting depth. Seasonal values of Et are presented and combined to allow a boundary analysis of annual Et for each species. Et was found to be influenced by the amount and timing of biomass production, and by the rooting depth. The median annual evapotranspiration of annual pasture was shown to be the least (339 mm), and lupins the most (471 mm). The site environment combined high rainfall and low evaporative demand in winter, and low moisture-holding capacity of duplex soils with preferred pathways through subsoil clays. In this context, the potential of deeper rooted, perennial species to use more water, was apparent. It is argued that the smaller the difference in annual evapotranspiration between alternative and current agricultural practice (annual pasture), the larger the proportion of a catchment likely to be required for treatment to affect groundwater levels. Recharge manipulation alone, using the species tested, may not be sufficient for catchment salinity control. A wide range of other strategies exist; a combination of these, to suit the practical and economic constraints of the farmers, together with recharge manipulation, offers the best solutions for catchment management to control salinity in the southwest of Western Australia.
C o r r e s p o n d e n c e to: P,R. Scott, Western Australian D e p a r t m e n t of Agriculture, Division of Plant Industries, B a r o n - H a y Court, South Perth, W.A. 6151, Australia.
0022-1694/93/$06.00
© 1993 - - Elsevier Science Publishers B.V. All rights reserved
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P.R. SCOTT A N D R.A. S U D M E Y E R
INTRODUCTION
Dryland salinity causes a major land degradation and water resource problem throughout southern Australia. The impact of land clearing on dryland salinity in southwest Western Australia is well documented (Schofield et al., 1988). The mechanism of soil and water salinization is also well understood (Schofield et al., 1988) and research has now focused on rehabilitation strategies to control salinity. Bell et al. (1990) used reforestation to lower saline groundwaters on and around discharge areas. This treatment has been utilized extensively in the Collie River catchment by the Water Authority of Western Australia (WAWA). Considerable farmer interest has been aroused in the potential of trees and salt-tolerant vegetation to control salinity and provide some secondary economic benefits. The potential use of vegetation to alter recharge rates to groundwater has been canvassed by several authors (Sedgely et al., 1981; Nulsen and Baxter, 1982). Differences in evapotranspiration (Et) between agricultural species (Nulsen, 1984) and between different agronomic practices for the same species (Anderson, 1980) have been shown to be important. The possibility of changing agronomic practice on recharge areas to help control salinity gives more options to farmers and water resource managers dealing with the salinity problem. However, any new agronomic system must be equally or more productive, in economic terms, than the current system for it to be widely adopted by farmers. This paper deals with the potential for agronomic manipulation of the water balance in higher rainfall areas of the southwest of Western Australia, where the implications for water resources are important, and where the range of potential agronomic changes is more extensive than for lower rainfall areas. Et measurements are related to above-ground biomass and rooting depth of the different agricultural species. The implications of the measured Et rate for groundwater recharge control are discussed. SITE
The study site was located 32 km southeast of Collie in the southwest of Western Australia (33°28'S, 116°29'E). The area is characterized by a moderately undulating landscape and lateritic soils. The site was in a mid to upper slope landscape position with a northerly aspect, bounded to the west and north by remnants of the original dry sclerophyll native forest ofjarrah (Eucalyptus marginata), marri (Eucalyptus calophylla) and wandoo (Eucalyptus wandoo). About half of the Collie River catchment is cleared and used mainly for livestock production on annual pastures.
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CLIMATE
The climate is Mediterranean with cool, wet winters and warm, dry summers. The long-term annual rainfall is 694mm, of which 75% falls between May and September; this leads to very seasonal conditions for plant growth. The annual pastures generally germinate in April, flower in September, and senesce by mid-December. Annual rainfalls for the years 1986, 1987 and 1988 were 537mm, 467mm and 743 mm, respectively. Mean long-term annual evaporation (class A pan) is about 1500mm of which 15% occurs between May and September. SOILS
The plots were on duplex yellow soils occasionally with a bleached A2 horizon. The A1 horizon ranged from loamy sand to sandy loam. Pisolitic gravel was present in most of the soil profiles. Subsoil clays were present at depths ranging from 20 to 180 cm and consisted of dense, mottled and pallid zone clays with poor structure and numerous remnant tree root channels. Johnston (1987) showed that fissures and old root channels through the clay can be important structures in the process of groundwater recharge. The soils are generally considered to have a low moisture-holding capacity (Sharma et al., 1987a). SAMPLING
Measurements were made on eight plots of 0.1 ha each. During the period 1986-1989, crop species were planted in rotation to assist in weed and disease control. Lucerne (Medicago sativa var. WL318, Sheffield, Springfield, Siriver and Trifecta mixture) was sown in 1986. Phalaris (Phalaris aquatica var.) was sown in 1987. The resultant 'perennial' pastures were a mix of perennial and annual species. Annual pasture species murex (Medicago murex - - a mixture of experimental varieties) and subterranean clover (Trijblium subterraneum var. Junee, Seaton Park, Dalkeith) were sown in 1986. Crop species lupins (Lupinus albus var. Yandee), oats (Avena sativa vars. Echidna and Mortlock) and barley (Hordeum vulgare var. Stirling) were sown in 1986, 1987 and 1988. Rape (Brassica napus var. Westbrook) was sown in 1987 and 1988, and wheat (Triticum aestivum var. Aroona) was sown in 1986 and 1988. The existing annual pasture consisting of a mixture of bromegrass (Bromus diandrus), ryegrass (Lolium rigidum), silvergrass (Vulpia bromoides), subterranean clover, cape weed (Arctotheca calendula) and erodium (Erodium spp.) was also measured. Annual pastures were set stocked after an initial establishment
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period, and perennial pastures crash grazed when around 3-4t ha ~ of dry matter was available in spring, and about every 6-8 weeks for the remainder of the year. The ventilated chamber, a single representative quadrat was located in the plot being measured at the time. Et measurements were made periodically during the growing season (May-December) of 1986 and 1987, throughout 1988, and January and February of 1989. Four species were sampled at each measurement during the daylight hours. METHODS
Evapotranspiration The ventilated chamber (VC) method (see for example, Dunin and Greenwood, 1986; Farrington et al., 1990) was chosen as an appropriate method to compare Et rates from small, agricultural plots. The method was particularly suitable because the gravelly soils at the site made methods reliant upon soil moisture measurement difficult. Ventilated chambers 3 m x 5 m in ground area and 1.7m high enclosed the vegetation to be measured. The chambers were covered using Solarweave R, a fabric with about 80% light transmittance and good resistance to tearing. In 1986 a single infra-red gas analyser was coupled to an automatic sampling device and the results were recorded on a chart recorder. During 1987-1989, a pair of infra-red gas analysers were coupled to a computerdriven sampler and the results were logged on to a floppy disc. Both of these systems also recorded temperature (shaded platinum resistance thermometers) and ambient windspeed (Rimco cup anemometer). Estimates of windspeed through the chamber were made from measurements using an Alnor velometer (pressure differential anemometer) within a pitot shaft adjacent to the fan intake. Windspeed through the chambers averaged 0.23ms ~ for the 1987 and 1988 seasons, and 0.45ms ~ for the 1986 season (using different fans and chamber inlet configurations). The accuracy of the VC method of estimating Et has been recently reviewed by Dunin and Greenwood (1986) and Leuning and Foster (1990). The method has been used successfully by a number of wcrkers (see, for example, Farrington et al., 1990). The expertise gained by these workers was utilized in this study, resulting in a very similar measurement system. Recovery tests of a known amount of water from within the chambers resulted in 96%, 98% and 97% accuracy. The recovery test technique is recommended as a check of the accuracy of the VC system in measuring evaporation from vegetation within the chambers. Periods of missing data accounting for less than 25% of the daylight hours were interpolated or extrapolated using average daily
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transpiration curves and a computer aided spline fitting procedure described by Fox (1987a, b). Biornass
Above-ground biomass from the plots was measured using replicated small quadrat cuts each time Et was measured. The dry weights of six replicates of a 30cm × 30cm quadrat cut were determined and averaged to give a dry matter estimate for each Et measurement date. All the green material in the quadrat was cut, including any weed species present. Rooting patterns
The rooting patterns of the different species were examined by visual inspection and root mapping of an exposed face of a backhoe pit (Bohm, 1979) in 1986 and 1988. The exposed pit face was divided into a 10cm × 10cm grid and root maps were drawn. Four replicates of a 10cm wide × 200 cm deep strip were averaged and compared. The top 20 cm was not measured because of the large number of roots in this zone and the interest in rooting depth rather than root quantity. Data are presented as a percentage of the total number of roots recorded. RESULTS
Seasonal and yearly Et
Seasonal (1986 and 1987) and yearly (1988) Et for each crop and pasture species are given in Table 1. Seasonal Et was determined by integrating the daily Et values and assuming that there was no Et before 29 May or after 12 December 1986 and 22 December 1987. Et for 1988 is the integral of measurements taken throughout the year. Also shown in Table 1 is the number of data points from which the annual or yearly Et estimate is derived. The Et data for the crop species show that oats, lupins and rape used more water than barley or wheat. Note that in 1987 the Et and grain yield (see Table 2) of barley, and to a lesser extent oats, were reduced by damage caused by birds stripping the seed heads during grain filling and ripening. The seasonal data for the pasture species show that in 1986, the improved subterranean clover pasture used most water. The result is viewed with some caution as it is generated from only five data points, one of which, in August, was an unusually high reading; this exerted a large influence on the integrated Et value for the season. Et from the existing pasture and from newly established
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P.R. SCOTT AND R.A. SI:DMI~YER
TABLE 1
Seasonal and yearly Et estimates for crop and pasture species (n is the number of observations from which the integration was integrated) Species
Seasonal Et (200 day growing season)
Yearly Et
1986
1988
1987
Et (mm)
n
Et (ram)
n
326 342 294 226
9 9 8 7
285 272 200
6 7 8
306
7
169 209 214 269
8 6 8 7
Et (ram)
n
226 322 258
17 15 16
Crops Lupins Oats
Barley Wheat Rape Pasture
Existing Sub. clover M urex
Lucerne Phalaris Rainfall for period (ram)
228 336 310 244
9 5 6 6 370
350
733
lucerne was low. In 1987 the lucerne was well established (see Biomass) and had the highest seasonal Et, with the existing pasture again being the lowest. In 1988 Et was measured throughout the year on lucerne, phalaris and murex. Lucerne showed the highest Et, and murex the lowest. Seasonal Et values were generally higher in 1986 than 1987. There was 20 mm more growing season rainfall in 1986, but both years were of below average seasonal rainfall. In 1988, rainfall was slightly above average. Biomass
Table 2 gives biomass yields for crops and pastures and grain yields for crops. For the crops, biomass yields generally reached 10-12 t ha- Jin October or November. This translated to grain yields of 1.7 and 1.6 t ha-~ for lupins, 2.5 and 2.2 t ha-~ for oats, 2.8 and 0 t ha-T for barley, 1.0 t ha- 1 for wheat and 2.0 t ha-I for rape for 1986 and 1987, respectively. Lower than expected yields in 1987 for barley and to a lesser extent oats were caused by bird damage to the grain heads. The pasture species generally attained higher biomass peaks of 2.04.1 t ha ~ in 1986, with the exception of lucerne (0.8 t ha -~), which was very
EVAPOTRANSPIRATIONFROMAGRICULTURALPLANTCOMMUNITIES
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TABLE 2 Peak integrated biomass for crop and pasture species and grain yield of crops Species
Lupins Oats Barley Wheat Rape Pasture" Sub. clover Murex Lucerne Phalaris
1986
1987
1988
Peak biomass (tha 1)
Grain yield (tha l)
Peak biomass (tha-l)
Grain yield (tha 1)
11.3 9.7 10.1 9.9
1.7 2.5 2.8 1.0
6.6 12.3 10.4
1.6 2.2 0
10.4
2.0
4.1 2.0 2.6 0.8
2.1 2.4 1.9 2.8
Peak biomass (tha i)
1.9 2.6 2.8
Pasture is the existing annual subterranean clover-based pasture on the site.
slow to grow in its establishment year. In 1987 biomass peaks were 1.92.8 t ha -1, consistent with the lower seasonal Et measurements in that season. In 1987 and 1988 lucerne biomass reached 2.8 and 2.6tha -~ peaks; it maintained a green biomass of between 0.1 and 0.7 t ha-~ over the summer/ autumn period and had a density of around 7 crowns m -2 in November 1987. The phalaris acted more like an extended seasonal annual; it maintained only 0.1 t h a -I over much of the summer, but as much as 0.4t ha -1 in March 1989 in response to summer rainfall. During the spring flush, the phalaris carried 2.8 t ha-r of biomass. Roots
The rooting patterns are shown in Tables 3 and 4 for pastures and crops, respectively. The surface roots (0-20 cm depth) were too numerous to record accurately using the root mapping technique, therefore the proportion of the remaining root system for each depth increment is shown. The information for most species is the average of four replicates from each of 1, 2 or 3 sample pits. The shallow rooted nature of the annual pasture species is immediately evident; very few roots extended beyond 70 cm. The crops generally attained rooting depths of 110-150 cm, with some variation being noted where samples came from different soil types. The apparently deep rooting pattern of oats was rather suprising compared with that of the reputedly deep rooting lupins.
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P.R. SCOTTAnD R.A. SUDMEYER
TABLE 3 Percentage of total number of roots in each l0 cm layer below 20 cm (n is the number of sample pits) Depth
Species
(cm)
20-30 40 50 60 70 80 90 100 110 120 130
n=2 Murex (%)
n=3 Lucerne (%)
n=2 sub. clover (%)
n=2 Pasture (%)
n= I Phalaris (%)
22 20 16 15 15 7 3 2
16 16 I1 19 15 8 5 2 2 2 I
36 32 22 8 I 1
31 24 16 13 7 5 3 1
14 14 25 19 14 4 2 3 2 3
TABLE 4 Percentage of total number of roots in each 10cm layer below 20cm (n is the number of sample pits) Depth
Species
(cm)
20-30 40 50 60 70 80 90 100 I10 120 130 140 150
n=2 Barley
n=2 Oats
n= 1 Lupins
n= 1 Rape
(%)
(%)
(%)
~°/o)
20 15 15 10 8 6 4 3 2 1 1
I1 10 12 8 11 8 9 6 6 4 7 2 4
29 20 19 15 11 2 2 I I
38 24 15 6 4 4 4 4 0 4
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The perennial pasture species were deeper rooted than the annual pasture species, generally attaining depths of 120-130cm. It should be noted that lucerne roots were observed in the bottom of one pit at a depth of 200 cm, and appeared to be following old root channels and fissures in the clay. The same trend was noted to a lesser extent in the other deep rooted species. DISCUSSION
The results of the seasonal E t measurements support those of Nulsen (1984) who showed differences in seasonal E t between crop species and clover-based pastures in a lower rainfall environment in Western Australia. The range of crop species, currently able to be grown successfully in the higher rainfall zone, is wider than that tested by Nulsen (1984). Crops of oats, lupins and rape, in particular, are likely to use more water than the existing annual pasture, or currently low-yielding crops such as wheat, in this environment. The relationship between crop yield and E t is important, and will be discussed later. Currently, however, cropping is only conducted on about a one year in eight rotation in the area. For cropping to have a significant impact on catchment hydrology, it will need to be carried out over a large proportion of the catchment. Matching cropping areas with soil types on which high yields can be expected is a logical approach to both profitability and groundwater recharge control for farmers to take. Farms in the area are predominantly sheep grazing enterprises based on annual subterranean clover pastures which have been profitable and productive (particularly on the better soil types). The low seasonal and annual Et of the existing pastures have resulted in increased rates of groundwater recharge, rising saline groundwaters and saline discharge areas that have decreased land productivity and increased stream salinity (Peck and Williamson, 1987). The case for farmers to grow perennial-based pastures to increase catchment Et is supported by this study. Phalaris, and in particular, lucerne, showed higher E t than annual pastures. Further work is required to ensure that farmers can establish and manage extensive stands of these perennials profitably before they will be adopted. Rates of groundwater recharge beneath different vegetation types may be calculated when the other elements of the water balance equation (eqn. (1)) are known P
=
E t + u + As + R O
(1)
where P is precipitation, E t is evapotranspiration, u is drainage, As is change in soil moisture storage, and R O is runoff. It may be assumed for some conditions that R O = 0 and that on a seasonal basis As = 0. Therefore,
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P.R. SCOTT AND R.A. SUDMEYER
drainage can be estimated simply by measuring Et and P. Brouwer and Van de Graaff (1988), however, showed that the assumption that RO = 0 can be misleading, and that differences in Et do not necessarily result in an equivalent difference in recharge. Differences in RO between perennial and annual pastures are highlighted by Brouwer (1989). It is important, therefore, to view the differences in Et as being only a guide to possible effects on catchment hydrology. The likely range of the variables in eqn. (1) on an annual basis is also of interest. In particular, years with low Et and high P are likely to result in one or more of the other terms being larger than usual. In the southwest of Western Australia, where crops and pastures rely heavily on growing season rainfall, P and Et are linked to some degree. In higher rainfall areas, however, P is often well in excess of the Et requirements of the crop or pasture species. RO is frequently initiated in these areas after saturation of the coarse surface layer of the duplex soils (Sharma et al., 1987b) and a significant proportion of recharge may be through preferred water flow paths in the subsoil clays (Johnston, 1987). Given this set of circumstances the data set was examined using a boundary analysis technique to give some idea of the maximum likely range of Et on an annual basis. Figures l(a) and l(b) show individual Et measurements plotted against day of the year for all of the data. The maximum and minimum possible annual Et (Table 5) have been generated by integrating the area beneath the upper and lower limit curves. The median Et is the integrated area beneath the curve that incorporates all the points: it represents the best estimate of annual Et for the species in this environment. Rates of Et over the summer/autumn period were assumed to be the same for senesced, grazed crop stubble and for senesced, grazed pasture. This assumption is not unreasonable given that the presence of stubble has a minimal effect on net long-term evaporation from bare soil (Freebairn et al., 1986). The lack of data over this period makes it difficult to determine the magnitude of the effect of variable summer/autumn conditions on annual Et. The median and upper limit curves coincide for this period due to the lack of data. High daily rates of Et ( 1 . 5 m m d a y ~) from senesced pasture were measured immediately following rainfall events when soil evaporation was the only contributing component. These high rates of daily Et were deemed to have lasted only temporarily and then fallen rapidly, consistent with the soil evaporation data of Winter (1974) and with data presented by Greenwood et al. (1985) of annual pasture Et after cyclonic summer rainfall. Over a dry summer period, lower daily Et rates of 0-0.3 m m d a y ~ could be expected to occur for extended periods between sporadic rainfall events. Thus, the estimated summer component of the mean annual Et for the annual species is likely to be excessive for an average year. For the period January to May,
EVAPOTRANSPIRATION
(a)
FROM
AGRICULTURAL
PLANT
(b)
6-
6
wheat
5
5-
E E
31 1
COMMUNITIES
3
E
3
,v,
2.
g
2
•
i
-
i
•
annual
i
,
i
oats
5
pasture
4 E vE
3-
g 2
2-
1
i
•
i
•
0
i
,7,
2
•
i
i
lupins
4
4 3
i
5
lucerne
E vE
-
E E 2-
1
i
i
5
=
0
i
3-
,T,
2-
i
,
5
phalaris
i
barley
4
4E E
•
2 1
i
100
i
200
i
i
300
400
Days since January 1
0
i
i
1 O0
200
300
400
Days since January 1
Fig. I. (a) Median, minimum and maximum annual Et curves from Et measurements during the period 1986-1989. Results for rape, annual pasture, lucerne and phalaris. (b) Median, minimum and maximum annual Et curves from Et measurements during the period 1986-1989. Results for wheal, oats, lupins and barley.
the median and maximum estimated Et for the annual species was about 95 mm, while the minimum is about 50 mm. In general terms, there were two periods of high potential Et when conditions of high evaporative demand coincided with adequate soil moisture
312
P.R. SCOTT AND R.A. SUDMEYER
700 600
I
500 400 E E
300 200 100 0
I
wheat
I
I
i
annual phalaris barley pasture
I
I
I
I
oats
lupins
rape
lucerne
Species
Fig. 2. Results of the boundary analysis on Et measurements. The upper and lower limits and the average annual Et are shown for each species.
conditions (see Fig. l(a), lucerne for example). The first was for a short period early in the growing season during April and May; early autumn rains provided moisture and warm temperatures and moderate solar radiation provided high evaporative demand. Perennial pasture species were well placed to utilize these early rains as annual species did not begin to germinate until later in the season. The second peak in potential Et occurred in spring, when high evaporative demand conditions coincided with adequate soil moisture and high biomass (particularly for crop species) around October. In general the 'spring flush' of the annual species resulted in more biomass (and greater TABLE 5 Upper and lower limits and median annual Et values calculated from Et measurements Species
kupins Oats Lucerne Rape Barley Phalaris Wheat Annual pasture
Annual Et (mm) Average
Minimum
Maximum
471 458 444 430 420 349 346 339
230 264 281 254 211 261 229 159
580 583 608 585 554 553 450 501
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potential Et) than for the perennial pasture species. However, limited access to soil moisture by the shallower rooted annual species became evident by December when the perennial species again showed higher daily Et rates. Et rates of about 0.5mmday -I from crops after harvest are similar to those recorded by Wallace et al. (1981) in conditions of high evaporative demand after crop harvest in India. Median annual Et for the plots ranged from 339 mm for the annual pasture species, to 471 mm for lupins (Table 5). The simultaneous effect of increasing evaporative demand, high biomass and adequate soil moisture conditions during spring is evident for all species (Figs. l(a) and (b)). The magnitude and timing of the 'spring flush' on Et is obviously important in determining annual Et. This is particularly evident for the crop species lupins, which carried large amounts of" actively transpiring biomass well into the spring period. The annual species all exhibited approximately the same length of growing season, thus differences in annual Et are determined solely by their seasonal Et. The effect of the deep root system and the potential of the perennial pasture species to utilize incident summer rainfall is illustrated by the maximum Et for lucerne for the January to May period being about 100mm, and the minimum, 20 mm (Fig. 2). Phalaris, at this site, acted as an extended season annual species (largely dormant over summer) and the upper limit for its summer/autumn Et is likely to be an overestimate. However, on moisturegaining sites with residual summer moisture, phalaris has been observed to have maintained high rates of growth (and therefore potential Et rates) over summer/autumn. The current practice of grazing annual pastures can be seen to have low minimum, mean and maximum Et (Table 5). Wheat varieties that can produce high yields in high rainfall environments are currently being tested, but the varieties used in this experiment were subject to disease and consequently showed little evaporative potential. The other crop species, rape, barley, oats and lupins, are currently better adapted to the environment and showed much higher evaporation. Of these crops, oats yielded most consistently as reflected by the high minimum annual Et. Lupins and rape provide useful high Et alternatives to oats and barley, and their agronomic role as legumes and 'break' crops to reduce grass disease levels is important. Phalaris showed a low median annual Et, but has high Et potential because of its perennial nature. The lack of data points warrants caution in using this result. Lucerne has been used successfully in the Great Plains of America to lower groundwater levels (Halvorson and Reule, 1980). Carbon et al. (1982) found that recharge under lucerne was less than that under annual pasture species on a deep sand. Because of the productivity and profitability of the current annual pasture system, lucerne may only be adopted over relatively
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small areas of an individual farm, which may limit its role in manipulating groundwater levels. Further work on the agronomy, adaptation, productivity and management of lucerne in this area is necessary before it can be introduced on a broad scale. Greenwood (1986) discusses the rationale for using vegetation to lower water tables and, in Australia, a great deal of salinity reclamation work using vegetation is being conducted at the moment. The relatively small reductions in recharge rates likely to result from changing agronomic practice need to be achieved on a broad scale if they are to be effective in salinity management. This data set confirms that there are more management options in catchment Et manipulation than just reforestation. The most effective and socially acceptable salinity control techniques are likely to be derived from a range of options: from increased cropping, through perennial pastures, drainage of perched water fresh aquifers, to different scales of reforestation as discussed by Schofield et al. (1989). Agronomic manipulation of recharge areas, using annual species alone, is unlikely to be successful in reducing salinity because of several factors. First, rainfall exceeding potential evaporation over the period May to September means that water is available for groundwater recharge. Secondly, the low soil moisture-holding capacity of the root zone and the presence of preferred pathways allows this water to move rapidly to the groundwater system. Therefore, only perennial species addressing both recharge and discharge areas are likely to have sufficient impact on catchment hydrology to control salinity.
Rooting depth Maximum root depths ranged from 80 cm for annual pasture to 150 cm for oats. The cropped plots were generally located on sites more favourable for root penetration resulting in rooting depths of 110-150cm. Lucerne roots were observed in remnant root channels at greater than 200cm depth elsewhere in the soil pit. The deep taproot of lucerne enables it to reach subsoil moisture not available to the annual species. Summer dormancy is probably necessary for phalaris given its limited rooting depth for a perennial which limits access to subsoil moisture in summer at this site. Brown et al. (1983) related rooting depth to soil water depletion (R 2 = 0.75) for plants with a large range of rooting depths. Figure 3 shows the regression of effective rooting depth (RD) (mm) against median Et (ram) for this data set; (Et = 0 . 1 6 3 ( R D ) + 217, R2 = 0.31). The regression implies that each additional 10 mm of rooting depth contributes about 1.6 mm of annual Et. At greater depth Sharma et al. (1987a) found that native forest extracts about 0.5mm per 10ram of subsoil clay. Regressions of maximum root depth
3 15
EVAPOTRANSPIRAT1ON FROM A G R I C U L T U R A L PLANT COMMUNITIES
5°°i 450 t
I 4004
j/
E
/
350-
300 Et = 1.63RD +217 r2= 0.31 250
60
8'0
160 --
120
1 ,0
160
180
200
Rooting depth (cm) Fig. 3. Regression of effective rooting depth and average annual Et for the species lucerne, phalaris, oats, lupins, barley, rape and annual pasture.
against maximum and minimum Et result in somewhat better correlations (R 2 = 0.43 and 0.51, respectively). Rooting depth is obviously not the only factor influencing Et, but in general terms, greater rooting depth leads to potentially higher annual Et. Biomass
Leaf area index has been found to account for 84% of the variation in annual Et from trees (Greenwood et al., 1985). Looking at the canopy resistance term, the Penman-Monteith equation for calculating crop Et reveals that minimum canopy resistances are generally achieved at leaf area index 3-4 (Szeicz and Long, 1969). The biomass of crops and pastures has also been related to Et (Tanner and Sinclair, 1983). Singh and Virmani (1990) related Et to seasonal biomass production in irrigated chickpea; the relationship was improved by normalizing Et using vapour pressure deficit to allow for different seasonal conditions. A simple regression of seasonal biomass production against Et using all species for this data set results in a very poor correlation (Et(mm) = 3.41 B(tha -I) + 246, R 2 = 0.074). A regression of daily Et against biomass on that day for all species results in a better relationship (Fig. 4; E t ( m m d a y -1) = 0.187B(tha -t) + 0.937, R2 = 0.27). An attempt to normalize the Et relationship to biomass using vapour pressure
316
P.R. SCOTT AND R.A. SUDMEYER
Et = 0.187B + 0.937 2 r =0.27
•mm
E 3-
•
t 1 1 ~ / ~ Nmm mm •m
2- %
muam
•
w n I
an• mma
o
lo
1'2
14
Biomass (t/ha) Fig. 4. Combined regression of above-ground biomass and daily Et for the species lucerne, phalaris, annual pasture, oats, wheat, lupins, rape and barley. Dry stubble and pasture are not included.
deficit did not result in a better correlation. Different canopy structures and soil moisture availability probably account for most of the variation. The difficulty of resolving the complex relationships between Et, biomass, radiation, rooting depth, soil moisture availability and canopy structure was beyond the scope of this work. Within the range of agricultural species studied, however, it is realistic to expect that crops and pastures with high biomass production will use more water. Numerous workers have shown that within a species, biomass production and Et are related. A strategy of matching productive plant species (preferably deep rooted and perennial) with soil and moisture conditions to which they are well adapted, and giving them the best possible agronomic inputs is likely to result in large biomass accumulations, leaf areas, and consequently Et. Apart from the benefits in catchment management for salinity control, this strategy generally conforms with the farmers' intention of maximizing yield and income from the land. No attempt has been made here to investigate the effect on annual Et of manipulating grazing of annual pastures. Maintaining more biomass on pastures is likely to result in somewhat higher rates of annual Et but, to achieve a noticeable effect in groundwater recharge rates it would have to be carried out over a large proportion of a catchment.
EVAPOTRANSPIRAT1ON FROM AGRICULTURAL PLANT COMMUNITIES
317
CONCLUSIONS
Estimates of seasonal Et from daily measurements using the ventilated chamber technique reveal that lupins, oats and rape can be expected to use more water than barley or wheat. Perennial pasture species phalaris and lucerne used more water on an annual basis than did annual pasture species. Rooting depth and biomass production influence seasonal and daily Et to a limited extent. A boundary analysis predicted the likely range of Et for each species. The variability in the seasons and the physical and biological characteristics that may influence elements of the water balance necessitate caution in attributing, by extrapolation, differences in Et to differences in recharge. Furthermore, careful consideration by catchment managers must be given to the economic and practical problems associated with changing farm practice. Changes in catchment vegetation which result in only small reductions in recharge to groundwater must be made over large areas to have a measurable impact on salinity. A combination of options that offers the most practical and economic solutions for salinity control is likely to be adopted by catchment managers and farmers. ACKNOWLEDGEMENTS
The authors are grateful for the assistance of numerous staff from the Western Australian Department of Agriculture in conducting the trial and processing the data. In particular, the efforts of K. Burke, G. Heady and J. Sudmeyer are acknowledged. The technical advice of J. Beresford, Dr. E. Greenwood, Dr. R. Nulsen, A. Passchier and G. Watson is gratefully acknowledged. The loan of equipment from Alcoa of Australia and CSIRO was invaluable. Land was made available by the Water Authority of Western Australia. REFERENCES Anderson, W.K., 1980. Some water use responses of barley, lupin and rapeseed. Aust. J. Exp. Agric. Anim. Husb., 20: 202-209. Bell, R.W., Schofield, N.J., Lob, C. and Bari, M.A., 1990. Groundwater response to reforestation in the Darling Range of Western Australia. J. Hydrol., 115: 297-317. Bohm, W., 1979. Methods of Studying Root Systems. Ecological Studies, Vol. 33. Springer, Berlin, 188 pp. Brouwer, J., 1989. Reducing groundwater recharge - - is increasing plant water use enough? Salt Force News, 12: 8-9. Brouwer, J, and van de Graaff, R.H.M., 1988. Readjusting the water balance to combat dryland salting in southern Australia: changing the hydrology of a texture contrast soil by deep ripping. Agric. Wat. Man., 14: 287-298.
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Brown, P.L., Halvorson, A.D., Siddoway, F.H., Mayland, H.F. and Miller, M.R., 198Y Saline seep diagnosis, control and reclamation. Conservation Research Rep. No. 30, Agricultural Research Service, United States Department of Agriculture, Washington, DC, 22 pp. Carbon, B.A., Roberts, F.J., Farrington, P. and Beresford, ,I.D., 1982. Deep drainage and water use of forests and pastures grown on deep sands in a Mediterranean environment. J. Hydrol., 55: 53-64. Dunin, F.X. and Greenwood, E.A.N., 1986. Evaluation of the ventilated chamber for measuring evaporation from a forest. Hydrol. Process., 1: 47-62. Farrington, P., Watson, G.D., Bartle, G.A. and Greenwood, E.A.N., 1990. Evaporation from dampland vegetation on a groundwater mound. J. Hydrol., 115: 65-75. Fox, D.R., 1987a. Evaporative transpiration research project (Data acquisition and modelling). Internal Rep. No. 7/87, Curtin University School of Mathematics and Computing, Perth, W.A., 72 pp. Fox, D.R., 1987b. Evaporative transpiration research project. Software documentation: Programs AVPLOT and PLOTFL. Internal Rep. No. 8/87, Curtin University School of Mathematics and Computing, Perth, W.A., 60 pp. Freebairn, D.M., Hancock, N.H. and Lott, S.C., 1986. Soil evaporation studies using shallow weighing lysimeters: Techniques and preliminary results. In: Conference on Agricultural Engineering, Adelaide, 24-28 August 1986. Inst. Engineers, Australia, pp. 106-110. Greenwood, E.A.N., 1986. Water use by trees and shrubs for lowering saline groundwater. Reclam. Reveg. Res., 5: 423-434. Greenwood, E.A.N., Klein, L., Beresford, ,I.D. and Watson, G.D., 1985. Differences in annual evaporation between grazed pasture and Eucalyptus species in plantations on a saline farm catchment. J. Hydrol., 78: 261-278. Halvorson, A.D. and Reule, C.A. 1980. Alfalfa to control saline seeps. Soil Sci. Soc. Am. J., 44: 370-374. Johnston, C.D., 1987. Preferred water flow and localized recharge in a variable regolith. J. Hydrol., 94: 129-142. Leuning, R. and Foster, I.J., 1990. Estimation of transpiration by single trees: comparison of a ventilated chamber, leaf energy budgets and a combination equation. Agric. For. Meteorol., 5 I: 63-86. Nulsen, R.A., 1984. Evapotranspiration of four major agricultural plant communities in the south-west of Western Australia measured with large ventilated chambers. Agric. Water Manage., 8: 191-202. Nulsen, R.A. and Baxter, I.N., 1982. The potential of agronomic manipulation for controlling salinity in Western Australia. J. Aust. Inst. Agric. Sci., 48: 222-226. Peck, A.,I. and Williamson, D.R., (Editors), 1987. Hydrology and salinity in the Collie River Basin, Western Australia. J. Hydrol., 94 (Special Issue). Schofleld, N.J., Ruprecht, J.K. and Loh, I.C., 1988. The impact of agricultural development on the salinity of surface water resources in south-west Western Australia. Rep. No. WS27, Water Authority of Western Australia, Perth, W.A., 83 pp. Schofield, N.J., Loh, I.C., Scott, P.R., Bartle, J.R., Ritson, P., Bell, R.W., Anson, B. and Moore, R., 1989. Vegetation strategies to reduce stream salinities of water resource catchments in south-west Western Australia. Rep. No. WS33, Water Authority of Western Australia, Perth, W.A., 81 pp. Sedgley, R.H., Smith, R.E. and Tennant, D., 1981. Management of soil water budgets of recharge areas for control of salinity in south-western Australia. Agric. Water Manage., 4: 313-334.
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Sharma, M.L., Barron, R.J.W. and Williamson, D.R., 1987a. Soil water dynamics of lateritic catchments as affected by forest clearing for pasture. J. Hydrol., 94: 29-46. Sharma, M.L,, Barron, R.J.W. and Fernie, M.S., 1987b. A real distribution of infiltration parameters and some soil physical properties in lateritic catchments. J. Hydrol., 94: 89-107, Singh, P. and Virmani, S.M., 1990. Evapotranspiration and yield of irrigated chickpea. Agric, For. Meteorol., 52: 333-345. Szeicz, G. and Long, I.F., 1969. Surface resistance of crop canopies. Water Resour. Res., 5: 622-633. Tanner, C.B. and Sinclair, T.R., 1983. Efficient water use in crop production: Research or re-search? In: Limitations to Efficient Water Use in Crop Production. ASA-CSSA-SSSA, Madison, WI, pp. 1-27. Wallace, J.S., Batchelor, C.H. and Hodnett, M.G., 1981. Crop evaporation and surface conductance calculated using soil moisture data from central India. Agric. Meteorol., 25: 83-96. Winter, E.J., 1974. Water, Soil and the Plant. MacMillan, London, 141 pp.
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[3]
Evapotranspiration from agricultural plant communities in the high rainfall zone of the southwest of Western Australia P.R. Scott and R.A. Sudmeyer
Western Australian Department ~f Agriculture, Division of Plant Industries, Baron-Hay Court, South Perth, W.A. 6151, Australia (Received 27 March 1992; accepted 4 November 1992)
ABSTRACT Scott, P.R. and Sudmeyer, R.A., 1993. Evapotranspiration from agricultural plant communities in the high rainfall zone of the southwest of Western Australia. J. Hydrol., 146: 301-319. The clearing of native vegetation and its replacement with shallow rooted, annual crops and pastures has resulted in rising groundwater levels and concentration of salts in the surface soils of resulting groundwater discharge areas in the southwest of Western Australia. The potential to manipulate the recharge rates to groundwaters by using agronomic techniques to change catchment evapotranspiration (Et), has been the subject of much discussion. From 1986 to 1989, annual Et was estimated from daytime measurements of Et from annual pasture (existing pasture, subterranean clover, Medicago murex), crops (lupins, oats, rape, barley and wheat) and two perennial pastures (lucerne and phalaris) at a site near Collie in the southwest of Western Australia. The ventilated chamber technique was used to measure Et rates, together with ancillary measurements of above ground biomass and rooting depth. Seasonal values of Et are presented and combined to allow a boundary analysis of annual Et for each species. Et was found to be influenced by the amount and timing of biomass production, and by the rooting depth. The median annual evapotranspiration of annual pasture was shown to be the least (339 mm), and lupins the most (471 mm). The site environment combined high rainfall and low evaporative demand in winter, and low moisture-holding capacity of duplex soils with preferred pathways through subsoil clays. In this context, the potential of deeper rooted, perennial species to use more water, was apparent. It is argued that the smaller the difference in annual evapotranspiration between alternative and current agricultural practice (annual pasture), the larger the proportion of a catchment likely to be required for treatment to affect groundwater levels. Recharge manipulation alone, using the species tested, may not be sufficient for catchment salinity control. A wide range of other strategies exist; a combination of these, to suit the practical and economic constraints of the farmers, together with recharge manipulation, offers the best solutions for catchment management to control salinity in the southwest of Western Australia.
C o r r e s p o n d e n c e to: P,R. Scott, Western Australian D e p a r t m e n t of Agriculture, Division of Plant Industries, B a r o n - H a y Court, South Perth, W.A. 6151, Australia.
0022-1694/93/$06.00
© 1993 - - Elsevier Science Publishers B.V. All rights reserved
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INTRODUCTION
Dryland salinity causes a major land degradation and water resource problem throughout southern Australia. The impact of land clearing on dryland salinity in southwest Western Australia is well documented (Schofield et al., 1988). The mechanism of soil and water salinization is also well understood (Schofield et al., 1988) and research has now focused on rehabilitation strategies to control salinity. Bell et al. (1990) used reforestation to lower saline groundwaters on and around discharge areas. This treatment has been utilized extensively in the Collie River catchment by the Water Authority of Western Australia (WAWA). Considerable farmer interest has been aroused in the potential of trees and salt-tolerant vegetation to control salinity and provide some secondary economic benefits. The potential use of vegetation to alter recharge rates to groundwater has been canvassed by several authors (Sedgely et al., 1981; Nulsen and Baxter, 1982). Differences in evapotranspiration (Et) between agricultural species (Nulsen, 1984) and between different agronomic practices for the same species (Anderson, 1980) have been shown to be important. The possibility of changing agronomic practice on recharge areas to help control salinity gives more options to farmers and water resource managers dealing with the salinity problem. However, any new agronomic system must be equally or more productive, in economic terms, than the current system for it to be widely adopted by farmers. This paper deals with the potential for agronomic manipulation of the water balance in higher rainfall areas of the southwest of Western Australia, where the implications for water resources are important, and where the range of potential agronomic changes is more extensive than for lower rainfall areas. Et measurements are related to above-ground biomass and rooting depth of the different agricultural species. The implications of the measured Et rate for groundwater recharge control are discussed. SITE
The study site was located 32 km southeast of Collie in the southwest of Western Australia (33°28'S, 116°29'E). The area is characterized by a moderately undulating landscape and lateritic soils. The site was in a mid to upper slope landscape position with a northerly aspect, bounded to the west and north by remnants of the original dry sclerophyll native forest ofjarrah (Eucalyptus marginata), marri (Eucalyptus calophylla) and wandoo (Eucalyptus wandoo). About half of the Collie River catchment is cleared and used mainly for livestock production on annual pastures.
EVAPOTRANSPIRATION FROM A G R I C U L T U R A L PLANT C O M M U N I T I E S
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CLIMATE
The climate is Mediterranean with cool, wet winters and warm, dry summers. The long-term annual rainfall is 694mm, of which 75% falls between May and September; this leads to very seasonal conditions for plant growth. The annual pastures generally germinate in April, flower in September, and senesce by mid-December. Annual rainfalls for the years 1986, 1987 and 1988 were 537mm, 467mm and 743 mm, respectively. Mean long-term annual evaporation (class A pan) is about 1500mm of which 15% occurs between May and September. SOILS
The plots were on duplex yellow soils occasionally with a bleached A2 horizon. The A1 horizon ranged from loamy sand to sandy loam. Pisolitic gravel was present in most of the soil profiles. Subsoil clays were present at depths ranging from 20 to 180 cm and consisted of dense, mottled and pallid zone clays with poor structure and numerous remnant tree root channels. Johnston (1987) showed that fissures and old root channels through the clay can be important structures in the process of groundwater recharge. The soils are generally considered to have a low moisture-holding capacity (Sharma et al., 1987a). SAMPLING
Measurements were made on eight plots of 0.1 ha each. During the period 1986-1989, crop species were planted in rotation to assist in weed and disease control. Lucerne (Medicago sativa var. WL318, Sheffield, Springfield, Siriver and Trifecta mixture) was sown in 1986. Phalaris (Phalaris aquatica var.) was sown in 1987. The resultant 'perennial' pastures were a mix of perennial and annual species. Annual pasture species murex (Medicago murex - - a mixture of experimental varieties) and subterranean clover (Trijblium subterraneum var. Junee, Seaton Park, Dalkeith) were sown in 1986. Crop species lupins (Lupinus albus var. Yandee), oats (Avena sativa vars. Echidna and Mortlock) and barley (Hordeum vulgare var. Stirling) were sown in 1986, 1987 and 1988. Rape (Brassica napus var. Westbrook) was sown in 1987 and 1988, and wheat (Triticum aestivum var. Aroona) was sown in 1986 and 1988. The existing annual pasture consisting of a mixture of bromegrass (Bromus diandrus), ryegrass (Lolium rigidum), silvergrass (Vulpia bromoides), subterranean clover, cape weed (Arctotheca calendula) and erodium (Erodium spp.) was also measured. Annual pastures were set stocked after an initial establishment
304
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period, and perennial pastures crash grazed when around 3-4t ha ~ of dry matter was available in spring, and about every 6-8 weeks for the remainder of the year. The ventilated chamber, a single representative quadrat was located in the plot being measured at the time. Et measurements were made periodically during the growing season (May-December) of 1986 and 1987, throughout 1988, and January and February of 1989. Four species were sampled at each measurement during the daylight hours. METHODS
Evapotranspiration The ventilated chamber (VC) method (see for example, Dunin and Greenwood, 1986; Farrington et al., 1990) was chosen as an appropriate method to compare Et rates from small, agricultural plots. The method was particularly suitable because the gravelly soils at the site made methods reliant upon soil moisture measurement difficult. Ventilated chambers 3 m x 5 m in ground area and 1.7m high enclosed the vegetation to be measured. The chambers were covered using Solarweave R, a fabric with about 80% light transmittance and good resistance to tearing. In 1986 a single infra-red gas analyser was coupled to an automatic sampling device and the results were recorded on a chart recorder. During 1987-1989, a pair of infra-red gas analysers were coupled to a computerdriven sampler and the results were logged on to a floppy disc. Both of these systems also recorded temperature (shaded platinum resistance thermometers) and ambient windspeed (Rimco cup anemometer). Estimates of windspeed through the chamber were made from measurements using an Alnor velometer (pressure differential anemometer) within a pitot shaft adjacent to the fan intake. Windspeed through the chambers averaged 0.23ms ~ for the 1987 and 1988 seasons, and 0.45ms ~ for the 1986 season (using different fans and chamber inlet configurations). The accuracy of the VC method of estimating Et has been recently reviewed by Dunin and Greenwood (1986) and Leuning and Foster (1990). The method has been used successfully by a number of wcrkers (see, for example, Farrington et al., 1990). The expertise gained by these workers was utilized in this study, resulting in a very similar measurement system. Recovery tests of a known amount of water from within the chambers resulted in 96%, 98% and 97% accuracy. The recovery test technique is recommended as a check of the accuracy of the VC system in measuring evaporation from vegetation within the chambers. Periods of missing data accounting for less than 25% of the daylight hours were interpolated or extrapolated using average daily
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transpiration curves and a computer aided spline fitting procedure described by Fox (1987a, b). Biornass
Above-ground biomass from the plots was measured using replicated small quadrat cuts each time Et was measured. The dry weights of six replicates of a 30cm × 30cm quadrat cut were determined and averaged to give a dry matter estimate for each Et measurement date. All the green material in the quadrat was cut, including any weed species present. Rooting patterns
The rooting patterns of the different species were examined by visual inspection and root mapping of an exposed face of a backhoe pit (Bohm, 1979) in 1986 and 1988. The exposed pit face was divided into a 10cm × 10cm grid and root maps were drawn. Four replicates of a 10cm wide × 200 cm deep strip were averaged and compared. The top 20 cm was not measured because of the large number of roots in this zone and the interest in rooting depth rather than root quantity. Data are presented as a percentage of the total number of roots recorded. RESULTS
Seasonal and yearly Et
Seasonal (1986 and 1987) and yearly (1988) Et for each crop and pasture species are given in Table 1. Seasonal Et was determined by integrating the daily Et values and assuming that there was no Et before 29 May or after 12 December 1986 and 22 December 1987. Et for 1988 is the integral of measurements taken throughout the year. Also shown in Table 1 is the number of data points from which the annual or yearly Et estimate is derived. The Et data for the crop species show that oats, lupins and rape used more water than barley or wheat. Note that in 1987 the Et and grain yield (see Table 2) of barley, and to a lesser extent oats, were reduced by damage caused by birds stripping the seed heads during grain filling and ripening. The seasonal data for the pasture species show that in 1986, the improved subterranean clover pasture used most water. The result is viewed with some caution as it is generated from only five data points, one of which, in August, was an unusually high reading; this exerted a large influence on the integrated Et value for the season. Et from the existing pasture and from newly established
306
P.R. SCOTT AND R.A. SI:DMI~YER
TABLE 1
Seasonal and yearly Et estimates for crop and pasture species (n is the number of observations from which the integration was integrated) Species
Seasonal Et (200 day growing season)
Yearly Et
1986
1988
1987
Et (mm)
n
Et (ram)
n
326 342 294 226
9 9 8 7
285 272 200
6 7 8
306
7
169 209 214 269
8 6 8 7
Et (ram)
n
226 322 258
17 15 16
Crops Lupins Oats
Barley Wheat Rape Pasture
Existing Sub. clover M urex
Lucerne Phalaris Rainfall for period (ram)
228 336 310 244
9 5 6 6 370
350
733
lucerne was low. In 1987 the lucerne was well established (see Biomass) and had the highest seasonal Et, with the existing pasture again being the lowest. In 1988 Et was measured throughout the year on lucerne, phalaris and murex. Lucerne showed the highest Et, and murex the lowest. Seasonal Et values were generally higher in 1986 than 1987. There was 20 mm more growing season rainfall in 1986, but both years were of below average seasonal rainfall. In 1988, rainfall was slightly above average. Biomass
Table 2 gives biomass yields for crops and pastures and grain yields for crops. For the crops, biomass yields generally reached 10-12 t ha- Jin October or November. This translated to grain yields of 1.7 and 1.6 t ha-~ for lupins, 2.5 and 2.2 t ha-~ for oats, 2.8 and 0 t ha-T for barley, 1.0 t ha- 1 for wheat and 2.0 t ha-I for rape for 1986 and 1987, respectively. Lower than expected yields in 1987 for barley and to a lesser extent oats were caused by bird damage to the grain heads. The pasture species generally attained higher biomass peaks of 2.04.1 t ha ~ in 1986, with the exception of lucerne (0.8 t ha -~), which was very
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307
TABLE 2 Peak integrated biomass for crop and pasture species and grain yield of crops Species
Lupins Oats Barley Wheat Rape Pasture" Sub. clover Murex Lucerne Phalaris
1986
1987
1988
Peak biomass (tha 1)
Grain yield (tha l)
Peak biomass (tha-l)
Grain yield (tha 1)
11.3 9.7 10.1 9.9
1.7 2.5 2.8 1.0
6.6 12.3 10.4
1.6 2.2 0
10.4
2.0
4.1 2.0 2.6 0.8
2.1 2.4 1.9 2.8
Peak biomass (tha i)
1.9 2.6 2.8
Pasture is the existing annual subterranean clover-based pasture on the site.
slow to grow in its establishment year. In 1987 biomass peaks were 1.92.8 t ha -1, consistent with the lower seasonal Et measurements in that season. In 1987 and 1988 lucerne biomass reached 2.8 and 2.6tha -~ peaks; it maintained a green biomass of between 0.1 and 0.7 t ha-~ over the summer/ autumn period and had a density of around 7 crowns m -2 in November 1987. The phalaris acted more like an extended seasonal annual; it maintained only 0.1 t h a -I over much of the summer, but as much as 0.4t ha -1 in March 1989 in response to summer rainfall. During the spring flush, the phalaris carried 2.8 t ha-r of biomass. Roots
The rooting patterns are shown in Tables 3 and 4 for pastures and crops, respectively. The surface roots (0-20 cm depth) were too numerous to record accurately using the root mapping technique, therefore the proportion of the remaining root system for each depth increment is shown. The information for most species is the average of four replicates from each of 1, 2 or 3 sample pits. The shallow rooted nature of the annual pasture species is immediately evident; very few roots extended beyond 70 cm. The crops generally attained rooting depths of 110-150 cm, with some variation being noted where samples came from different soil types. The apparently deep rooting pattern of oats was rather suprising compared with that of the reputedly deep rooting lupins.
308
P.R. SCOTTAnD R.A. SUDMEYER
TABLE 3 Percentage of total number of roots in each l0 cm layer below 20 cm (n is the number of sample pits) Depth
Species
(cm)
20-30 40 50 60 70 80 90 100 110 120 130
n=2 Murex (%)
n=3 Lucerne (%)
n=2 sub. clover (%)
n=2 Pasture (%)
n= I Phalaris (%)
22 20 16 15 15 7 3 2
16 16 I1 19 15 8 5 2 2 2 I
36 32 22 8 I 1
31 24 16 13 7 5 3 1
14 14 25 19 14 4 2 3 2 3
TABLE 4 Percentage of total number of roots in each 10cm layer below 20cm (n is the number of sample pits) Depth
Species
(cm)
20-30 40 50 60 70 80 90 100 I10 120 130 140 150
n=2 Barley
n=2 Oats
n= 1 Lupins
n= 1 Rape
(%)
(%)
(%)
~°/o)
20 15 15 10 8 6 4 3 2 1 1
I1 10 12 8 11 8 9 6 6 4 7 2 4
29 20 19 15 11 2 2 I I
38 24 15 6 4 4 4 4 0 4
EVAPOTRANSPIRATION FROM AGRICULTURAL PLANT COMMUNITIES
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The perennial pasture species were deeper rooted than the annual pasture species, generally attaining depths of 120-130cm. It should be noted that lucerne roots were observed in the bottom of one pit at a depth of 200 cm, and appeared to be following old root channels and fissures in the clay. The same trend was noted to a lesser extent in the other deep rooted species. DISCUSSION
The results of the seasonal E t measurements support those of Nulsen (1984) who showed differences in seasonal E t between crop species and clover-based pastures in a lower rainfall environment in Western Australia. The range of crop species, currently able to be grown successfully in the higher rainfall zone, is wider than that tested by Nulsen (1984). Crops of oats, lupins and rape, in particular, are likely to use more water than the existing annual pasture, or currently low-yielding crops such as wheat, in this environment. The relationship between crop yield and E t is important, and will be discussed later. Currently, however, cropping is only conducted on about a one year in eight rotation in the area. For cropping to have a significant impact on catchment hydrology, it will need to be carried out over a large proportion of the catchment. Matching cropping areas with soil types on which high yields can be expected is a logical approach to both profitability and groundwater recharge control for farmers to take. Farms in the area are predominantly sheep grazing enterprises based on annual subterranean clover pastures which have been profitable and productive (particularly on the better soil types). The low seasonal and annual Et of the existing pastures have resulted in increased rates of groundwater recharge, rising saline groundwaters and saline discharge areas that have decreased land productivity and increased stream salinity (Peck and Williamson, 1987). The case for farmers to grow perennial-based pastures to increase catchment Et is supported by this study. Phalaris, and in particular, lucerne, showed higher E t than annual pastures. Further work is required to ensure that farmers can establish and manage extensive stands of these perennials profitably before they will be adopted. Rates of groundwater recharge beneath different vegetation types may be calculated when the other elements of the water balance equation (eqn. (1)) are known P
=
E t + u + As + R O
(1)
where P is precipitation, E t is evapotranspiration, u is drainage, As is change in soil moisture storage, and R O is runoff. It may be assumed for some conditions that R O = 0 and that on a seasonal basis As = 0. Therefore,
310
P.R. SCOTT AND R.A. SUDMEYER
drainage can be estimated simply by measuring Et and P. Brouwer and Van de Graaff (1988), however, showed that the assumption that RO = 0 can be misleading, and that differences in Et do not necessarily result in an equivalent difference in recharge. Differences in RO between perennial and annual pastures are highlighted by Brouwer (1989). It is important, therefore, to view the differences in Et as being only a guide to possible effects on catchment hydrology. The likely range of the variables in eqn. (1) on an annual basis is also of interest. In particular, years with low Et and high P are likely to result in one or more of the other terms being larger than usual. In the southwest of Western Australia, where crops and pastures rely heavily on growing season rainfall, P and Et are linked to some degree. In higher rainfall areas, however, P is often well in excess of the Et requirements of the crop or pasture species. RO is frequently initiated in these areas after saturation of the coarse surface layer of the duplex soils (Sharma et al., 1987b) and a significant proportion of recharge may be through preferred water flow paths in the subsoil clays (Johnston, 1987). Given this set of circumstances the data set was examined using a boundary analysis technique to give some idea of the maximum likely range of Et on an annual basis. Figures l(a) and l(b) show individual Et measurements plotted against day of the year for all of the data. The maximum and minimum possible annual Et (Table 5) have been generated by integrating the area beneath the upper and lower limit curves. The median Et is the integrated area beneath the curve that incorporates all the points: it represents the best estimate of annual Et for the species in this environment. Rates of Et over the summer/autumn period were assumed to be the same for senesced, grazed crop stubble and for senesced, grazed pasture. This assumption is not unreasonable given that the presence of stubble has a minimal effect on net long-term evaporation from bare soil (Freebairn et al., 1986). The lack of data over this period makes it difficult to determine the magnitude of the effect of variable summer/autumn conditions on annual Et. The median and upper limit curves coincide for this period due to the lack of data. High daily rates of Et ( 1 . 5 m m d a y ~) from senesced pasture were measured immediately following rainfall events when soil evaporation was the only contributing component. These high rates of daily Et were deemed to have lasted only temporarily and then fallen rapidly, consistent with the soil evaporation data of Winter (1974) and with data presented by Greenwood et al. (1985) of annual pasture Et after cyclonic summer rainfall. Over a dry summer period, lower daily Et rates of 0-0.3 m m d a y ~ could be expected to occur for extended periods between sporadic rainfall events. Thus, the estimated summer component of the mean annual Et for the annual species is likely to be excessive for an average year. For the period January to May,
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i
i
300
400
Days since January 1
0
i
i
1 O0
200
300
400
Days since January 1
Fig. I. (a) Median, minimum and maximum annual Et curves from Et measurements during the period 1986-1989. Results for rape, annual pasture, lucerne and phalaris. (b) Median, minimum and maximum annual Et curves from Et measurements during the period 1986-1989. Results for wheal, oats, lupins and barley.
the median and maximum estimated Et for the annual species was about 95 mm, while the minimum is about 50 mm. In general terms, there were two periods of high potential Et when conditions of high evaporative demand coincided with adequate soil moisture
312
P.R. SCOTT AND R.A. SUDMEYER
700 600
I
500 400 E E
300 200 100 0
I
wheat
I
I
i
annual phalaris barley pasture
I
I
I
I
oats
lupins
rape
lucerne
Species
Fig. 2. Results of the boundary analysis on Et measurements. The upper and lower limits and the average annual Et are shown for each species.
conditions (see Fig. l(a), lucerne for example). The first was for a short period early in the growing season during April and May; early autumn rains provided moisture and warm temperatures and moderate solar radiation provided high evaporative demand. Perennial pasture species were well placed to utilize these early rains as annual species did not begin to germinate until later in the season. The second peak in potential Et occurred in spring, when high evaporative demand conditions coincided with adequate soil moisture and high biomass (particularly for crop species) around October. In general the 'spring flush' of the annual species resulted in more biomass (and greater TABLE 5 Upper and lower limits and median annual Et values calculated from Et measurements Species
kupins Oats Lucerne Rape Barley Phalaris Wheat Annual pasture
Annual Et (mm) Average
Minimum
Maximum
471 458 444 430 420 349 346 339
230 264 281 254 211 261 229 159
580 583 608 585 554 553 450 501
EVAPOTRANSP1RATION FROM AGRICULTURAL PLANT COMMUNITIES
313
potential Et) than for the perennial pasture species. However, limited access to soil moisture by the shallower rooted annual species became evident by December when the perennial species again showed higher daily Et rates. Et rates of about 0.5mmday -I from crops after harvest are similar to those recorded by Wallace et al. (1981) in conditions of high evaporative demand after crop harvest in India. Median annual Et for the plots ranged from 339 mm for the annual pasture species, to 471 mm for lupins (Table 5). The simultaneous effect of increasing evaporative demand, high biomass and adequate soil moisture conditions during spring is evident for all species (Figs. l(a) and (b)). The magnitude and timing of the 'spring flush' on Et is obviously important in determining annual Et. This is particularly evident for the crop species lupins, which carried large amounts of" actively transpiring biomass well into the spring period. The annual species all exhibited approximately the same length of growing season, thus differences in annual Et are determined solely by their seasonal Et. The effect of the deep root system and the potential of the perennial pasture species to utilize incident summer rainfall is illustrated by the maximum Et for lucerne for the January to May period being about 100mm, and the minimum, 20 mm (Fig. 2). Phalaris, at this site, acted as an extended season annual species (largely dormant over summer) and the upper limit for its summer/autumn Et is likely to be an overestimate. However, on moisturegaining sites with residual summer moisture, phalaris has been observed to have maintained high rates of growth (and therefore potential Et rates) over summer/autumn. The current practice of grazing annual pastures can be seen to have low minimum, mean and maximum Et (Table 5). Wheat varieties that can produce high yields in high rainfall environments are currently being tested, but the varieties used in this experiment were subject to disease and consequently showed little evaporative potential. The other crop species, rape, barley, oats and lupins, are currently better adapted to the environment and showed much higher evaporation. Of these crops, oats yielded most consistently as reflected by the high minimum annual Et. Lupins and rape provide useful high Et alternatives to oats and barley, and their agronomic role as legumes and 'break' crops to reduce grass disease levels is important. Phalaris showed a low median annual Et, but has high Et potential because of its perennial nature. The lack of data points warrants caution in using this result. Lucerne has been used successfully in the Great Plains of America to lower groundwater levels (Halvorson and Reule, 1980). Carbon et al. (1982) found that recharge under lucerne was less than that under annual pasture species on a deep sand. Because of the productivity and profitability of the current annual pasture system, lucerne may only be adopted over relatively
314
P.R. SCOTT A N D R.A. S U I ) M E Y E R
small areas of an individual farm, which may limit its role in manipulating groundwater levels. Further work on the agronomy, adaptation, productivity and management of lucerne in this area is necessary before it can be introduced on a broad scale. Greenwood (1986) discusses the rationale for using vegetation to lower water tables and, in Australia, a great deal of salinity reclamation work using vegetation is being conducted at the moment. The relatively small reductions in recharge rates likely to result from changing agronomic practice need to be achieved on a broad scale if they are to be effective in salinity management. This data set confirms that there are more management options in catchment Et manipulation than just reforestation. The most effective and socially acceptable salinity control techniques are likely to be derived from a range of options: from increased cropping, through perennial pastures, drainage of perched water fresh aquifers, to different scales of reforestation as discussed by Schofield et al. (1989). Agronomic manipulation of recharge areas, using annual species alone, is unlikely to be successful in reducing salinity because of several factors. First, rainfall exceeding potential evaporation over the period May to September means that water is available for groundwater recharge. Secondly, the low soil moisture-holding capacity of the root zone and the presence of preferred pathways allows this water to move rapidly to the groundwater system. Therefore, only perennial species addressing both recharge and discharge areas are likely to have sufficient impact on catchment hydrology to control salinity.
Rooting depth Maximum root depths ranged from 80 cm for annual pasture to 150 cm for oats. The cropped plots were generally located on sites more favourable for root penetration resulting in rooting depths of 110-150cm. Lucerne roots were observed in remnant root channels at greater than 200cm depth elsewhere in the soil pit. The deep taproot of lucerne enables it to reach subsoil moisture not available to the annual species. Summer dormancy is probably necessary for phalaris given its limited rooting depth for a perennial which limits access to subsoil moisture in summer at this site. Brown et al. (1983) related rooting depth to soil water depletion (R 2 = 0.75) for plants with a large range of rooting depths. Figure 3 shows the regression of effective rooting depth (RD) (mm) against median Et (ram) for this data set; (Et = 0 . 1 6 3 ( R D ) + 217, R2 = 0.31). The regression implies that each additional 10 mm of rooting depth contributes about 1.6 mm of annual Et. At greater depth Sharma et al. (1987a) found that native forest extracts about 0.5mm per 10ram of subsoil clay. Regressions of maximum root depth
3 15
EVAPOTRANSPIRAT1ON FROM A G R I C U L T U R A L PLANT COMMUNITIES
5°°i 450 t
I 4004
j/
E
/
350-
300 Et = 1.63RD +217 r2= 0.31 250
60
8'0
160 --
120
1 ,0
160
180
200
Rooting depth (cm) Fig. 3. Regression of effective rooting depth and average annual Et for the species lucerne, phalaris, oats, lupins, barley, rape and annual pasture.
against maximum and minimum Et result in somewhat better correlations (R 2 = 0.43 and 0.51, respectively). Rooting depth is obviously not the only factor influencing Et, but in general terms, greater rooting depth leads to potentially higher annual Et. Biomass
Leaf area index has been found to account for 84% of the variation in annual Et from trees (Greenwood et al., 1985). Looking at the canopy resistance term, the Penman-Monteith equation for calculating crop Et reveals that minimum canopy resistances are generally achieved at leaf area index 3-4 (Szeicz and Long, 1969). The biomass of crops and pastures has also been related to Et (Tanner and Sinclair, 1983). Singh and Virmani (1990) related Et to seasonal biomass production in irrigated chickpea; the relationship was improved by normalizing Et using vapour pressure deficit to allow for different seasonal conditions. A simple regression of seasonal biomass production against Et using all species for this data set results in a very poor correlation (Et(mm) = 3.41 B(tha -I) + 246, R 2 = 0.074). A regression of daily Et against biomass on that day for all species results in a better relationship (Fig. 4; E t ( m m d a y -1) = 0.187B(tha -t) + 0.937, R2 = 0.27). An attempt to normalize the Et relationship to biomass using vapour pressure
316
P.R. SCOTT AND R.A. SUDMEYER
Et = 0.187B + 0.937 2 r =0.27
•mm
E 3-
•
t 1 1 ~ / ~ Nmm mm •m
2- %
muam
•
w n I
an• mma
o
lo
1'2
14
Biomass (t/ha) Fig. 4. Combined regression of above-ground biomass and daily Et for the species lucerne, phalaris, annual pasture, oats, wheat, lupins, rape and barley. Dry stubble and pasture are not included.
deficit did not result in a better correlation. Different canopy structures and soil moisture availability probably account for most of the variation. The difficulty of resolving the complex relationships between Et, biomass, radiation, rooting depth, soil moisture availability and canopy structure was beyond the scope of this work. Within the range of agricultural species studied, however, it is realistic to expect that crops and pastures with high biomass production will use more water. Numerous workers have shown that within a species, biomass production and Et are related. A strategy of matching productive plant species (preferably deep rooted and perennial) with soil and moisture conditions to which they are well adapted, and giving them the best possible agronomic inputs is likely to result in large biomass accumulations, leaf areas, and consequently Et. Apart from the benefits in catchment management for salinity control, this strategy generally conforms with the farmers' intention of maximizing yield and income from the land. No attempt has been made here to investigate the effect on annual Et of manipulating grazing of annual pastures. Maintaining more biomass on pastures is likely to result in somewhat higher rates of annual Et but, to achieve a noticeable effect in groundwater recharge rates it would have to be carried out over a large proportion of a catchment.
EVAPOTRANSPIRAT1ON FROM AGRICULTURAL PLANT COMMUNITIES
317
CONCLUSIONS
Estimates of seasonal Et from daily measurements using the ventilated chamber technique reveal that lupins, oats and rape can be expected to use more water than barley or wheat. Perennial pasture species phalaris and lucerne used more water on an annual basis than did annual pasture species. Rooting depth and biomass production influence seasonal and daily Et to a limited extent. A boundary analysis predicted the likely range of Et for each species. The variability in the seasons and the physical and biological characteristics that may influence elements of the water balance necessitate caution in attributing, by extrapolation, differences in Et to differences in recharge. Furthermore, careful consideration by catchment managers must be given to the economic and practical problems associated with changing farm practice. Changes in catchment vegetation which result in only small reductions in recharge to groundwater must be made over large areas to have a measurable impact on salinity. A combination of options that offers the most practical and economic solutions for salinity control is likely to be adopted by catchment managers and farmers. ACKNOWLEDGEMENTS
The authors are grateful for the assistance of numerous staff from the Western Australian Department of Agriculture in conducting the trial and processing the data. In particular, the efforts of K. Burke, G. Heady and J. Sudmeyer are acknowledged. The technical advice of J. Beresford, Dr. E. Greenwood, Dr. R. Nulsen, A. Passchier and G. Watson is gratefully acknowledged. The loan of equipment from Alcoa of Australia and CSIRO was invaluable. Land was made available by the Water Authority of Western Australia. REFERENCES Anderson, W.K., 1980. Some water use responses of barley, lupin and rapeseed. Aust. J. Exp. Agric. Anim. Husb., 20: 202-209. Bell, R.W., Schofield, N.J., Lob, C. and Bari, M.A., 1990. Groundwater response to reforestation in the Darling Range of Western Australia. J. Hydrol., 115: 297-317. Bohm, W., 1979. Methods of Studying Root Systems. Ecological Studies, Vol. 33. Springer, Berlin, 188 pp. Brouwer, J., 1989. Reducing groundwater recharge - - is increasing plant water use enough? Salt Force News, 12: 8-9. Brouwer, J, and van de Graaff, R.H.M., 1988. Readjusting the water balance to combat dryland salting in southern Australia: changing the hydrology of a texture contrast soil by deep ripping. Agric. Wat. Man., 14: 287-298.
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Brown, P.L., Halvorson, A.D., Siddoway, F.H., Mayland, H.F. and Miller, M.R., 198Y Saline seep diagnosis, control and reclamation. Conservation Research Rep. No. 30, Agricultural Research Service, United States Department of Agriculture, Washington, DC, 22 pp. Carbon, B.A., Roberts, F.J., Farrington, P. and Beresford, ,I.D., 1982. Deep drainage and water use of forests and pastures grown on deep sands in a Mediterranean environment. J. Hydrol., 55: 53-64. Dunin, F.X. and Greenwood, E.A.N., 1986. Evaluation of the ventilated chamber for measuring evaporation from a forest. Hydrol. Process., 1: 47-62. Farrington, P., Watson, G.D., Bartle, G.A. and Greenwood, E.A.N., 1990. Evaporation from dampland vegetation on a groundwater mound. J. Hydrol., 115: 65-75. Fox, D.R., 1987a. Evaporative transpiration research project (Data acquisition and modelling). Internal Rep. No. 7/87, Curtin University School of Mathematics and Computing, Perth, W.A., 72 pp. Fox, D.R., 1987b. Evaporative transpiration research project. Software documentation: Programs AVPLOT and PLOTFL. Internal Rep. No. 8/87, Curtin University School of Mathematics and Computing, Perth, W.A., 60 pp. Freebairn, D.M., Hancock, N.H. and Lott, S.C., 1986. Soil evaporation studies using shallow weighing lysimeters: Techniques and preliminary results. In: Conference on Agricultural Engineering, Adelaide, 24-28 August 1986. Inst. Engineers, Australia, pp. 106-110. Greenwood, E.A.N., 1986. Water use by trees and shrubs for lowering saline groundwater. Reclam. Reveg. Res., 5: 423-434. Greenwood, E.A.N., Klein, L., Beresford, ,I.D. and Watson, G.D., 1985. Differences in annual evaporation between grazed pasture and Eucalyptus species in plantations on a saline farm catchment. J. Hydrol., 78: 261-278. Halvorson, A.D. and Reule, C.A. 1980. Alfalfa to control saline seeps. Soil Sci. Soc. Am. J., 44: 370-374. Johnston, C.D., 1987. Preferred water flow and localized recharge in a variable regolith. J. Hydrol., 94: 129-142. Leuning, R. and Foster, I.J., 1990. Estimation of transpiration by single trees: comparison of a ventilated chamber, leaf energy budgets and a combination equation. Agric. For. Meteorol., 5 I: 63-86. Nulsen, R.A., 1984. Evapotranspiration of four major agricultural plant communities in the south-west of Western Australia measured with large ventilated chambers. Agric. Water Manage., 8: 191-202. Nulsen, R.A. and Baxter, I.N., 1982. The potential of agronomic manipulation for controlling salinity in Western Australia. J. Aust. Inst. Agric. Sci., 48: 222-226. Peck, A.,I. and Williamson, D.R., (Editors), 1987. Hydrology and salinity in the Collie River Basin, Western Australia. J. Hydrol., 94 (Special Issue). Schofleld, N.J., Ruprecht, J.K. and Loh, I.C., 1988. The impact of agricultural development on the salinity of surface water resources in south-west Western Australia. Rep. No. WS27, Water Authority of Western Australia, Perth, W.A., 83 pp. Schofield, N.J., Loh, I.C., Scott, P.R., Bartle, J.R., Ritson, P., Bell, R.W., Anson, B. and Moore, R., 1989. Vegetation strategies to reduce stream salinities of water resource catchments in south-west Western Australia. Rep. No. WS33, Water Authority of Western Australia, Perth, W.A., 81 pp. Sedgley, R.H., Smith, R.E. and Tennant, D., 1981. Management of soil water budgets of recharge areas for control of salinity in south-western Australia. Agric. Water Manage., 4: 313-334.
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Sharma, M.L., Barron, R.J.W. and Williamson, D.R., 1987a. Soil water dynamics of lateritic catchments as affected by forest clearing for pasture. J. Hydrol., 94: 29-46. Sharma, M.L,, Barron, R.J.W. and Fernie, M.S., 1987b. A real distribution of infiltration parameters and some soil physical properties in lateritic catchments. J. Hydrol., 94: 89-107, Singh, P. and Virmani, S.M., 1990. Evapotranspiration and yield of irrigated chickpea. Agric, For. Meteorol., 52: 333-345. Szeicz, G. and Long, I.F., 1969. Surface resistance of crop canopies. Water Resour. Res., 5: 622-633. Tanner, C.B. and Sinclair, T.R., 1983. Efficient water use in crop production: Research or re-search? In: Limitations to Efficient Water Use in Crop Production. ASA-CSSA-SSSA, Madison, WI, pp. 1-27. Wallace, J.S., Batchelor, C.H. and Hodnett, M.G., 1981. Crop evaporation and surface conductance calculated using soil moisture data from central India. Agric. Meteorol., 25: 83-96. Winter, E.J., 1974. Water, Soil and the Plant. MacMillan, London, 141 pp.