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Evaluation of throughflow interceptors for controlling secondary soil and water salinity in dryland agricultural areas of southwestern Australia: I. Questionnaire surveys
29
Applied Geography (19831, 3, 2944
Evaluation of throughflow interceptors for controlling secondary soil and water salinity in dryland agricultural areas of southwestern Australia : I. Questionnaire surveys A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan Department Australia
of Geography,
University of Western Australia,
Nedlands,
WA 6009,
Abstract Increasing soil and water salinity in southwestern Australia is a major social, economic and environmenta problem. Hydrological imbal~c~s following extensive clearing of the natural vegetation for agriculture have redistributed soluble salts into soils and streams. Interceptors have been designed by farmers to control the throughflow component of this hydrological imbalance, to reduce waterIogging of low-lying areas, and thus to ameliorate salinity problems. Farmer interviews show that, to date, interceptors have only partly achieved these objectives. Many interceptor systems are insufficiently comprehensive and leakages are common. Further, too short a period of time has elapsed since interceptor construction to have reversed the severe deterioration of soil properties.
The salinization of soils and streams in southwestern Australia, following European settlement from 1829 and removal of the indigenous vegetation mainly for agricultural purposes, is causing increasing concern. The nature, extent and causes of the problem
have been reviewed by Peck (1978), Mulcahy (1978) and Conacher (1979,1982, and in press) and are summarized only briefly here. These summaries are followed by a discussion of possible remedial measures. Specific methods used depend to some extent on the causal mechanisms which are perceived as being responsible for the salt problem. A large and in~uential group of farmers considers the causal mechanism to be the development of perched soiI waters in low-lying areas, with the main water input being valley-side throughflow. Consequently these farmers have constructed interceptor systems to divert throughflow from salt-affected areas. The substantive part of this paper assesses farmers’ perceptions of the effectiveness of interceptor systems. Nature and extent of salinity problems in southwestern Australia: summary By 1979,263 752 ha of previously productive land in southwestern Australia had been rendered useless for agriculture due to high salt concentrations, predominantly sodium chloride. By far the greater part of this salt-affected land-256 314 ha-is in the wheatbeh (Fig. l), and it is the equivalent of about 250 wheatbelt farms removed from production (using the average cleared area per property). Estimates by the authors suggest that, by 1978, the capital losses involved were of the order of $116 million and 0143-6228/83/010029-16SO3.00 cc? 1983 Butterworth & Co (Publishers) Ltd
30
Throughflow
interceptors:
Figure 1. The Western
I. questionnaire
Australian
suraeys
wheatbelt
and places referred
to in the text.
that annual production losses exceeded $20 million. Although the percentage ofcleared wheatbelt soils which are now salt-affected is only 2.02 per cent, the majority of farmers have no salt problems at all while others may have up to 30 per cent or more of their farms rendered unproductive (3852 of more than 11 500 wheatbelt farmers had some salt-affected land in 1979). The area of soil salinization had increased by 255 per cent from the time the first salt land survey was carried out by the Department of Agriculture in 1955, and the rate of increase appears to be accelerating. Between the 1974 and 1979 salt land surveys an area equivalent to 18 wheatbelt farms became unproductive each year, whereas prior to 1974 the annual rate of increase was the equivalent of only 4.6 farms. Increasing water salinity may well be a more serious problem than that of soil salinization. Since all the major streams and rivers of southwestern Australia have their headwaters in the cleared wheatbelt areas, they have all been polluted by soluble salts and their waters are now unfit for human consumption. Historical records, and interviews, show that all these rivers were previously fresh. Moreover, there are a number of saline lakes in the wheatbelt which had good quality water prior to clearing, and a proportion of farm water supplies from wells, bores and farm dams has become too salty for stock (which have a much higher salt tolerance than the acceptable limits for human beings). In addition, some clearing for agriculture has taken place in the forested catchments (Fig. 1) of some of the smaller streams which have been dammed or which will be required for future water supplies. The resulting threat of water
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
31
salinization was recognized by the government when, between 1976 and 1979, bans on further agricultural clearing were imposed on five of these catchments where significant areas of land are privately owned. In summary, the wheatbelt comprises a cleared area of 126 906 km2 in which virtually all surface waters have become unfit for human consumption, mostly within the last 70 years. There can be few other parts of the world where such extensive and severe water and soil pollution has occurred over such a short period of time. Causal mechanisms:
summary
It is widely accepted that the underlying cause of secondary salinization in Western Australia is the replacement of indigenous vegetation with introduced crops and pastures. The native vegetation has deep rooting systems which enable it to draw on deep soil- and groundwater storages throughout the year. In contrast the exotic species grow only from late autumn through to early summer and have shallow roots. Consequently less water is now being transpired by plants in the agricultural areas than previously, leading to water build-up in the generally featureless landscape. Most wheatbelt soils have considerable quantities of soluble salts in their deeply weathered profiles. Groundwaters, which are characteristically semi-confined beneath siliceous hardpans and clay horizons, are also highly saline in many areas. Most of these salts are thought to have accumulated from millennia of rain- and dustfall rather than from weathering or, with some exceptions, from marine sediments. It is generally believed-and recent work has provided supporting evidence (Williamson and Bettenay 1979tthat the reduced uptake of water by plants has resulted in a rise of groundwater tables either to the surface, causing saline seepages, or to within the capillary fringe of the soil surface. However, there is also evidence which suggests that in some instances groundwaters may not be present beneath salt-affected soils, or that the groundwater tables are too far below the surface for capillarity to be effective, or that the rates of upward movement of water through the confining hardpans, in response to hydraulic gradients, are too slow to explain the surface waterlogging that characterizes many ‘salt scalds’ (secondary salt-affected soils). Other work has shown that, in some areas, more rainwater is redistributed by ephemeral downslope throughflow than by overland flow or deep infiltration to groundwater, and widespread observations indicate that throughflow is a ubiquitous hydrological process throughout the wheatbelt. Although throughflow water is relatively fresh, it has been suggested that it accumulates as shallow perched water bodies in lower slope positions and in valley floors, and that salt concentrations are increased over time by high evaporation rates during summers (Conacher, in press). This work has also drawn attention to the three-dimensional nature of the landscape, stressing that upvalley inputs of water and salts by ephemeral or seasonal streamllowwhich in turn is supplied by throughflow, groundwater seepages and overland flowmust also be considered; and that soil and water salinization must be modelled and treated on a catchment and not a two-dimensional valley-side basis. In this context, calculations have shown that insufficient soluble salts are translocated by throughflow in certain catchments to account for the amount of salts being exported from those catchments by streamflow, and it is clear that some mixing of throughflow with more saline groundwaters is taking place. Thus there is a complex set of mechanisms which needs to be understood if the total system is to be manipulated in order to control the salt problem. These mechanisms include rain- and saltfall, transpiration, evaporation, overland flow, throughflow,
32
Throughflow interceptors: I. questionnaire surveys
streamflow, shallow and deep infiltration, groundwater movements, capillary action and shallow salt leaching in salt-affected soils, Vegetation, water, salt and soil properties-especially hydraulic conductivity-are intimately interrelated with climatic constraints and land management practices in a three-dimensional land surface. Remediai measures
Revegetation with indigenous, eucalypt woodland species in order to restore hydrological equilibrium is not a practicable, economic proposition for the wheatbelt farmer. Empirical evidence suggests that at least 50 per cent of salt scald or saline stream catchments would have to be so treated. The alternative of using commercial crop (including tree) and pasture species which would simulate the hydrological behaviour of the indigenous vegetation is an attractive and perhaps ideal alternative. However, there is insufficient knowledge about such species at present for this to be more than a theoretical option. Since the 1920s the rising water table mechanism has been the most widely accepted cause of soil salinization. Consequently the Western Australian Department of Agriculture has in essence considered the cause to be untreatable and has instead focused its attention on the effects. Thus, for more than 30 years the Department’s standard recommendation to farmers has been to fence the salt scald-to control grazing-and to establish salt-tolerant species, mainly grasses and shrubs. At best this approach provides some light grazing for stock, reduces water and wind erosion, and improves the aesthetics of the previously bare scalds. But the establishment of salttolerant pastures is not easy and, even if successfully achieved, it does not solve the problem. Not only do saline waters continue to seep from many scalds into streams, or continue their upslope expansion, but in many cases the salt scalds were amongst the farmers’ more productive areas. Salt-tolerant plants do not return these areas to full production. Not surprisingly, many farmers have become disillusioned with what they perceive as being a negative measure and have implemented alternative methods. A few have claimed that good crop and pasture management-including chisel ploughing to improve soil structure. organic matter content and hence water-holding capacities+ombined with judicious tree planting, have cured their salt problems. It is, however, a difficult method to implement without expert assistance; moreover there are no monitored data which would support these farmers’ assessments. Many farmers have attempted-largely unsuccessfully-to improve plant root environments on salt scalds by adding sand, hay or straw mulches, or by deep ripping, or by constructing shallow drainage systems. In all three instances it appears that the depths to which soil aeration is improved are either too shallow, or persist for too short a period, to provide sufficient improvement for plant germination and growth. A third group has excavated deep drains to lower water tables beneath the salt scalds. One example is near Watheroo (Fig. l), where significant success-partly supported by Department of Agriculture assessment-has been claimed. However the low hydraulic conductivities of most wheatbelt subsoils would necessitate a close spacing of such drains (within 40-50 m). Indeed deep drains near Bakers Hill (Fig. 1) have not resulted in any improvement in plant growth even immediately adjacent to one of the drains after four years, and although the potentiometric head of the groundwater has been lowered considerably from up to three metres above the ground surface, it is still within a few centimetres of the soil surface next to the drain, In general, therefore, deep drainage may not be even technically feasible in some locations; and it would certainly not appear to be an economic proposition for wheatbelt farmers. The value of their production per unit area is considerably less than in irrigated areas where deep
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
33
drainage to control waterlogging and salinity is a common practice. Moreover, this approach is inappropriate in water supply catchments since the draining groundwaters are highly saline. The fourth and most widespread approach is an attempt to de-water perched soilwaters in low-lying positions by intercepting and/or diverting valley-side throughflow on a catchment basis. Throughflow interceptors
Interceptors were first constructed in the Western Australian wheatbelt by a Brookton (Fig. 1) farmer, H. S. Whittington, in 1954. Both salt-tolerant plants and grade banks (shallow ditches and banks on valley sides, recommended by the Department of Agriculture as an erosion control measure), had been used previously in unsuccessful attempts to combat a serious salt problem. Interceptors were then tried, drawing on advice obtained from the United States Department of Agriculture. The design of the interceptors reflected Whittington’s perception that the salt problem was caused primarily by throughflow, leading to the development of perched water systems and waterlogging in lower slope positions, and not by a rising groundwater table. Following observations of the initial interceptors’ performance, progressive extensions were constructed until today the entire property is controlled by a comprehensive interceptor system. Each interceptor was constructed along the contour. A bulldozer was used to gouge a gentle-sided open ditch to a depth below the subsoils (which have a higher clay-sized content, and above which most of the throughflow occurs), and to push the clayey materials up against the bank of excavated spoil below the ditch (Plate I). This was
Plate I. Throughflow interceptor, showing the method of construction.
34
Throughflow
interceptors:
I. questionnaire
surceys
done to prevent leakages through the bank when the interceptor fills with water. The first interceptor was constructed approximately three vertical metres below the interfluve (water parting) or at the nearest point where soil materials were suitable and it was possible for a bulldozer to operate. Downslope interceptors were then constructed progressively at vertical height distances apart of between three and four metres, subject to the presence of suitable subsoil materials. Lateral spacing rarely exceeded 400 metres. The objectives of the system were: (a) to prevent overland flow and throughflow from reaching lower slope positions, thereby preventing waterlogging from extending further upvalley and upslope; (b) to de-water the perched soil-water systems in lowlying positions, and (c) to then improve overall soil properties in order to return the land to full production. Side benefits have included excellent erosion control, enhanced awareness by the farmer of his soils and the physical processes operating on his property, and improved farm management practices generally. In the early 1970s three near neighbours, impressed by Whittington’s results, started to construct their own interceptor systems. Within a few years a number of other wheatbelt farmers had followed suit, placing severe demands on Whittington’s time. On 15 March 1978, a meeting held in Quairading (Fig. 1) formed a farmer association named WISALTS-Whittington Interceptor Salt Affected Land Treatment Society- ~ and by October 1981 the society had attracted over 1000 financial members (more than 25 per cent of all farmers with salt-affected land). The society trains operators (who are farmer members), awarding A, B and C-class licences, to assist other members with the design and construction of interceptor systems. It is not known how many farmers have actually constructed such systems, but in one local government area alone there are 33 such farmers and the total number must be several hundred. Not all farmers have constructed comprehensive interceptors on the contour. One constraint which militates against fully comprehensive systems is cost; another is that many farmers do not have control over the catchments of their salt-affected land; and a third is concern that contour interceptors may increase deep infiltration, thereby raising groundwater tables or increasing hydraulic pressure gradients and aggravating the salt problem. A fourth problem is that level interceptors encourage water accumulation and hence leakages through the downslope banks. As a result of these constraints most farmers are now constructing interceptors on very slight gradients (with a fall of lo-20 mm in 30 m), diverting the intercepted water to a stream or, an added bonus, to farm dams. Some farmers have combinations of contoured and graded interceptors, while many others have constructed only one or two graded interceptors above a salt scald, largely for economic reasons. In general, therefore, there are a number of possible approaches available depending on the nature of, and the problems posed by, each property. The Department of Agriculture is opposed to the WISALTS approach mainly because it considers that interceptors are ineffective as a means of controlling soil salinity, and partly because of concern that contour interceptors may exacerbate the salt problem. In both cases the Department’s view is based on its understanding of the mechanisms responsible for salinization. Clearly, if groundwater is the main cause then the Department’s view is correct. WISALTS, equally clearly, considers that the Department is incorrect and the society has claimed significant success on a number of properties. In response to this situation the authors evaluated the effectiveness of interceptors as a means of combating salinity problems by interviewing farmers, repeated after a fouryear period, in an attempt to gauge farmer perceptions of the success or otherwise of their interceptor systems.
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
35
Evaluation of interceptors: farmer interviews Methods In 1977 discussions with Mr Whittington identified 18 other farmers who had constructed interceptors prior to that date. Whittington himselfwas not included in the survey as the results of his work had been presented and discussed elsewhere (Conacher 1974, 1975; Whittington 1975). Two other farmers did not respond to preliminary requests for their cooperation, leaving 16 farmers who were finally interviewed. They are located in the Brookton, Wickepin, Quairading, Tammin and Gingin districts (Fig. l), with the majority in the Brookton-Quairading area (Vaux and Morony 1977). It was intended to conduct the interviews in person, using a questionnaire. However, four farm visits could not be arranged and the questionnaires were completed by the farmers and returned by mail. Each of the remaining 12 properties was examined in the field, sketch maps of the interceptor systems were produced-with the aid of aerial photographs-and photographs of selected sites were taken. However, the focus of the survey was on the farmers’ perceptions of interceptor effectiveness. of soil Questions were asked on: farm size; extent and dates of occurrence salinization in relation to clearing; previous measures used to combat salinity; details concerning the design and construction of the interceptors; the degree of success obtained and the criteria used for that assessment; the cost of interceptor construction, and future intentions. In 1981 the same 16 farmers were re-interviewed, with only one not being visited personally. New maps were drawn for each property and additional photographs were taken. A new questionnaire was designed with a similar set ofquestions to those used in 1977, but requesting more detailed information on dates of various events, the degree of effort put into previously attempted control measures (especially those recommended by the Department of Agriculture), and the extent to which Whittington’s advice had been followed in designing and constructing the interceptors. More specific questions were asked on the criteria used by the farmers in assessing the effectiveness of their interceptors, drawing on the findings of the 1977 survey. Also in 1981, an attempt was made to interview all farmers who had constructed WISALTS interceptor systems in Kellerberrin Shire (Fig. l), using a very similar questionnaire. The purpose of this survey was to provide base data for future comparative studies. There are 33 such farmers in the Shire, of whom 28 were interviewed.
Results and discussion
The average size of the 16 farms in 1981 was 1376 ha (range 526-3036 ha) of which an average of 1250 ha or 91 per cent of the total property have been cleared (range 526-2834 ha; see Table 1). These farms are therefore generally larger than the average wheatbelt farm. The total area of salt-affected land on the 16 properties in 1981 was 2399 ha, constituting 12 per cent of the total cleared area. Scalded areas ranged between 16 and 486 ha per property, with a mean of 150 ha (Table 1). These averages are well in excess of the equivalent total wheatbelt data. Clearing of the natural vegetation commenced in the 19th century on one property (farmer 1) but in most cases the land was not settled until early this century or later, and clearing continued progressively to as recently as 1980 in some cases. An attempt was therefore made to identify scald catchments and to determine the clearing dates of these specific areas (Table 1, column 5), in relation to the dates when salt first became a problem
mean range
,I
9 10 11 12 13 14 15 16 Total Mean
526 955 2024 785 1349 561 2834 1102 3036 22018 1376
526 931 1740 783 1734 544 2672 726 2834 19996 1250
1920s 1920s 1930 1910.-14 1940s 1920 1920 40 1955-60 1920-30
‘7 1920-30 1930s 1920s 1920s ‘, 1940s
Dates of scald catchment clearmg (5)
28
1284 437-5387
area (ha)
28 115 O&486
20 23.5 3.-50
1976 81 197680 1976.-78 1976-H1 1976-8 1 1976-81 1977.-79 1977-8 1 1977 81
1971 80 1973-75 1974-75 197’7..78 1976 1976 81 1976-81 3300 545 9800 21.50 llooo loooc) 3000 5600 28000 11530 7369
‘? 8000 1200 2ooO 800 9800 14735
28 6.2 0.5 20
Yes NO Yes
3 I 4
with Table 1
Yes No Yes Yes
NO
Yes No No No
NO
1
4 1 2 2 3 2 1 3 4 39 2.4
Yes Yes Yes
(12)
Improvement to salt land
4 2 2
Dates of Total Success interceptor cost score construction to 1981 ($1 (IO) (TiI (9)
Shire, (1981) for comparison
16 121 485 137 161 105 80 283 486 2399 150
55 40 60 80 121 40 49 150
Extent of salt affected area (ha) (8)
(refer text for full discussion)
in Kelierberrin
30 10 19 31 20 15 30 7 20 307 22
‘1 30 30 20 40 ‘f ‘5
(7)
Lag tyr) (6) (5)
year salt first noticed (yrt
farmers
1950s 1933 1949 1943 1963 193.5 1960 1965 1945
1960s 1950s 1969 1940s 1960s 1955 1947
(6)
Dates of salt appearance
Table 2. Selected data from WISALTS
1387 446-5670
28
Quairading Quairading Quairadmg Quairading Quatrading Quairading Wickepin Gingin Tammin
8
5
6 7
1093 1215 809 1149 568 1672 1000
Brookton Brookton Brookton Brooktan Brookton Brookton Brookton
1
1227 1215 967 1457 516 2077 I321
(4)
(3)
12)
(1)
2 3 4
Area cleared (ha1
Farm size (ha)
District
Farmer
Table 1. Results of 198i farmer interviews
Yes Yes Yes Yes Yes Yes Yes Yes Yes
Possibly Yes Possibly
Yes
No Possibly Possibly
03)
Future interceptors
? .G !z 3. % 5 a. z g % ‘3
.3.
d 5
R 2
Y S 2
w
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
37
(column 6). Calculation of the lag between clearing and the onset of safinization (column 7) shows periods ranging between five and 40 years, with a mean of 22 years, which corresponds with previous results from the York-Mawson and Dalwallinu (Fig. 1) areas (Conacher and Murray 1973; Conacher 1975). Six of the I4 properties from which these data were obtained had a lag period greater than the average with an additional four having a lag only slightly below the average, and it can be concluded that the lag period generally exceeds 20 years. Comparative data from the 28 farms in Kellerberrin Shire are presented in Table 2. Farm sizes and lag periods are similar to those in Table 1 but the extent of salt-affected land is less. Prior to the construction of interceptors, six of the 16 farmers had attempted to establish salt-tolerant plants allied with grazing control by fencing. None were satisfied with the results although some plant cover was established. A further eight farmers had attempted various measures in&ding salt-tolerant plant establishment (mostly Puccjnellia sp.) but without grazing control, planting Ta~aris~ sp., deep ripping, grade banks, drainage, and applying different fertilizers. In one instance some ~uccinell~a grew, a few Tarnarisks were successfully established on other properties, and some improvement in erosion control was obtained on two properties. But none of the farmers considered that the salt problem had improved and in several cases it became worse. The final two farmers had not attempted any remedial measures prior to interceptor construction. Initial advice on interceptor construction was obtained from Whittington in all cases except one (farmer 6). Whittington also carried out soil tests, surveyed the interceptors and supervised their construction for 11 of the 16 farmers in the initial stages. Two farmers have contour interceptors only; six have both contour and graded interceptors, and the remaining eight have only graded interceptors. One farmer completed his (present) interceptor system in one year and two others in two years, but most worked on their systems for several years, observing results from one set before progressing further (Table 1, column 9). Eight farmers were still constructing interceptors in 1981, and 11 definitely intended to install additional interceptors in the future, with four more ‘possibles’ (column 13). Thus 15 of the 16 farmers presumably do not regard their interceptor systems as being complete. There is in fact a wide variation in the comprehensiveness of the systems amongst the 16 properties. Space precludes including a map of each system, but two extreme examples are presented in Figs 2 and 3. The amount expended on interceptor construction (Table 1, column 10) is indicative of the relative comprehensiveness of the 16 systems, with amounts ranging between $545 and $28 000 (mean $7369). Comparison of columns (5) and (6) in Table 2 gives an indication of the relationship between expenditure and extent of the interceptor systems. Cost is not an entirely reliable criterion, however, due to inflationary trends and differences in construction costs caused by varying terrain. All 16 farmers have experienced problems with their interceptors, the most common difficulty being leakages through imperfectly sealed banks (10 farmers), Three farmers specifically referred to inexperienced bulldozer operators. There is virtually no room for error with either type of interceptor and many have had to be rebuilt. Two farmers referred to problems with gullying and another two experienced bank bursts following heavy rain. Four farmers had difficulty with contour interceptors, where excessive water accumulation encouraged leakages and necessitated opening up one end of the interceptor or complete reconstruction on a slight gradient. ‘Sand seams’ caused problems for four farmers. These are thought to be palaeo-drainage channels which can be up to 3 m or more deep. They have been infilled by aeolian or colluvial sands and, in some instances, covered over by finer-textured materials making them difficult to
38
Throughjow
interceptors:
2
Creek
_._
Graded
lnterceplw
-__
Future
IntcrccfNcx
‘&
salt-affected
0 L-b_
0.5
1. questionnaire surveys
sm.3 lkm
Figure 2. Sketch map showing
a small interceptor
system (farmer 2)
identify by normal soil-testing procedures. Considerable quantities of fresh water are translocated downslope through the porous sands in these ‘seams’, causing major problems during interceptor construction (but providing a welcome addition to farm water supplies). Often the only practicable solution is to remove the sand and fill the excavation with clay brought in from elsewhere, in order to maintain the level or graded base of the interceptor and to seal the downslope bank. A wide range of interceptor situations was therefore encountered on the 16 properties. They varied in terms of: dates of interceptor construction, which commonly vary in different parts of the same farm; varying combinations of interceptor types; the care with which soils were tested and the interceptors surveyed, especially after construction of the initial interceptors; and the quality and comprehensiveness of the interceptors themselves. Turning now to the question of whether the interceptors have fulfilled their objectives, all 16 farmers relied on visual observations for their assessments, supported in only three cases (farmers 1,15 and 16) by some sequential ground-level photographs. No objective on-site monitoring has been carried out by the farmers. A further difficulty is that observed changes do not occur uniformly in different parts of the same property or even around a single salt scald. Figure 4, completed and annotated by farmer 1 without the interviewers’ presence, indicates this spatial variability. Some parts of a property may even deteriorate whilst improvements are observed elsewhere. Evaluation of such detailed spatial differences requires accurate
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
z
Creek zzraed
EDI 0
Salt-affected 1
39
interceptor area 2km
1
Joins A above
Figure 3. Sketch map showing a large interceptor
system (farmer 16).
monitoring. Instead, an attempt was made to obtain an overall assessment by seeking the farmers’ responses to four improvement criteria drawn from the 1977 survey. These were : 1. 2. 3. 4.
decreased area improvements improvements improved crop
of waterlogging in soil properties to pasture production
An affirmative response to a criterion scored one point. Thus possible scores range from 0 to 4 (Table 1, column 11). It must be stressed, however, that a maximum score of 4 does not mean that complete rehabilitation has been obtained or observed. No farmer
40
Throughflow interceptors:
1. questionnaire
surveys
contour
‘“tarc*ptors
Proposed
___._--.
.
posstble
water
harvesting
__~__
i
Sketch map of an interceptor system (farmer 1) annotated by the farmer, indicating the spatial variation of perceived interceptor effectiveness. Figure4.
has claimed or achieved this. The ‘success score’ indicates the number of criteria for which some improvement has been observed and not the degree or extent of that improvement. Nevertheless, in general those farmers who claimed the greatest degree of improvement also noted beneficial changes in the largest number of criteria. (It should be noted that farmer 15 is disadvantaged by this scoring system since his farm is used for grazing only.) From the 1977 survey only three of the 16 farmers claimed any success (farmers 1,2 and 3, the only ones to have commenced interceptor construction before 1975). By 1981, all farmers claimed some improvements (Table 1, column 11). However, only four farmers recorded the maximum ‘success score’ while a further four scored only one point. It is interesting to note that the two top spenders both score 4, with the third highest spender scoring 3, while the lowest spender scores only 1. However, the next lowest scores 3, and between the highest and lowest spending extremes there is no relationship between expenditure on interceptors and the success scores. Likewise there is no relationship between success scores and the time when the first interceptors were constructed, but there does seem to be some connection with the duration of the period taken for interceptor construction (and therefore, possibly, comprehensiveness of the system and degree of care taken over its construction). For example, all farmers scoring 4 have taken at least four years to construct their systems (columns 9 and 1 1),
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
41
while some of the less ‘successful’ systems were constructed over shorter periods (with farmer 5 being the main exception). Fifteen of the 16 farmers reported that waterlogging had been reduced to varying extents following interceptor construction. However, the period 1977-81 (before the relatively wet 1981 winter) corresponded with a run of dry seasons, and several farmers were sceptical of the validity of this criterion. Nevertheless the farmers’ observations are supported by Negus (Department of Agriculture, Narrogin 1981, personal communication), who surveyed 35 interceptors during the wettest part of the 1981 winter. He found that, on average, a 50 m strip of land below the interceptors showed improved drainage and crop growth. Moreover 12 farmers reported some improvements in pasture growth over the 1977-81 period, which would be difficult to attribute to low rainfalls. In two cases this improvement referred only to an increased spread of barley grasses-a salt-tolerant species-into previously bare areas; but a number of farmers reported observing the following sequences of vegetation changes : Farmer 5 7 8 12 15 16
barley grass-cape weed+wimmera rye grass bare+barley grass+rye grass-tcape weed-tclover barley grass+rye grass+clover bare+samphire+barley grass-+rye grass+clover barley grass+gingin clover+subterranean clover barley grass-tcape weed+clover
These sequences are essentially a reversal of the trends noted as waterlogging and soil salinization develop, and the reappearance of the highly salt-sensitive clovers is rightly regarded by the farmers as a particularly significant occurrence. Six farmers observed improvements in soil properties other than waterlogging, increased humus and increased earthworm referring to reduced compaction, populations. It is evident, however, that any improvements to severely salt-affected land will take considerably longer than in peripheral or marginally affected areas. Reversal of severe deterioration of soil physical and morphological properties will take time and conscious effort to achieve, even with complete water control. With regard to the degree of improvement following interceptor construction, six farmers observed relatively significant successes. Farmer 1 reported dramatic improvements in crop yields on parts of his property, supported by production data. Farmer 8 observed an 80 per cent decrease of his waterlogged areas and improved crops in one location. Farmers 12 and 16 reported that 24 ha and 16-20 ha, respectively, have been returned to production. Farmer 13 observed that a paddock which was previously 75 per cent bare ground was only 25 per cent bare in 1981. Farmer 15 noted marked improvements in pasture growth over previously bare and waterlogged areas; his assessment was independently confirmed by three neighbours. Many of the above criteria and observations refer partly to areas which are peripheral to salt scalds, and it was difficult to obtain a clear distinction. Column 12 of Table 1 summarizes the responses obtained to a question which asked whether the economic or productive value of the salt scalds themselves had improved after interceptor construction. Again, a ‘yes’ response merely indicates that some improvement was observed. As can be seen, all farmers who scored 4 and 3 (except farmer 12) in column 11 provided a ‘yes’ answer. None of the farmers who scored 1 answered ‘yes’, but three of those who scored 2 did so. Two of the latter (farmers 2 and 3) are amongst those with the longest period of interceptor establishment, suggesting that time is a significant factor despite their relatively low scores on other criteria, The
42
Throughfiox
interceptors:
I. questionnairr
Lsurve_w
third ‘yes’ farmer with a score of 2, farmer 13, stated that part of his salt-affected land would be put under crop in 1982. If this crop is successful then his success score would improve to at least 3. Table 3, columns 2 and 3, shows the extent of salt land reported by the 16 farmers in the 1977 and 1981 surveys. Nine farmers reported increases in the areas of their saltaffected land-in most cases by quite considerable amounts. Comparison with columns 11 and 12 of Table 1 shows that some farmers claiming relatively significant improvements have nevertheless reported increases in salt land, while others who have claimed relatively little improvement nevertheless reported decreases in the extent of their salt land. With the partial exception of farmer 10, increases in the extent of salt land cannot be explained by increased farm sizes over the four-year period (Table 3, column 7-where minor changes are attributed to incorrect conversions by farmers from acres to hectares). Whilst concluding from these data that farmer perceptions are unreliable~specially of the area1 extent of salt-affected land-it is stressed again that many of the improvements observed and reported by farmers have taken place only on certain parts of their properties, and often to marginally salt-affected land only (cf. Fig. 4). With certain exceptions, the unreliability of farmers’ perceptions as suggested by the discrepancies between the data in Table 3, column 4, and Table 1, columns 11 and 12, may be more apparent than real. It is also possible that subconsciously farmers may justify their sometimes considerable expenditure and effort on interceptors by claiming positive results. The data in column 10 of Table 1, which were used previously to suggest some relationship between comprehensiveness of interceptor systems and their effectiveness, may be interpreted as providing partial support for the above suggestion. Nevertheless, most farmers gave interviewers the impression of having a very good observational knowledge of their properties even though area estimates may be unreliable. It is most Table
3. Areas of salt-affected land and farm sizes (1977, 1981)
Area of salt-affected Farmer
1977 (2)
(1) 1 2 3 4 5 6 7 8 9 IO 11 12 13 14 15 16 Total Mean
?
(55) 24 80 60 12 60 104 20 144 200 160 160 120 40 20 80 1339” 84”
Farm size (ha)
land (ha)
1981 (3? 55 40 60 80 121 40 49 150 16 121 485 137 161 105 80 283 486 2399 150
n Where 1977 data were not obtained, have been the same as in 1981
Change
1977 (5)
(4) ?
(O)O + 16 - 10 +61
+28 -1) +46 -4 -23 s285 -23 fl -15 -i-40 +263 +406 + 1060” + 66” for the purposes
?
(1227)” 1200 946 1480 569 1734 1312 ? (526)Q 616 1800 392 1320 546 2800 1080 3000 20548” 1284”
1981 (6) 1227 1215 967 1487 576 2077 1327 526 955 2024 785 1349 561 2834 1102 3036 2201X 1376
of summatioi~
Change (7) ? (0) +15 f21 -23 +7 + 343 + IS ? (0)” + 339 + 224 + 393 129 t15 t 34 +22 t36 + 1470” + 92”
they are assumed
to
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
43
unfortunate that WISALTS has not undertaken detailed monitoring of crop and pasture conditions to provide objective, quantitative support for its members’ observations. Finally, given the correctness of farmers’ perceptions of improvements, it is still difficult to assess the extent to which throughflow water control has in fact been achieved. All farmers experienced problems with their interceptors, especially leakages. Likewise, few if any of the 16 farmers have wholly comprehensive systems. Nevertheless, if throughflow is the mechanism solely responsible for all soil salinization, then better results than those found during the 1981 survey might have been anticipated. Four farmers had a success score of only 1 and a further two had a score of 2 and also reported no change to their salt land. Although four of these six farmers had expenditures on interceptor construction well below the average, suggesting that relatively little effort had been put into the work, two of the six farmers had above-average expenditures. On the other hand, even the most successful farmers had achieved only partial successes on parts of their properties. While the work of the 16 farmers demonstrates that throughflow is an important mechanism operating on all properties, and despite qualifications concerning the adequacy of the interceptor systems and the generally short period that has elapsed since their construction, it appears that another mechanism (or mechanisms) causing soil salinization is probably operating, at least on some parts of the 16 properties. Conclusions Soil and water salinity is a major environmental problem in southwestern Australia. Extensive clearing-up to 400 000 ha per annum-of natural vegetation for agricultural purposes took place in the middle and late 1960s and smaller-scale clearing has continued throughout the agricultural areas to the present day. In 1981 a further large-scale expansion of agriculture was announced by the state government. Since the onset of soil salinization generally occurs more than 20 years after clearing, and continues to expand in area for up to 40 years thereafter, or longer, further large increases in the areas of salt-affected land are likely to occur after the middle and late 1980s and well into the 21st century, unless preventative measures are implemented. The introduction by WISALTS ofinterceptors throughout the wheatbelt is designed to prevent throughflow from reaching salt-affected land, either by holding throughflow water on the slope (contour interceptors) or, more commonly, by diverting it to streams and farm dams (graded interceptors). The evidence presented in this paper suggests that interceptor systems generally achieve the above objectives. The major problems encountered are inadequacies in soil testing and in interceptor design and construction, resulting in leakages through and beneath the interceptors. The evidence further suggests that, following interceptor construction, the amelioration of salt-affected land is only partial. In diminishing order of success, improvements are reported after at least two years to: (a) the extent of waterlogging, (b) pasture quality, (c) crop yields, and (d) soil properties other than waterlogging. Such improvements have generally occurred on some but not all parts of individual farms, and in general there has been little improvement of severely salt-affected land. Soil salinization commences after three and generally more than 20 years following clearing, and is associated with a progressive deterioration of soil properties. It can be anticipated therefore that similar periods will be required for significant soil improvements to take place-and probably longer-ven if complete throughflow control is obtained. Such complete control has not been achieved on any of the 16
44
Throughflow
interceptors:
I. questionnaire
surwys
properties investigated, and in addition most of the interceptors had been installed for a period of five years or less. Nevertheless, it is debatable whether throughflow control alone will result in significant improvements to all salt-affected areas over the longer term. Acknowledgements The authors particularly wish to thank the farmers for willingly contributing and energies to the interview surveys.
their time
References Conacher, A. J. (1974) Rehabilitation
of salt scalds in the Western Australian wheatbelt by the interception and diversion of overland flow and throughflow on valley-side slopes. Scirnctl and Australian Technolog!, fl(8), 14-16. Conacher, A. J. (1975) Throughflow as a mechanism responsible for excessive soil salinization in non-irrigated, previously arable lands in the Western Australian wheatbelt: a field study. Catenu 2, 3 I- 68. Conacher, A. J. (1979) Water quality and forests in southwestern Australia: review and evaluation. Australian Geographer 14, 150- 159. Conacher. A. J. (1982) Dryland agriculture and secondary salinity. In Man and the Austrulian environment (W. S. Hanley and M. J. M. Cooper. eds) pp. 113 125. Sydney: McGraw-Hill. Conacher, A. J. (in press) Salt scalds and subsurface water: a special case of badland development in southwestern Australia. In Bud[undgeomorpholog~ and piping (R. Bryan and A. Yair, eds). Norwich: Geo Books. Conacher, A. J. and Murray, I. D. (1973) Implications and causes of salinity problems in the Western Australian wheatbelt: the York -Mawson area. Australian Geographical Studies I I, 40-6
I.
Mulcahy, M. J. (1978) Salinization in the southwest of Western Australia. Search 9, 269 -272. Peck, A. J. (1978) Salinization ofnon-irrigated soils and associated streams: a review. Austrulian Journal
of’soil
Research
Vaux, A. and Morony, hunks
16, 157- 168.
D. (1977) General
in salt-qj’J&ted
areuS
of
reriw, qfsult problems W. A. Perth: Department
and application
qfinrerceptor
of Geography,
University
Western Australia (unpublished report). Whittington, H. S. (1975) A battle& surcicul against salt encroachment at ‘Springhill’, Brookton, Western Australia. PO Box 9, Brookton, Western Australia (published by author). Williamson, D. R. and Bettenay, E. (1979) Agricultural land use and its effect on catchment output of salt and water- evidence from southern Australia. Progress in Water Technoloy~ I I. 463480. (Receiwd
1 I April
1981)
of
Applied Geography (19831, 3, 2944
Evaluation of throughflow interceptors for controlling secondary soil and water salinity in dryland agricultural areas of southwestern Australia : I. Questionnaire surveys A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan Department Australia
of Geography,
University of Western Australia,
Nedlands,
WA 6009,
Abstract Increasing soil and water salinity in southwestern Australia is a major social, economic and environmenta problem. Hydrological imbal~c~s following extensive clearing of the natural vegetation for agriculture have redistributed soluble salts into soils and streams. Interceptors have been designed by farmers to control the throughflow component of this hydrological imbalance, to reduce waterIogging of low-lying areas, and thus to ameliorate salinity problems. Farmer interviews show that, to date, interceptors have only partly achieved these objectives. Many interceptor systems are insufficiently comprehensive and leakages are common. Further, too short a period of time has elapsed since interceptor construction to have reversed the severe deterioration of soil properties.
The salinization of soils and streams in southwestern Australia, following European settlement from 1829 and removal of the indigenous vegetation mainly for agricultural purposes, is causing increasing concern. The nature, extent and causes of the problem
have been reviewed by Peck (1978), Mulcahy (1978) and Conacher (1979,1982, and in press) and are summarized only briefly here. These summaries are followed by a discussion of possible remedial measures. Specific methods used depend to some extent on the causal mechanisms which are perceived as being responsible for the salt problem. A large and in~uential group of farmers considers the causal mechanism to be the development of perched soiI waters in low-lying areas, with the main water input being valley-side throughflow. Consequently these farmers have constructed interceptor systems to divert throughflow from salt-affected areas. The substantive part of this paper assesses farmers’ perceptions of the effectiveness of interceptor systems. Nature and extent of salinity problems in southwestern Australia: summary By 1979,263 752 ha of previously productive land in southwestern Australia had been rendered useless for agriculture due to high salt concentrations, predominantly sodium chloride. By far the greater part of this salt-affected land-256 314 ha-is in the wheatbeh (Fig. l), and it is the equivalent of about 250 wheatbelt farms removed from production (using the average cleared area per property). Estimates by the authors suggest that, by 1978, the capital losses involved were of the order of $116 million and 0143-6228/83/010029-16SO3.00 cc? 1983 Butterworth & Co (Publishers) Ltd
30
Throughflow
interceptors:
Figure 1. The Western
I. questionnaire
Australian
suraeys
wheatbelt
and places referred
to in the text.
that annual production losses exceeded $20 million. Although the percentage ofcleared wheatbelt soils which are now salt-affected is only 2.02 per cent, the majority of farmers have no salt problems at all while others may have up to 30 per cent or more of their farms rendered unproductive (3852 of more than 11 500 wheatbelt farmers had some salt-affected land in 1979). The area of soil salinization had increased by 255 per cent from the time the first salt land survey was carried out by the Department of Agriculture in 1955, and the rate of increase appears to be accelerating. Between the 1974 and 1979 salt land surveys an area equivalent to 18 wheatbelt farms became unproductive each year, whereas prior to 1974 the annual rate of increase was the equivalent of only 4.6 farms. Increasing water salinity may well be a more serious problem than that of soil salinization. Since all the major streams and rivers of southwestern Australia have their headwaters in the cleared wheatbelt areas, they have all been polluted by soluble salts and their waters are now unfit for human consumption. Historical records, and interviews, show that all these rivers were previously fresh. Moreover, there are a number of saline lakes in the wheatbelt which had good quality water prior to clearing, and a proportion of farm water supplies from wells, bores and farm dams has become too salty for stock (which have a much higher salt tolerance than the acceptable limits for human beings). In addition, some clearing for agriculture has taken place in the forested catchments (Fig. 1) of some of the smaller streams which have been dammed or which will be required for future water supplies. The resulting threat of water
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
31
salinization was recognized by the government when, between 1976 and 1979, bans on further agricultural clearing were imposed on five of these catchments where significant areas of land are privately owned. In summary, the wheatbelt comprises a cleared area of 126 906 km2 in which virtually all surface waters have become unfit for human consumption, mostly within the last 70 years. There can be few other parts of the world where such extensive and severe water and soil pollution has occurred over such a short period of time. Causal mechanisms:
summary
It is widely accepted that the underlying cause of secondary salinization in Western Australia is the replacement of indigenous vegetation with introduced crops and pastures. The native vegetation has deep rooting systems which enable it to draw on deep soil- and groundwater storages throughout the year. In contrast the exotic species grow only from late autumn through to early summer and have shallow roots. Consequently less water is now being transpired by plants in the agricultural areas than previously, leading to water build-up in the generally featureless landscape. Most wheatbelt soils have considerable quantities of soluble salts in their deeply weathered profiles. Groundwaters, which are characteristically semi-confined beneath siliceous hardpans and clay horizons, are also highly saline in many areas. Most of these salts are thought to have accumulated from millennia of rain- and dustfall rather than from weathering or, with some exceptions, from marine sediments. It is generally believed-and recent work has provided supporting evidence (Williamson and Bettenay 1979tthat the reduced uptake of water by plants has resulted in a rise of groundwater tables either to the surface, causing saline seepages, or to within the capillary fringe of the soil surface. However, there is also evidence which suggests that in some instances groundwaters may not be present beneath salt-affected soils, or that the groundwater tables are too far below the surface for capillarity to be effective, or that the rates of upward movement of water through the confining hardpans, in response to hydraulic gradients, are too slow to explain the surface waterlogging that characterizes many ‘salt scalds’ (secondary salt-affected soils). Other work has shown that, in some areas, more rainwater is redistributed by ephemeral downslope throughflow than by overland flow or deep infiltration to groundwater, and widespread observations indicate that throughflow is a ubiquitous hydrological process throughout the wheatbelt. Although throughflow water is relatively fresh, it has been suggested that it accumulates as shallow perched water bodies in lower slope positions and in valley floors, and that salt concentrations are increased over time by high evaporation rates during summers (Conacher, in press). This work has also drawn attention to the three-dimensional nature of the landscape, stressing that upvalley inputs of water and salts by ephemeral or seasonal streamllowwhich in turn is supplied by throughflow, groundwater seepages and overland flowmust also be considered; and that soil and water salinization must be modelled and treated on a catchment and not a two-dimensional valley-side basis. In this context, calculations have shown that insufficient soluble salts are translocated by throughflow in certain catchments to account for the amount of salts being exported from those catchments by streamflow, and it is clear that some mixing of throughflow with more saline groundwaters is taking place. Thus there is a complex set of mechanisms which needs to be understood if the total system is to be manipulated in order to control the salt problem. These mechanisms include rain- and saltfall, transpiration, evaporation, overland flow, throughflow,
32
Throughflow interceptors: I. questionnaire surveys
streamflow, shallow and deep infiltration, groundwater movements, capillary action and shallow salt leaching in salt-affected soils, Vegetation, water, salt and soil properties-especially hydraulic conductivity-are intimately interrelated with climatic constraints and land management practices in a three-dimensional land surface. Remediai measures
Revegetation with indigenous, eucalypt woodland species in order to restore hydrological equilibrium is not a practicable, economic proposition for the wheatbelt farmer. Empirical evidence suggests that at least 50 per cent of salt scald or saline stream catchments would have to be so treated. The alternative of using commercial crop (including tree) and pasture species which would simulate the hydrological behaviour of the indigenous vegetation is an attractive and perhaps ideal alternative. However, there is insufficient knowledge about such species at present for this to be more than a theoretical option. Since the 1920s the rising water table mechanism has been the most widely accepted cause of soil salinization. Consequently the Western Australian Department of Agriculture has in essence considered the cause to be untreatable and has instead focused its attention on the effects. Thus, for more than 30 years the Department’s standard recommendation to farmers has been to fence the salt scald-to control grazing-and to establish salt-tolerant species, mainly grasses and shrubs. At best this approach provides some light grazing for stock, reduces water and wind erosion, and improves the aesthetics of the previously bare scalds. But the establishment of salttolerant pastures is not easy and, even if successfully achieved, it does not solve the problem. Not only do saline waters continue to seep from many scalds into streams, or continue their upslope expansion, but in many cases the salt scalds were amongst the farmers’ more productive areas. Salt-tolerant plants do not return these areas to full production. Not surprisingly, many farmers have become disillusioned with what they perceive as being a negative measure and have implemented alternative methods. A few have claimed that good crop and pasture management-including chisel ploughing to improve soil structure. organic matter content and hence water-holding capacities+ombined with judicious tree planting, have cured their salt problems. It is, however, a difficult method to implement without expert assistance; moreover there are no monitored data which would support these farmers’ assessments. Many farmers have attempted-largely unsuccessfully-to improve plant root environments on salt scalds by adding sand, hay or straw mulches, or by deep ripping, or by constructing shallow drainage systems. In all three instances it appears that the depths to which soil aeration is improved are either too shallow, or persist for too short a period, to provide sufficient improvement for plant germination and growth. A third group has excavated deep drains to lower water tables beneath the salt scalds. One example is near Watheroo (Fig. l), where significant success-partly supported by Department of Agriculture assessment-has been claimed. However the low hydraulic conductivities of most wheatbelt subsoils would necessitate a close spacing of such drains (within 40-50 m). Indeed deep drains near Bakers Hill (Fig. 1) have not resulted in any improvement in plant growth even immediately adjacent to one of the drains after four years, and although the potentiometric head of the groundwater has been lowered considerably from up to three metres above the ground surface, it is still within a few centimetres of the soil surface next to the drain, In general, therefore, deep drainage may not be even technically feasible in some locations; and it would certainly not appear to be an economic proposition for wheatbelt farmers. The value of their production per unit area is considerably less than in irrigated areas where deep
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
33
drainage to control waterlogging and salinity is a common practice. Moreover, this approach is inappropriate in water supply catchments since the draining groundwaters are highly saline. The fourth and most widespread approach is an attempt to de-water perched soilwaters in low-lying positions by intercepting and/or diverting valley-side throughflow on a catchment basis. Throughflow interceptors
Interceptors were first constructed in the Western Australian wheatbelt by a Brookton (Fig. 1) farmer, H. S. Whittington, in 1954. Both salt-tolerant plants and grade banks (shallow ditches and banks on valley sides, recommended by the Department of Agriculture as an erosion control measure), had been used previously in unsuccessful attempts to combat a serious salt problem. Interceptors were then tried, drawing on advice obtained from the United States Department of Agriculture. The design of the interceptors reflected Whittington’s perception that the salt problem was caused primarily by throughflow, leading to the development of perched water systems and waterlogging in lower slope positions, and not by a rising groundwater table. Following observations of the initial interceptors’ performance, progressive extensions were constructed until today the entire property is controlled by a comprehensive interceptor system. Each interceptor was constructed along the contour. A bulldozer was used to gouge a gentle-sided open ditch to a depth below the subsoils (which have a higher clay-sized content, and above which most of the throughflow occurs), and to push the clayey materials up against the bank of excavated spoil below the ditch (Plate I). This was
Plate I. Throughflow interceptor, showing the method of construction.
34
Throughflow
interceptors:
I. questionnaire
surceys
done to prevent leakages through the bank when the interceptor fills with water. The first interceptor was constructed approximately three vertical metres below the interfluve (water parting) or at the nearest point where soil materials were suitable and it was possible for a bulldozer to operate. Downslope interceptors were then constructed progressively at vertical height distances apart of between three and four metres, subject to the presence of suitable subsoil materials. Lateral spacing rarely exceeded 400 metres. The objectives of the system were: (a) to prevent overland flow and throughflow from reaching lower slope positions, thereby preventing waterlogging from extending further upvalley and upslope; (b) to de-water the perched soil-water systems in lowlying positions, and (c) to then improve overall soil properties in order to return the land to full production. Side benefits have included excellent erosion control, enhanced awareness by the farmer of his soils and the physical processes operating on his property, and improved farm management practices generally. In the early 1970s three near neighbours, impressed by Whittington’s results, started to construct their own interceptor systems. Within a few years a number of other wheatbelt farmers had followed suit, placing severe demands on Whittington’s time. On 15 March 1978, a meeting held in Quairading (Fig. 1) formed a farmer association named WISALTS-Whittington Interceptor Salt Affected Land Treatment Society- ~ and by October 1981 the society had attracted over 1000 financial members (more than 25 per cent of all farmers with salt-affected land). The society trains operators (who are farmer members), awarding A, B and C-class licences, to assist other members with the design and construction of interceptor systems. It is not known how many farmers have actually constructed such systems, but in one local government area alone there are 33 such farmers and the total number must be several hundred. Not all farmers have constructed comprehensive interceptors on the contour. One constraint which militates against fully comprehensive systems is cost; another is that many farmers do not have control over the catchments of their salt-affected land; and a third is concern that contour interceptors may increase deep infiltration, thereby raising groundwater tables or increasing hydraulic pressure gradients and aggravating the salt problem. A fourth problem is that level interceptors encourage water accumulation and hence leakages through the downslope banks. As a result of these constraints most farmers are now constructing interceptors on very slight gradients (with a fall of lo-20 mm in 30 m), diverting the intercepted water to a stream or, an added bonus, to farm dams. Some farmers have combinations of contoured and graded interceptors, while many others have constructed only one or two graded interceptors above a salt scald, largely for economic reasons. In general, therefore, there are a number of possible approaches available depending on the nature of, and the problems posed by, each property. The Department of Agriculture is opposed to the WISALTS approach mainly because it considers that interceptors are ineffective as a means of controlling soil salinity, and partly because of concern that contour interceptors may exacerbate the salt problem. In both cases the Department’s view is based on its understanding of the mechanisms responsible for salinization. Clearly, if groundwater is the main cause then the Department’s view is correct. WISALTS, equally clearly, considers that the Department is incorrect and the society has claimed significant success on a number of properties. In response to this situation the authors evaluated the effectiveness of interceptors as a means of combating salinity problems by interviewing farmers, repeated after a fouryear period, in an attempt to gauge farmer perceptions of the success or otherwise of their interceptor systems.
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
35
Evaluation of interceptors: farmer interviews Methods In 1977 discussions with Mr Whittington identified 18 other farmers who had constructed interceptors prior to that date. Whittington himselfwas not included in the survey as the results of his work had been presented and discussed elsewhere (Conacher 1974, 1975; Whittington 1975). Two other farmers did not respond to preliminary requests for their cooperation, leaving 16 farmers who were finally interviewed. They are located in the Brookton, Wickepin, Quairading, Tammin and Gingin districts (Fig. l), with the majority in the Brookton-Quairading area (Vaux and Morony 1977). It was intended to conduct the interviews in person, using a questionnaire. However, four farm visits could not be arranged and the questionnaires were completed by the farmers and returned by mail. Each of the remaining 12 properties was examined in the field, sketch maps of the interceptor systems were produced-with the aid of aerial photographs-and photographs of selected sites were taken. However, the focus of the survey was on the farmers’ perceptions of interceptor effectiveness. of soil Questions were asked on: farm size; extent and dates of occurrence salinization in relation to clearing; previous measures used to combat salinity; details concerning the design and construction of the interceptors; the degree of success obtained and the criteria used for that assessment; the cost of interceptor construction, and future intentions. In 1981 the same 16 farmers were re-interviewed, with only one not being visited personally. New maps were drawn for each property and additional photographs were taken. A new questionnaire was designed with a similar set ofquestions to those used in 1977, but requesting more detailed information on dates of various events, the degree of effort put into previously attempted control measures (especially those recommended by the Department of Agriculture), and the extent to which Whittington’s advice had been followed in designing and constructing the interceptors. More specific questions were asked on the criteria used by the farmers in assessing the effectiveness of their interceptors, drawing on the findings of the 1977 survey. Also in 1981, an attempt was made to interview all farmers who had constructed WISALTS interceptor systems in Kellerberrin Shire (Fig. l), using a very similar questionnaire. The purpose of this survey was to provide base data for future comparative studies. There are 33 such farmers in the Shire, of whom 28 were interviewed.
Results and discussion
The average size of the 16 farms in 1981 was 1376 ha (range 526-3036 ha) of which an average of 1250 ha or 91 per cent of the total property have been cleared (range 526-2834 ha; see Table 1). These farms are therefore generally larger than the average wheatbelt farm. The total area of salt-affected land on the 16 properties in 1981 was 2399 ha, constituting 12 per cent of the total cleared area. Scalded areas ranged between 16 and 486 ha per property, with a mean of 150 ha (Table 1). These averages are well in excess of the equivalent total wheatbelt data. Clearing of the natural vegetation commenced in the 19th century on one property (farmer 1) but in most cases the land was not settled until early this century or later, and clearing continued progressively to as recently as 1980 in some cases. An attempt was therefore made to identify scald catchments and to determine the clearing dates of these specific areas (Table 1, column 5), in relation to the dates when salt first became a problem
mean range
,I
9 10 11 12 13 14 15 16 Total Mean
526 955 2024 785 1349 561 2834 1102 3036 22018 1376
526 931 1740 783 1734 544 2672 726 2834 19996 1250
1920s 1920s 1930 1910.-14 1940s 1920 1920 40 1955-60 1920-30
‘7 1920-30 1930s 1920s 1920s ‘, 1940s
Dates of scald catchment clearmg (5)
28
1284 437-5387
area (ha)
28 115 O&486
20 23.5 3.-50
1976 81 197680 1976.-78 1976-H1 1976-8 1 1976-81 1977.-79 1977-8 1 1977 81
1971 80 1973-75 1974-75 197’7..78 1976 1976 81 1976-81 3300 545 9800 21.50 llooo loooc) 3000 5600 28000 11530 7369
‘? 8000 1200 2ooO 800 9800 14735
28 6.2 0.5 20
Yes NO Yes
3 I 4
with Table 1
Yes No Yes Yes
NO
Yes No No No
NO
1
4 1 2 2 3 2 1 3 4 39 2.4
Yes Yes Yes
(12)
Improvement to salt land
4 2 2
Dates of Total Success interceptor cost score construction to 1981 ($1 (IO) (TiI (9)
Shire, (1981) for comparison
16 121 485 137 161 105 80 283 486 2399 150
55 40 60 80 121 40 49 150
Extent of salt affected area (ha) (8)
(refer text for full discussion)
in Kelierberrin
30 10 19 31 20 15 30 7 20 307 22
‘1 30 30 20 40 ‘f ‘5
(7)
Lag tyr) (6) (5)
year salt first noticed (yrt
farmers
1950s 1933 1949 1943 1963 193.5 1960 1965 1945
1960s 1950s 1969 1940s 1960s 1955 1947
(6)
Dates of salt appearance
Table 2. Selected data from WISALTS
1387 446-5670
28
Quairading Quairading Quairadmg Quairading Quatrading Quairading Wickepin Gingin Tammin
8
5
6 7
1093 1215 809 1149 568 1672 1000
Brookton Brookton Brookton Brooktan Brookton Brookton Brookton
1
1227 1215 967 1457 516 2077 I321
(4)
(3)
12)
(1)
2 3 4
Area cleared (ha1
Farm size (ha)
District
Farmer
Table 1. Results of 198i farmer interviews
Yes Yes Yes Yes Yes Yes Yes Yes Yes
Possibly Yes Possibly
Yes
No Possibly Possibly
03)
Future interceptors
? .G !z 3. % 5 a. z g % ‘3
.3.
d 5
R 2
Y S 2
w
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
37
(column 6). Calculation of the lag between clearing and the onset of safinization (column 7) shows periods ranging between five and 40 years, with a mean of 22 years, which corresponds with previous results from the York-Mawson and Dalwallinu (Fig. 1) areas (Conacher and Murray 1973; Conacher 1975). Six of the I4 properties from which these data were obtained had a lag period greater than the average with an additional four having a lag only slightly below the average, and it can be concluded that the lag period generally exceeds 20 years. Comparative data from the 28 farms in Kellerberrin Shire are presented in Table 2. Farm sizes and lag periods are similar to those in Table 1 but the extent of salt-affected land is less. Prior to the construction of interceptors, six of the 16 farmers had attempted to establish salt-tolerant plants allied with grazing control by fencing. None were satisfied with the results although some plant cover was established. A further eight farmers had attempted various measures in&ding salt-tolerant plant establishment (mostly Puccjnellia sp.) but without grazing control, planting Ta~aris~ sp., deep ripping, grade banks, drainage, and applying different fertilizers. In one instance some ~uccinell~a grew, a few Tarnarisks were successfully established on other properties, and some improvement in erosion control was obtained on two properties. But none of the farmers considered that the salt problem had improved and in several cases it became worse. The final two farmers had not attempted any remedial measures prior to interceptor construction. Initial advice on interceptor construction was obtained from Whittington in all cases except one (farmer 6). Whittington also carried out soil tests, surveyed the interceptors and supervised their construction for 11 of the 16 farmers in the initial stages. Two farmers have contour interceptors only; six have both contour and graded interceptors, and the remaining eight have only graded interceptors. One farmer completed his (present) interceptor system in one year and two others in two years, but most worked on their systems for several years, observing results from one set before progressing further (Table 1, column 9). Eight farmers were still constructing interceptors in 1981, and 11 definitely intended to install additional interceptors in the future, with four more ‘possibles’ (column 13). Thus 15 of the 16 farmers presumably do not regard their interceptor systems as being complete. There is in fact a wide variation in the comprehensiveness of the systems amongst the 16 properties. Space precludes including a map of each system, but two extreme examples are presented in Figs 2 and 3. The amount expended on interceptor construction (Table 1, column 10) is indicative of the relative comprehensiveness of the 16 systems, with amounts ranging between $545 and $28 000 (mean $7369). Comparison of columns (5) and (6) in Table 2 gives an indication of the relationship between expenditure and extent of the interceptor systems. Cost is not an entirely reliable criterion, however, due to inflationary trends and differences in construction costs caused by varying terrain. All 16 farmers have experienced problems with their interceptors, the most common difficulty being leakages through imperfectly sealed banks (10 farmers), Three farmers specifically referred to inexperienced bulldozer operators. There is virtually no room for error with either type of interceptor and many have had to be rebuilt. Two farmers referred to problems with gullying and another two experienced bank bursts following heavy rain. Four farmers had difficulty with contour interceptors, where excessive water accumulation encouraged leakages and necessitated opening up one end of the interceptor or complete reconstruction on a slight gradient. ‘Sand seams’ caused problems for four farmers. These are thought to be palaeo-drainage channels which can be up to 3 m or more deep. They have been infilled by aeolian or colluvial sands and, in some instances, covered over by finer-textured materials making them difficult to
38
Throughjow
interceptors:
2
Creek
_._
Graded
lnterceplw
-__
Future
IntcrccfNcx
‘&
salt-affected
0 L-b_
0.5
1. questionnaire surveys
sm.3 lkm
Figure 2. Sketch map showing
a small interceptor
system (farmer 2)
identify by normal soil-testing procedures. Considerable quantities of fresh water are translocated downslope through the porous sands in these ‘seams’, causing major problems during interceptor construction (but providing a welcome addition to farm water supplies). Often the only practicable solution is to remove the sand and fill the excavation with clay brought in from elsewhere, in order to maintain the level or graded base of the interceptor and to seal the downslope bank. A wide range of interceptor situations was therefore encountered on the 16 properties. They varied in terms of: dates of interceptor construction, which commonly vary in different parts of the same farm; varying combinations of interceptor types; the care with which soils were tested and the interceptors surveyed, especially after construction of the initial interceptors; and the quality and comprehensiveness of the interceptors themselves. Turning now to the question of whether the interceptors have fulfilled their objectives, all 16 farmers relied on visual observations for their assessments, supported in only three cases (farmers 1,15 and 16) by some sequential ground-level photographs. No objective on-site monitoring has been carried out by the farmers. A further difficulty is that observed changes do not occur uniformly in different parts of the same property or even around a single salt scald. Figure 4, completed and annotated by farmer 1 without the interviewers’ presence, indicates this spatial variability. Some parts of a property may even deteriorate whilst improvements are observed elsewhere. Evaluation of such detailed spatial differences requires accurate
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
z
Creek zzraed
EDI 0
Salt-affected 1
39
interceptor area 2km
1
Joins A above
Figure 3. Sketch map showing a large interceptor
system (farmer 16).
monitoring. Instead, an attempt was made to obtain an overall assessment by seeking the farmers’ responses to four improvement criteria drawn from the 1977 survey. These were : 1. 2. 3. 4.
decreased area improvements improvements improved crop
of waterlogging in soil properties to pasture production
An affirmative response to a criterion scored one point. Thus possible scores range from 0 to 4 (Table 1, column 11). It must be stressed, however, that a maximum score of 4 does not mean that complete rehabilitation has been obtained or observed. No farmer
40
Throughflow interceptors:
1. questionnaire
surveys
contour
‘“tarc*ptors
Proposed
___._--.
.
posstble
water
harvesting
__~__
i
Sketch map of an interceptor system (farmer 1) annotated by the farmer, indicating the spatial variation of perceived interceptor effectiveness. Figure4.
has claimed or achieved this. The ‘success score’ indicates the number of criteria for which some improvement has been observed and not the degree or extent of that improvement. Nevertheless, in general those farmers who claimed the greatest degree of improvement also noted beneficial changes in the largest number of criteria. (It should be noted that farmer 15 is disadvantaged by this scoring system since his farm is used for grazing only.) From the 1977 survey only three of the 16 farmers claimed any success (farmers 1,2 and 3, the only ones to have commenced interceptor construction before 1975). By 1981, all farmers claimed some improvements (Table 1, column 11). However, only four farmers recorded the maximum ‘success score’ while a further four scored only one point. It is interesting to note that the two top spenders both score 4, with the third highest spender scoring 3, while the lowest spender scores only 1. However, the next lowest scores 3, and between the highest and lowest spending extremes there is no relationship between expenditure on interceptors and the success scores. Likewise there is no relationship between success scores and the time when the first interceptors were constructed, but there does seem to be some connection with the duration of the period taken for interceptor construction (and therefore, possibly, comprehensiveness of the system and degree of care taken over its construction). For example, all farmers scoring 4 have taken at least four years to construct their systems (columns 9 and 1 1),
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
41
while some of the less ‘successful’ systems were constructed over shorter periods (with farmer 5 being the main exception). Fifteen of the 16 farmers reported that waterlogging had been reduced to varying extents following interceptor construction. However, the period 1977-81 (before the relatively wet 1981 winter) corresponded with a run of dry seasons, and several farmers were sceptical of the validity of this criterion. Nevertheless the farmers’ observations are supported by Negus (Department of Agriculture, Narrogin 1981, personal communication), who surveyed 35 interceptors during the wettest part of the 1981 winter. He found that, on average, a 50 m strip of land below the interceptors showed improved drainage and crop growth. Moreover 12 farmers reported some improvements in pasture growth over the 1977-81 period, which would be difficult to attribute to low rainfalls. In two cases this improvement referred only to an increased spread of barley grasses-a salt-tolerant species-into previously bare areas; but a number of farmers reported observing the following sequences of vegetation changes : Farmer 5 7 8 12 15 16
barley grass-cape weed+wimmera rye grass bare+barley grass+rye grass-tcape weed-tclover barley grass+rye grass+clover bare+samphire+barley grass-+rye grass+clover barley grass+gingin clover+subterranean clover barley grass-tcape weed+clover
These sequences are essentially a reversal of the trends noted as waterlogging and soil salinization develop, and the reappearance of the highly salt-sensitive clovers is rightly regarded by the farmers as a particularly significant occurrence. Six farmers observed improvements in soil properties other than waterlogging, increased humus and increased earthworm referring to reduced compaction, populations. It is evident, however, that any improvements to severely salt-affected land will take considerably longer than in peripheral or marginally affected areas. Reversal of severe deterioration of soil physical and morphological properties will take time and conscious effort to achieve, even with complete water control. With regard to the degree of improvement following interceptor construction, six farmers observed relatively significant successes. Farmer 1 reported dramatic improvements in crop yields on parts of his property, supported by production data. Farmer 8 observed an 80 per cent decrease of his waterlogged areas and improved crops in one location. Farmers 12 and 16 reported that 24 ha and 16-20 ha, respectively, have been returned to production. Farmer 13 observed that a paddock which was previously 75 per cent bare ground was only 25 per cent bare in 1981. Farmer 15 noted marked improvements in pasture growth over previously bare and waterlogged areas; his assessment was independently confirmed by three neighbours. Many of the above criteria and observations refer partly to areas which are peripheral to salt scalds, and it was difficult to obtain a clear distinction. Column 12 of Table 1 summarizes the responses obtained to a question which asked whether the economic or productive value of the salt scalds themselves had improved after interceptor construction. Again, a ‘yes’ response merely indicates that some improvement was observed. As can be seen, all farmers who scored 4 and 3 (except farmer 12) in column 11 provided a ‘yes’ answer. None of the farmers who scored 1 answered ‘yes’, but three of those who scored 2 did so. Two of the latter (farmers 2 and 3) are amongst those with the longest period of interceptor establishment, suggesting that time is a significant factor despite their relatively low scores on other criteria, The
42
Throughfiox
interceptors:
I. questionnairr
Lsurve_w
third ‘yes’ farmer with a score of 2, farmer 13, stated that part of his salt-affected land would be put under crop in 1982. If this crop is successful then his success score would improve to at least 3. Table 3, columns 2 and 3, shows the extent of salt land reported by the 16 farmers in the 1977 and 1981 surveys. Nine farmers reported increases in the areas of their saltaffected land-in most cases by quite considerable amounts. Comparison with columns 11 and 12 of Table 1 shows that some farmers claiming relatively significant improvements have nevertheless reported increases in salt land, while others who have claimed relatively little improvement nevertheless reported decreases in the extent of their salt land. With the partial exception of farmer 10, increases in the extent of salt land cannot be explained by increased farm sizes over the four-year period (Table 3, column 7-where minor changes are attributed to incorrect conversions by farmers from acres to hectares). Whilst concluding from these data that farmer perceptions are unreliable~specially of the area1 extent of salt-affected land-it is stressed again that many of the improvements observed and reported by farmers have taken place only on certain parts of their properties, and often to marginally salt-affected land only (cf. Fig. 4). With certain exceptions, the unreliability of farmers’ perceptions as suggested by the discrepancies between the data in Table 3, column 4, and Table 1, columns 11 and 12, may be more apparent than real. It is also possible that subconsciously farmers may justify their sometimes considerable expenditure and effort on interceptors by claiming positive results. The data in column 10 of Table 1, which were used previously to suggest some relationship between comprehensiveness of interceptor systems and their effectiveness, may be interpreted as providing partial support for the above suggestion. Nevertheless, most farmers gave interviewers the impression of having a very good observational knowledge of their properties even though area estimates may be unreliable. It is most Table
3. Areas of salt-affected land and farm sizes (1977, 1981)
Area of salt-affected Farmer
1977 (2)
(1) 1 2 3 4 5 6 7 8 9 IO 11 12 13 14 15 16 Total Mean
?
(55) 24 80 60 12 60 104 20 144 200 160 160 120 40 20 80 1339” 84”
Farm size (ha)
land (ha)
1981 (3? 55 40 60 80 121 40 49 150 16 121 485 137 161 105 80 283 486 2399 150
n Where 1977 data were not obtained, have been the same as in 1981
Change
1977 (5)
(4) ?
(O)O + 16 - 10 +61
+28 -1) +46 -4 -23 s285 -23 fl -15 -i-40 +263 +406 + 1060” + 66” for the purposes
?
(1227)” 1200 946 1480 569 1734 1312 ? (526)Q 616 1800 392 1320 546 2800 1080 3000 20548” 1284”
1981 (6) 1227 1215 967 1487 576 2077 1327 526 955 2024 785 1349 561 2834 1102 3036 2201X 1376
of summatioi~
Change (7) ? (0) +15 f21 -23 +7 + 343 + IS ? (0)” + 339 + 224 + 393 129 t15 t 34 +22 t36 + 1470” + 92”
they are assumed
to
A. J. Conacher, P. L. Combes, P. A. Smith and R. C. McLellan
43
unfortunate that WISALTS has not undertaken detailed monitoring of crop and pasture conditions to provide objective, quantitative support for its members’ observations. Finally, given the correctness of farmers’ perceptions of improvements, it is still difficult to assess the extent to which throughflow water control has in fact been achieved. All farmers experienced problems with their interceptors, especially leakages. Likewise, few if any of the 16 farmers have wholly comprehensive systems. Nevertheless, if throughflow is the mechanism solely responsible for all soil salinization, then better results than those found during the 1981 survey might have been anticipated. Four farmers had a success score of only 1 and a further two had a score of 2 and also reported no change to their salt land. Although four of these six farmers had expenditures on interceptor construction well below the average, suggesting that relatively little effort had been put into the work, two of the six farmers had above-average expenditures. On the other hand, even the most successful farmers had achieved only partial successes on parts of their properties. While the work of the 16 farmers demonstrates that throughflow is an important mechanism operating on all properties, and despite qualifications concerning the adequacy of the interceptor systems and the generally short period that has elapsed since their construction, it appears that another mechanism (or mechanisms) causing soil salinization is probably operating, at least on some parts of the 16 properties. Conclusions Soil and water salinity is a major environmental problem in southwestern Australia. Extensive clearing-up to 400 000 ha per annum-of natural vegetation for agricultural purposes took place in the middle and late 1960s and smaller-scale clearing has continued throughout the agricultural areas to the present day. In 1981 a further large-scale expansion of agriculture was announced by the state government. Since the onset of soil salinization generally occurs more than 20 years after clearing, and continues to expand in area for up to 40 years thereafter, or longer, further large increases in the areas of salt-affected land are likely to occur after the middle and late 1980s and well into the 21st century, unless preventative measures are implemented. The introduction by WISALTS ofinterceptors throughout the wheatbelt is designed to prevent throughflow from reaching salt-affected land, either by holding throughflow water on the slope (contour interceptors) or, more commonly, by diverting it to streams and farm dams (graded interceptors). The evidence presented in this paper suggests that interceptor systems generally achieve the above objectives. The major problems encountered are inadequacies in soil testing and in interceptor design and construction, resulting in leakages through and beneath the interceptors. The evidence further suggests that, following interceptor construction, the amelioration of salt-affected land is only partial. In diminishing order of success, improvements are reported after at least two years to: (a) the extent of waterlogging, (b) pasture quality, (c) crop yields, and (d) soil properties other than waterlogging. Such improvements have generally occurred on some but not all parts of individual farms, and in general there has been little improvement of severely salt-affected land. Soil salinization commences after three and generally more than 20 years following clearing, and is associated with a progressive deterioration of soil properties. It can be anticipated therefore that similar periods will be required for significant soil improvements to take place-and probably longer-ven if complete throughflow control is obtained. Such complete control has not been achieved on any of the 16
44
Throughflow
interceptors:
I. questionnaire
surwys
properties investigated, and in addition most of the interceptors had been installed for a period of five years or less. Nevertheless, it is debatable whether throughflow control alone will result in significant improvements to all salt-affected areas over the longer term. Acknowledgements The authors particularly wish to thank the farmers for willingly contributing and energies to the interview surveys.
their time
References Conacher, A. J. (1974) Rehabilitation
of salt scalds in the Western Australian wheatbelt by the interception and diversion of overland flow and throughflow on valley-side slopes. Scirnctl and Australian Technolog!, fl(8), 14-16. Conacher, A. J. (1975) Throughflow as a mechanism responsible for excessive soil salinization in non-irrigated, previously arable lands in the Western Australian wheatbelt: a field study. Catenu 2, 3 I- 68. Conacher, A. J. (1979) Water quality and forests in southwestern Australia: review and evaluation. Australian Geographer 14, 150- 159. Conacher. A. J. (1982) Dryland agriculture and secondary salinity. In Man and the Austrulian environment (W. S. Hanley and M. J. M. Cooper. eds) pp. 113 125. Sydney: McGraw-Hill. Conacher, A. J. (in press) Salt scalds and subsurface water: a special case of badland development in southwestern Australia. In Bud[undgeomorpholog~ and piping (R. Bryan and A. Yair, eds). Norwich: Geo Books. Conacher, A. J. and Murray, I. D. (1973) Implications and causes of salinity problems in the Western Australian wheatbelt: the York -Mawson area. Australian Geographical Studies I I, 40-6
I.
Mulcahy, M. J. (1978) Salinization in the southwest of Western Australia. Search 9, 269 -272. Peck, A. J. (1978) Salinization ofnon-irrigated soils and associated streams: a review. Austrulian Journal
of’soil
Research
Vaux, A. and Morony, hunks
16, 157- 168.
D. (1977) General
in salt-qj’J&ted
areuS
of
reriw, qfsult problems W. A. Perth: Department
and application
qfinrerceptor
of Geography,
University
Western Australia (unpublished report). Whittington, H. S. (1975) A battle& surcicul against salt encroachment at ‘Springhill’, Brookton, Western Australia. PO Box 9, Brookton, Western Australia (published by author). Williamson, D. R. and Bettenay, E. (1979) Agricultural land use and its effect on catchment output of salt and water- evidence from southern Australia. Progress in Water Technoloy~ I I. 463480. (Receiwd
1 I April
1981)
of