5 Discussion
The overall aim of this project was to test the previously developed set of indicators of sustainable irrigated agriculture. Specific outcomes related to determining ease of determination, usefulness and practicality, interpretation, farmers' perceptions, possible alternatives, and overall value.
5.1 EASE OF DETERMINATION
Overall the process of determining the indicators reported here was straightforward with one major exception. It was realised early on in the project that water metering would be a complex and expensive exercise on all the farms selected. As a result no continuous water measurements were made on the three control farms (1A, 2A, 3A) and only limited measurements were made on two of the BMG farms (1B, 2B). The inability to provide accurate measurement of the total amount of water flowing onto a property has compromised the accuracy of all those indicators that are subsequently based on total water volume.
As a result of the difficulty encountered with installing water meters it was decided to calculate the total seasonal water use based on average application depth, area, and number of applications. The limiting factor to accurate determination of total flow by this technique was the estimates of average application depth. The farmers, based on their personal knowledge of their irrigation systems, supplied these. Under spray irrigation, the accuracy of these estimates could be improved by measuring application depths using 'catch can' techniques or installing low cost flow meters on each irrigator. Unfortunately, similar techniques for flood irrigation are either not available or expensive.
Most other information, required to determine the end of season indicators, was easily obtained by interviewing the farmers. This process took about one hour on average for the dairy farms. Information gathering for each arable farm took approximately half a day, on average, due to the number of crops to be analysed. Preparation of the gross margins required another day per farm.
Collection of soil samples was a straightforward operation. Determining energy consumption was often more difficult and in some cases involved obtaining electricity use records from the energy supply company.
Overall, the ability to meter the total amount of water coming onto a property on a daily or seasonal basis was the only real limiting factor in determining the indicators.
5.2 USEFULNESS AND PRACTICALITY
5.2.1 Dairy Farms
From the dairy farms a number of the historically determined indicators appear to be useful and have practical value.
Production per unit effective area is already used as an industry benchmark of performance. The results from this trial showed a considerable spread between farm 2A, a top local performer, and 1B a new conversion that experienced difficulties with pasture establishment on new borders.
The production per unit of water use (or its inverse), even taking into account possible inaccuracies, highlighted the variation in overall water use efficiency between the different systems. In this respect the spray-irrigated systems appear to use water more efficiently than the surface irrigation systems. However, it should be borne in mind that trends in this indicator between farms reflect both the total depth of application and the production per unit area. Thus the very high water use on farm 1B reflects not only the assumption of a relatively high total application rate (1875 mm) but also the lower production (630 kg MS/ha). Furthermore, the flow monitoring conducted by Lincoln Environmental suggested that the assumed application rate might be too high due to the variability of the incoming flow. If an average application rate of 70 mm per irrigation was assumed this would make the water use per unit of production 21 m3/kg MS, a figure more comparable with that of farm 1A (18 m3/kg MS in 1997/98).
The total energy per unit of water also highlighted the different energy requirements of the various irrigation systems used. These varied from zero for the border strip systems to 0.82 3 for the diesel-powered big gun. The poor relative performance of the big gun can be attributed to the higher pressure requirement and the lower energy conversion efficiency of the diesel motor compared to electric motors. Differences in performance between the centre pivot and the travelling irrigators are largely attributable to the differences in pressure requirement.
A number of likely reasons can be proposed for the 12.5% increase in the energy cost per unit of water on farm 2A, from 1996/97 to 1997/98. One is a suspected drop off in performance of one of the submersible pumps. This is supported by a 97% increase in electricity used by this pump when overall water use only increased by 14%. Another possible contributing factor was the general drop in ground water levels during the 1997/98 drought season.
Useful insights were also gained about differences between the irrigation systems from the labour requirement indicators. Labour requirements, in terms of hours per hectare were highest for the big gun and travelling irrigators. The older border strip system, on property 1B, also had a high labour requirement. The results show quite clearly the labour advantages of the new wider border strips (1B) and the centre pivot (2B).
The ratios D/ET and D/ET*, while probably not as useful as the original proposed indicator of daily percentage of water flowing onto the farm that is stored in the root zone, did show that all these irrigation systems were being operated close to the design conditions. That is D/ET was close to 1. During a drought situation, as in the 1997/98 season, when rainfall is low and ET* is close to ET, D/ET* is also close to 1. In a wetter summer, as experienced in 1996/97 then the ratio D/ET* was in the order of 2, showing that these farms were potentially over-watered.
Unfortunately what the seasonal ratios D/ET or D/ET* do not show is the day by day situation. A property with a D/ET* of 1 could still be over-watering. This is best illustrated with an example. Imagine an irrigation system applying 60 mm, within a day, on a 12 day return cycle. If the daily water demand is constant at 5 mm over the twelve-day period then the total demand of 60 mm should be perfectly satisfied by the irrigation system. However, if the soil water holding capacity was only 50 mm, then 10 mm of each irrigation will be lost below the root zone, at the time of application, and the pasture will be under water stress for the last two days of each cycle.
The purpose of the fertiliser and agri-chemical use indicators was to highlight potential problems associated with leaching of nutrients or other pollutants into ground or surface water. The fertiliser use indicators showed that farms in the trial were all applying similar quantities of N, P, K, and S. These levels are considered to be normal for dairy farming in the Canterbury area. Property 1B applied significantly more sulphur, based on soil test recommendations. A limitation of this type of overall indicator is that it does not show whether all the nutrient was applied at one time or more evenly spread, and if so how the timing of nutrient application fitted in with expected periods of rapid grass growth and hence rapid uptake. Nor does it show how the timing of application compared with the irrigation period, or periods of high rainfall, when maximum leaching would be expected.
Agricultural chemical use was particularly high on farm 2A compared to the other dairy farms. Two factors contributed to this. One was the use of herbicides to combat a problem with thistles. The other was the high stocking density and hence the high use of bloat oil and cleaning chemicals per effective area.
Therefore, the fertiliser and agri-chemical use indicators, should be used with caution. While excessive use may be an indicator of potential pollution problems, seemingly acceptable application rates may cause significant problems if application timing is poor.
The water holding capacity indicator is potentially useful when compared to the mean depth applied as an indicator of potential over-watering. This information suggests that the application rate of the centre pivot would be well matched to the water holding capacity of the soil, although over-watering could occur when applying 50 mm with a double revolution of the pivot. Lincoln Environmental confirmed this using continuous soil moisture monitoring. On farm 2A the application rate of the travelling irrigators is within the band of measured water holding capacity suggesting that over-watering is a potential problem, in some areas. For the two border strip farms (1A and 1B) it is most likely that over-watering occurs in parts of the system, particularly at the top end of borders.
The soil indicators revealed very few differences between farms. Due to the fact that variability was greater within farms than between farms it is not possible to make any comparisons between irrigation systems. As soil properties are expected to change slowly the real use of these indicators would be to identify particularly poor soils, or reflect gradual deterioration, or improvement, over a number of years.
5.2.2 Arable farms
As mentioned in section 4.3, the yield and economic indicators calculated for the arable crops were highly variable and no significant patterns emerged from the data. This is likely to be a result of the large number of factors, other than irrigation that determine final yield and economic return. In particular this would include variety, soil (paddock) and climatic conditions, management and economic factors. For example, variety, climate and management are likely to have a greater effect on yield per unit of water than variation in applied water. Similarly, price variability between seasons is likely to have a greater influence on the gross margin per unit of water
The soil indicators showed that the arable soils had generally poor aggregate stability and lower total organic carbon and total nitrogen levels than those under pasture. Total organic carbon and total nitrogen levels were significantly lower on 3B than 3A but it would not be appropriate to attribute this to variations in the irrigation systems.
In order for the indicators to be useful in the arable context it will be necessary to track individual paddocks and crops over much longer time periods. Furthermore, the most meaningful indicators will probably require some integration up to a whole farm level. This will be complicated when livestock are included.
5.3 FARMERS' PERCEPTIONS
The results of the indicator study were discussed with the dairy farmer group in some detail. All those involved expressed the opinion that the results would be useful and wished to continue to be involved in monitoring in future years. In particular this information would be useful in assessing future decisions regarding irrigation development and management. Illustrative examples are outlined below.
Farm 1A is faced with future decisions about the use of the diesel pump and big gun. The area currently watered by this system is under development and makes only a marginal contribution to the overall milk production. However, this area uses a disproportionate amount of energy and labour. As soils and pastures in this area improve, it is expected to contribute an appropriate proportion of the total production, however energy and labour costs will still be high be comparison with the border strip system. Options already considered are changing to an electric pump and/or rearranging paddocks and buying a travelling irrigator to cut down on labour. The cost of installing power lines is prohibitive and the cost of a new irrigator is hard to justify economically.
Farm 1B, being at a formative stage, is interested to use the information to track improvements in performance as both milk production increases and 'teething-problems' with the new irrigation system are overcome.
Farm 2A has a complex irrigation system with operating characteristics that are not well understood by the owner or sharemilker. The audit showed that the system is suffering from poor pump selection and performance. Rectifying these should result in reduced energy use and hence running cost. Other improvements may also be possible to improve the hydraulic performance and the operational management with resulting gains in water use efficiency.
Farm 2B, also at a formative stage, will be using the irrigation indicators in conjunction with the Best Management Guideline to improve the operational management of the centre pivot next season. In the long terms this system is expected to be capable of providing superior water use efficiency compared to the other systems considered.
5.4 INTERPRETATION
There are a number of ways that the indicator data developed in this project could be used. These include:
- Comparison of time series data from the same farm. This could be used to show whether a farm is becoming more sustainable in its use of water with time.
- Comparisons between farms of the same type with similar irrigation systems to identify differences in management
- Comparison between farms of the same type with different irrigation systems to identify effects of the irrigation system type
- Comparison between different types of farms to identify differences in water use between different outputs.
There are a number of limitations to each of these approaches. These are discussed below.
With regard to time series analysis, it is not immediately obvious how useful these indicators are likely to be given that only two successive years have been evaluated on two dairy farms. Results from the arable farms were inconclusive. However, there are promising signs. On farm 2A the energy indicator correctly picked up a drop-off in performance of one of the pumps. The D/ET* indicator also responded as expected to the drought conditions of 1997/98.
Tracking the performance of an individual property in time is seen as the most appropriate use of the indicators. A technique to successfully aggregate indicators for different crops is needed before this can be applied to arable farms.
Comparison of indicators between different irrigation systems should be approached with a considerable degree of caution. In particular, it is important that a balanced view is taken when making comparisons. For example it would be tempting to surmise, on the basis of these limited results, that spray irrigation has superior water use efficiency to surface irrigation. On the dairy farms with spray the water use per unit of production appears to be in the order of 10 m3/kg MS or less compared to 15m3/kg MS or more under surface irrigation. However, this ignores the other benefits of surface irrigation, lower energy costs and potentially lower labour requirements, which have economic and environmental consequences. This also ignores any valid arguments about the sustainability of different water sources. Before any general statements about the sustainability of different irrigation practices was made it would be necessary to collect indicators from a statistically significant sample of farms and apply some rigorous analysis to eliminate the effects of endogenous (e.g. soils and climate) and management factors from irrigation.
Similar words of caution must be expressed regarding the comparison of different farms of the same type. Once again, soil, climate and management factors, as well as the goals of the individual farm owners and operators, are expected to have as much or more influence on the irrigation indicators as management of the irrigation system itself. Considerable detail and effort would be required to isolate irrigation management from other effects.
This study has shown that it is relatively straightforward to determine the historical irrigation indicators for a mono-cultural situation such as a dairy farm. In situations with multiple outputs there are considerable problems. This often leads to aggregation at an economic level. It is debatable whether any valid comparison, of indicators containing an economic component, can be made between different types of enterprise.
5.5 OVERALL VALUE
Overall the indicators that appear to have the greater short-term value are the economic indicators. In particular, production per unit of water, energy use per unit of water, and labour per unit of irrigated area. These are factors that farmers can relate to the benefits and costs of irrigation and are more likely to play a part in decision-making processes in the short term.
The results of this study with respect to environmental indicators are less conclusive. A number of reasons for this can be proposed. Some indicators, such as those for soil, change only slowly. Therefore, several years' consecutive data would be required to show trends conclusively. Therefore this limited study was unlikely to yield any firm conclusions.
Indicators such as fertiliser and agri-chemical use are not directly related to irrigation use and issues of application timing and intensity complicate their effects. Other indicators could not be determined historically and require daily observation by the irrigation operator, which may be a problem unless they are particularly enthusiastic about the process. Still other indicators require expensive monitoring equipment such as lysimeters and can only be measured in limited cases.
Despite these problems there is anecdotal evidence to suggest that many farmers are interested in tracking indicators of the environmental type. The high level of interest shown in sustainability projects in North Otago, Selwyn, and Pukekohe, for example illustrates this. The value of these indicators, however, relates to the long term sustainability goals of farmers. Farmers often have more difficulty expressing these goals than their short-term economic goals – which often relate to survival from one year to the next.
Thus, this project highlights a problem that we believe may be common to all sustainability indicator projects. Indicators with an economic component, that relate to easily measured physical and monetary flows, are readily calculated and relate well to the readily expressed short-term goals of farmers. On the other-hand, indicators with an environmental emphasis are likely to involve slowly evolving processes and complex ecological phenomenon. These indicators are either difficult to measure or highly intractable. They also relate to long term farm goals that are at best 'fuzzy' within the farmer's perception. The result of this is likely to be a concentration on the technical, measurable and tractable at the expense of the overall goals of sustainability, which after all are still rather ill defined or 'fuzzy', a point that is illustrated by the variety of definitions of sustainability that exist internationally.
5.6 COMMENTS ON SCOPE OF USE OF INDICATORS
While it would be tempting to compare indicators between different properties and irrigation systems we would strongly caution against this practise for several reasons. First, the accuracy of the indicators is limited by the inability to measure water use directly. Second, there is insufficient baseline data at present to state definitively how particular values of an indicator relate to sustainability. Third, we feel strongly that using inter-farm comparison to 'point-the-finger' at particular farmers or type of irrigation that appear to be operating in a less sustainable manner than others is not within the 'spirit' of sustainable management. Rather these indicators should be used as 'check' measures within a 'Plan-Do-Check-Act' cycle of implementing more sustainable irrigation practice.
It is also important that indicators of sustainable irrigation are considered within a wider suite of indicators that include land, air, water, bio-diversity, energy and financial sustainability. A farmer should not be criticised for using a particular type of irrigation practice or management technique on the basis of one 'snap shot' measurement compared to a limited number of other 'snap shot' measures. Rather we see that an irrigator should be able to use these indicators, in conjunction with a system audit, to identify the most effective way of improving system efficiency through improved management and possible system upgrades within physical and financial limits.
Having stated that these indicators should not be used for inter-farm comparison we also believe that it is inevitable that farmers will make these comparisons on an informal basis, as they have always compared production information. Thus it is important that there are reliable benchmarks available to make comparisons against. This highlights a need to collect data from a much wider sample of farms and to set more reliable values on the range of each indicator. Information on the range of indicator values then needs to be widely published so that irrigators have an accurate 'yardstick' for their own comparison.
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