|Page tools: Print Page Print All|
This document was added or updated on 22/11/2013.
FEATURE ARTICLE 2: EXPERIMENTAL ESTIMATES OF SOIL WATER USE IN AUSTRALIA
2. Soil water estimates are proposed to be introduced to the 2012-13 WAA which will be published in late 2014. The methods and results described in this article are considered to be experimental. Discussion, comments or suggestions on the methodology and results are welcome and can be sent to <email@example.com>.
3. Soil water is described in both the System of Environmental-Economic Accounts - Water (SEEA-W) and the 2012 version of the System of Environmental-Economic Accounts (SEEA 2012). This method prefers the SEEA 2012 framework for measuring soil water abstraction which excludes evaporation and any water retained in the soil. Evaporation and water retained in soil may be measured in future extensions to the WAA.
4. Soil water is defined in SEEA-Water as:
“Soil water consists of water suspended in the uppermost belt of soil, or in the zone of aeration near the ground surface, that can be discharged into the atmosphere by evapotranspiration.” (para. 6.17)
5. Soil water estimates are defined in SEEA 2012 as:
“Abstraction of soil water refers to the uptake of water by plants and is equal to the amount of water transpired by plants plus the amount of water that is embodied in the harvested product” (SEEA 2012 para. 3.198)
6. There are three estimates that measure soil water abstraction and use:
7. Soil water estimates appear in physical supply and use tables as a source of abstracted water from the environment and as a use of the sources of abstracted water by industry and households. Water transpiration from soil water and water incorporated into products are supplied by industry and households. In this model, transpired soil water flows to the environment and reflects the quantity of water no longer available for use by the economy. Water incorporated into products is defined by the accumulated water in the economy embodied in harvested plant material. Water embodied in exports and imports are not included in this methodology.
8. There are two potential sources of double counting in these results, but they have a minor impact on estimates. Hay and silage consumed by livestock should be offset in the pasture component of the model, but it is not known if the hay and silage is consumed in the same financial year or in different financial years. Hay and silage soil water abstraction is approximately 2% of total soil water abstracted by the activities of grazing in 2011-12. The other component of potential double counting in the WAA are irrigation on crops and pasture. Irrigation is used to supply water to productive activities where there is a lack of rainfall or rainfall does not occur at times suitable for plant growth. Irrigation estimates will be netted out when soil water estimates are incorporated into the water flow tables. Irrigation accounted for 2% of soil water abstraction in 2011-12.
9. Water Use Efficiency (WUE) defines the efficiency with which water - rainfall or irrigation - is converted through a farming system into a saleable commodity, for example, grain, meat or fibre (http://www.csiro.au/en/Organisation-Structure/Divisions/Ecosystem-Sciences/Water-Use-Efficiency.aspx, extracted June 2013). One of the core components of modelling soil water is plant material extracting water from the environment. Evapotranspiration describes a plant's need for water as a part of the growth process. In this process, water is taken up from the environment, stored and used to combine with other bio-chemical processes for growth. These processes have been described in many scientific studies and generally use a coefficient (WUE) based on yield of plant material, the water applied and the area used for planting. For each agricultural or forestry commodity the coefficient used (WUE) has the characteristics:
(see: French, RJ., Schultz, J.E. (1984). Water use efficiency of wheat in a Mediterranean type environment: The relation between yield water use and climate. Australian Journal of Agricultural Research (35) 743-764. )
10. The French & Schultz article discusses the minimum soil moisture requirement for crop plants (wheat in the article's case study) to establish themselves. The WUE formula assumes that there is sufficient water moisture content in the soil to allow transpiration and plants to establish. In their case study plants established at about 100 mm of rainfall (or about 1 ML per hectare), though other studies have shown that this base moisture level for crops can vary between 80 and 140 mm.
11. The coefficient is an extremely useful measure for calculating soil water use. With the assumption that there is sufficient soil moisture content before farmers decide to plant means that the WUE coefficient can be used to calculate the soil water required for a particular plant:
12. For some plants (for example in plantations and horticulture) the water use efficiency coefficient is not widely available. This means a slight change in context and the formula for water use is needed to align for own account water use (per year for horticulture and per tree for orchards).
Note that area planted for orchards is calculated by the total number of trees divided by number of trees per hectare.
13. Measurement of water incorporated into products is a simple calculation once the weight of any harvested material is known. Water incorporated into products can be calculated through the formula:
14. As water transpired is the residual flows of water after growth processes have taken place then its calculation can be the net of soil water and the water stored in the products. Water transpired back to the environment is calculated through the identity recommended in SEEA 2012:
i.e. Soil water abstracted = 1 000 x 1/18 x 1 000 000 = 55 555 555 L (or over 55 ML)
i.e. Water incorporated into plants = 1 000 kg x 10 g ÷ 100 g = 100 L
2.1 Data sources
16. Soil water use estimates are derived from the sources detailed in Table 1. These named sources need a small degree of transformation to make them suitable for the estimation process. Some of the detailed WUE coefficients are determined using assumptions based on neighbouring states or assuming that the water use coefficient is consistent across the state. While there are likely to be some differences, the contribution of those products impacted by the assumption were found to be quite small (less than 1% of the total estimate). More recent estimates of water use by horticultural species (vegetables, fruits and nuts, other plantations) would be useful but their contribution to total soil water is quite small (less than 1%) and any changes are likely to be insignificant.
2.2 Transformations and assumptions
17. The two main transformations convert estimates of weight and area to standard units of kilograms for weight and hectares for area. Water use efficiency was typically measured in kg/mm.ha, except for annual water use for horticultural species which were measured in mm/year. These calculations use the fact that one mm per metre squared is equivalent to one litre of water (or 1 mm translates to 10 000 L over a hectare).
18. The area for fruits and nuts was modelled based on the number of live trees at the end of the period by the average number of trees for that particular species. The average number of trees per hectare is available on state and territory primary industry websites.
19. Pasture weight estimates were considered to be the harvested pasture by livestock on that land. Native pasture and modified pasture have widely differing characteristics. Modified pasture yields on average about 30 kg per hectare of plant material for every 5 kg of plant material per hectare of native pasture. The amount of modified pasture used is small (Figure 2) in comparison to native pasture. Cattle and sheep were considered to be the primary source of harvesting livestock for pasture. Their consumption rates were assumed to be consistent across Australia. The model used approximated consumption by cattle to be 16 kg per day and consumption by sheep to be 2.5 kg per day. These values are consistent with Eurostat's measure of biomass consumed by livestock for their material flow accounts. The slightly elevated intake reflects the drier conditions and quality of grass stocks in Australia compared to Europe.
20. Pasture area estimates were modelled for some years using total pasture as a constraining value or interpolated when data were missing. Figure 2 demonstrates the effects of modelling.
22. Grass and other plant material harvested in urban and rural residential land uses was modelled. Harvesting activities in urban and rural residential generally describe the removal of organic material from green spaces which is predominantly from mowing lawns and other fields. These harvesting activities in urban and rural residential regions are equivalent to own account production of plant material and should be included in the economy. The weight was calculated in two ways. Weights of removed material from urban land use types was based on watering rates by State and expected material produced from lawn mowing activity approximated by hay and silage yields seen on modified pasture (that is about 30 kg per hectare per year). Rural residential was engineered from the water use coefficient and applying a standard yield per hectare based on the native grass production. Values for soil water abstraction by urban gardens was not included in these estimates. Ovals and parklands were considered to be in the estimate as the area used to calculate soil water abstraction included green space in urban environments.
23. Area for urban and rural residential land use is modelled based on the Australian Collaborative Land Use and Management Program (ACLUMP) and movements in household estimates from Household and Family Projections, Australia 2006 to 2031 (ABS cat. no 3236.0). It was assumed that capital city household growth rates were sufficient for urban land use and the balance of state household growth rates were sufficient for rural residential land uses.
24. Estimates of gross soil water include water applied as irrigation for agricultural, household and parkland management activities. These need to be netted out before they are applied to supply use tables due to the risk of double counting abstraction.
2.3 Water incorporated into products
25. Water incorporated into products in these estimates are the moisture remaining in plant material after it is harvested. The moisture content varies widely between plant species. For example some species of lettuce are 90% water whereas wheat is only around 10%. These moisture contents are not published here but are available from food content websites such as the Australian Food Standards site (http:\\www.foodstandards.gov.au).
26. Water incorporated in products is a simple transformation:
2.4 Water transpiration
27. Water transpired is calculated as a residual to the calculation of soil water and water incorporated into products. The soil water identity recommended in SEEA Central Framework (SEEA-CF) is the most efficient way of producing transpiration data.
28. Water transpiration is calculated using the identity:
3.1 Soil water abstraction
29. Gross soil water abstraction describes the amount of soil water extracted by industry and households for the purposes of growing organic material. These activities include pasture grazed by cattle and sheep. Figure 1 presented the results for soil water noting that there is a significant decline in soil water supply between 2005-06 and 2009-10. This coincides with a long period of drought and a steep decline in sheep numbers over the same period. Figure 3 shows water applied in irrigation which would typically supplement agricultural production activities. As can be seen in Figures 1 and 3, both soil water abstraction and irrigation decline with prolonged drought conditions.
3.2 Soil water use by industry
31. Table 3 shows the allocation of soil water to industry. Sheep, Beef Cattle and Grain Farming has the bulk of soil water use (88% of total soil water use) due to grazing and broadacre crops such as wheat. A substantial part of the decrease in soil water use from 2005-06 onwards was from sheep stocking decreasing steadily due to drought conditions.
32. Water incorporated into products and water transpired are components of final water use since they represent flows of water that are no longer available in the economy. Final use estimates are not provided in this article as evaporation should be included but was not measured as a part of soil water abstraction. Water incorporated into products represents the moisture present in organic material as it is harvested from the economy and used over 91,000 ML of water in 2011-12. Harvested material includes pasture grazed by livestock and grass cut in urban environments. Water incorporated in products is quite low compared to soil water supplied, but a significant component of the total biomass recorded under current collection methodology. These data represent a significant component of the plant mass moving through the economy.
33. Water transpired (over 327,000 GL of soil water use) is the amount lost to the atmosphere as organic material releases oxygen and water back to the environment through its evapotranspiration processes needed for growth. Under accounts principles this water becomes lost to the economy as it cannot be recovered or re-used despite water entering the atmosphere will eventually be deposited somewhere. Water transpired shows similar changes to soil water supply as only a small amount of water is incorporated in the product harvested.
3.4 Conclusions and further improvements
34. Soil water supply and use can be regularly measured in the Water Account Australia (WAA) (ABS cat. no. 4610.0). Due to the scope of the changes proposed in this article, incorporation into the existing WAA will be challenging. It is recommended that Australia adopts the international standard for presentation of water accounts from either SEEA 2012 or SEEA-Water. This would allow clearer separation of supply of natural inputs from residual flows back to the environment and improve comprehension of the economic concepts of inputs into the economy (gross water input) and losses which cannot be recovered by the economy (final use of water). Further work on calculating evaporation of abstracted water would also be useful. Comments on the methods and sources of data used in this article can be submitted at <firstname.lastname@example.org>.
35. Bibliography available on request from <email@example.com>.
These documents will be presented in a new window.