Higher evapotranspiration rates, reduced rainfall and increased water scarcity have led to a need for improved irrigation water use efficiency. Evaporation is a significant component of the total evapotranspiration (ET) and high evaporation losses reduce the amount of available water for transpiration, resulting in reduced plant water availability and hence increased irrigation. Values for soil evaporation vary widely in the literature, from 27-65% of total ET. In order to increase transpiration relative to evaporation, a reduction in evaporative losses is needed. Spatial variability of evaporation is an important factor that needs to be taken into consideration when improving water use efficiency. Soil physical properties control evaporation by influencing both the transport of water toward the soil or root surface and soil water storage. The effect of variability in soil properties on evaporation is likely to be larger in cases with water deficiency. Cultural practices such as use of narrow row spacing, mulch and minimum tillage can reduce evaporation however, they are not always effective. The potential for major savings of water depends on the length of drying interval following irrigation or rain. There is the potential for reductions in evaporation losses of up to 60% through the use of improved management techniques, enabling more water to be used by the plant for transpiration. In order to improve water use, more research in quantifying evaporation variability at the field scale needs to be completed.

This paper addresses how the design of subsurface drainage systems impacts on the drainage volume and salinity of drainage water generated. The effects of drain depth and spacing are reviewed and discussed. The conceptualization of a subsurface drainage system which incorporates drainage water quality into the design is presented. This system, known as a Multi-Level Drainage System, aims to minimize offside impacts associated with subsurface drainage while still providing adequate protection from water logging and salinity of the plant root zone. This is achieved through the use of a shallow closely spaced drainage system (0.7m deep at 3.3m) underlain by a deeper, widely spaced drainage system (1.8m deep at 20m). Field investigations show that the shallow drains had approximately five times lower salinity than deep drains, with median values being 5.5 dS/m and 28 dS/m respectively. The results indicate that, by re-thinking subsurface drainage design to incorporate water quality aspects, alternative designs can be formulated which go some way to meet the present day environmental constraints placed on subsurface drainage systems.

Surface irrigation currently accounts for 70-80%of irrigation water use in Australia and surfaceapplication is by far the dominant irrigationmethod applied throughout the world. However,water use efficiencies with surface irrigationmethods tend to be low. In recent years a numberof surface irrigation simulation models forassessing surface irrigation system performancehave been developed. One of the most commonlyused models SIRMOD, developed by Utah StateUniversity, has seen wide use and evaluationthroughout the world particularly by researchersand has been shown to offer potential forincreasing surface irrigation water useefficiencies.Considerable efforts are now being undertaken tomove use of the model from the realm of aresearch domain to the farmer domain. Maximumbenefit from the use of such models will onlyoccur when farmers have the ability to useDecision Support Systems (DSS) such asSIRMOD in a near real time environment i.e. forindividual irrigations.

This paper introduces the concept of controlled drainage and presents results from a study investigating the potential of controlled drainage for reducing drainage volumes and salt loads in an irrigated vineyard in the Murrumbidgee Irrigation Area of South Eastern Australia. This study compared traditional unmanaged drainage systems with controlled drainage systems utilizing weirs to maintain water tables and minimise drainage volumes. The results from the field experiments indicated that controlled drainage has the potential to significantly reduce drainage volumes and salt loads compared to unmanaged subsurface drainage systems. However, careful management is needed to ensure that rootzone salinity levels are maintained at optimum levels for plant production.

Recent community based actions to ensure the sustainability of irrigation and protection of associated ecosystems in the Murrumbidgee Irrigation Area (MIA) of Australia has seen the implementation of a regional Land and Water Management Plan. This aims to improve land and water management within the irrigation area and minimise downstream impacts associated with irrigation. One of the plan objectives is to decrease current salt loads generated from subsurface drainage in perennial horticulture within the area from 20 000 tonnes/year to 17 000 tonnes/year. In order to meet such objectives Controlled Water table Management (CWM) is being investigated as a possible ‘Best Management Practice’, to reduce drainage volumes and salt loads. During 2000–2002 a trial was conducted on a 15 ha subsurface drained vineyard. This compared a traditional unmanaged subsurface drainage system with a controlled drainage system utilizing weirs to maintain water tables and changes in irrigation scheduling to maximize the potential crop use of a shallow water table. Drainage volumes, salt loads and water table elevations throughout the field were monitored to investigate the effects of controlled drainage on drain flows and salt loads. Results from the experiment showed that controlled drainage significantly reduced drainage volumes and salt loads compared to unmanaged systems. However, there were marked increases in soil salinity which will need to be carefully monitored and managed.

Many coastal floodplain wetlands in northern NSW have been drained and fitted with one-way floodgates. These wetlands are often underlain by acid sulfate soils (ASS) that can release acidic by-products into the groundwater and surface water if oxidised. Little Broadwater, on the Clarence River, is typical of these altered wetlands. An ongoing restoration trial focussing on increased tidal exchange, initiated in June 2003, provided the opportunity to study the rehabilitation process taking place within the wetland. This study aimed to investigate the hydrology and changes in water quality characteristics during the reestablishment of tidal exchange at Little Broadwater. Discharge water quality (pH, electrical conductivity (EC) and dissolved oxygen (DO)) were compared pre- and post-rehabilitation to determine if restoring tidal exchange improved discharge water quality. Monitoring of surface water and groundwater quality (pH, EC, DO, temperature, acidic cations, basic anions, total nitrogen and total phosphorus) was conducted over a 28-month period. Short-term monitoring was also conducted at two reference wetlands to compare spatial patterns and factors that affected water quality. The results of this study were used to develop a conceptual model of coastal floodplain wetland functioning, with particular reference given to Little Broadwater.

An enclosed portable chamber was constructed and calibrated to measure actual evapotranspiration (ET) from crop and pasture and then evaluated against established methods that are used to determine evapotranspiration. The chamber was equipped with variable speed electric fans to mix the air within the chamber during each ET measurement. The most appropriate fan speed was investigated.Pasture ET measured using the enclosed portable chamber compared well with predicted water loss using the water balance method for a 6-day period during winter 1997 in Armidale (NSW, Australia). Mean cumulative pasture ET for the 6-day period was 5.8 and 5.9 mm measured with the enclosed portable chamber and water balance method, respectively.Wheat crop ET measured using the enclosed portable chamber was compared with that estimated by the Bowen ratio (BR) method for a 2-day period in the early growth stages of the crop. Mean ET using the enclosed portable chamber was 2.4 mm per day compared with 2.3 mm per day using the BR method.Results from the enclosed portable chamber method showed sensitivity to the choice of fan speed. A slow fan speed that produced an air velocity of 2.7 km/h, gave the closest agreement with the Bowen ratio method (2.3 mm per day versus 2.2 mm per day) for the wheat crop.The main attractions of the enclosed portable chamber method include: (1) its suitability for ET measurement within small areas (<1 m2), which enables ET measurement from individual plant communities within small areas; (2) its main principles of measuring the actual water flux from transpiring vegetation rather than inferring it from climatic parameters; (3) the speed by which an instantaneous ET rate is obtained (less than 1 min); (4) instantaneous ET measurement can be repeated throughout the day from the same plant communities; and (5) the portability of the enclosed portable chamber. ET measurement using the enclosed portable chamber method may be combined with the existing soil water balance models for comparing alternative crop and pasture systems in terms of their water balance.

This paper describes a multi-level drainage system, designed to improve drainage water quality. Results are presented from a field scale land reclamation experiment implemented in the Murrumbidgee Irrigation Area of New South Wales, Australia. A traditional single level drainage system and a multi-level drainage system were compared in the experiment in an irrigated field setting. The single level drainage system consisted of 1.8 m deep drains at 20 m spacing. This configuration is typical of subsurface drainage system design used in the area. The multi-level drainage system consisted of shallow closely spaced drains (3.3 m spacing at 0.75 m depth) underlain by deeper widely spaced drains (20 m spacing at 1.8 m depth). Data on drainage flows and salinity, water table regime and soil salinity were collected over a 2-year period. Comparisons of water and solute movement between the multi-level drainage system and a single level drainage system are presented. Differences in the performance of the multi-level and single level drainage systems were found in the water table regime, drain water salinity and soil salinity.