Growing more rice with less water: Increasing water productivity in rice-based cropping systems

• CSIRO Land and Water, Australia
• Wuhan University of Hydraulic and Electric Engineering, China
• International Rice Research Institute, Philippines

Rice is a vital food staple in China with a large part of the population dependant on rice production. Most of this rice is produced using irrigation, using valuable water resources. Water use in China is rising, prompting China to promoting water-saving irrigation (WSI) techniques. Such techniques must not reduce yields, which must remain at a high level to produce sufficient food. One of these techniques is alternate wetting and drying of rice paddies, rather than the traditional method of continuous flooding. In the Zhanghe Irrigation System (ZIS), a part of the Yangtze River Basin, authorities are balancing demands for irrigation against increased demand for hydro-power. Farmers operating within the reaches of the system rely on rainfall, with surface ponds used to store water. Irrigated supplies supplement this source of water and are used to overcome water scarcity during rice cropping. Managing the conflict between farmers and the hydro-power sector is the task of ZIS authorities. A second system, the Liu Yuan Kuo Irrigation System (LIS), in the Yellow River Basin contrasts that of the ZIS by being water poor, that is rainfall is insufficient to match demand.

Field experiments in China have shown that in most of the rice cropping systems WSI is a potential solution to such conflict. It does not reduce the yield of the rice, but does reduce the irrigation input to rice fields by about 10-15 per cent compared to continuous flooding. This has led to questions about the nature, success, and adaptability of the alternate wetting and drying technique, and how it can aid water managers in systems such as those of the ZIS and LIS. Also can this technique be transferred to other areas. Australia is facing problems similar to those of China. The declining water availability, increasing water prices, and environmental degradation of agricultural lands and riverine ecosystems are driving the demand for water savings.

Past research has indicated that there is a potential to save irrigation water by using alternate wetting and drying and saturated soil culture. However, because these techniques lead to changes in irrigation demand, the potential benefits to river flow patterns have yet to be determined.

The project aimed to address these issues by linking Chinese and Australian laboratories to promote water management techniques in rice-based irrigation systems that reduce water consumption and do not decrease crop production. In particular this project aims to further investigate the technique of alternate wetting and drying, and to compare it to the techniques of saturated soil culture and aerobic rice in China.

Field experiments and surveys were conducted in the two irrigation systems in China (the Yangtze River Basin's Zhanghe System and the Liu Yuan Kuo System of the Yellow River Basin) and one region in Australia (Murrumbidgee River Basin).

At the two study sites in China, four water management treatments were tested: continuous flooded rice (control), alternate wetting and drying, saturated soil culture and planting of aerobic rice. These were assessed with crop growth and water balance models to assess the impact of treatments in different hydrological and soil conditions. Farmer surveys about all aspects of crop management were also conducted.

Data on water diversion was collected for a large number of irrigation groups in China to assess different variables. For example surveys and interviews at the village, system and basin level were conducted and combined with this data from experimental sites to compare treatments across sites.

Water saving irrigation using alternative wetting and drying is a viable option for farmers in the Zhanghe Irrigation System (ZIS). Studies confirmed that alternate wetting and drying can be practiced without yield losses, while saving significant irrigation water. Farm surveys revealed that at the farm level the maintenance of yields does not affect profitability. The key finding was that farmers will practice water saving only by default. At the catchment level the artificial creation of water scarcity will prompt farmers to adopt water saving irrigation without reducing yields or profits, providing farmers know how to implement such techniques.

The main source of water loss from fields is water seepage through bunds. These bunds separate fields but can be reinforced to reduce seepage. However the adoption of alternate wetting and drying has been shown to reduce standing water and with it the opportunity for seepage.

All of these findings were confirmed through use of the ORYZA2000 crop model. The model was used to assess water saving irrigation under differing water management scenarios, notably differing hydrological conditions and the use of aerobic rice. Testing at the ZIS catchment level revealed that limited flows out of the basin are present, but field level management is very effective. These findings revealed that surface outflow cannot be further exploited without impinging on water availability downstream. Through this project the Tuanlin Research Station in the ZIS has become a centre for irrigation and water saving research excellence. To date an estimated 1.5 million hectares of farming land is now under alternate wetting and drying.

Findings for the Lin Yuan Kuo Irrigation System were different, due largely to the diversity of characteristics. Unlike the ZIS catchment where evaporation and rainfall are almost equal, in the LIS evaporation demand is greater than rainfall. The shortfall in rainwater and the lack of places to store water result in more water scarcity. Aerobic rice, that does not require flooding, was trialled resulting in a yield reduction. Despite this farmers indicated in surveys that aerobic rice has value, reducing labour inputs by up to 50 per cent with the benefit of not needing sufficient water to flood fields.

At the catchment level hydrological-economic analysis determined water savings could be achieved by pumping groundwater through canals. Lining the canals to prevent seepage was needed. Redistributing irrigation water from nearer the river to those fields further from the river, in concert with groundwater pumping for those farmers closer to the river helped more farmers' access sufficient water. Farmers nearer the river could be aided in utilising groundwater pumping through a subsidy scheme to install pumps.

The institutional arrangements and policies practices in the ZIS are more conducive to water savings than those of the LIS. In the LIS farmers pay a flat rate for water, with managers having to return savings to the Yellow River. The lack of incentives contrasts to the ZIS, where farmers pay a volumetric rate for water consumed. This encourages water savings at the farm-level, on-farm storage and a willingness to try new techniques. System managers too benefit through savings by being able to allocate water for non-agricultural uses.

Aligning system objectives and incentives to save water so that all users are working together helps create a culture of efficiency. The policy ramifications are clear for the LIS, which has moved to work this approach into its 11th five year planning for the modernisation and rehabilitation of the system.

In Australia's Murrumbidgee Irrigation Area methodologies for tracking accounted for water flows have been developed as a result of this research. Modelling to quantify water savings at the farm and system levels was conducted revealing potential savings that have a net benefit of between $64 to $344/mega litres saved each year. Water savings identified will be adopted to guide infrastructure investments in the Murrumbidgee catchment, and will provide inputs into the National Water Initiative, aimed at introducing irrigation and other water efficiency improvements to recapture 500 giga litres.