Impact of water saving irrigation techniques in China

• Wuhan University of Hydraulic and Electric Engineering, China
• Zhejiang Agricultural University, China

In Asia, demand for water is growing rapidly, with little potential for increasing the supply. As the region develops, more and more water is being diverted to non-agricultural uses. But at the same time the demand for food is growing and rice, the staple crop, needs irrigation water.
In China, shortage of water (rather than of land) is the biggest constraint to agriculture. Irrigation has played a vital role in keeping the pace of growth in rice production aligned with growth of the population. Currently, 96 per cent of the annual rice production of 130.6 million tonnes comes from irrigated systems. In the future rice production in China will need to double while water throughout the country will be in increasing short supply. Per capita freshwater availability, already among the lowest in Asia, is declining. Therefore saving water in rice-based irrigation systems deserves more attention. In essence, the country needs to produce more rice with less water.
The Chinese government has already pioneered water-saving irrigation (WSI) techniques to help achieve greater water efficiency in rice-based farming systems. The techniques, aimed mainly at reducing seepage and percolation from farms, have been widely adopted in China. However, several scientific issues need to be better understood before going any further with the development and implementation of WSI practices. For example, does the drier soil in WSI lead to water stress in rice plants during their reproductive phase? Is the efficiency of nitrogen uptake altered? Are there any effects on the population dynamics of weeds?
This project attempted to answer some of these questions and to address broader technical issues underlying the successful application of WSI.

The main aim was to develop new, readily-implemented irrigation strategies for rice-growing systems to improve water efficiency, without losing productivity or cost-effectiveness.

The work was divided into four subprojects. The first focused on assessing WSI on farms. The team quantified and compared crop yield and nutrient balance in rice grown either with WSI techniques or with continuously ponded water.
The second subproject focused on the financial assessment of WSI. The team carried out surveys and studied farm budgets. They quantified farmers' use of various inputs (including labour) and of water. For comparison, data were also collected from similar farms that were not using WSI. The aim was to determine the financial benefits accruing to farmers using WSI.
The third subproject assessed the impact of WSI on water efficiency and productivity. Again, similar pairs (with and without WSI) were selected. A water accounting and water productivity study was carried out at various scales to form a complete water balance for the studied areas. The scientists quantified components of flow (such as irrigation diversions, rainfall, seepage, evapotranspiration and water recycling from drainage canals). They also measured rice yields.
The final subproject focused on training Chinese professionals and disseminating the outputs of the research work through seminars and publications.

As the research progressed, the decision was made to focus on alternate wetting and drying (AWD) irrigation, a WSI technique that is being popularised in China. Also, it was found essential to obtain detailed information on institutions and on the management of water resources in the Zhanghe Irrigation System (ZIS) system, including the change in allocation of water between irrigation and other sectors over time. In short, researchers concluded that it was impossible to separate technical and biophysical issues from management and socioeconomic issues.
Results of both the experiments and farm surveys showed that there was no difference in yields between AWD and conventional shallow-flooded irrigation practices. However, there were gains in water productivity. Both experimental and farmers' monitoring confirmed that AWD tended to use less irrigation water. Mid-season drainage and intermittent soil drying were not prerequisites for high yield. Thus in years with high rainfall, farmers do not have to drain out rainwater to achieve soil drying, and therefore can conserve more water. There was no significant water x nutrient interaction on grain yields, biomass and nitrogen (N) uptake. Thus AWD does not require a different N-fertiliser management regime from continuous flooding.
Two townships were surveyed. The township where farmers practiced less AWD had higher water use and higher cost of irrigation per farm. However, AWD was practiced much more widely in the dry year (2000) than in the wet year (1999). It is thus difficult to determine whether farmers are voluntarily accepting AWD or being forced to practice AWD due to reduced deliveries in the system.
In subproject 3 the researchers tested a water accounting methodology to study irrigation system and basin-wide implications of on-farm AWD. They found that, in addition to micro and mezzo scales, even larger scales were needed to better understand the degree to which water savings were achieved at the system and basin levels.
A prerequisite for AWD is a reliable supply system. This is achieved in ZIS by a combination of system management, institutional arrangement, and infrastructure. The infrastructure includes numerous small ponds and reservoirs permitting storage and control of water releases near the farm. Also important is the volumetric pricing of water at the village level, providing incentive for the farmers to reduce irrigation costs by using less water.
This combination of factors has permitted the ZIS to divert water to higher-valued uses. The amount of water supplied for irrigation from the main reservoir has dropped from 80 to 25 per cent of its volume over the past 25 years. However, this combination of factors is not readily found in other developing countries, and researchers should be cautious about the applicability of these findings in other regions.