T.P. Tuong

Field water management to save water and increase its productivity in irrigated lowland rice

Abstract Rice production in Asia needs to increase to feed a growing population whereas water for irrigation is getting scarcer. Major challenges are to (i) save water; (ii) increase water productivity and (iii) produce more rice with less water. This study analyzes the ways in which water-saving irrigation can help to meet these challenges at the field level. The analyses are conducted using experimental data collected mostly in central–northern India and the Philippines. Water input can be reduced by reducing ponded water depths to soil saturation or by alternate wetting/drying. Water savings under saturated soil conditions were on average 23% (±14%) with yield reductions of only 6% (±6%). Yields were reduced by 10–40% when soil water potentials in the root zone were allowed to reach −100 to −300xa0mbar. In clayey soils, intermittent drying may lead to shrinkage and cracking, thereby risking increased soil water loss, increased water requirements and decreased water productivity. Water productivity in continuous flooded rice was typically 0.2–0.4xa0g grain per kg water in India and 0.3–1.1xa0g grain per kg water in the Philippines. Water-saving irrigation increases water productivity, up to a maximum of about 1.9xa0g grain per kg water, but decreases yield. It therefore does not produce more rice with less water on the same field. Field-level water productivity and yield can only be increased concomitantly by improving total factor productivity or by raising the yield potential. Total rice production can be increased by using water saved in one location to irrigate new land in another. If this is not done, a strategy of saving water at the field level potentially threatens total rice production at large.

Rice production in water-scarce environments

Rice production in Asia needs to increase to feed a growing population. Though a complete assessment of the level of water scarcity in Asian rice production is still lacking, there are signs that declining quality of water and declining availability of water resources are threatening the sustainability of the irrigated rice-based production system. Drought is one of the main constraints for high yield in rain-fed rice. Exploring ways to produce more rice with less water is essential for food security and sustaining environmental health in Asia. This chapter reviews the International Rice Research Institute (IRRI)’s integrated approach, using genetics, breeding and integrated resource management to increase rice yield and to reduce water demand for rice production. Water-saving irrigation, such as saturated-soil culture and alternate wetting and drying, can drastically cut down the unproductive water outflows and increase water productivity. However, these technologies mostly lead to some yield decline in the current lowland rice varieties. Other new approaches are being researched to increase water productivity without sacrifice in yield. These include the incorporation of the C4 photosynthetic pathway into rice to increase rice yield per unit water transpired, the use of molecular biotechnology to enhance drought-stress tolerance and the development of ‘aerobic rice’, to achieve high and sustainable yields in non-flooded soil. Through the adoption of water-saving irrigation technologies, rice land will shift away from being continuously anaerobic to being partly or even completely aerobic. These shifts will have profound changes in water conservation, soil organic-matter turnover, nutrient dynamics, carbon sequestration, soil productivity, weed ecology and greenhouse-gas emissions. Whereas some of these changes can be perceived as positive, e.g. water conservation and decreased methane emission, some are perceived as negative, e.g. release of nitrous oxide from the soil and decline in soil organic matter. The challenge will be to develop effective integrated natural-resource-management interventions, which allow profitable rice cultivation with increased soil aeration, while maintaining the productivity, environmental services and sustainability of rice-based ecosystems.

Rice and Water

The Comprehensive Assessment of Water Management in Agriculture (CA) seeks answers to the question of how freshwater resources can be developed and managed to feed the worlds population and reduce poverty, while at the same time promoting environmental security. The CA pays particular attention to rice as this crop is the most common staple food of the largest number of people on Earth (about 3 billion people) while receiving an estimated 24–30% of the worlds developed freshwater resources. Rice environments also provide unique—but as yet poorly understood—ecosystem services such as the regulation of water and the preservation of aquatic and terrestrial biodiversity. Rice production under flooded conditions is highly sustainable. In comparison with other field crops, flooded rice fields produce more of the greenhouse gas methane but less nitrous oxide, have no to very little nitrate pollution of the groundwater, and use relatively little to no herbicides. Flooded rice can locally raise groundwater tables with subsequent risk of salinization if the groundwater carries salts, but is also an effective restoration crop to leach accumulated salts from the soil in combination with drainage. The production of rice needs to increase in the coming decades to meet the food demand of growing populations. To meet the dual challenges of producing enough food and alleviating poverty, more rice needs to be produced at a low cost per kilogram grain (ensuring reasonable profits for producers) so that prices can be kept low for poor consumers. This increase in rice production needs to be accomplished under increasing scarcity of water, which threatens the sustainability and capability to provide ecosystem services of current production systems. Water scarcity is expected to shift rice production to more water‐abundant delta areas, and to lead to crop diversification and more aerobic (nonflooded) soil conditions in rice fields in water‐short areas. In these latter areas, investments should target the adoption of water‐saving technologies, the reuse of drainage and percolation water, and the improvement of irrigation supply systems. A suite of water‐saving technologies can help farmers reduce percolation, drainage, and evaporation losses from their fields by 15–20% without a yield decline. However, greater understanding of the adverse effects of increasingly aerobic field conditions on the sustainability of rice production, environment, and ecosystem services is needed. In drought‐, salinity‐, and flood‐prone environments, the combination of improved varieties with specific management packages has the potential to increase on‐farm yields by 50–100% in the coming 10 years, provided that investment in research and extension is intensified.

More rice, less water - integrated approaches for increasing water productivity in irrigated rice-based systems in Asia.

Abstract The water crisis is threatening the sustainability of the irrigated rice system and food security in Asia. Our challenge is to develop novel technologies and production systems that allow rice production to be maintained or increased in the face of declining water availability. This paper introduces principles that govern technologies and systems for reducing water inputs and increasing water productivity, and assesses the opportunities of such technologies and systems at spatial scale levels from plant to field, to irrigation system, and to agro-ecological zones. We concluded that, while increasing the productivity of irrigated rice with transpired water may require breakthroughs in breeding, many technologies can reduce water inputs at the field level and increase field-level water productivity with respect to irrigation and total water inputs. Most of them, however, come at the cost of decreased yield. More rice with less water can only be achieved when water management is integrated with (i) germplasm selection and other crop and resource management practices to increase yield, and (ii) system-level management such that the water saved at the field level is used more effectively to irrigate previously un-irrigated or low-productivity lands. The amount of water that can be saved at the system level could be far less than assumed from computations of field-level water savings because there is already a high degree of recycling and conjunctive use of water in many rice areas. The impact of reducing water inputs for rice production on weeds, nutrients, sustainability, and environmental services of rice ecosystems warrants further investigation.

Drought-stress responses of two lowland rice cultivars to soil water status.

The physiological and morphological responses of two semi-dwarf lowland rice cultivars to transient drought were studied in three greenhouse experiments. These responses were related to root-zone soil water status for use in a rainfed-rice simulation model. Results were very similar for both varieties. Drought responses in young plants occurred at a lower soil water status than in older plants. The first observed effect in a drought period in the vegetative phase was a decline in leaf expansion rate compared to well-watered plants. Leaf expansion stopped completely with root-zone soil water pressure potential h in the range −50 to −250 kPa, depending on crop age and growing season. The rate of transpiration, corrected for differences in LAI, remained roughly equal to that of well-watered plants in the range 0 > h > −100 kPa, depending on crop age. As the soil water status declined further, relative transpiration rate decreased with increasing values of log(|h|), following a logistic function. Leaf rolling and early senesence started at h < −200 kPa or lower and were linearly related with log(|h|). Yield differences between plants that were transiently stressed in the early vegetative phase and well-watered plants were not significant. However, flowering and maturity were delayed. Severe drought in the reproductive phase resulted in large yield reductions, mainly caused by an increase in the percentage of unfilled grains and also in grain weight.

Effect of irrigation method and N-fertilizer management on rice yield, water productivity and nutrient-use efficiencies in typical lowland rice conditions in China

Alternate wetting and drying irrigation (AWD) has been reported to save water compared with continuous flooding (CF) in rice cultivation. However, the reported effects on yield varied greatly and detailed agro-hydrological characterization is often lacking so that generalizations are difficult to make. Furthermore, it is not known how AWD modifies nutrient use efficiencies and if it requires different N-fertilizer management compared with CF. This study quantified the agro-hydrological conditions of the commonly practiced AWD and compared the impact of AWD and CF irrigations at different N-fertilizer management regimes on rice growth and yield, water productivity, and fertilizer-use efficiencies in five crop seasons in 1999 and 2000xa0at two typical lowland rice sites in China (Jinhua, Zheijang Province and Tuanlin, Hubei Province), with shallow groundwater tables.Grain yields varied from 3.2 to 4.5xa0txa0ha–1 with 0xa0kgxa0Nxa0ha–1 to 5.3–8.9xa0txa0ha–1 with farmers’ N-rates (150xa0kgxa0Nxa0ha–1 in Jinhua and 180 in Tuanlin). In both sites, no significant water by nitrogen interaction on grain yields, biomass, water productivity, nutrient uptakes and N-use efficiency were observed. Yield and biomass did not significantly differ (P >0.05) between AWD and CF and among N timings. The productivity of irrigation water in AWD was about 5–35% higher than in CF, but differences were significant (P <0.05) only when the rainfall was low and evaporation was high. Increasing the number of splits to 4–6 times increase the total N uptake, but not total P-uptake, and total K-uptake compared with farmers’ practices of two splits. Apparent Nitrogen recovery (ANR) increased as the number of splits increased, but there was no significant difference in ANR between AWD and CF. During the drying cycles of AWD irrigation, the perched water table depths seldom went deeper than – 20xa0cm and the soil in the root zone remained moist most of the time. The results suggest that in typical irrigated lowlands in China, AWD can reduce water input without affecting rice yields and does not require N-fertilizer management differently from continuous flooding. The results can be applied to many other irrigated lowland rice areas in Asia which have a shallow groundwater table.

Comparing water input and water productivity of transplanted and direct-seeded rice production systems

Increasing water scarcity threatens food production in irrigated rice systems in Asia. It is, therefore, important to identify rice production systems that require less irrigation water than traditional transplanted (TP) rice. This study investigated the effect of crop establishment methods on irrigation input and water productivity (weight of produce per unit volume of water used) in three irrigation service units (ISU) from 1988 to 1994 in the Muda Irrigation Scheme, Malaysia. Water balance components, crop establishment method, the progress of farming activities, and rice yield of individual farmers and over the whole ISU were monitored. Yields in TP ISU were higher than in wet-seeded (WS) ISU and those in WS ISU higher than in dry-seeded (DS) ISU, but the difference was significant (P<5%) only between TP and DS ISU. Land preparation duration was significantly reduced in DS and WS ISU compared with TP ISU. This led to a significant reduction in irrigation and total water input (rainfall and irrigation) before crop establishment. However, during the crop growth period in the main field, TP ISU had a significantly shorter crop growth duration and total water input than DS and WS ISU. Over the whole crop season, the three crop establishment methods had a similar total water input, but DS ISU had significantly less irrigation water and higher water productivity with respect to irrigation water than WS ISU and TP ISU. This was attributed to the ability of DS rice to capture more rainfall after crop establishment. Site-specific management of WS rice versus TP rice has to be taken into account in assessing their relative advantage for water input and water productivity.

Increasing water-use efficiency in rice production : farm-level perspectives

Abstract This paper describes the components of water use in rice-based production systems and identifies water used during land preparation, and seepage and percolation during crop growth as important sources of water `loss from the system. Strategies for increasing farm-level water-use efficiency are discussed, including the problems of up-scaling from on-farm to system-level water savings.

Response to Salinity in Rice: Comparative Effects of Osmotic and Ionic Stresses

Abstract The Effects Of The Osmotic Component Of Salt Stress On Rice Cultivar Ir64 Were Examined. Treatments Were Four Combinations Of Two Levels Of Osmotic Stress At Two Developmental Stages: Medium- And High-Level Stress Applied At The Vegetative And Reproductive Stages Using Salt (Nacl) And Polyethylene Glycol-6000 (Peg) As Sources Of Osmotic Stress. Both Peg And Nacl Reduced The Total Above Ground Biomass And Delayed Flowering And Maturity, With A Longer Delay Observed With The High-Level Stress. The Reduction In Number Of Filled Spikelets, 1,000-Grain Weight, And Hence Grain Yield Was Significantly Greater When They Were Applied During The Reproductive Stage Than During The Vegetative Stage. The Sodium Concentration In Plant Tissues Also Increased In Plants Treated With Nacl, Indicating That Besides Osmotic Stress, Plants Were Also Subjected To Ionic Stress. Treatment With Nacl Decreased The Potassium Concentration In Plant Tissues But Did Not Cause Significant Differences In Phenology, Biomass Accumulation, Yield Or N Uptake Compared With Peg. We Concluded That The Response Of Ir64 To Nacl Was Attributed To The Osmotic Component. These Findings May Be Specific To Ir64, Which Has A Medium Tolerance To Salinity Stress. Further Studies Are Needed With Longer Stress Durations To Achieve A Higher Na+ Concentration In Plant Tissues In Several Varieties With Contrasting Tolerance To Salt Stress To Further Establish The Relative Importance Of Osmotic Versus Ionic Components Of Salt Stress In Rice.

Water use efficiency of flooded rice fields II. Percolation and seepage losses

The concept of a constant seepage and percolation (SP) rate in monitoring the water balance of flooded rice fields, as often used in e.g. irrigation system design and management, was investigated. First, magnitude and variability of percolation rate were studied for different combinations of soil-hydraulic properties and hydrologic conditions using the validated water balance model SAWAH. Percolation losses from fields with relatively low subsoil permeability (ks,sub < 10−1 cm d−1) are either limited by a poorly permeable plow sole (ks, top < 10−2 cm d−1) or by the low hydraulic conductivity of the subsoil itself. Typical percolation losses of 0–0.5 and 1–1.5 cm·d−1 respectively are hardly affected by ponded water depth, subsurface water content and depth of ground water table. Percolation losses from fields with relatively high subsoil permeability (ks,sub101 cm·d−1) may vary from 0–0.5 cm d−1 with a poorly permeable plow sole, to 1–5 cm·d−1 or more for a relatively permeable plow sole (ks, top10−2 cm d−1). Only in the latter case, percolation rates are largely affected by the depth of ponded water. Next, the constancy of combined SP rates was studied in a field experiment on a permeable subsoil. Simple book-keeping of the water balance using a fixed SP rate proved accurate to predict the depth of ponded water in time in case of a poorly permeable plow sole and a small seepage component. A decision tree was suggested based on soil-hydraulic properties and characteristics of bunds to estimate the magnitude and variation of SP rates, and to decide whether book-keeping with a fixed SP rate is an appropriate tool in monitoring the water balance of paddy fields.