Assessment of irrigation efficiencies and water productivity in paddy fields in the lower Mekong River Basin

Improving irrigation performance is a crucial issue for agriculture and irrigation development in the Lower Mekong River Basin to secure food production for people’s livelihoods. Irrigation efficiency is the most important indicator to determine the performance of an irrigation scheme. This study looks at water management practices and irrigation efficiency in three pilot sites in the Lower Mekong River Basin: the Numhoum scheme in Laos, the Huay Luang scheme in Thailand, and the Komping Pouy scheme in Cambodia. Irrigation efficiency and water productivity were analyzed using a water balance approach at the irrigation scheme level and results in the pilot areas show efficiencies that are definitely higher using this approach than by using the classical concept. Lower water productivity was observed at pilot schemes in areas of single cropping and higher productivity in areas where multiple agricultural activities were practiced. Strict and active water management is required to control and save water to meet agricultural demand and have sufficient water to expand cultivation areas while avoiding shortages. Promoting multiple uses of water for various agricultural activities in command area will increase water productivity.

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 2000 at 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 4.5 t ha^–1 with 0 kg N ha^–1 to 5.3–8.9 t ha^–1 with farmers N-rates (150 kg N ha^–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 – 20 cm 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.     

Experimental study on the influence of percolation patterns on the removal of soluble elements in stratified alluvial-soil paddy fields for rice

The dynamics of substances in percolating water was investigated using paddy field models planted with rice and fertilized. Percolation patterns mimicking the plow layer and plow sole were set up in these models for closed system percolation. In one of these models, in which the groundwater level was kept high, the percolation pattern of the subsoil layer was set up as closed system percolation. In another model with a lower groundwater level, the percolation pattern of the portions above and below the groundwater level was set up as open and closed system percolation, respectively.The results show that the concentrations of nitrate nitrogen and nitrite nitrogen in percolating water were higher in the upper portion of the subsoil layer with open system percolation than in the plow layer and plow sole with closed system percolation, while these concentrations tended to be lower in the lower portion of the subsoil layer with closed system percolation than in the plow layer and plow sole. Iron and manganese concentrations in the subsoil layer were higher in the portion with closed system percolation than those in the portion with open system percolation. The concentrations of bases such as calcium tended to increase with the increase in iron concentrations in soil water.     

Evaluation and application of ORYZA2000 for irrigation scheduling of puddled transplanted rice in north west India

Water-saving technologies that increase water productivity of rice are urgently needed to help farmers to cope with irrigation water scarcity. This study tested the ability of the ORYZA2000 model to simulate the effects of water management on rice growth, yield, water productivity (WP), components of the water balance, and soil water dynamics in north-west India. The model performed well as indicated by good agreement between simulated and measured values of grain yield, biomass, LAI, water balance components and soil water tension, for irrigation thresholds ranging from continuous flooding (CF) to 70 kPa soil water tension.Using weather data for 40 different rice seasons (1970-2009) at Ludhiana in Punjab, India, the model predicted that there is always some yield penalty when moving from CF to alternate wetting and drying (AWD). With an irrigation threshold of 10 kPa, the average yield penalty was 0.8 t ha⁻¹ (9%) compared with CF, with 65% irrigation water saving, which increased to 79% at 70 kPa with a yield penalty of 25%. The irrigation water saving was primarily due to less drainage beyond the root zone with AWD compared to CF, with only a small reduction in evapotranspiration (ET) (mean 60 mm).There were tradeoffs between yield, irrigation amount and various measures of WP. While yield was maximum with CF, water productivity with respect to ET (WP_ET) was maximum (1.7 g kg⁻¹) for irrigation thresholds of 0 (CF) to 20 kPa, and irrigation water productivity (WP_I) increased to a maximum plateau (1.3 g kg⁻¹) at thresholds ≥30 kPa.Because of the possibility of plant stress at critical stages known to be sensitive to water deficit (panicle initiation (PI) and flowering (FL)), treatments with additional irrigations were superimposed for 2 weeks at one or both of these stages within the 10, 20 and 30 kPa AWD treatments. Ponding for two weeks at FL was more effective in reducing the yield penalty with AWD than ponding at PI, but the biggest improvement was with ponding at both stages. This reduced the average yield loss from 9% (0.8 t ha⁻¹) to 5% (0.5 t ha⁻¹) for AWD with thresholds of 10 and 20 kPa. However, maximum WP_I (1.1 g kg⁻¹) was achieved with an irrigation threshold of 20 kPa combined with more frequent irrigation at FL only, but with a greater yield penalty (8%). Thus the optimum irrigation schedule depends on whether the objective is to maximise yield, WP_ET or WP_I, which depends on whether land or water are most limiting. Furthermore, the optimum irrigation schedule to meet the short term needs of individual farmers may differ from that needed for sustainable water resource management.     

A process model for explaining genotypic and environmental variation in growth and yield of rice based on measured plant N accumulation

The objective of this study was to develop a mechanistic model for simulating the genotypic and environmental variation in rice growth and yield based on measured plant N accumulation. The model calibrations and evaluations were conducted for rice growth and yield data obtained from a cross-locational experiment on 9 genotypes at 7 climatically different locations in Asia. The rough dry grain yield measured in the experiment ranged from 71 to 1044 g m⁻² over the genotypes and locations. An entire process model was developed by integrating sub-models for simulating the processes of leaf area index development, partitioning of nitrogen within plant organs, vegetative biomass growth, spikelet number determination, and yield. The entire process model considered down-regulation of photosynthesis caused by limited capacity for end-product utilization in growing sink organs by representing canopy photosynthetic rate as a function of sugar content per unit leaf nitrogen content. The model well explained the observed genotypic and environmental variation in the dynamics of above-ground biomass growth (for validation dataset, R² = 95), leaf area index development (R² = 0.82) and leaf N content (R² = 0.85), and spikelet number per unit area (R² = 0.67) and rough grain yield (R² = 0.66), simultaneously. The model calibrations for each sub-model and the entire process model against observed data identified 10 genotype-specific model parameters as important traits for determining genotypic differences in the growth attributes. Out of the 10 parameters, 5 were related to the processes of phenological development and spikelet sterility, considered to be major determinants of genotypic adaptability to climate. The other 5 parameters of stomatal conductance, radiation extinction coefficient, nitrogen use efficiency in spikelet differentiation, critical leaf N causing senescence, and potential single grain mass had significant influence on the yield potential of genotypes under given climate conditions.