Comparisons of energy balance and evapotranspiration between flooded and aerobic rice fields in the Philippines

The seasonal and annual variability of sensible heat flux (H), latent heat flux (LE), evapotranspiration (ET), crop coefficient (K_c) and crop water productivity (WP_ET) were investigated under two different rice environments, flooded and aerobic soil conditions, using the eddy covariance (EC) technique during 2008-2009 cropping periods. Since we had only one EC system for monitoring two rice environments, we had to move the system from one location to the other every week. In total, we had to gap-fill an average of 50-60% of the missing weekly data as well as those values rejected by the quality control tests in each rice field in all four cropping seasons. Although the EC method provides a direct measurement of LE, which is the energy used for ET, we needed to correct the values of H and LE to close the energy balance using the Bowen ratio closure method before we used LE to estimate ET. On average, the energy balance closure before correction was 0.72 ± 0.06 and it increased to 0.99 ± 0.01 after correction. The G in both flooded and aerobic fields was very low. Likewise, the energy involved in miscellaneous processes such as photosynthesis, respiration and heat storage in the rice canopy was not taken into consideration.Average for four cropping seasons, flooded rice fields had 19% more LE than aerobic fields whereas aerobic rice fields had 45% more H than flooded fields. This resulted in a lower Bowen ratio in flooded fields (0.14 ± 0.03) than in aerobic fields (0.24 ± 0.01). For our study sites, evapotranspiration was primarily controlled by net radiation. The aerobic rice fields had lower growing season ET rates (3.81 ± 0.21 mm d⁻¹) than the flooded rice fields (4.29 ± 0.23 mm d⁻¹), most probably due to the absence of ponded water and lower leaf area index of aerobic rice. Likewise, the crop coefficient, K_c, of aerobic rice was significantly lower than that of flooded rice. For aerobic rice, K_c values were 0.95 ± 0.01 for the vegetative stage, 1.00 ± 0.01 for the reproductive stage, 0.97 ± 0.04 for the ripening stage and 0.88 ± 0.03 for the fallow period, whereas, for flooded rice, K_c values were 1.04 ± 0.04 for the vegetative stage, 1.11 ± 0.05 for the reproductive stage, 1.04 ± 0.05 for the ripening stage and 0.93 ± 0.06 for the fallow period. The average annual ET was 1301 mm for aerobic rice and 1440 mm for flooded rice. This corresponds to about 11% lower total evapotranspiration in aerobic fields than in flooded fields. However, the crop water productivity (WP_ET) of aerobic rice (0.42 ± 0.03 g grain kg⁻¹ water) was significantly lower than that of flooded rice (1.26 ± 0.26 g grain kg⁻¹ water) because the grain yields of aerobic rice were very low since they were subjected to water stress.The results of this investigation showed significant differences in energy balance and evapotranspiration between flooded and aerobic rice ecosystems. Aerobic rice is one of the promising water-saving technologies being developed to lower the water requirements of the rice crop to address the issues of water scarcity. This information should be taken into consideration in evaluating alternative water-saving technologies for environmentally sustainable rice production systems.     

Water use efficiency and economic feasibility of growing rice and wheat with sprinkler irrigation in the Indus Basin of Pakistan

With a population of more than 150 million, Pakistan cannot meet its need for food, if adequate water is not available for crop production. Per capita water availability has decreased from 5600 m³ in 1947 to 1000 m³ in 2004. Water table has gone down by more than 7 m in most parts of the country. Present need is to identify and adopt measures, that will reduce water use and increase crop production. This study was conducted in farmers’ fields during 2002–2004 to evaluate the water use efficiency and economic viability of sprinkler irrigation system for growing rice and wheat crops. Yields and water use were also measured on adjacent fields irrigated by basin flooding, which were planted with the same crop varieties. Sprinkler irrigation of rice produced 18% more yield, while reducing consumption of water to 35% of that used in the traditional irrigation system. Sprinkler irrigation of wheat resulted in a water use efficiency of 5.21 kg of grain per cubic meter of water used compared to 1.38 kg/m³ in the adjacent flooded basins. Benefit–cost analysis showed that adoption of rain-gun sprinkler irrigation for rice and wheat is a financially viable option for farmers. While these findings show large potentials for improving water use efficiency in crop production they also indicate that a large portion of the water applied in traditional flooded basin irrigation is going to groundwater recharge, which has high value near large cities which draw their water from the aquifer.     

Integrated effect of transplanting date, cultivar and irrigation on yield, water saving and water productivity of rice (Oryza sativa L.) in Indian Punjab: Field and simulation study

Individual effect of different field scale management interventions for water saving in rice viz. changing date of transplanting, cultivar and irrigation schedule on yield, water saving and water productivity is well documented in the literature. However, little is known about their integrated effect. To study that, field experimentation and modeling approach was used. Field experiments were conducted for 2 years (2006 and 2007) at Punjab Agricultural University Farm, Ludhiana on a deep alluvial loamy sand Typic Ustipsamment soils developed under hyper-thermic regime. Treatments included three dates of transplanting (25 May, 10 June and 25 June), two cultivars (PR 118 inbred and RH 257 hybrid) and two irrigation schedules (2-days drainage period and at soil water suction of 16 kPa). The model used was CropSyst, which has already been calibrated for growth (periodic biomass and LAI) of rice and soil water content in two independent experiments. The main findings of the field and simulation studies conducted are compared to any individual, integrated management of transplanting date, cultivar and irrigation, sustained yield (6.3-7.5 t ha⁻¹) and saved substantial amount of water in rice. For example, with two management interventions, i.e. shifting of transplanting date to lower evaporative demand (from 5 May to 25 June) concomitant with growing of short duration hybrid variety (90 days from transplanting to harvest), the total real water saving (wet saving) through reduction in evapotranspiration (ET) was 140 mm, which was almost double than managing the single, i.e. 66 mm by shifting transplanting or 71 mm by growing short duration hybrid variety. Shifting the transplanting date saved water through reduction in soil water evaporation component while growing of short duration variety through reduction in both evaporation and transpiration components of water balance. Managing irrigation water schedule based on soil water suction of 16 kPa at 15-20 cm soil depth, compared to 2-day drainage, did not save water in real (wet saving), however, it resulted into apparent water saving (dry saving). The real crop water productivity (marketable yield/ET) was more by 17% in 25th June transplanted rice than 25th May, 23% in short duration variety than long and 2% in irrigation treatment of 16 kPa soil water suction than 2-days drainage. The corresponding values for the apparent crop water productivity (marketable yield/irrigation water applied) were 16, 20 and 50%, respectively. Pooled experimental data of 2 years showed that with managing irrigation scheduling based on soil water suction of 16 kPa at 15-20 cm soil depth, though 700 mm irrigation water was saved but the associated yield was reduced by 277 kg ha⁻¹.     

InfoCrop: A dynamic simulation model for the assessment of crop yields,losses due to pests,and environmental impact of agro-ecosystems in tropical environments. II. Performance of the model

InfoCrop, a generic crop model, simulates the effects of weather, soils, agronomic management (planting, nitrogen, residues and irrigation) and major pests on crop growth, yield, soil carbon, nitrogen and water, and greenhouse gas emissions. This paper presents results of its evaluation in terms of its validation for rice and wheat crops in contrasting agro-environments of tropics, sensitivity to the key inputs, and also illustrates two typical applications of the model. Eleven diverse field experiments, having treatments of location, seasons, varieties, nitrogen management, organic matter, irrigation, and multiple pest incidences were used for validation. Grain yields in these experiments varied from 2.8 to 7.2 ton ha⁻¹ in rice and from 3.6 to 5.5 ton ha⁻¹ in wheat. The results indicated that the model was generally able to explain the differences in biomass, grain yield, emissions of carbon dioxide, methane and nitrous oxides, and long-term trends in soil organic carbon, in diverse agro-environments. The losses in dry matter and grain yield due to different pests and their populations were also explained satisfactorily. There were some discrepancies in the simulated emission of these gases during first few days after sowing/transplanting possibly because of the absence of tillage effects in the model. The sensitivity of the model to change in ambient temperature, crop duration and pest incidence was similar to the available field knowledge. The application of the model to quantify multiple pests damage through iso-loss curves is demonstrated. Another application illustrated is the use of InfoCrop for analyzing the trade-offs between increasing crop production, agronomic management strategies, and their global warming potential.     

Yield gap of rainfed rice in farmers’ fields in Central Java, Indonesia

Yield constraint analysis for rainfed rice at a research station gives insight into the relative role of occurring yield-limiting factors. However, soil nutrient status and water conditions along toposequences in rainfed farmers’ fields may differ from those at the research station. Therefore, yield constraints need to be analyzed in farmers’ fields in order to design management strategies to increase yield and yield stability.We applied production ecological concepts to analyze yield-limiting factors (water, N) on rice yields along toposequences in farmers’ fields using data from on-farm experiments conducted in 2000-2002 in Indonesia. Potential, water-limited, and N-limited yields were simulated using the ORYZA2000 crop growth model. Farmers’ fields showed large spatial and temporal variation in hydrology (354-1235 mm seasonal rainfall, −150 to 50 cm field-water depth) and fertilizer doses (76-166 N, 0-45 P, and 0-51 kg K ha⁻¹). Farmers’ yields ranged from 0.32 to 5.88 Mg ha⁻¹. The range in yield gap caused by water limitations was 0-28% and that caused by N limitations 35-63%, with large temporal and spatial variability.The relative limitations of water and N in farmers’ fields varied strongly among villages in rainfed rice areas and toposequence positions, with yield gaps due to water and N at the top and upper middle positions higher than at the lower middle and bottom toposequence positions, and yield gaps in late wet seasons higher than those in early wet seasons. Management options (e.g. crop establishment dates, shortening turnaround time, using varieties with shorter duration, supplemental irrigation) to help the late-season crop escape, or minimize the negative effects of, late-season droughts and supplying adequate N-fertilizer are important for increasing yield in rainfed lowland rice in Indonesia. More N-fertilizer should be given to upper toposequence positions than to lower positions because the former had a lower indigenous nutrient supply and hence a better response to N-fertilizer inputs. Systems approaches using production ecological concepts can be applied in yield constraint analysis for indentifying management strategies to increase yield and yield stability in farmers’ fields in other rainfed lowland areas.     

Integrating remote sensing, census and weather data for an assessment of rice yield, water consumption and water productivity in the Indo-Gangetic river basin

Crop consumptive water use and productivity are key elements to understand basin water management performance. This article presents a simplified approach to map rice (Oryza sativa L.) water consumption, yield, and water productivity (WP) in the Indo-Gangetic Basin (IGB) by combining remotely sensed imagery, national census and meteorological data. The statistical rice cropped area and production data were synthesized to calculate district-level land productivity, which is then further extrapolated to pixel-level values using MODIS NDVI product based on a crop dominance map. The water consumption by actual evapotranspiration is estimated with Simplified Surface Energy Balance (SSEB) model taking meteorological data and MODIS land surface temperature products as inputs. WP maps are then generated by dividing the rice productivity map with the seasonal actual evapotranspiration (ET) map. The average rice yields for Pakistan, India, Nepal and Bangladesh in the basin are 2.60, 2.53, 3.54 and 2.75 tons/ha, respectively. The average rice ET is 416 mm, accounting for only 68.2% of potential ET. The average WP of rice is 0.74 kg/m³. The WP generally varies with the trends of yield variation. A comparative analysis of ET, yield, rainfall and WP maps indicates greater scope for improvement of the downstream areas of the Ganges basin. The method proposed is simple, with satisfactory accuracy, and can be easily applied elsewhere.     

Performance of temperate aerobic rice under different water regimes in North China

Since the late 1990s, aerobic rice varieties have been released to farmers in the North China Plain to grow rice as a supplementary-irrigated upland crop to cope with water scarcity. Little is known about their yield potential, water use, water productivity (WP), and flood tolerance. In 2001–2002, experiments with aerobic rice varieties HD502 and HD297 and lowland rice variety JD305 were conducted under aerobic and flooded conditions. Under aerobic conditions, five irrigation treatments were implemented. Under flooded conditions, JD305 yielded up to 8.8 t ha⁻¹, HD502 up to 6.8 t ha⁻¹, and HD297 up to 5.4 t ha⁻¹. Under aerobic conditions, the aerobic varieties yielded higher than the lowland variety. HD502 produced 3–3.5 t ha⁻¹ with 450–500 mm total water input and 5.3–5.7 t ha⁻¹ with 650 mm water input and more. HD297 produced 3–3.5 t ha⁻¹ with 450–500 mm total water input and 4.7–5.3 t ha⁻¹ with 650 mm water input and more. The water productivity of aerobic rice under aerobic conditions was higher or on a par with that of the lowland variety under flooded conditions, reaching values of 0.6–0.8 g grain kg⁻¹ water. The relatively high yields of the aerobic varieties under aerobic soil conditions were obtained under “harsh” conditions for growing rice. The soil contained more than 80% sand, was permeable, and held water above field capacity for a few hours after irrigation only. The groundwater table was deeper than 20 m, the soil moisture content in the rootzone was mostly between 50 and 80% of saturation, and soil moisture tension went up to 90 kPa. We conclude that the aerobic rice varieties HD502 and HD297 are suitable for water-scarce environments, and can stand being periodically flooded.     

Drip irrigation of tea (Camellia sinensis L.): 1. Yield and crop water productivity responses to irrigation

The effects of drip irrigation on the yield and crop water productivity responses of four tea (Camellia sinensis (L.) O. Kuntze) clones were studied four consecutive years (2003/2004-2006/2007), in a large (9 ha) field experiment comprising of six drip irrigation treatments (labelled: I₁-I₆) and four clones (TRFCA PC81, AHP S15/10, BBK35 and BBT207) planted at a spacing of 1.20 m × 0.60 m at Kibena Tea Limited (KTL), Njombe in the Southern Tanzania in a situation of limited water availability. Each clone × drip irrigation treatment combination was replicated six times in a completely randomized design with 144 net plots each with an area of 72 m². Clone TRFCA PC81 gave the highest yields (range: 5920-6850 kg dried tea ha⁻¹) followed by clones BBT207 (5010-5940 kg dried tea ha⁻¹), AHP S15/10 (4230-5450 kg dried tea ha⁻¹) and BBK35 (3410-4390 kg dried tea ha⁻¹) and drip irrigation treatment I₂ gave the highest yields, ranging from 4954 to 6072 kg dried tea ha⁻¹) compared with those from other treatments (4113-5868 kg dried tea ha⁻¹). Most of these yields exceeded those (4200 kg dried tea ha⁻¹) obtained from overhead sprinkler irrigation system in Mufindi also Southern Tanzania, and Kibena Estate itself. Results showed that drip irrigation of tea not only increased yields but also gave water saving benefits of up to 50% from application of 50% less water to remove the cumulative soil water deficit (treatment I₂), and with labour saving of 85% for irrigation. The yield of dried tea per mm depth of water applied, i.e., “the crop water productivity” for drip irrigation of clones TRFCA PC81, BBT207 and BBK35, in 2003/2004 for instance, were 9.3, 8.5 and 7.1 kg dried tea [ha mm]⁻¹, respectively. The corresponding values in 2004/2005 were 2.7, 4.5 and 2.0 kg dried tea [ha mm]⁻¹ while the yield responses from clone AHP S15/10 were linear decreasing by 1 and 1.6 kg dried tea [ha mm]⁻¹ in 2003/2004 and 2004/2005, respectively. In 2005/2006 the crop water productivity from clones TRFCA PC81, AHP S15/10, BBK35 and BBT207 were 4.5, 0.4, 5.2 and 6.9 kg dried tea [ha mm]⁻¹, respectively with quadratic yield response functions to drip irrigation depth of water application. The results are presented and recommendations and implications made for technology-transfer scaling-up for increased use by large and smallholder tea growers.     

Sugarcane growth and yield responses to a 3-month summer flood

Sugarcane (Saccharum spp.) in south Florida is often subjected to flooding due to interacting effects of soil subsidence, pumping restrictions, and tropical storms. While there has been considerable research on the response of sugarcane cultivars to high water tables and periodic flooding, there is a lack of information on commercial cultivar yield response to long-term flooding. An experiment was established in Belle Glade, FL to examine the effect of a 3-month summer flood (July-September) on the growth and yield of cultivars CP 80-1743 and CP 72-2086 during the plant cane (2003) and second ratoon (2005) crop. Harvest samples were taken early-, mid-, and late-season. Flooding sugarcane in the summer caused sequentially greater yield reductions throughout the harvest season in plant cane. Sucrose yields for flooded cane, compared with the non-flooded control, were 9.6 t sucrose ha⁻¹ versus 11.7 t sucrose ha⁻¹ early, 9.2 t sucrose ha⁻¹ versus 12.8 t sucrose ha⁻¹ mid-season and 7.8 t sucrose ha⁻¹ versus 12.3 t sucrose ha⁻¹ at late harvest. In the second ratoon crop, flooding reduced sugarcane tonnage and sucrose yield by 54-64% across sampling dates, and preliminary results indicated that flooding reduced leaf nutrient content by 10-78%. Yield reductions due to flooding in both crops were attributed more to reduced tonnage rather than sucrose content. CP 72-2086 yielded 18-28% greater sucrose than CP 80-1743 when harvested late. However the flood × cultivar interaction was not significant as both cultivars recorded similar yield reductions under flooded conditions. Our results identified severe yield losses caused by a 3-month summer flood in these cultivars, particularly in ratoon crops. Strategies to increase summer on-farm water storage in Florida should focus on short-duration periodic flooding rather than long-term flooding.     

Rice growth, yield and water productivity responses to irrigation scheduling prior to the delayed application of continuous flooding in south-east Australia

The majority of rice grown in south-east Australia is continuously flooded for much of its growing season, but reduced irrigation water availability brought about by a combination of drought and environmental flow legislation has presented a need to maintain (or even increase) rice production with less irrigation water. Delaying the application of continuous flooding until prior to panicle initiation can increase input water productivity by reducing non-beneficial evaporation losses from free water and the soil. A field experiment was conducted over two growing seasons, 2008/9 and 2009/10, comparing a conventional dry seeded treatment (the control - continuous flooding from the 3 leaf stage) with delayed continuous flooding (10-20 days prior to panicle initiation) with several irrigation scheduling treatments prior to flooding commencement. In the first year, the delayed water treatments were irrigated at intervals of 40, 80 and 160 mm of cumulative reference evapotranspiration (ETo) prior to delayed continuous flooding, thereby imposing differing degrees of crop water stress. In year 2, the 80 and 160 mm treatments were modified by use of a crop factor (Kc) when the plants were small and the 40 mm treatment was replaced with a continuously flooded treatment throughout the crop duration.Decreases in net water input (irrigation + rain − surface drainage) and increases in input water productivity were achieved by reducing the flush irrigation frequency during the pre-flood period. Savings of 150 and 230 mm (10 and 15%) were achieved in Year 1 from the 80 and 160 mm cumulative ETo irrigation frequency treatments, respectively, in comparison to the control. In the second year, net water input savings of 230 and 330 mm (15 and 22%) were achieved with the 80/Kc and 160/Kc mm treatments, respectively. Input water productivity of the 160 mm treatment was 0.06 kg/m³ (8%) higher than the control in Year 1, while in Year 2 a 0.15 kg/m³ (17%) increase in input water productivity above the control was achieved by the 160/Kc mm treatment. Delaying the application of continuous flooding in the second year greatly extended the period of crop growth suggesting the need for earlier sowing (by 7-10 days) to ensure pollen microspore still occurs at the best time to minimise yield loss due to cold damage. Nitrogen fertiliser management is an important issue when delaying continuous flooding, and nitrogen losses appeared to increase with the frequency of irrigation prior to continuous flooding. This was likely due to increased denitrification from alternate wetting and drying of the soil. Further research is required to determine the most appropriate nitrogen management strategies, and to also better define the optimal pre-flood irrigation frequency.     

Yield and water productivity of rice as affected by time of transplanting in Punjab, India

Early planting of rice crop during the period of peak evaporative demand results in substantial mining of ground water and threats the sustainability of rice production in Punjab, northwest India. In order to increase yield and water productivity, arrest the mining of ground water, and achieve sustainability of rice production, there is need to adopt water-saving management practices. The present investigation in the Indian Punjab was aimed at investigating the effect of date of transplanting in four rice cultivars varying in growth duration (short-duration RH-257 and PR-115, and medium-duration PR-113 and PAU-201) on yield and water productivity. Delaying in transplanting from 15 June to 25 June or 5 July resulted in reduction in mean grain yield of the four cultivars by 7.2% and 15.9%, respectively. PAU-201, a photoperiod-sensitive cultivar, had higher mean grain yield (7.8 t ha⁻¹) by 14.1%, 12.8% and 11.5% over the photoperiod-insensitive cultivars, PR-113, PR-115 and RH-257, respectively. Irrespective of transplanting dates, short-duration cultivars, RH-257 and PR-115, respectively, resulted in 18.9% and 16.6% saving of water, as compared to medium-duration cultivar PR-113. With delayed transplanting after 15 June, both yield and water productivity decreased for all photoperiod insensitive cultivars, but yields remained statistically similar and water productivity greater for a photoperiod sensitive cultivar. Mean irrigation water productivity (WP_I) was highest for 15 June transplanting (0.66 kg m⁻³) and lowest for 5 July transplanting (0.57 kg m⁻³), and was highest for RH-257 (0.68 kg m⁻³) and lowest for PR-113 (0.50 kg m⁻³). Total water productivity (WP_I+R; irrigation plus rainfall) decreased by 9.1% for 5 July transplanting compared with 15 June transplanting, and was highest for RH-257 (0.49 kg m⁻³) and lowest for PR-113 (0.38 kg m⁻³). Real crop water productivity (WP_ET) of the photoperiod insensitive cultivars decreased (1.10-1.40 kg m⁻³), but that of a photoperiod sensitive cultivar increased (1.63 kg m⁻³), with delayed transplanting. We conclude that substantial amount of water can be saved and yield increased by transplanting short-duration cultivars during the period of peak evaporative demand, or water saved and yield maintained by transplanting a photoperiod-sensitive cultivar late in the season when the evaporative demand is low.     

Can soil bunds increase the production of rain-fed lowland rice in south eastern Tanzania?

Rain-fed lowland rice is by far the most common production system in south eastern Tanzania. Rice is typically cultivated in river valleys and plains on diverse soil types although heavy soil types are preferred as they can retain moisture for a longer period. To assess the effects of soil bunds on the production of rain-fed lowland rice, the crop was cultivated in bunded and non-bunded farmers’ plots under the common agronomic practices in the region, in three successive seasons on Grumic Calcic Vertisols (Pellic). For the three seasons and for the two plot types, crop transpiration was simulated with the BUDGET soil water balance model by using the observed weather data, soil and crop parameters. Comparison between the observed yields and the simulated crop transpiration yielded an exponential relationship with a determination factor of 0.87 and an RMSE of 0.15 tonnes ha⁻¹. With the validated soil water balance model crop yields that can be expected in bunded and non-bunded fields were subsequently simulated for wet, normal and dry years and various environmental conditions. Yield comparison shows that soil bunds can appreciably increase the production of rain-fed lowland rice in south eastern Tanzania in three quarters of the years (wet and normal years) when the soil profile is slow draining (K_SAT equal to or less than 10 mm day⁻¹). In normal years a minimum yield increase of 30% may be expected on those soil types. In wet years and when the soil hardly drains (drainage class of 0–5 mm day⁻¹), the yield may even double. In dry years the yield increase will be most of the time less than 10% except for plots with a percolation rate of 0–5 mm day⁻¹.     

Effect of brackish water irrigation and straw mulching on soil salinity and crop yields under monsoonal climatic conditions

Fresh water shortages are severally restricting sustainable agriculture development in the North China Plain. The scarcity of fresh water has forced farmers to use brackish water from shallow underground sources, which helps to overcome drought and increase crop yields but also increases the risk of soil salinization. To identify safe and effective ways of using brackish water in this region, field experiments were conducted to evaluate the effect of brackish water irrigation and straw mulching on soil salinity and crop yield in a winter wheat-summer maize double cropping system. The experiment was in a split-plot design. Six rates of straw mulching (0, 4.5, 6.0, 7.5, 15.0 and 30.0 Mg/ha) were assigned to the main plots and two irrigation water qualities (i.e. brackish water with salt content of 3.0-5.0 g/L and fresh water with only 1.27 g salt/L) were applied to subplots. The brackish water irrigation significantly increased the salt content at different soil depths in the upper 1 m soil layer during the two growing seasons. Straw mulching affected the vertical distribution of salt in the brackish water irrigation plots and the average salt content of straw mulch treatments (4.5, 6.0, 7.5, 15.0 and 30.0 Mg/ha) within the 0-20, 20-40 and 0-100 cm soil depths was 10.2, 14.0 and 1.8% lower than that without straw mulch (A₀). No salt accumulation occurred to a depth of 1 m in the brackish water irrigation plots and there was no correlation between the value of SAS (salt accumulated in 1 m of soil) and straw mulch rate. In 2000 and 2001, the salt content within the 0-40 cm soil layer in brackish water irrigation plots increased due to high evaporation rates during April-June, and then decreased up to September as salts were leached by rain. For the fresh water irrigation plots, the salt content remained relatively stable. Straw mulching affected the salt content in the 0-40 cm soil layer in brackish water irrigation plots in different periods of 2000 and 2001, but no correlation between salt content and straw mulch rates was observed except in September of 2000. Unlike for wheat, the yield of maize increased as the straw mulch rate increased according to the equation, y = 0.1589x + 5.3432 (R² = 0.6506). Our results would be helpful in adopting brackish water irrigation and straw mulching in ways that enhance crop yields and reduce the risk of soil salinization. However, long-term effects of brackish water irrigation and straw mulching on soil salinity and crop yield need to be further evaluated for sustainability of the system.     

A new look at an old practice: Benefits from soil water accumulation in long fallows under Mediterranean conditions

The practice of long fallowing, by omitting a year of cropping, is gaining renewed focus in the low rainfall zone of the northern agriculture region of Western Australia. The impetus behind this practice change has been a reduced use of pasture breaks in cereal crop rotations, and the belief that a fallow can improve soil water accumulation and thus buffer the negative effects of dry seasons on crop yields. We evaluated the benefits of long fallowing (full stubble retention, no weed growth allowed) in a continuous wheat sequence via simulation modelling with APSIM at two rainfall locations and five soil types. The simulated benefits to long fallowing were attributable to soil water accumulation only, as the effects on soil nitrogen, diseases or weeds were not evaluated.The long-term (100 years) mean wheat yield benefit to fallowing was 0.36-0.43 t/ha in clay, 0.20-0.23 t/ha in sand and loam, and 0-0.03 t/ha in shallow sand and shallow loams. Over the range of seasons simulated the response varied from −0.20 to 3.87 t/ha in the clay and −0.48 to 2.0 t/ha for the other soils. The accumulation of soil water and associated yield benefits occurred in 30-40% of years on better soils and only 10-20% on poorer soils. For the loam soil, the majority of the yield increases occurred when the growing-season (May-September) rainfall following the fallow was low (<210 mm) and the difference in plant available soil water at sowing between fallowed and continuously cropped soil was high (>30 mm), although yield increase did occur with other combinations of growing-season rainfall and soil water. Over several years of a crop sequence involving fallow and wheat, the benefits from long fallowing due to greater soil water accumulation did not offset yield lost from omitting years from crop production, although the coefficient of variation for inter-annual farm grain production was reduced, particularly on clay soils during the 1998-2007 decade of below-average rainfall. We conclude that under future drying climates in Western Australia, fallowing may have a role to play in buffering the effects of enhanced inter-annual variability in rainfall. Investigations are required on the management of fallows, and management of subsequent crops (i.e. sowing earlier and crop density) so as to maximise yield benefits to subsequent crops while maintaining groundcover to prevent soil erosion.     

Modelling of water and nitrogen balances in the ponded water and soil profile of rice fields in Northern Greece

This paper presents a water and nitrogen balance model for the surface ponded water and soil profile system of rice (Oryza sativa L.) fields. The model estimates the daily water balance components, as well as, the daily losses and transformations of nitrogen. Data from two neighbouring rice fields during the growing season of 2005 in the Thessaloniki plain of Northern Greece were used for the application of the model. The data set of field A was used for the calibration of the model, while the data set from the field B for validation of model. Simulation results of total inorganic nitrogen in the soil and runoff water exhibited reasonable agreement with the measured data during calibration and verification of the model. Significant amounts of applied irrigation water were lost through surface runoff and deep percolation into the groundwater. The sum of nitrogen inputs from fertilization, mineralization and irrigation water were 292.7 and 280.4 kg ha⁻¹ for field A and B, respectively. Nitrogen uptake by algae in ponding water and plants was one of the main processes of nitrogen reduction in the rice field systems with an amount of 125.7 and 131.8 kg ha⁻¹ for field A and B, respectively. Leaching through percolated water was the other significant process with 118.3 and 120.8 kg ha⁻¹, respectively. Gaseous losses of nitrogen (via volatilization and denitrification) were also substantial processes of nitrogen reduction in the flooded compartment. The study showed that the simple model presents important results for the water and nitrogen management in rice fields. This information can be used for irrigation water saving and prevention of water resources contamination in rice-based agroecosystems.     

Using vegetation indices from satellite remote sensing to assess corn and soybean response to controlled tile drainage

Controlled tile drainage (CTD) is a management practice designed to retain water and nutrients in the field for crop use. CTD has shown promise for improving water quality and augmenting crop yields but findings are often restricted to field and plot scales. Remote sensing is one of the alternatives to evaluate crop responsiveness to CTD at large spatial scales. This study compared normalized and green normalized difference vegetation indices (NDVI and GNDVI) for corn (Zea mays L.) and soybean (Glycine max L.) among CTD and uncontrolled tile drainage (UCTD) fields in a ∼950 ha experimental watershed setting in Ontario, Canada from 2005 to 2008. The indices were derived from Landsat-5 and SPOT-4 satellite imagery. Log-transformed NDVI and GNDVI for soybean (R3-R6 growth stage) and corn (VT to R5-R6 growth stage) crops were higher significantly (p ≤ 0.05) for CTD, relative to UCTD for 50% (soybean) and 72% (corn) of both the log-transformed NDVI and GNDVI image acquisitions compared; only 17% and 13% were significant (p ≤ 0.05) in the reverse direction (UCTD > CTD). Log-transformed NDVI and GNDVI standard errors for CTD, relative to UCTD fields, were lower for 65% of the significant corn and 71% of the significant soybean NDVI and GNDVI comparisons for the growth stages noted above. This finding suggested overall more uniform crop growth for CTD fields relative to UCTD fields. Observed yields from a subset of commonly managed CTD and UCTD fields in the study area were not significantly different from each other (p > 0.05) with respect to tile drainage management practice; however, 87% of these paired yield comparisons indicated that CTD mean corn/soybean grain yields were greater than or equal to those for UCTD. On average, CTD observed corn and soybean grain yields were 3% and 4%, respectively, greater than those from UCTD. From observed yield and NDVI and GNDVI observations, vegetation indices vs. yield linear regression models were developed to predict grain yields over a broader land base in the experimental watershed area. Here, predicted mean yields were 0.1-11% higher for CTD corn and −5% to 4% higher for CTD soybean, relative to UCTD crops; but results varied between manured and non-manured fertilizer practices. Eighty-nine percent of the standard deviations for these yield predictions were lower for CTD relative to UCTD. The results of this study indicate that at a minimum, CTD did not adversely impact corn and soybean grain yields over the time span and field environments of the study, and based on the weight of evidence presented here, CTD shows general promise for augmenting crop performance. Finally, remote sensing derived vegetation indices such as NDVI and GNDVI can be used to assess the impact of agricultural drainage management practices on crop response and production properties.     

华南地区覆盖旱种水稻节水及其水分利用效率研究

大田试验条件下设置常规水稻(T11)、覆膜旱作水稻(T12)、覆草旱作水稻(T13)和裸地旱作水稻(T14)4个处理,研究旱作水稻的节水效率及水分利用效率。水稻本田期T11用水总量545.40 mm,显著高于各旱作处理水稻,以此为参照,T12、T13和T14节水效率分别为35.1%、22.8%和21.2%。以裸地旱作处理T14为参照,T12节水率17.7%,而T13无明显节水效果。旱作水稻的节水效果主要发生在水稻移栽后前2个月,之后无明显节水效果。各处理以籽粒产量/耗水量为基础计算的水分利用效率(WUE),T12、T13、T14和T11处理分别为1.47、1.31、1.21和1.05 g/kg,以干物质量/耗水量为基础计算的WUE各处理之间的趋势与以籽粒产量/耗水量计算的WUE基本一致。     

Evapotranspiration, crop coefficient and growth of two young pomegranate (Punica granatum L.) varieties under salt stress

Pomegranate (Punica granatum L.) is a drought-hardy crop, suited to arid and semi-arid regions, where the use of marginal water for agriculture is on the rise. The use of saline water in irrigation affects various biochemical processes. For a number of crops, yields have been shown to decrease linearly with evapotranspiration (ET) when grown in salt-stressed environments. In the case of pomegranate, little research has been conducted regarding the effect of salt stress. Our study focused on the responses of ET, crop coefficient (K_c) and growth in pomegranate irrigated with saline water. Experiments were conducted using lysimeters with two varieties of pomegranate, P. granatum L. vars. Wonderful and SP-2. The plants were grown with irrigation water having an electrical conductivity (EC_iw) of 0.8, 1.4, 3.3, 4.8 and 8 dS m⁻¹. Plants were irrigated with 120% of average lysimeter-measured ET. Seasonal variation in ET, crop coefficient (K_c) and growth were recorded. Variation in daily ET was observed 1 month after initiation of the treatments. While significant seasonal ET variation was observed for the EC-0.8 treatment, it remained more stable for the EC-8 treatment. Salinity treatment had a significant effect on both daily ET (F = 131, p < 0.01) and total ET (F = 112.68, p = 0.001). Furthermore, the electrical conductivity of the drainage water (EC_dw) in the EC-8 treatment was five times higher than that of the EC-0.8 treatment in the peak season. Fitting the relative ET (ET_r) to the Maas and Hoffman salinity yield response function showed a 10% decrease in ET per unit increase in electrical conductivity of the saturated paste extract (EC_e) with a threshold of 1 dS m⁻¹. If these parameters hold true in the case of mature pomegranate trees, the pomegranate should be listed as a moderately sensitive crop rather than a moderately tolerant one. Fitting 30-day interval ET_r data to the Maas and Hoffman salinity yield response function showed a reduction in the slope as the season progressed. Thus using a constant slope in various models is questionable when studying crop-salinity interactions. In addition, both of the varieties showed similar responses under salt stress. Moreover, the calculated value of K_c is applicable for irrigation scheduling in young pomegranate orchards using irrigation water with various salinities.     

Estimating yield of sorghum using root zone water balance model and spectral characteristics of crop in a dryland Alfisol

This study investigated the relationship between sorghum grain yield for a range of soil depths, with the seasonal crop water stress index based on relative evapotranspiration deficits and spectral vegetation indices. A root zone water balance model was used to evaluate seasonal soil water fluctuations and actual evapotranspiration within a toposequence; soil depth varied between 30 and 75 cm and available water capacity ranged from 6.9 to 12.6% (v/v, %). An empirical model was used to determine root growth. Runoff was estimated from rainfall data using the curve number techniques of the Soil Conservation Services, combined with a soil water-accounting procedure. The high r² values between modeled and observed values of soil water in the root zone (r² > 0.70, significant at P < 0.001) and runoff (r² = 0.95, significant at P < 0.001) indicated good agreement between the model output and observed values. Canopy reflectance was measured during the entire crop growth period and the following spectral indices were calculated: simple ratio, normalized difference vegetation index (NDVI), green NDVI, perpendicular vegetation index, soil adjusted vegetation index (SAVI) and modified SAVI (MSAVI). All the vegetation indices, except for the perpendicular vegetation index, measured from booting to anthesis stage, were positively correlated with leaf area index (LAI) and yield. The correlation coefficient for spectral indices with dry biomass was relatively less than for LAI and yield. Modified SAVI recorded from booting to milk-grain stage gave the highest average correlation coefficient with grain yield. Additive and multiplicative forms of water-production functions, as well as water stress index calculated from water budget model, were used to predict crop yield. A multiple regression was carried out with yield, for the years 2001–2003, as the dependent variable and MSAVI, from the booting to the milk-grain stage of crop and relative yield values, calculated using both additive and multiplicative water production functions as well as water stress index, as the independent variables. The multiplicative model and MSAVI, recorded during the heading stage of crop growth, gave the highest coefficient of determination (r² = 0.682, significant at P < 0.001). The multiple regression equation was tested for yield data recorded during 2004; the deviation between observed and estimated yields varied from −6.2 to 9.4%. The water budget model, along with spectral vegetation indices, gave satisfactory estimates of sorghum grain yields and appears to be a useful tool to estimate yield as a function of soil depth and available water.     

Improving drought tolerance in rainfed lowland rice: An example from Thailand

A large portion of the world's poor farm in rainfed systems where the water supply is unpredictable and droughts are common. In Thailand there are approximately 6.2 million ha of rain fed lowland rice, which account for 67% of the country's total rice-growing area. This rice system is often characterised by too much and too little water in the same season. Farmers’ estimates of their annual losses to drought are as high as 45% in the upper parts of the toposequence. In contrast to irrigated rice systems, gains from crop improvement of rainfed rice have been modest, in part because there has been little effort to breed and select for drought tolerance for the target rainfed environments. The crop improvement strategy being used in Thailand considers three mechanisms that influence yield in the drought prone targets: yield potential as an important mechanism for mild drought (where yield loss is less than 50%), drought escape (appropriate phenology) and drought tolerance traits of leaf water potential, sterility, flower delay and drought response index for more severe drought conditions. Genotypes are exposed to managed drought environments for selection of drought tolerant genotypes. A marker assisted selection (MAS) scheme has been developed and applied for selection of progenies in the backcrossing program. The plant breeding program uses rapid generation advance techniques that enable early yield testing in the target population of environments (TPE) through inter-station (multi-location yield testing) and on-farm trials. A farmer participatory approach has been used to identify the TPE for the breeding program. Four terrace paddy levels have been identified, upper (drought), middle (drought prone to favorable) and lower (flooded). This paper reports the change in the breeding program for the drought prone rainfed lowland rice environments of North and Northeast Thailand by incorporating our knowledge on adaptation and on response of rice to drought.