Bachelor’s thesis
Geography, 15 Credits
Rice yields under water-saving irrigation management
A meta-analysis
Amanda Åberg
GG 198 2017
Department of Physical Geography
Preface
This Bachelor’s thesis is Amanda Åberg’s degree project in Geography at the Department of Physical Geography, Stockholm University. The Bachelor’s thesis comprises 15 credits (a half term of full-time studies).
Supervisor has been Stefano Manzoni at the Department of Physical Geography, Stockholm University. Examiner has been Steve Lyon at the Department of Physical Geography, Stockholm University.
The author is responsible for the contents of this thesis.
Stockholm, 16 June 2017
Steffen Holzkämper
Director of studies
Abstract
Water scarcity combined with an increasing world population is creating pressure to develop new methods for producing food using less water. Rice is a staple crop with a very high water demand. This study examined the success in maintaining yields under water-‐
saving irrigation management, including alternate wetting and drying (AWD). A meta-‐
analysis was conducted examining yields under various types of water-‐saving irrigation compared to control plots kept under continuous flooding. The results indicated that yields can indeed be maintained under AWD as long as the field water level during the dry cycles is not allowed to drop below -‐15 cm, or the soil water potential is not allowed to drop below -‐
10 kPa. Yields can likewise be maintained using irrigation intervals of 2 days, but the variability increases. Midseason drainage was not found to affect yield, though non-‐flooded conditions when maintained throughout most of the crop season appeared to be detrimental to yields. Increasingly negative effects on yields were found when increasing the severity of AWD or the length of the drainage periods. Potential benefits and drawbacks of water-‐saving irrigation management with regards to greenhouse gas emissions, soil quality and nutrient losses were discussed to highlight the complexity of the challenges of saving water in rice production.
Keywords: rice, yield, water scarcity, water-‐saving irrigation, WSI, alternate wetting and drying, AWD, soil organic matter, soil organic carbon.
Table of contents
Abbreviations ... 4
1. Introduction ... 5
2. Research questions and problem formulation ... 6
3. Water management in rice farming ... 7
3.1 Field level water flows ... 7
3.2 Field level irrigation management approaches ... 7
3.2.1 Alternate wetting and drying, submergance-‐nonsubmergance and intermittent irrigation ... 8
3.2.2. Saturated soil culture ... 9
3.2.3. Controlled irrigation ... 9
3.2.4. Midseason drainage ... 9
3.3 Influence of water-‐saving irrigation management on rice yields and water savings ... 9
4. Methodology ... 10
4.1. Data collection ... 10
4.2. Data compilation and evaluation ... 11
4.3. Data analysis ... 13
5. Results ... 17
5.1. Yields under WSI management ... 17
5.2. Regional differences in WSI yield ... 18
5.3. Influence of severity of WSI management on yield ... 19
5.4. Nitrogen fertilization effect on yields ... 21
6. Discussion ... 22
6.1. Rice yields under varying irrigation managements ... 22
6.2. Environmental implications of water-‐saving irrigation ... 24
6.2.1. Greenhouse gas emissions and soil quality ... 24
6.2.2. Nutrient and herbicide losses ... 26
6.3. Implementation of water-‐saving irrigation management ... 27
6.4. Methodological and data weaknesses ... 27
6.5. Future research ... 28
7. Conclusions ... 29
Acknowledgements ... 29
References ... 29
Appendix A ... 34
Abbreviations
ASNS alternate submergance-‐nonsubmergance AWD alternate wetting and drying
CF continuous flooding FWL field water level SOC soil organic carbon SOM soil organic matter SWP soil water potential WSI water-‐saving irrigation WUE water use effiency
1. Introduction
Rice is a staple food for a large number of the human population and constitutes the largest food source as well as a significant income for inhabitants of developing countries (GRiSP, 2013). Worldwide rice-‐farming environments are oftentimes divided into four types:
lowland irrigated rice, lowland rainfed rice, flood-‐prone rice and upland rice. Of these environments, the irrigated lowlands, covering approximately 93 million hectares of land, produce 75% of the total world rice production (Bouman et al., 2007; GRiSP, 2013). Rice receives about two to three times more water at the field level than most other crops, and these irrigated rice environments consume approximately 24-‐30% of the world's freshwater withdrawals. Meanwhile, decreases in water resources and declines in water quality are resulting in water scarcity. Combined with increased competition from urban and industrial sectors, this water scarcity poses a threat to the sustainability of rice production. New methods are thus required to deal with the challenges posed by water scarcity (Bouman et al., 2007).
Saving water is rarely a voluntary decision made by farmers. It is more often either an imposed decision made at a higher level or a necessity dictated by physical water scarcity (Bouman et al., 2007). Due to the pivotal role of rice as a staple food for a large part of the world's population, many studies have examined the effect of different irrigation regimes on rice yields (Carrijo et al., 2017). Many of these studies have found a small decrease in yield accompanied by a significant increase in water productivity when water-‐saving irrigation (WSI) methods were used (see e.g. Bouman and Tuong, 2001 for a summary). Other studies found an insignificant difference in yield between continuous flooding and water-‐saving methods (e.g. Cabangon et al., 2001; Belder et al., 2004).
The purpose of this study is to examine patterns in the relationship between rice yield and water management by systematically collating data from a number of studies. Bouman and Tuong published a meta-‐analysis in 2001, examining water-‐saving irrigation at the field level and its impact on yields. Their study provides an excellent opportunity to examine whether yield improvements under WSI management, relative to continuous flooding, have been documented in the 16 years that have passed since. A recently published meta-‐analysis conducted by Carrijo et al. (2017) will serve as a contemporary comparison.
As has been stated by Linquist et al. (2015), though many studies have examined potential benefits of water-‐saving irrigation, the consequences are rarely evaluated concomitantly.
Water management in rice farming has several environmental implications, aside from the challenge of water scarcity. Rice farming emits approximately four times as much
greenhouse gas as wheat or maize and therefore has significant potential in terms of
mitigating agricultural greenhouse gas contributions (Linquist et al., 2012). Reducing the
amount of time the soil is kept under flooded anaerobic conditions has been found to
decrease emissions of the strong greenhouse gas methane. However, the conversion to
aerobic conditions instead leads to increased microbial activity and increased soil organic
matter (SOM) decomposition and CO
2emissions (Sahrawat, 2005; Haque et al., 2016a). SOM
has great importance for soil health and agricultural sustainability. The conversion to more
aerobic conditions may therefore have significant implications for long-‐term soil fertility and
rice farming sustainability. Furthermore, the implementation of water-‐saving irrigation has
also been found to affect nutrient availability in the soil, as well as losses of fertilizers through surface runoff and seepage (Sahrawat, 2005; Yang et al., 2015). Hence, the
implementation of water-‐saving irrigation has many implications that should be considered in addition to the challenges of water scarcity.
2. Research questions and problem formulation
The aim of this project is to examine the relationship between yields and water-‐saving irrigation in rice farming systems using a meta-‐analysis approach. This study will attempt to collate information from multiple studies to examine said relationship. Specifically, this project will attempt to answer the following questions:
• Is rice yield consistently higher under continuous flooding compared to alternate wetting and drying and other water-‐saving forms of irrigation management?
• Can a spatial pattern be discerned, in which water-‐saving irrigation has been more successful in any certain region of Asia?
Through literature studies, some of the environmental implications of employing water-‐
saving irrigation management will also be qualitatively examined and discussed. This study wishes to place water-‐saving irrigation in a larger context by providing a summary of both benefits and drawbacks of its implementation. Whether or not different levels of nitrogen fertilizer input affect the success of water-‐saving irrigation will also be briefly examined.
There are many ways to save water aside from changing irrigation practices, such as proper land preparation and bund construction (Bouman et al., 2007), but these measures are largely outside the scope of this paper. No attempt will be made to quantitatively assess actual water savings, though potential water savings will be briefly discussed. The study will be limited to rice systems in East, South, and Southeast Asia. The relatively large spatial extent of field experiments included in this analysis is partly the result of the need to keep the collection of data objective and systematic. Limiting the spatial extent by using search words such as "Southeast Asia" resulted in a very limited results list. Furthermore, this approach enables an examination of whether the effects of WSI are the same over a range of different environmental conditions.
Due to the necessity of being able to control the water input to implement WSI management, the focus is inevitably placed primarily on irrigated lowlands. In these environments farmers may, depending on the structure of the irrigation system, have the opportunity to influence not only drainage of water from the fields, but also the input of water (Bouman et al., 2007).
Farmers can therefore to a certain degree influence the amount of water-‐stress experienced by plants during the growth period. A focus on these rice systems is deemed suitable for this study since irrigated lowlands are responsible for such a large part of the global rice
production.
3. Water management in rice farming
3.1 Field level water flows
There are various ways in which water can enter and leave a rice field. Inflow occurs through rainfall, irrigation, and capillary rise, and outflow through percolation, seepage underneath bunds, overbund flow, evaporation and transpiration. Transpiration is the only type of outflow that contributes to crop growth and is therefore termed 'productive water use'.
Capillary rise is generally negated by the constant downward flow of percolation in flooded rice fields. In a series of fields, both seepage and overbund flow can contribute to adjoining farmers' fields before draining into ditches or the groundwater. Even after entering the groundwater, this water may remain reusable through pumping (Bouman et al., 2007).
The high water demand for rice differs from dryland crops and is the result of the daily percolation and seepage of water that occurs in flooded rice fields, along with evaporation from exposed water surfaces. The profuse percolation rates over long periods of time have in many places served to locally raise the groundwater surface. In some locations, the
groundwater table is found within 20 cm from the soil surface, and the water is therefore available for direct uptake by the rice roots (Bouman et al., 2007). When the field water level (FWL) is at or above the soil surface and the soil is saturated, such as in flooded paddies, the soil water potential (SWP) near the surface will equal 0 kPa. When the soil is saturated, most of the water is held in large pores where the molecules are not strongly bound by the soil solids and are therefore able to easily move around. As the soil dries, the remaining water is increasingly held in smaller pores closer to the soil solids, where they are more tightly bound and harder for plant roots to extract, This change is measured as an increasingly negative SWP (Brady and Weil, 2008). If there is not enough water available, the rice plant will experience drought stress, expressed, for example, in the closing of stomata and ceasing of transpiration, which can in turn result in yield declines (Bouman et al., 2007).
3.2 Field level irrigation management approaches
Before rice is transplanted or seeded, the field is normally ploughed and puddled under wet conditions (Bouman et al., 2007). Puddling is a type of harrowing or rotavating that helps in controlling weeds, but also reduces soil permeability by destroying soil aggregates and creating a plough pan, usually at a depth of approximately 10 to 20 cm. The hydraulic
conductivity decreases, and therefore also the loss of water through percolation (Arora et al., 2006; Bouman et al., 2007). Following puddling, fields are usually kept flooded before
transplanting for a period ranging from a few days to four weeks, though it has been known to stretch as long as two months in large-‐scale systems. Once transplanted or seeded, the crop is traditionally kept flooded at a depth of 5 to 10 cm until one or two weeks before harvesting. Flooding following crop establishment helps to control weeds and pests
(Bouman et al., 2007). Figure 1 provides an overview of the different growth stages of rice.
Figure 1. Schematic overview of the different growth stages of the rice plant. Adapted from CGIAR, n.d.
For water-‐saving irrigation to be a feasible alternative, losses of water through seepage, percolation and evaporation must be addressed. Efforts can be made during land
preparation by constructing appropriate field channels that enable the control of water levels in individual fields, maintaining good bunds, levelling the field, implementing tillage and minimising the time passing between land preparation and crop establishment (Bouman et al., 2007).
Bouman et al. (2007) describe three types of water-‐saving irrigation; alternate wetting and drying (AWD), saturated soil culture (SSC), and aerobic rice (not covered here,
predominantly used in upland environments). Which form is implemented depends on the type and severity of water scarcity, socioeconomical situation and how much control individual farmers can exercise over their irrigation. The implementation of AWD requires that a farmer can control water levels in their own field, or that a communal effort is made.
With reduced water availability, saturated soil culture may be the first option, followed by AWD and then aerobic rice when faced with severe shortages (Bouman et al., 2007).
3.2.1 Alternate wetting and drying, submergance-‐nonsubmergance and intermittent irrigation
Alternate wetting and drying, sometimes referred to as alternate submergence-‐
nonsubmergance (ASNS) (Belder et al., 2004) or intermittent irrigation (Lin et al., 2012), utilizes cycles of alternating flooded conditions and dry periods when the water is allowed to drop below field level. The length of the dry periods can vary from as little as one day to longer than 10 days (Bouman et al., 2007). Cabangon et al. (2001) state that AWD normally includes a midseason drainage of 10-‐15 days in the late tillering stage, and that the dry cycles between irrigation events are normally kept at lengths of two to four days. In practice, however, the pre-‐designed timing and length of drainages and dry cycles can be difficult to achieve due to the variability of rainfall events. Carrijo et al. (2017) have, in their meta-‐
analysis, chosen to define AWD as any irrigation management that contains a minimum of one single dry cycle with soil conditions below saturation. Their definition differs from those found in most other sources.
AWD primarily reduces water use by lessening the amount lost through seepage and percolation. In terms of practical implementation, the use of a field water tube to monitor water levels is recommended (Bouman et al., 2007; Yang et al., 2017). The field water tube also allows farmers to detect 'hidden' groundwater sources (Lampayan et al., 2015). When the water drops to a depth of -‐15 cm, the field should be re-‐irrigated to a ponded depth of approximately 5 cm. The AWD cycles can be implemented starting a few days after
transplanting, after two to three weeks if weeds are prolific, or following panicle initiation
which occurs around 50 days after sowing (Bouman et al., 2007; Cabangon et al., 2001;
CGIAR, n.d.). Bouman et al. (2001; 2007) state that the timing of dry cycles with regards to growth stages generally has little to no effect on yield, with the exception that ponded water during flowering is required to avoid yield loss. According to Yang et al. (2017), however, different thresholds should be used at different growth stages due to the variable sensitivity of rice at different points in the crop cycle.
The -‐15 cm field water depth is often referred to as 'safe AWD', because it keeps the root zone saturated. The water savings are generally around a modest 15%, but yield loss is avoided, and depending on local conditions farmers can experiment with longer dry cycles (Bouman et al., 2007). Though irrigation in AWD treatments is often scheduled based on FWL, other indicators are also in use, such as SWP thresholds or simply a set number of days following disappearance of previous irrigation from the soil surface.
3.2.2. Saturated soil culture
In saturated soil culture (SSC), irrigation is applied to achieve a water depth of
approximately 1 cm following disappearance of the previous irrigation. The goal is to keep the soil as close to saturation as possible, which requires very frequent irrigation. The practice reduces the hydraulic head, resulting in decreased seepage and percolation (Bouman et al., 2007). Though examples of similar practices can be found in the academic literature, the term 'SSC' was rarely encountered during this study.
3.2.3. Controlled irrigation
The term 'controlled irrigation' is sometimes employed in the literature without a firm definition. When Yang et al. (2013, 2015) and Hou et al. (2012) employ the term, the management regime is described as including irrigation to keep the soil moist. However, graphs presented in their articles show that irrigation has been applied to reach a FWL of 1 to 4 cm in between regular dry cycles, in practice appearing to make the approach very similar to AWD.
3.2.4. Midseason drainage
'Midseason drainage' or 'intermittent drainage' are concomitantly used to describe the practice of draining the rice paddy midseason for an extended period, often lasting for about 30 days. The approach is mainly used as a means to achieve decreased methane emissions (Haque et al., 2016a, b), but has also been used as a water-‐saving measure (Rahman et al., 2013).
3.3 Influence of water-‐saving irrigation management on rice yields and water savings
Based on a number of studies, Bouman et al. (2007) concluded that although AWD has in
some instances been found to increase yield, it more often decreases yield. Bouman and
Tuong (2001) conducted a meta-‐analysis based on 31 field experiments using AWD or SSC
conducted under various conditions. They found that average water savings under SSC
amounted to 23% with small yield reductions of approximately 6%. When SWPs in the root
zone were allowed to drop to -‐10 to -‐30 kPa, however, yield penalties of 10-‐40% were
recorded (Bouman and Tuong, 2001). The variability in results identified in the study is
attributed to soil and hydrological conditions and the varying length of dry periods in
different experiments (Bouman et al., 2007). Carrijo et al. (2017) found a similar water use reduction of 23.4% under AWD across a selection of 56 studies, but with SWPs maintained above -‐20 kPa or FWL above -‐15 cm, with no yield penalty.
Commenting on a several studies conducted in areas with shallow groundwater tables and fine textured soils, Bouman et al. (2007) concluded that the nearness of groundwater to the field level meant that the root zone remained saturated, supplying a hidden water source. A 15-‐30% lower water input could therefore be achieved without a significant penalty to rice yield. Where the groundwater table is very high and within reach of the roots, potentially negative effects of water-‐saving irrigation can be mitigated, and yields in relation to
irrigation can therefore appear superficially high. The water savings in these environments are relatively small due to the losses already being low when using continuous flooding (CF) under such conditions. A number of studies conducted in loamier soils with deep
groundwater tables presented higher water savings, exceeding 50%, but heavy yield penalties in excess of 20% (Bouman et al., 2007).
Ye et al. (2013) draws on a number of studies to reason that modern rice varieties have been adapted to semi-‐aquatic conditions with only intermittent flooding. The aerated conditions assist in SOM mineralization and inhibition of N immobilization, promoting nutrient release and favouring good yields. Furthermore, based on recently conducted studies Yang et al.
(2017) draw the conclusion that AWD within certain limits can increase yield by reducing redundant vegetative growth, elevating hormonal levels, improving canopy structure and root growth and enhancing carbon remobilization from vegetative tissues to grains.
Yields have been found to increase in China under AWD, and decrease in tropical locations such as India and the Philippines; a difference that Belder et al. (2004) and Cabangon et al.
(2004) reason may be the result of variable WSI practices, soil properties, groundwater depths, rice variety and crop management. AWD and other forms of WSI have been widely adopted in China where per capita fresh water availability is amongst the lowest in Asia, and is being recommended in parts of India and the Philippines (Cabangon et al., 2001; Bouman et al., 2007; Yang et al., 2013).
4. Methodology
4.1. Data collection
Meta-‐analyses provide a tool for examining the results of studies in the context of other studies (Borenstein et al., 2009), and has been used for purposes similar to those presented in this paper by e.g. Bouman and Tuong (2001) and Carrijo et al. (2017). In this study, a search of published studies was conducted to obtain raw data on rice yield, water management method, N fertilizer input, water input, soil organic carbon or soil organic matter (SOC/SOM), crop duration, number of dry cycles, rice variety, soil texture and/or classification, some climatic variables, and whether the crop was transplanted or direct-‐
seeded, creating a varied dataset with potential for many applications.
The article search was conducted in the Web of Science database, using the search term combinations "rice yield" AND water management AND irrigation, and "rice yield" AND water AND flood*. The abstracts of all results produced through these searches were
examined, and those deemed likely to contain relevant information were obtained for more detailed study. Specific criteria considered relevant for inclusion into this study included the studies being original research based on field experiments conducted in East, Southeast or South Asia, and containing quantitative data on rice yields and information about water management methods, of which at least one had to be continuous flooding. Various WSI types have been included in the study, but in each case the irrigation approach had to be paired with a control in the form of aforementioned continuous flooding, where all other factors but water management were the same. The WSI treatment had to have a minimum of either one extended dry period, which should be more significant than the ~10-‐day drainage during tillering that is recommended in some locations for optimal yields under CF
management (see e.g. Yang et al., 2013; 2015), or multiple shorter cycles where FWL was allowed to drop below the soil surface. The work process for the searches is visualized in figure 2.
Figure 2. Flowchart describing the stages of the data collection process.
The terms 'AWD' or 'alternate wetting and drying' could not be used during the data collection process, as many other terms are often employed for similar water-‐saving irrigation techniques that are likely to be relevant for the purpose of this meta-‐analysis.
Examples of these terms include 'alternate submergence-‐nonsubmergence', 'intermittent irrigation', and 'controlled irrigation'.
The environmental implications of water-‐saving irrigation management were qualitatively assessed based on a literature review, and the findings are summarized and discussed in section 6.2. The review is mainly based on articles encountered during the data collection for the meta-‐analysis and is not intended to be exhaustive. Rather, the goal is to highlight the complexity of the interactions that are affected by WSI management.
4.2. Data compilation and evaluation
The data was compiled in Microsoft Excel, wherein all the analyses were conducted. Many of
the included studies placed primary focus on issues such as greenhouse gas emissions or
identifying optimal fertilizer regimes, but were oftentimes useful for providing the quantitative data needed for this analysis. Data on irrigation management and yield was frequently presented despite any differences having been deemed to be statistically
insignificant by the author(s), due to the irrigation data simply being complementary to the main focus of the study.
Belder et al. (2007) promised to contain valuable data, but the focus was placed on
simulation using the ORYZA2000 model. For this reason, an additional search was made to acquire the original field data, which was then used in the meta-‐analysis (i.e. Belder et al., 2004). A few of the articles generated by the search were found to contain data based on the same set of experiments. This was the case with Hou et al. (2012), Xu et al. (2013), Yang et al.
(2013) and Yang et al. (2015). Data was primarily taken from Yang et al. (2013, 2015), and these are therefore the articles that are referred to in the henceforth. A summary of all studies included in the analysis is displayed in table 1.
In some instances, only part of the data came from plots fulfilling the above stated criteria.
Plots that used relevant water management methods but deviated in other management
aspects, thereby invalidating any comparison with a continuous flooding control plot, were
excluded. Data was digitalized in those few cases where it was only presented in graphical
form. A total of 21 articles, equalling 19 original studies, were included in the analysis,
covering 41 sets of comparative field trials, and 179 side-‐by-‐side comparisons of WSI with
CF, in a wide range of locations (figure 3).
Figure 3. Approximate locations of the sites used for field experiments in all studies included in the meta-‐
analysis.
4.3. Data analysis
The collected data was analysed using simple quantitative methods. Due to the variability in field conditions between different experiments, actual yields and water input values are generally not directly comparable across studies (Bouman and Tuong, 2001). For this reason, the relative differences between WSI treatments and corresponding CF treatments have been used. For each study, yield data for every WSI-‐plot (Y
WSI) was normalized by the corresponding CF control plot (Y
CF).
Y
N= Y
WSI/ Y
CF(eq. 1)
Due to the normalization, Y
Nvalues >1 indicate that the yield was higher in the WSI plot compared to the corresponding CF plot, and values <1 indicate that WSI treatment resulted in a decreased yield. The mean normalized yield, used as 'effect size' or alternately
'treatment effect', was calculated for each study, along with the standard deviation, standard
error of the mean and 95% confidence intervals. The mean normalized yield is a simple measure of the effect of specific WSI treatments on yield. The method has weaknesses, but will be used as an indicator in this study. The treatment effect and therefore the differences in yield between WSI and CF were considered significant if the 95% confidence intervals did not overlap the value 1.
A summary effect was calculated for all studies included in the meta-‐analysis. The summary effect is based on the mean normalized yields for all WSI/CF pairs, and not on the mean effect of each study. This approach results in the weight of each study in the summary effect being proportional to the sample size. Using effect sizes has some significant advantages over statistical significance testing. Unlike significance testing, which can only tell us whether the effect is or is not zero and which is also affected by sample size, using effect sizes allows an estimation of the magnitude of that effect (Borenstein et al., 2009). In this study, it means that we can not only tell if WSI management affects the yield, but also how large that effect is, and if certain types of WSI have a greater effect.
Figure 4. Distribution of all normalized yield values from the 19 field studies.
Oftentimes in meta-‐analyses, the log of the normalized yield is the preferred metric (see e.g.
Vico et al., 2016; Carrijo et al., 2017), as the log helps make a skewed distribution of values
more Gaussian, and therefore more suitable for calculating confidence intervals. The effect
size used here is essentially a response ratio, as described by Borenstein et al. (2009), who
also state that the log should be used for all calculations. Both the log and the exponential of
the normalized yields were considered for use in this meta-‐analysis, but did not achieve a
more Gaussian distribution of values than the normalized yields (figure 4) and were
therefore dismissed.
Table 1. Summary of the 19 experiments included in the meta-‐analysis.
Table 1 continued.
5. Results
5.1. Yields under WSI management
Of the 19 studies, eight (1, 5, 7, 9, 10, 15, 18, 19) showed a significant difference in yield between the WSI and CF treatments (not including 6 and 16 that lacked confidence intervals) (figure 5). Of these eight studies, seven displayed a significant decline in yields under WSI treatment, and only one (18) showed an increase in yields under WSI
management. Of the remaining 11 studies, where the differences were not considered significant due to confidence intervals overlapping with 1, seven had a treatment effect below 1, potentially indicating a tendency toward decreased yields. Three lay above 1, and one had a treatment effect of exactly 1. The summary effect lay slightly below 1, indicating a trend of decreased yields under WSI management, and the confidence intervals indicated that this effect was significant.
Figure 5. Treatment effects with 95% confidence intervals for each study, along with the summary effect
size. Studies 6 and 16 only had one side-‐by-‐side comparison of WSI and CF each, and therefore do not have any confidence intervals.
5.2. Regional differences in WSI yield
The scatter plot in figure 6 displays the relationship between WSI and CF yields for each side-‐by-‐side comparison identified in the 19 field studies. The overall distribution appears to align well with the 1:1 line, though scattering is seen both above and below the line. When the yields deviate from the 1:1 line, the deviation tends to be more pronounced in the direction of higher yields under CF. The chart indicates that though yields were oftentimes maintained under WSI, they rarely increased. The values in figure 6 have also been categorized depending on if the field experiment was conducted in East Asia (China, Taiwan, Japan, South Korea), Southeast Asia (Vietnam, Philippines) or South Asia (India). The scattering indicates no obvious pattern in terms of the ability of WSI management to maintain yields in different regions. The highest yields appear to have been achieved in East Asia, but since the various studies' yields are not directly comparable due to varying environmental conditions and management approaches, the actual yields are not reliable values for analysis. Some yields produced in experiments in South Asia appear fictitiously low, with yields below 2 t ha
-‐1. These low yields have been attributed to the rice variety used (Bhaduri, 2017, personal communication).
Figure 6. Relationship between WSI yields and corresponding CF yields in three major
regions in Asia.
Figure 7 is based on the same data as figure 6, but provides the summary effects for the three regions. As expected based on figures 5 and 6, the overall effect of WSI treatment was a decrease in yield, though this effect was not significant for the experiments conducted in South Asia. The summary effect for East Asia was very similar to South Asia, but with lower variability. Southeast Asia displayed a significant decrease in yields under WSI management.
Figure 7. Summary effects for groupings of side-‐by-‐side comparison into regions; South, East and Southeast Asia. Error bars correspond to a 95% confidence interval. The summary effects contain 72 side-‐by-‐side comparisons for South Asia, 48 for Southeast Asia, and 59 for East Asia.
5.3. Influence of severity of WSI management on yield
The different WSI treatments have been classified into a number of categories (figure 8).
The mild, moderate and severe AWD treatments have been grouped with treatments using drainage periods of a maximum of 2, 4, and 7 days, respectively. Figure 8 demonstrates a very close alignment between the regression line for 'mild AWD/<=2-‐
day drainage' and the 1:1 line, indicating that very similar yields were attained in these WSI treatments as compared to corresponding CF treatments. As the severity of the AWD management and the length of the drainage periods increased, the scattering and corresponding regression lines became increasingly displaced from the 1:1 line. The high R
2values for all three categories indicate that the regression lines incorporate much of the variability.
Figure 8. Patterns in the relationship between yield and WSI method used. Mild AWD -‐ SWP
potential >-‐10kPa or FWL >-‐15 cm, moderate AWD -‐ SWP between -‐10 and -‐30 kPa or FWL between -‐15 and -‐30 cm, severe AWD -‐ SWP <-‐30 kPa. Data from studies 7, 8, 11, 13, 16, 17, 19 and parts of study 14 was excluded due to not suiting any of the designated categories.
The category 'mild AWD' corresponds to the safe AWD defined by Bouman et al. (2007), whereby the field water level should stay within -‐15 cm from the surface (figure 8). The -‐15 cm FWL has been paired with a SWP limit of -‐10 kPa, as the SWP normally stays above -‐10 kPa (measured at 15 cm depth) at a FWL of -‐15 cm (Lampayan et al., 2015).
Bouman and Tuong (2001) found that yield decreases often became noticeable at SWPs between -‐10 and -‐30 kPa, and Brady and Weil (2008) have stated that field capacity often corresponds to SWPs ranging from -‐10 to -‐30 kPa. As rice is classified as a semiaquatic plant (GRiSP, 2013) and in the examined lowland settings is most
commonly grown under submerged conditions (Lampayan et al., 2015), SWPs at field capacity have been classified as 'moderate AWD'.
When using only the data from plots that were specifically stated to have been kept under 'mild AWD' conditions, the yields corresponded almost perfectly to those achieved under CF management (figure 9).
Figure 9. AWD treatments specifically stated to have been kept above a SWP of
-‐10kPa and field water level depth of -‐15 cm.
When all categories were examined individually, some additional variability was discovered. Mild AWD and midseason drainage displayed yields on par with CF plots with relatively high precision (figure 10). It is worth noting that the midseason drainage category was based on a small sample made up of seven side-‐by-‐side comparisons.
Yields appeared to have increased under the <=2-‐day drainage treatments, and have been maintained almost on par with CF yields under 4-‐day drainage treatments, though neither of these treatment effects were deemed significant at the 95% level. Significant yield decreases were seen for moderate and severe AWD, as well as for 7-‐day drainage periods. It is clear that the yields achieved under WSI management gradually decreased from mild, to moderate, to severe AWD, as well as when drainage periods were
increased from 2, to 4, to 7-‐day intervals. Likewise, though yields were maintained under midseason drainage treatment, they decreased when the non-‐flooded conditions were maintained throughout the growing season.
Figure 10. Summary effects and 95% confidence intervals for various WSI categories. Mild
AWD -‐ SWP potential >-‐10kPa or FWL >-‐15 cm, moderate AWD -‐ SWP between -‐10 and -‐30 kPa or FWL between 15 and 30 cm, severe AWD -‐ SWP <-‐30 kPa. Data from studies 13 and 17 that lacked the necessary information to categorize the treatments were excluded.
Number of side-‐by-‐side comparisons: mild AWD -‐ 53, moderate AWD -‐ 12, severe AWD -‐ 21,
<=2-‐day drainage -‐ 36, 3 to 4-‐day drainage -‐ 25, 5 to 7-‐day drainage -‐ 6, midseason drainage -‐ 7, non-‐flooded -‐ 7.
5.4. Nitrogen fertilization effect on yields
Plotting yield against nitrogen (N) input showed an overall increase in yield with increasing N inputs, though the variability was large (figure 11). The regression lines indicate that the trend does not differ between CF and WSI management, with both irrigation treatments showing yields increasing at similar rates under increased N input.
Figure 11. Relationship between nitrogen input and yield in all CF and WSI plots. Studies 3,
15 and 19 lacked data on nitrogen and are not represented.
