Wastewater irrigation and crop yield:A meta-analysis

WANG Han-jie,Jingjing WANG,Xiaohua YU

1 College of Economics and Management,Southwest University,Chongqing 400715,P.R.China

2 Department of Economics,University of New Mexico,NM 87131,USA

3 Department of Agricultural Economics and Rural Development,University of Goettingen,Goettingen 37073,Germany

Abstract Although wastewater irrigation in agriculture could be a potential adaptation to water scarcity,its effect on crop yield varies in the literature,making it difficult to evaluate its role in global food security comprehensively.Using agronomic experiment data from 62 studies between 1987 and 2021,we employ a meta-analysis to analyze the factors contributing to the heterogeneous effects of wastewater irrigation on crop yield.Our findings can be summarized as (1) the mean yield growth effect of wastewater irrigation is 19.7%;(2) domestic and breeding wastewater irrigation could significantly increase crop yield,while industrial wastewater has a negative effect although not significant;(3) high nutrients concentration of domestic wastewater is significantly positively correlated with crop yield;(4) agronomic experiment designs in terms of field experiment,experiment times,and fertilizer use could contribute to the divergent crop yield effects across the studies;(5) there is a publication bias of the research results between the English and Chinese literature;(6) the literature mainly sheds light on the short-run effect,and the long-run impact shall be an important research question in the future.

Keywords:wastewater irrigation,crop yield,food security,meta-analysis

1.Introduction

Water scarcity will be one of the most significant challenges in the twenty-first century,as more than twothirds of the world population will be living in countries with“stress”water scarcity by 2050.Climate change will further intensify the scarcity (Falcoet al.2019;UNESCO 2020).In company with population growth,irrigation land area continues to expand globally to ensure food security.In the past half-century,the global irrigation area has more than doubled (UNESCO 2017),competing for the limited water resources as more than 69% of all freshwater withdraws is currently used for irrigation.Meanwhile,around 80% of all wastewater is directly discharged into the world’s waterways,where it creates health,environmental and climate-related hazards.More than 50% of the global population still depends on polluted water sources for various uses (UNESCO 2003;IWA 2018).For instance,wastewater is widely used for irrigation,particularly in developing countries,which is a compromised measure for tackling water scarcity(Drechsel and Alexandra 2010).

In many parts of the world,especially in developing countries such as China,India,and Mexico,wastewater irrigation is prevalent for crop production.Roughly,it is estimated that 20 million hectares of cropland are irrigated with wastewater globally,accounting for nearly 10% of the irrigated agricultural land (Goyal 2016).A recent study by Theboet al.(2017) further points out that the area of wastewater irrigated cropland worldwide is at least 50%more than previously estimated.Although wastewater irrigation,particularly industrial sewage without processes,could potentially bring health and environmental problem,it has several benefits for crop production.On the one hand,most importantly,wastewater irrigation could reduce water scarcity to some extent,especially in water-stressed countries.With the rapid development of urbanization and industrialization,a large volume of wastewater in terms of domestic,municipal,and industrial sewage,provides an alternative water supply and a lowcost choice for farmers.On the other hand,wastewater irrigation is economically valuable for farmers as it is a cheap replacement for artificial fertilizer.The wastewater components include many kinds of valuable nutrients for crop production,such as potassium,nitrogen,and phosphorous.For this reason,wastewater irrigation can effectively increase crop irrigation supplies and result in environmental improvement regarding the reduction of artificial fertilizer use.

Nevertheless,although a strand of literature has identified the advantages of wastewater irrigation,the effect of wastewater irrigation on crop production remains controversial.For instance,several studies argue that wastewater irrigation could significantly promote crop production because wastewater contains many nutrient components (e.g.,Salakinkop and Hunshal 2014;Mojidet al.2016;Tranet al.2019).However,experiments such as those conducted by Begumet al.(2011) and Rinaldiet al.(2003) offer contradictory findings that wastewater irrigation has a significant negative effect on crop production.Such a considerable divergence makes it difficult for policymakers to assess the actual value of wastewater irrigation policies regarding wastewater irrigation’s contribution to food security.

To date,why such a significant divergence exists in the current literature is still unanswered.In light of this,a central issue of this study is to clearly explain the variation of results regarding the effect of wastewater irrigation on crop production.We conduct a metaanalysis,a quantitative literature analysis that combines the results of many studies,which reveals the factors that drive the heterogeneous results.This study could help better understand the effect of wastewater irrigation on crop production,which is particularly important for policymakers to make sustainable agricultural irrigation policies to ensure future food security.

The rest of the paper is structured as follows:Section 2 introduces the background and summarizes the related literature.Section 3 presents the data,variables,and estimation strategy for the meta-analysis.Section 4 presents the empirical results and discussions.Finally,the paper ends with Section 5,drawing conclusions of our analysis and offering some policy implications.

2.Background and literature

2.1.Recent wastewater irrigation trends

Irrigation is vital for securing food supplies worldwide.According to the World Bank,agriculture accounts for over 70% of freshwater consumption (https://www.worldbank.org/en/topic/water-in-agriculture).In the past decades,wastewater irrigation in crop production has been far more widespread than previously estimated (Theboet al.2017).Wastewater irrigation plays a vital role in water reuse,particularly alleviating water scarcity pressure in arid and semi-arid areas.Accordingly,wastewater reuse has become an important water source for agricultural irrigation (Zhang and Shen 2019;UNESCO 2020).Nearly 45% of municipal wastewater is reused for agricultural irrigation (Bixioet al.2005) and 47% of reused water is for agricultural irrigation globally (IWA 2018).

Given the vital role of wastewater irrigation,several international organizations,such as FAO (the Food and Agriculture Organization of the United Nations)and WHO (the World Health Organization),have published a series of guidelines regarding the safe use of wastewater in irrigation for human health and environmental protection.In practice,wastewater irrigation regulations develop differently worldwide due to different levels of economic development and water resource.Generally,most developed countries have strict regulations for wastewater irrigation,while developing countries,especially in Africa,have lax or no regulations for wastewater irrigation.In developed countries,wastewater irrigation is a strategy to improve the environment and sustainable agricultural production.As such,the process of wastewater treatment has a high ecological standard.However,raw wastewater irrigation is still widespread in most developing countries,as they concern more about water shortage in agricultural production.Overall,wastewater irrigation has proven to be an effective adaption to water scarcity in crop production.It is estimated that the water resources for more than 10% of irrigated cropland are from wastewater(Drechsel and Alexandra 2010;Goyal 2016).

2.2.Effect of wastewater irrigation on crop yield

A much-debated question among the existing literature is whether wastewater irrigation significantly increases crop yield.Although some agronomic experiments have been conducted to analyze the effect of wastewater irrigation on crop yield,the conclusions are somewhat divergent.Most of the studies conclude that wastewater irrigation positively affect crop yield.For instance,Mojidet al.(2016) find domestic wastewater irrigation significantly increases wheat yield compared with groundwater irrigation.Similarly,current literature points out that such an effect is significantly positive in corn and rice yield(e.g.,Tsadilas and Vakalis 2003;Junget al.2014;Tranet al.2019).However,although the effect of wastewater irrigation on crop yield is significantly positive,yield increase varies considerably,ranging from 2% to ten times.

On the contrary,experiments from other studies suggest that wastewater irrigation cannot increase crop yield.Rinaldiet al.(2003) indicate that industrial wastewater irrigation could decrease wheat yield.Also,the agronomic experiments by Huanget al.(2006) and Huet al.(2009) find the same conclusion by using domestic wastewater irrigation.Although most of the experiments mainly focus on domestic and industrial wastewater irrigation,a recent study by Almeidaet al.(2018) provides evidence that breeding wastewater significantly increases crop yield.Likewise,the negative effect on crop yield ranges from 1 to 50%.Additionally,a strand of literature further indicates that the effect of wastewater irrigation on crop yield is insignificant (Chakrabarti and Chakrabarti 1998;Juet al.2010;Sunet al.2015;Zhang 2016).

As is evident above,the effect of wastewater irrigation on crop yield varies.What causes divergent conclusions across studies? One possible explanation may be that wastewater quality is different across experiments.Wastewater investigated in previous studies mainly includes domestic,industrial,and breeding wastewater whose distinct components might have caused different effects on crop yield.In addition,the concentration of wastewater might lead to the gap of crop yield related to the nutrients of wastewater.Determining the factors of wastewater affecting crop yield is critical for policymaking.The following section describes the meta-analysis design to answer this question.

Meta-analysis has been widely used to identify the factors affecting the preliminary research results in the literature (Tian and Yu 2012;Chenet al.2016;Yuet al.2016).Nelson and Kennedy (2009) and Tian and Yu(2012) summarized the factors into five categories:(1)characteristics of the results in primary studies (e.g.,real or nominal,marginal or average);(2) characteristics of the analysts (e.g.,published or unpublished);(3)characteristics of the research (e.g.,geographic region or wastewater types);(4) evaluation characteristics (e.g.,ex-post or ex-ante,field or experimental data);and (5)random measurement errors.We collected 62 papers on studying the yield effect of wastewater irrigation and quantitatively identified the heterogeneous effects.

3.Data and methods

3.1.Data

The effect of wastewater irrigation on crop yield has been well studied by many researchers using agronomic experiments or field data since the 1980s.Hence,the data used in this study are taken from the existing literature,covering 154 agronomic experiments results drawn from 62 studies between 1987 and 2021.We collected related literature using Google Scholar and China National Knowledge Infrastructure (CNKI).The detailed information of all primary studies is listed in Appendix A.

3.2.Variables and descriptive statistics

Regarding the variables involved in this study,Table 1 presents their detailed definition,while Table 2 shows the results of the descriptive statistics.

Crop yieldThe outcome variable for meta-analysis in this study is crop yield.The units of crop yield are different across the studies,which makes it challenging to capture a comparable effect of wastewater irrigation on crop yield.One reasonable way to make the experiment results more comparable is to calculate the percentage change in crop yield.More specifically,the percentage change in crop yield between the treatment group (wastewater irrigation)and the control group is used.As shown in Table 2,the average percentage change in crop yield is 19.73%.However,the range is extensive,with the maximum value being 1 089.00%,while the minimum being -50%.It highlights the necessity to clearly explain the divergence of crop yield effect to provide valuable information for crop irrigation policymaking.Fig.1 also depicts the density of crop yield effects.

Fig.1 Crop yield effect.

Wastewater typeWastewater type is used as one of the explanatory variables.Theoretically,wastewater type might affect crop yield due to the differences in nutrient components.In general,municipal wastewater mainly includes domestic wastewater and industrial wastewater.Domestic wastewater is the effluent produced from daily human activities in households,while industrial wastewater is the effluent discharged by industrial activities.Compared with domestic wastewater,industrial wastewater contains more heavy metals,including mercury,lead,chromium,copper,nickel,and zinc.In order to reveal the heterogeneous effect on crop yield from different kinds of wastewater,we classified wastewater into three types based on the 154 experiments:domestic wastewater,industrial wastewater,and breeding wastewater.Table 2 indicates that domestic wastewater ranks first across studies,accounting for 65.8%,while industrial wastewater and breeding wastewater consist of 26.3 and 16.4%,respectively.

Table 1 Definition of variables

Table 2 Descriptive statistics of variables

Wastewater concentrationAs discussed above,benefitting from the nutrients contained in wastewater,wastewater irrigation has played an increasingly important role in crop production.The nutrients in terms of potassium,nitrogen,and phosphorous are essential to crop yield,suggesting that wastewater concentration might affect crop yield.Thus,we further controlled the wastewater concentration variable.Specifically,some agronomic experiments mixed wastewater and clean water to adjust the wastewater concentration1For instance,if the experiment mixes the wastewater and clean water in a ratio of 1 to 1,the wastewater concentration goes down to 50%..Table 2 shows that the average concentration of wastewater is approximately 80.6%.The maximum concentration is 100%,while the minimum is 20.0%.

Crop typeThe growth conditions for crops regarding the required nutrients are different to some extent (Maskinaet al.1988).As a result,the effect of wastewater irrigation on crop yield might vary among different crops.Table 2 reveals that crops selected in agronomic experiments vary across studies.Nearly half of the experiments study the effect of wastewater irrigation on wheat,while corn and rice account for 21.1 and 26.3%,respectively.The meta-analysis sets soybean as the reference group.

Experiment designThe experimental design may affect the results.Thus,this study controls the following variables:(1) the control group for wastewater is clean water;(2) the location of agronomic experiments includes field experiments and laboratory experiments;(3) the conducted year of the experiment;(4) number of replicates of the experiment;and (5) the use of fertilizers.Table 2 suggests that the experimental design varies across the studies,with 74.3% being in the field,the average trials of experiments being 2.76,and half using fertilizers for crops.

Journal characteristicsPublication bias is one of the vital concerns for meta-analysis (Tian and Yu 2002).For instance,it is easier for more significant results to be published.Accordingly,the characteristics of the journal might contribute to the divergent conclusions.To control for the publication bias,we used two dummy variables to measure the characteristics of the journal.The first one is whether the journal is peer-reviewed,while the second one is whether the journal is published in English.Table 2 shows that 93.4% of the studies are peer-reviewed and approximately half of the studies are published in English,with the rest in Chinese.

3.3.Estimation strategy

Meta-analysis is a quantitative method widely used in the economics literature and aims to explain the heterogeneous findings from similarly related primary studies.This study uses the meta-regression to conduct a meta-analysis,with the outcome variable being the effect of wastewater irrigation on crop yield (percentage of crop yield growth) and the explanatory variables covering factors that might lead to the heterogeneous effect on crop yields such as wastewater type,wastewater concentration,crop type,experiment design,and journal characteristics.

Notably,one of the main challenges for meta-regression is the heteroskedasticity problem,including samples size heterogeneity or heteroskedasticity of effect-size variances.This study employs the weighted least squares regression(WLS) by taking sample sizes as the weights to tackle the heteroskedasticity problem as the sample size is correlated with the variance (Tian and Yu 2012).

4.Results and discussion

4.1.Meta-analysis results

We pooled all the 154 results from 62 primary researches and employed the WLS econometric approach as it performs better than the OLS in the case of heteroskedasticity.Table 3 reports the full-sample results.

First of all,as shown in Table 3,column (1) reveals that the coefficient of wastewater is significantly positive.This conclusion suggests that wastewater could significantly increase crop yield,demonstrating the potential of wastewater irrigation.Particularly,in terms of wastewater type,column (2) indicates that the coefficients of domestic and breeding wastewater are positive and statistically significant,while the coefficient of industrial wastewater is not significant.This result confirms that wastewater type would significantly affect its effect on crop yield,causing divergent findings.Specifically,column (2) shows that crop yields irrigated by domestic and breeding wastewater are,respectively,16.8 and 22.7% higher than that in the clean water control group.

Table 3 Meta-regression with full sample

A possible explanation for the negative but insignificant effect of industrial wastewater might be due to the hazardous components of industrial wastewater.As already mentioned,compared with domestic and breeding wastewater,industrial wastewater,mainly produced bymanufactories,contains more non-degradable heavy metals such as mercury,lead,and chromium.In recent years,more literature concerns the environmental problem severely polluted by heavy metals (Paul 2017;Jacobet al.2018) as it is particularly harmful to soil quality.Consequently,crops irrigated by industrial wastewater cannot increase crop yield.In this scenario,the European Union has limited the reuse of wastewater with heavy metals in agriculture (Tervahautaet al.2014).

The estimated results show that the coefficient of wastewater concentration is significantly positive,indicating that the higher the wastewater concentration is,the higher the crop yield is.This is easy to understand because wastewater contains many nutrients,such as potassium,nitrogen,and phosphorous.Thus,a higher wastewater concentration indicates more abundant nutrients essential to crop growth.

Similarly,the coefficients of corn and wheat are significantly positive,while the coefficient of rice is not significant.It implies that the corn and wheat groups have a higher crop yield than the soybean group.The reason might be that the primary growth conditions regarding water and nutrients might differ for different crops.

Notably,the coefficients of variables related to experiment design are statistically significant,suggesting that different experiment designs contribute to the divergent effect on crop yield.Specifically,the experiments conducted in the field have a lower crop yield.It might be because the field experiments are difficult to provide an ideal environment for crop growth,such as temperature and humility.Besides,more replicates significantly decrease crop yield,suggesting that the effect of wastewater irrigation might be overestimated when there are not enough replicates.Also,the experiments using fertilizers have a lower crop yield.This is easy to understand as several wastewater components are similar to fertilizers.When fertilizers are used together with wastewater,the over-nutrition problem might lead to a decrease in crop yield.

Moreover,studies published in English have higher crop yields than those in Chinese.This inconsistency may be due to regional differences.Authors in China tend to publish significant results in English journals,as they are highly commended in the academic community in China.

4.2.lnteraction effect

Considering that the effect of wastewater concentration might vary across different types of wastewater,we used the interaction terms of wastewater types and concentration to test such an effect.Table 4 shows that only the coefficient of the domestic wastewater interaction term is significant.This implies that the domestic wastewater concentration has a significantly positive effect on crop yield,providing valuable insight into policymaking.

4.3.Robustness check

To ensure the robustness of our empirical results,we further excluded 5% of the most extreme values of the wastewater irrigation effect on crop yield and reran the meta-regression.Table 5 indicates the robustness of theprevious empirical findings as they are consistent with those in Table 3.

Table 4 Meta-regression with interaction effect

Table 5 Meta-regression with subsample excluding 5% of extreme values

4.4.Discussion of potential risk

Although the empirical analysis has substantiated the positive effect of wastewater irrigation on crop yield,the potential risk to human health still exists (e.g.,Drechselet al.2009;Hanjraet al.2012;Becerra-Castroet al.2015).Remarkably,such risk mainly comes from two aspects.On the one hand,wastewater irrigation,such as industrial wastewater,may lead to the heavy metal contamination problem in the soil.Accordingly,crops grown on heavy-metal polluted soil could cause possible health risks to humans (Balkhair and Ashraf 2016).On the other hand,domestic and breeding wastewater might carry some dangerous pathogens,including parasites and bacteria,far beyond the standard of safety irrigation.Such kind of untreated wastewater would harm human health as parasites and bacteria could lead to diseases like cholera and diarrhea.In this scenario,internationally,different regulations and guidelines for wastewater irrigation have been implemented,including FAO (Food and Agriculture Organization of the United Nations),WHO (World Health Organization),the United States EPA (Environmental Protection Agency),and the European Commission.These regulations provide the water quality standards and wastewater treatment processes,which could ensure the safe use of wastewater irrigation.Thus,to achieve the sustainable goal,the regulations of wastewater irrigation should be taken into consideration,especially in developing countries.

5.Conclusion and policy implications

Water scarcity is threatening world food security.Although a strand of literature consistently argues that wastewater irrigation in crop production could be a potential adaptation to water scarcity,the reported effect of wastewater irrigation on crop yield varies across studies,ranging from -50% to 1 089%.This makes it difficult to comprehensively evaluate the role of wastewater irrigation in crop production,which might hinder global food security policymaking.

Using agronomic experiment data from 62 studies between 1987 and 2021,we conducted a meta-analysis to reveal the factors contributing to wastewater irrigation’s divergent crop yield effects.Our results show that both wastewater quality differences and agronomic experiment design could cause a heterogeneous crop yield effect.First,crop yield effects are heterogeneous among different wastewater types.Domestic and breeding wastewater irrigation could significantly increase crop yield,while industrial wastewater has a negative effect,although insignificant.Second,increases in domestic wastewater concentration could significantly promote crop yield.Third,experiment design in terms of field experiment,experiment replicates,and fertilizer use could contribute to the divergent crop yield effects across studies.In addition,heterogeneities also exist between English and Chinese literature.

This study offers new insights into the role of wastewater irrigation in crop production.We explained the heterogeneous crop yield effect of wastewater irrigation,which could help governments make specific wastewater irrigation policies to ensure world food security.Notably,while wastewater irrigation has several advantages,it also causes the health and environmental problems,particularly industrial wastewater.In this scenario,governments need to carry out feasible wastewater treatment even though wastewater could be conducive to alleviating water scarcity.

Finally,a caveat is that the current results in the literature mainly shed light on the short-run effects of wastewater irrigation on crop yields.The long-run impact on soil fertility has not been well studied and should be an important research question in the future.

Acknowledgements

The contribution of Dr.Hanjie Wang (Southwest University,China) is supported by the National Social Science Fund of China (20CSH048,20AZD024 and 21ZDA062),the National Natural Science Foundation of China (71773099),the Humanities and Social Sciences Project Funded by the Ministry of Education,China (21YJC790110),and the Rural Finance Survey of the Ministry of Agriculture and Rural Affairs,China (05190084).

Declaration of competing interest

The authors declare that they have no conflict of interest.

Appendixassociated with this paper is available on http://www.ChinaAgriSci.com/V2/En/appendix.htm