Influence of different controlled drainage strategies on the water and salt environment of ditch wetland: A model-based study
Introduction
Direct human activities as well as climate change have resulted in the shrinkage and salinization of large areas of wetlands thoughout the world (Zhang, 2012). These changes pose a serious threat to the health and stability of wetland ecosystems (Jin, 2008; Huckelbridge et al., 2010; Clarke, 2015). Agriculture is the key driver of many of these changes (Defra, 2004). In particular, drainage from farmland is the main way that agricultural nonpoint source pollutants enter wetlands and other waters. Gulf waters such as the Baltic Sea (Larsson et al., 1985), the Black Sea (Tolmazin, 1985), the Adriatic Sea (Faganeli et al., 1985), the Gulf of Mexico (Rabalais et al., 1999), and wetlands such as the Zhalong Wetlands (Gao et al., 2011a) have all been severely affected by nonpoint source pollutants from farmland drainage.
The most effective, economic, and long-lasting engineering measure to reduce agricultural nonpoint source pollution and protect the water environment in drainage areas is to construct ditch wetland ecosystems or restore existing ones (Verhoeven et al., 2006; Mitsch and Day, 2006). These ditch wetland ecosystems reduce drainage output and increase the absorption of pollutants by plants (Kröger et al., 2009; Li et al., 2010), increase the interception of precipitation (Barber et al., 2003; Carluer and Marsily, 2004; Scholz and Trepel, 2004; Cotton et al., 2006), as well as increase hydraulic retention time leading to greater adsorption and microbial reactions (Kröger et al., 2009; Kangas and Mulbry, 2014; Li et al., 2016). Numerous research has shown that ditch wetland ecosystems in irrigated areas have the advantages of less investment, simple operation and management, low energy consumption, and decrease pollutants such as nitrogen and phosphorus (Kröger et al., 2008; Gill et al., 2008; Gregoire et al., 2009; Moore et al., 2010; Peng et al., 2010; Kröger et al., 2013; Vallée et al., 2015; Vymazal and Březinová, 2015; Luo et al., 2017; Tournebize et al., 2017).
Ditches in irrigated areas serve both a drainage function as well as a wetland function to retain agricultural nonpoint source pollution. The drainage ditches in arid areas are usually deep and wide to prevent salinization, which aids the formation of ditch wetlands. Controlling drainage is an engineering measure that has been increasingly recognized in recent years that can help form wetland. It is often possible to create such a wetland with minimal construction work such as retaining dams and gates installed at the exit of the original ditches. Controlled drainage in this manner is an environmental-friendly agricultural water management strategy (Wesström et al., 2001; Evans and Skaggs, 2004; Ale et al., 2012; Negm et al., 2017; Madramootoo et al., 1993). However, due to low rainfall in arid areas, the ditch wetland is vulnerable to the threat of salt of drainage water. For example, Jia et al. (2011) examined the process of salinity dynamics over time under conventional drainage in wetlands, the results showed that salinity may eventually build up to critical levels that are detrimental to the ecological service of the wetland system. Freshwater recharge and artificial irrigation are the two most recommended management measures to reduce the salinity level of ditch wetland (Lamontagne et al., 2006; Quinn, 2009; Jia et al., 2011). However, in addition to the salinization, agricultural drought disaster is another important threat in arid and semi-arid regions, especially in downstream of irrigation areas. In many cases the ditch wetland is located in here and even irrigation water is rarely guaranteed due to its disadvantage of being located far away from the water source. Therefore, it is difficult to guarantee recharging the wetlands with fresh water on time. Once the wetland salinity has breached a critical level, lack of fresh water may cause wetland salinity management to be in trouble.
However, change in the water content of wetlands affects their salt concentration (Raulings et al., 2010; Hudon et al., 2006; Milzow et al., 2010). Controlled drainage will inevitably affect the movement of water and salt through farmland and ditch wetland ecosystems, which offers a new possibility to keep the ditch wetlands at a lower salinity level. But if the drainage output of the ditch wetland ecosystem is too low, poor drainage of the farmland will increase salinity and threaten agricultural production Conversely, if the water output of ditch wetland ecosystems is too high, it will reduce the efficiency of water use and the removal of nonpoint source pollution. In addition, lower water levels in ditch wetlands will likely increase salt concentrations of downstream waters and adversely affect biodiversity (Brock, 1981; Hart et al., 1991; Nielsen et al., 2003). What then is the appropriate height for weirs? The answer to this question could affect the water and salt balance of the ditch wetland system and the field. Therefore, a solution is urgently needed to the problem of maximizing both crop production and environmental protection within land-use matrices of agriculture and controlled drainage ditch wetlands.
Although the field water table can be manipulated through controlled drainage practices, which can reduce drainage discharge and improve water use efficiency (Gilliam et al., 1979; Karimov et al., 2014; Wesström et al., 2014; Li et al., 2018a, 2018b), there are no consensuses on the shallow groundwater contribution and control depth currently (Ayars et al., 2006). However, among the depths proposed, water table control depth deeper than 1.2 m will generally not have a major adverse impact on agriculture (Ayars et al., 2009; Li et al., 2018b). Although numerous research has shown that changes in hydrological regimes and salinity can directly affect the distribution and succession of wetland vegetation (Magee and Kentula, 2005; Milzow et al., 2010; Todd et al., 2010; Nielsen et al., 2003; Goodman et al., 2010; Davis et al., 2003), few studies have fully addressed the problems of appropriate height for wetland drainage in arid and semi-arid regions. Therefore, it is not clear how to adjust controlled drainage depth to keep the ditch wetland in a good water and salt environment.
In this study, we dialectically evaluate the agricultural practice of using controlled drainage and ditch wetlands to manage water quality in arid areas using an analytical model based on the water-salt balance. We demonstrate an application of the model with a case study in the ditch wetland ecosystems of a semiarid irrigation area in northwestern China. The specific goals of this research were to:
- 1)
Develop an analytical approach for predicting the concentration of salt under different controlled drainage conditions;
- 2)
Demonstrate the application of the proposed method in a salt-impacted ditch wetland ecosystem;
- 3)
Compare the pros and cons of salt accumulation in ditch wetland ecosystems through controlled drainage; and
- 4)
Propose a controlled drainage scheme that takes into account both agricultural production and wetland environmental protection.
Section snippets
Field drainage process
Agricultural production is the primary goal of water management in irrigation areas. Therefore, the first step in drainage management is to control soil and water salinity. Assuming that the minimum amount of drainage that meets the salt control requirements of a field is h*, the minimum amount of drainage needed is h ≥ h*. When h = h*, the field water and salt by drainage output are at their minimum and there is minimal impact on the environment.
However, because of the difficulty of obtaining
Analysis of current situation of ditch wetland system in the study area and model validation
The field hydrological process during the monitoring period was studied using the DRAINMOD model. The results showed that the cumulative amount of drainage and return water from the upstream irrigation area into the ditch wetland was about 13.64 m3 per m of ditch, the cumulative amount of water discharged from the ditch wetland was about 8.07 m3per m of ditch. The rainfall input was only 5.29 m3 per m of ditch, the field underground drainage was 0.28 m3 per m of ditch, the cumulative reverse
Discussion
Compared with Jia et al. (2011), considering various inflows and outflows to establish a more perfect model, there is no doubt that it is helpful to improve the accuracy of model simulation. The verification of the model in our research proves this. Jia et al. (2011) assumed that agricultural drainage was the sole inflow to wetlands, but the low concentration inflow quickly lowered the salinity in the ditches. Therefore, a period without observed recharge events was chosen for verifying the
Conclusion
In summary, using the theoretical analysis model to predict the water and salt balance of ditch wetlands revealed the positive and negative impacts of different controlled drainage managements for the ditch wetlands. We showed that controlled drainage changes the hydrological processes of farmland and ditch wetlands, reduces drainage output as well as damage by high concentrations of salt, but also increases the risk of medium salt concentration. The water stored in the wetland is the key to
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
Funding for this research was partially supported by the National Natural Science Foundation of China (51909209), Shaanxi Provincial Department of Education Natural Science Special (18JK0573) and PhD early development foundation of Xi’an University of Technology (104-451118003).
We would like to thank Prof. Simon Queenborough at the Yale University for his assistance with English language editing of the manuscript.
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