3.4 Using return flow for irrigation
Return flow was considered as a management measure to mitigate the impact of SWI on agricultural production in the estuary. Due to the water scarcity and high SWI during dry periods, using return flow for irrigation is able to address the water shortage at IMU scale. According to recent studies, the average ratio of irrigation return flow of rice cultivation is relative high with variation from 12.4 to 87.9% under different conditions of soil texture, slope, climate, irrigation method and crop type (Fan et al., 2011; Chen et al., 2002; Dewandel et al., 2008). For example, study of Fan et al. (2011) found that return flow in Taiwan accounts for 24.4% total water applied, in which 67% is repeatedly used for irrigation 33% is not used yet. Apart of the drained water directly returns to the downstream rivers whereas the other is used to irrigate for lower paddies.
In 2012, the first on-farm irrigation structure was initially implemented in the TuCauIrrigation Management Unit (IMU) for using return flows for irrigation purposes. The existing Bau Sen Pond was enlarged, the drainage and irrigation canals within the TuCau irrigation scheme were reformed for using return flow. A temporal diesel-run pumping station was also installed at the Bau Sen Pond to pump water into the irrigation canal system. After implementing, the Bau Sen structure has demonstrated the potential in using return flow for irrigation in the region. The measures can minimise the damage to agricultural production caused by excess salt during the dry periods.
The return flow would be possible for reuse purpose only when it is available and captured through the storage structures. An integrated management framework for reuse system consisting of both hydraulic and institutional links should be established (Van, 2013 ; Stolpe et al., 2013). The institutional links indicate the cooperation between IMCs and other water user associations. To assess the feasibility of the measure, the Dong Quan and TuCau IMUs were selected to estimate the return-flow from paddies. The study has identified the potential of return flow by quantifying return flow coefficient, reuse potential, irrigation efficiency, total pumped volume, and irrigation needs of the investigated areas. Besides water quantity, quality of the return flow was also tested to indicate either the water is suitable for reuse or not. Selected parameters were pH, TDS, DO, total N, total P, sodium adsorption ratio, and salinity. The testing results were then compared to the standards applied for irrigation issued by MARD. In overall, the parameters of the testing points were below the thresholds of the standards to indicate that it is suitable for irrigation purposes (Van, 2013).
In order to collect return flow for irrigation, the drainage systems, storage tanks, on-farm irrigation scheme are suggested to reform similar to what were done in the TuCau irrigation scheme as shown in Figure 6. The measure was suggested to apply for TuCau, Cam Sa, Vinh Dien, and Xuyen Dong IMUs where the water is abnormally applied for irrigation during salt periods. Return flow of Ai Nghia, Cam Kim IMUs has been already reused for irrigation.
Figure 6: Suggested irrigation scheme to collect return flow
Return flows can additional supply water for irrigation. Applying return flows allows reducing the dependency on river water for irrigation. However, the most important role of using return flows in the study area is to supply valuable water amount for farms during excess salt periods.
Although there is high potential to use return flow for irrigation, it requires reforming the irrigation scheme within IMU from linear model to non-linear model, in which return water is reused. This measure can be carried out under the participation of IMCs and other water user associations, of which IMCs play the primary role.
3.5 Ensuring required minimal river flow
Together channel topography and tidal forcing, river inflow is the key factor that drives SWI in estuaries (Savenije, 2005; Nguyen, 2008; Shaha et al., 2011). Salinity is able to intrude further upstream in the drought periods when upstream flow diminished and the seasonal variation in river flows defines the seasonality of SWI (Horrevoetsa et al., 2004; Conrads et al., 2011). Maintaining a sufficient inflow is therefore able to retain SWI in estuaries.
The Vu Gia and Thu Bon are the two primary sources of freshwater for the estuary. Within the estuary, the Vinh Trinh, TraKieu, and Ly Ly rivers join the Thu Bon, and the Bau La and Tuy Loan join the Vu Gia but the flow of these tributaries is insignificant as discussed in previous chapters. In total, the upstream catchments of the Vu Gia and Thu Bon account for 86% area of entire basin to indicate the major role of upstream inflows of the Vu Gia and Thu Bon in pushing back SWI downstream.
Constructing hydropower reservoirs upstream that regulate and/or divert the flow of water will affect hydrological regime downstream as well as alter the seasonal and daily patterns. Currently, 44 hydroelectric dams are or will be constructed in the basin. Of these dams, six large-scale hydropower dams, specifically the A Vuong, Song Tranh 2, Song Con 2, Song Bung 2, Song Bung 6, and Dakmi 4 dams have been completed, and other two large dams should be completed until 2015. The operation of upstream dams play important role in pushing back saltwater downstream.
The goal of this measure was to estimate the required minimal flows of the Vu Gia and Thu Bon rivers for pushing back salinity, supplying for irrigation and domestic uses, as well as maintaining environmental flow for the VGTB estuary.Several numerical simulations of the MIKE 11 model have been run to define the amount of the Vu Gia and Thu Bon flows, which are required to push back SWI as well as providing water for irrigation, domestic use, and maintaining environment flow. The required minimal flows from upstream Vu Gia and Thu Bon rivers corresponding to current sea level are summarised in Table 3.
Table 3: Required minimal river flow corresponding to current sea level
|
Measures |
Required minimal river flow (m3/s) |
||
|
Vu Gia |
Thu Bon |
Total |
|
|
1. Modification of barrage operation |
91.6 |
42.3 |
133.9 |
|
2. Construction of Quang Hue Closure |
73.6 |
77.1 |
150.7 |
|
3. Construction of Tuyen Son Barrier |
37.2 |
38.1 |
75.3 |
|
4. Using return flow |
108.4 |
42.3 |
150.7 |
|
5. Ensuring required minimal flow in the case of without any measures applied |
108.4 |
42.3 |
150.7 |
The upstream required minimal flows vary correspondingly to proposed measures. With current sea level, when adaptation measures are not implemented, the flows of 108.4 m3/s and 42.3 m3/s are required from Vu Gia and Thu Bon, respectively to push back SWI downstream as well as to provide sufficient water for different purposes in the estuary.
As required minimal river flows are met, the adaptation measures addressing SWI are not required. However, maintaining minimal flows requires a close cooperation between upstream and downstream water users. Distinct seasonality with a long dry season results in low and very low discharge of the rivers in the basin during dry periods also pose a significant challenge to maintain substantial inflows during dry months.
4. DISCUSSION
The paper has described potential measures that can be applied to address SWI in the VGTB Estuary, Central Vietnam. These measures focus either to redistribute river flows; or block the immigration of seawater to rivers; reduce SWI impact at the irrigation units; or ensure required minimal river flow.
The water–redistribution measures are able to mitigate the intrusion of seawater into the system. They focus to optimise using upstream flows for pushing back SWI and therefore flows of the Vu Gia and Thu Bon become the prerequisite to influence to the possibility of these measures on pushing back saltwater intrusion. These measures are able to mitigate the short-term impact of saltwater intrusion. The application of every single measure is not effective to manage SWI in low-flow conditions. However, the problem can be solved by applying simultaneously all the proposed water–redistribution measures.
The blocking-saltwater-immigration measure can cut off the movement of seawater into the estuary. It was proposed as an adaptation measure to eliminate the long-term impact of SWI not only on irrigation but also on drinking water supply. This measure is able to protect the estuary from SWI in the extreme conditions of low flow. However, this measure might obstruct the navigation and disturb the aquatic life.
Return flow is incorporated as a measure to reduce the potential damage to agricultural production caused by SWI at IMU level. The high amount of return flow from paddy farms provides a potential solution to address salt-intrusion-water shortages in the dry periods.
Ensuring required minimal flows was proposed as a non-engineering measure to address SWI in estuaries. When the required minimal flows are met, the problems of SWI are solved without the participation of other measures.
5. CONCLUSION
Saltwater intrusion is a prominent concern in the VGTB Estuary, as it has caused adverse impacts on irrigation and urban water supply in recent years. To cope with saltwater intrusion, not only engineering measures but also non-engineering measures have been examined. The research has suggested possible measures, which might apply to mitigate the short-term impact or adapt the long-term impact of saltwater intrusion. These measures focus either to redistribute freshwater flows or to block the saltwater immigration. Another measure is to reduce the damage to agricultural production caused by saltwater intrusion. Although the features, requirements, feasibility as well as pros and cons of each measure has analysed and evaluated, further studies are required to investigate the overall influences of each proposed measure to natural and human environment of the region.
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