The climate in the Delta is tropical monsoon and is influenced by both the southwest and northeast monsoons. In general the dry season runs from December to April while the wet season spans May to November. Fig. 2.2 (redrawn from NEDECO, 1993) summarises the mean monthly rainfall and Penman evaporation of Vietnam’s lower Delta. Also shown is and the mean monthly stream flow for the Mekong at Phnom Penh. The pronounced seasonality of the rainfall and stream flow is obvious in Fig. 2.2. Average annual temperature in the Delta is close to 28°C. Mean monthly temperatures run from 25°C in January through to high of around 28.9°C in April. The mean monthly relative humidity varies from a low of around 74% in the dry to 83% in the wet season. The marked seasonality is also reflected in the volume of Cambodia’s Great Lake in the upper Delta. Fig. 2.3, using the invaluable data of (Carbonnel and Guiscafre, 1963) shows the variation in the volume of the Lake. The Lake is a natural flow regulator for the lower Mekong acting as a flood storage in the wet season until early October and a supply reservoir in the dry as the Lake drains from October on. The magnitude of its importance as a flow regulator for the Delta can be judged from Figs 2.3 and 1.2.
There is also a marked spatial variation in annual rainfall across the Delta which depends on the direction of the monsoon in the southwest and an orographic influence in the north (Fig 2.4, Minh 1995). The length of the rainfall season is also spatially dependent (Fig 2.5, Minh, 1995). It varies from 4 months in the north, to 7 months in the southwest and also reflects the direction of the monsoon.
Fig. 2.4 Distribution of annual rainfall across the Mekong Delta (Minh, 1995).
2.4.1 Floods and seawater intrusion
The marked seasonality in rainfall leads to both annual floods and water shortages in the Basin. In the wet season almost 50% of the Delta is flooded (1,900 km2). The maximum depth of inundation in the wet season is principally governed by the topography of the basin, the influence of the upstream flow from the Mekong and Bassac Rivers and tidal inundation in the south (Fig. 2.6, Minh, 1995). A smaller influence from the spatial variation of rainfall is also evident in Fig. 2.6. In the northern part of the Delta, in the Plain of Reeds, inundation depths can exceed 4 m. The original rice production systems in the Delta took advantage of wet season flooding by using floating rice crops.
Fig. 2.5. Distribution of length of rainfall season over the Mekong Delta (Minh, 1995).
In the dry season, flow in the Mekong is insufficient to prevent saline intrusion and extensive salinization of waterways occurs in the lower Delta. Fig. 2.7 (Minh, 1995) shows the extent of salinity intrusion at the beginning of the saline intrusion (BSI) and the end of the saline intrusion (ESI). The whole of the Ca Mau Peninsula in the Delta’s southwest, in Fig. 2.7, is salinized for 6 months during the dry as there is insufficient freshwater flow in the Mekong to displace saline intrusion from the southwestern sector of the Delta. Figures 2.6 and 2.7 exemplify two of the main hydrologic problems of the Delta, wet season floods and dry season saline intrusion.
The length of the wet season is an important factor in rice production, particularly for double and triple cropping. The frequency of occurrence of early season and mid season drought is
also critical. The delay of the onset of the wet season (early season drought), and the occurrence of mid season drought are key determinants of rice production. These droughts are also spatially distributed across the basin (Fig 2.8, Minh 1995). Proposed dams on upper Mekong, especially at the Tonle Sap, are designed to address this problem and provide water resource security during dry seasons and droughts.
Fig. 2.6. Mean annual depths of wet-season flooding across the Mekong Delta (Minh, 1995).
2.4.2 Tidal influences
Streams and canals in the Mekong Delta are influence by the tides of both the East and West Seas. In the East Sea the tide is semidiurnal but irregular and has a large tidal amplitude of 3 to 3.5m. The regime has a 15 day cycle average tidal level has a maximum in December and a minium in July. The tidal effects from the East Sea propagate over much of the Delta through the main and farm canal systems. Farmers use these tidal fluctuations to drain and flood their lands. Drainage of floodwaters can be impeded if wet season floods coincide with the spring tide.
Tides in the West sea are diurnal with a tidal range of about 0.8 to 1.2m. Canals in the Ca Mau Peninsula are influenced by both East and West Sea tides simultaneously. This can lead to a dead water zone which can prevent water movement from the Bassac River into the Ca Mau area (Ghassemi and Brennan, 2000).
2.4.3 Seawater intrusion floodgates
Studies of seawater intrusion (Mekong Secretariat, 1992a; 1992b;1993) developed a model to predict seawater intrusion into the lower Delta. Following these major studies, a proposal was developed to install saline intrusion floodgates on main canals along the Ca Mau Peninsula and South China Coast (NEDECO, 1993). The idea behind this scheme is to lengthen the growing season for rice from one to two crops in the saline intrusion affected areas. The discharges of the Mekong River are considered adequate to meet irrigation demand in the protected areas during the early periods of the dry and wet season, thus lengthening the growing season. In the dry season, Mekong flows are insufficient for irrigation in the protected areas (NEDECO. 1993).
Fig 2.7. Seawater intrusion into the Mekong Delta during the dry season. BSI and ESI are respectively the beginning and end of the seawater intrusion (Minh, 1995).
A series of 12 massive sluices or tidal floodgates have been installed on the major rivers and canals connected to the East and West Seas (Fig. 2.9) in an effort to prevent seawater intrusion into the Ca Mau Peninsula. The project, called the Quan Lo Phung Hiep Project cost over $US 12B and included the dredging of over 250 km of secondary canals. The project commenced in 1992 and was completed in 2001. The sluice gates are between 5 and 25m wide. They open automatically on the ebb and close on the spring tide. The objective of the project was to permit two rice crops per year to be grown in the irrigation area behind the sluices (Fig. 2.9) by decreasing the salinity ingress into the area behind the floodgates and increasing the flow of freshwater from the Bassac River during the dry season.
White et al. (1996) used the analogy of the impacts of saline floodgates in eastern Australia to point out the possible severe consequences that could arise in areas with acid sulfate soils.
This included the prevention of fish passage, the acidification of waterways behind floodgates and the loss of seawater species behind the floodgates. Since the brackish water canals are more productive that freshwater canals, this may have considerable consequences for subsistence farmers in the region, given the importance of fish as a protein source in the region. Our field trip to the area appeared to confirm these predictions. The impacts of already installed sluice gates near Soc Tranh were summed up succinctly by one farmer
“floodgates have given us a road (track) and electricity. But no crops and no fish!” The water in his field drains had a pH 3.5 and there was little tidal fluctuation to permit him to irrigate and drain his field.
A project is underway to examine the impacts of the saline intrusion floodgates (Tuong, 2002). It has found thus far that farmers on non-acid sulphate soils in the eastern part benefited from the salinity protection schemes, which allowed them to increase rice intensification. Farmers on acid sulphate soils in the western part had to abandon shrimp farming, which meant sharp decline in household incomes. Rice farmers in the eastern part desired to retain the salinity protection scheme while those in the western part preferred to dispose of the scheme such that brackish water could re-enter the area.
Fig 2.8 Spatial distribution of early season (ESD) and mid season drought (MSD) in the Mekong Delta (Minh, 1995).
Farmers perceived that the salinity prevention measures caused a decline in abundance of natural fishery products in rivers and canals. Trawling was carried out as part of the study in
six villages at the beginning of the rainy season (May and June) 2002. It found fish biomass declined sharply in area with pH < 6. This resulted in the decline in income earnings from capture fisheries, which was not only an important income source for the poor households but also an important protein source for them. Although the income decline from catching fish among the poor has been compensated for to some extent by other income-generating opportunities in several hamlets surveyed, this nevertheless reveals an important ecological consequence of preventing tidal ingress into the study area. As more sluices went into operation year by year, the rapid change in hydrological conditions had profound economic and social impacts on farmer’s livelihood (Tuong, 2002). As a result, conflicts over the operations of sluices have arisen.
Fig 2.9. Location of the 12 saline intrusion sluices in the Quan Lo Phung Hiep Project in the Ca Mau Peninsula. The irrigation area protected behind the floodgates is shaded in grey (Ghassemi and Brennan, 2000).