3. Responses to Water and Land Issues of the Delta
3.1 Completed Projects of the Mekong Secretariat
The Mekong Secretariat has adopted a long-term strategy plan for addressing the problems of the Delta as part of the broader management of the Mekong Basin. The Secretariat’s thrust has been hydrological, with the main effort concentrated on investigations of the impacts of river and tributary regulation, irrigation diversions and infrastructure development for hydropower and irrigation. Setting aside river regulation projects, there have several major completed projects with direct relevance to the Delta. These are the Salinity Intrusion Studies (Mekong Secretariat, 1992a; 1992b; 1993), Water Balance Study of the Lower Mekong Basin (Institute of Hydrology, 1982; 1984; 1988a; 1988b), the Management of Acid Sulfate Soils Project (Ministry of Agriculture and Rural development, 1995) and the water quality survey of the Lower Mekong (Mekong Secretariat, 1982). These studies were narrowly focussed.
3.1.1 Salinity intrusion forecasting
The Salinity Intrusion Study had its origin in the 1930’s. During 1935-1942 a dense network of salinity monitoring sites were established over the main branches of the Mekong and its canals to establish the extent of dry-season saline intrusion. The goal of the Mekong
Secretariat’s study was to develop at model of salinity intrusion which could be used to predict the impacts of upstream diversions on intrusion and asses the threats to agriculture in the Delta. Phase I (Mekong Secretariat, 1992a) modelled the canals and small systems of the Delta, Phase II (Mekong Secretariat, 1992b) studied the main river branches under low flow conditions, Phase III (Mekong Secretariat, 1993), extended the study to the whole Delta under all flow conditions. This study developed and tested the MERKSAL model and the VRSAP (Vietnam River System and Plains) and built up local capacity in their use. The VRSAP model is a one dimensional solution for solute transport for the river/canal network with runoff from rice fields.
The Salinity Intrusion Study concentrated on model development. The coupling of this model with a marine/freshwater fish population dynamics model may be of considerable benefit in determining the impact of management strategies on fish populations.
3.1.2 Water balance study
The Water Balance Study of the Lower Mekong Basin was designed to review the hydrology of the lower Basin and to determine the impacts of upstream regulation, diversions for irrigation and changes in land use on flow in the Mekong. The objective of the Phase I (Institute of Hydrology, 1982), study was to review available data, carry out a water balance study of selected tributary basins and investigate the variability of flows in the Mekong upstream of the Delta (as far as Pakse). A simple water balance model with rainfall, estimates of evapotranspiration, runoff and two part storage was developed.
Phase I identified the absence of long-term hydrometeorological data and the gaps in spatial coverage as significant limitations. The absence of available solar radiation was a major limitation for calculating potential evapotranspiration as was the inadequacies of estimating areal rainfall. The study demonstrated the difficulty of estimating evapotranspiration for such areas. The complementary approach was used (Brutsaert and Stricker, 1979; Brutsaert, 1982) together with Penman potential evaporation. The study’s use of Penman rather than the Priestly-Taylor approximation for potential evaporation appears to be an unnecessary complication for estimating monthly evaporation since it requires estimates of wind speed and humidity in addition to the principal driver, solar radiation. In work on estimating stream and groundwater flows in tropical northern Australia, Vardavas (1988) was able to predict successfully daily rainfall-runoff relations using equilibrium evaporation and estimated solar radiation. Such an approach would seem particularly appropriate for the Mekong Delta.
It was concluded that mainstream flows did not indicate any significant changes in the low- flow regime of the Mekong River due to upstream developments. Reasonable water balances could only be achieved by assuming a fixed soil storage at the end of the wet season.
Monthly water balances were more reliable than 5-day balances. The study found that the effects of land-use change on river flow were insignificant and suggested that statistical models were more appropriate than conceptual models.
The following Phase II project (Institute of Hydrology, 1984) had two primary objectives.
The first was to develop a network-routing model of the Lower Mekong Basin containing elements that represented storage reservoirs and irrigation schemes, and the second was to study problems inherent in estimating areal rainfall from point records. A network-routing model was produced which contained reservoir and irrigation submodels and could be used to investigate different water allocation policies. The analysis of rainfall was restricted to
monthly data from northwest Thailand were coverage of gauges was adequate in time and space and data was readily available. Data for the rest of the lower Basin was considered inadequate. Short period rainfall estimates were inaccurate due to sparse spatial distribution of raingauges. The study showed that the spatial correlation for monthly data dropped off within a few tens of kilometres.
The Phase III project (Institute of Hydrology, 1988a; 1988b), was required to determine a framework for the continuous monitoring of the flow regime of the lower Mekong Basin in order to quantify the effects of existing and ongoing developments, and to review and gather data for the effective modelling on planned developments in order to assess the impacts on downstream flow. This study is particularly relevant since it specifically addressed inflows to the Delta. Problems of data availability in Cambodia were noted. Simple regression models for wet and dry season flows of the Mekong at Kratie and Tonle Sap were used. It was concluded that dry season flows to the Delta had increased however at the expense of flood flows into the Great Lake. The importance of the Lake as a flow regulator for dry season flows into the Delta was emphasised. It was predicted that the Great Lake may not be filling to the same level as previously. Major reservoirs were identified as contributing to the changed flow regime. There were insufficient time series data to recognise impacts due to changes in land use or irrigation. The study also recognised the difficulty of estimating river discharges in the tidally influenced regions of the Delta.
The terms of reference of the water balance study were quite specific. Because of that the studies concentrated on surface waters with no reference to the recharge of groundwaters.
Since groundwater is an increasingly important component of domestic and urban supplies in the Delta (NEDECO, 1993). Recent developments in the stochastic analysis of temporal and spatial variations of rainfall which include orographic influences specifically and can be used to produce areal averages (Hutchinson, 1995a; 1995b) and the application of data-based mechanistic modelling to rainfall- runoff would seem to be of potential benefit in such water balance approaches.
3.1.3 Water quality monitoring
Earlier water quality monitoring in the Basin (Mekong Secretariat, 1982) concluded that there were insufficient samples taken on a regular basis and that chemical and biological measurements were inconsistent. These made it impossible to identify past trends in water quality.
A review of the Basin’s water quality monitoring network (Ongley et al., 1997) identified that there is no clear objective concerning the final use of information and that the programme is at risk of being diverted by irrelevant scientific or economic interests. General water quality objectives and criteria were found to be missing for the Mekong. The review believes that monitoring is a management tool and concludes that it should only be done when necessary information is needed by legislation or environmental managers. This curious conclusion seems to stem more from western economic rationalism than rational river management. One of the overwhelming conclusions of all Mekong studies is the importance of long term continuous monitoring data. The review believed that present and possible future effects of forestry /deforestation and fertilizer use in agriculture were well covered. It recommended that salinity and acidity monitoring in the Delta be continued only in so far as they contributed to larger issues as fish survival and on loadings of pollutants to the sea. The key finding of the Review was that the magnitude and complexity of water
quality concerns is increasing at a rate that exceeds the development capacity of the riparian countries and the Mekong Secretariat.
One of the surprising aspects of the review is the almost total absence of any suggested linkage between water quality monitoring and River hydrology or fish production. The linkage of these sectors into a chemical transport and biological production model would seem essential. As well, the failure to identify sediment transport as a major nutrient source and sink for both agriculture and fish production emphasises the compartmentalisation of projects in the Secretariat.
3.1.4 Management of acid sulfate soils
The overall objective of the Management of Acid Sulfate Soils Programme is to formulate environmentally sound reclamation and management strategies for acid sulfate soils. The project seeks to anticipate the reactions of acid sulfate soils to various development and management strategies through an understanding of the processes set in motion by these strategies (Ministry of Agriculture and Rural development, 1995). In order to achieve the overall objective two immediate objectives were set: to obtain information on the physical, chemical and biochemical reactions and the basis of soil acidification, such as their mechanisms, kinetics, interlinkage and dependence on environmental factors; and to develop a causal or semi-causal description of these processes, with the ultimate view to construct a dynamic mathematical model.
It is by no means clear how these objectives meet the condition in the overall objective for environmentally sound development. The claimed project outcomes are upgrading of project offices and laboratories; conducting plot-sized field experiments; conducting controlled soil column experiments; evaluating data; the commencement of a model for physical processes in acid sulfate soils; and training staff.
It would seem, with two exceptions, that the scale of this project is not commensurate with the scale of the problem in the Delta. The exceptions are the work of Tin and Wilander (1995) who studied the rate of transmission of acid drainage in the canal system, Truong et al. (1996) who simulated acid water movement in canals based on the assumption of acid sulfate and aluminium equilibrium with jurbanite (Al(OH)SO4, a mineral postulated but never observed in acid sulfate soils, and that of Tuong and Minh (1995) and Minh et al.
(1997) who studied rates of acid and aluminium production under different crop and soil management regimes. The last study showed that exports from rice crops were less than from upland and higher value crops which are not under flood irrigation. The outcomes of this project may be useful for crop production but they seem to be less useful in examining the broader scale environmental impacts. The pressing issue of the impact of massive saline intrusion floodgates on acidification of the large areas behind floodgates and dykes in the lower part of the Delta has not been examined nor its impact on fisheries.