More than 300,000 people living in Stung Staung river basin in Northern Cambodia rely on agriculture for their livelihoods. Access to a clean water supply is fundamental for wellbeing and health particularly to prevent transmission of the COVID-19 virus. Increased food security, higher income for farmers and improved health in the villages can be expected if the reliability and quality of water supplies in the province can be enhanced through improved sharing of existing water supplies.
Partnering with the World Bank and support from the Royal Government of Cambodia (RGC), eWater undertook a study to assess the challenges of water supply development and to establish an enabling environment, based on scientific evidence, for a water sharing dialogue between authorities, farmers and relevant water users and assist with developing a long-term investment plan for water supply utility for Staung district in Kompong Thom Province.
The Stung Staung river rises in the mountains of Northern Cambodia before flowing 213 km to the iconic Lake Tonle Sap, one of the largest freshwater lakes in the world. In its northern reaches, the river flows through forest before entering expansive cropland to the south. The majority of the 217 villages in the catchment are along the river in the farming districts which cover about a third of the catchment. More than a quarter of the people in the river basin live in poverty.
Competition between irrigation and village water supplies is increasing as climate change varies the annual rainfall pattern. Barrage ‘Samsep Kagna’ of 7,200 ha is the only large irrigation scheme in the catchment. Other existing schemes include 17 medium schemes and 8 small schemes (Figure 1). Currently, rainfed paddy rice is the main crop in the command areas, but further downstream field and garden crops are more common.
Figure 1: Irrigation schemes and water supply utility in the Stung Staung Catchment.
To help manage the water competition and cope with climate change, a large water reservoir of the Staung Water Resource Development Project with a potential area of 30,000 ha is planned.
Clean water supply in the coverage areas comes from the Stung Staung River, supplemented with groundwater from a 200m deep tube well next to the treatment plant near the river. A World Bank’s Water Supply and Sanitation Improvement Project (WaSSIP) aims to increase water supplies to villages by up to four times through a new water supply utility near Barrage ‘Samsep Kagna’.
Development of a Source model to assess demand and supply
Through 2019-2020, funded by the World Bank, eWater, in collaboration with the Institute of Technology Cambodia (ITC) and Ministry of Water Resources and Meteorology (MOWRAM), and Ministry of Industry, Science, Technology and Innovation (MISTI) undertook a comprehensive catchment assessment in the Stung Staung River and developed a Source model to support dialogue on a water sharing plan in the catchment. The outcome of the assessment will support the design of future water supply infrastructure and investment potential of the RGC and the World Bank in Staung district.
The data review included demography, geography, water sources, status and trends of water supply and demand, initial potential risks and mitigation measures. Additional hydrometeorological datasets were collected to help refine the existing country-wide Source model to form the Source Stung Staung Model, the model supports hydrological and water system modelling. The hydrological model was used to undertake a water balance assessment that characterised the natural condition (rainfall-runoff process) in the catchment. The water system model was run to quantify water demand from various sectors, mainly irrigation activities and municipal water requirements.
Once the hydrological characteristics for the catchment were defined, several future scenarios were formulated and later assessed using well calibrated models of the pre-development and future-development scenarios. This allowed for an assessment of potential hydrological risks. The simulations quantified surface water availability from the natural processes of rainfall-runoff and water resource use by different sectors. Hydrological baseline statistics were developed, which characterised the water supply/demand balance on a monthly, seasonal and annual basis.
Climate change impacts
An analysis of rainfall patterns over the Stung Staung Catchment reveals an observed change in rainfall between the past condition (2000-2011) and the present condition (2012-2019). The pattern of mono-modal with a peak in September has changed to bi-modal with peaks in July and September. A decrease in rainfall in August was clearly observed for all the key stations. Particularly, a big change in rainfall was observed at Stoung in the lower section of the river. Monthly rainfall in the wet season dropped about 100 mm. A significant drop was in September: decreasing from about 400 mm to below 200 mm. The climate change assessments suggest a decrease in all flow ranges from the present condition.
Pathway to resilient water resources
The project allows the Cambodian authorities to consider the developed workflow and application of the Source Modelling Platform for water balance analysis and catchment assessment as a scientific tool to support water allocation mechanisms.
The study recommended, through community discussions, establishing a minimum requirement for water quantity and quality as the priority for domestic water supply. Understanding community priorities will assist the large water storage managers in developing an equitable water sharing plan for domestic and irrigation water. While there is a potential for expansion of irrigation scale, the irrigation modernisation should be taken into account of water balance to improve the sustainability of the irrigation system, providing benefits to farmers and the downstream biodiversity and aquatic environment in the river. Crop intensification would lead to higher irrigation water demand resulting in water shortage so shifting the crop growing calendar could be considered as a way to ease water shortages in May and June.
With signs of less water available in the future, the villages in the river basin need to discuss their options, such as crop diversification to high value crops, which may generate greater income with less water. The study provides valuable details and guidance for the authorities and villagers in the river basin to mitigate risks and for Development Partners to investigate water resource development option and enhance water resource management benefits in the Stung Staung Basin.
Read this article by the CAVAC (Cambodia-Australia Agricultural Value Chain Program) for more on the use of water models in Cambodia https://cavackh.org/public/post/using-water-simulation-models-to-support-sustainable-water-resources-management