In Peril: The Freshwater Ecosystem of the Tonlé Sap Basin

Assessing the health of the Tonlé Sap Basin with remotely sensed data products

Lindsay Harmon, Keida Gaba, Susan Jarvis, Joey Lindsay

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Phnom Penh, Cambodia’s busy capital

Phnom Penh is the capital and chief city of Cambodia. Located over 180 miles from the sea, Phnom Penh is a major port of the Mekong River valley, where the city trades dried fish, corn (maize), cotton, and pepper (Britannica, 2021). 

Figure 1: The photo illustrates Phnom Penh, the capital city of Cambodia.

Let's take a trip to Tonlé!

Tonlé Sap is a floating village approximately 180km or 111.85 miles from Phnom Penh.

Figure 2: Bustling roadway in Cambodia, street-bikes line the route.

Study Area

Located within Cambodia, SE Asia, the study area for this project is the Tonlé Sap Lake and river basin. Our team focused on the region highlighted. 

Agriculture

Rice is Cambodia's major crop, its principal food, and one of its most important export commodities. The principal rice regions surround the Mekong and the Tonlé Sap. (Chandler D.P et al., 2021). 

Sustainably staying afloat

Tonle Sap is the most fertile and diverse freshwater ecosystem in SE Asia, recent development and climate change threaten its natural flow negatively impacting its diversity (Oeurng et al., 2019). 

Impact

Sorting the incoming catch, Tonlé Sap, Cambodia

Sorting the incoming catch, Tonlé Sap, Cambodia.  

  • Tonlé Sap Lake, located in the lower basin of the Mekong River, Cambodia, is a transboundary river that drains and crosses six countries: China, Myanmar, Laos, Thailand, Cambodia, and Vietnam (Wang et al., 2020).  
  • The Tonlé Sap Lake of Cambodia is one of the most productive freshwater ecosystems of the world (Varis et al., 2006).  
  • Over 1.2 million people depend on the resources from Tonlé Sap, which makes up more than two-thirds of Cambodia’s protein consumption and is worth an estimated US$ 2 billion annually (Dugan, 2008).

Problem

A boat stacked high with fish netting

A boat stacked high with fish netting floating on Tonlé Sap Lake.

  • Extreme weather driven by climate change paired with increased human activities put Tonlé Sap Lake’s ecological and socioeconomic health at risk (Wang et al., 2020). Floods, forest fires and degradation, overfishing, and agricultural development currently threaten the Tonlé Sap Lake and River basin. Agricultural pressures and ineffective land management (Varis et al., 2006) 
  • There are 93,000 total hectares of rice fields being irrigated, however, 57% of the field area is not in use or needs to be restored. It is estimated that around 1/9 of the irrigation potential is used in the Tonlé Sap region (Varis et al 2006).  Tonlé Sap currently faces agricultural pressures and ineffective land management.
  • Due to the COVID-19 pandemic and travel-related restrictions, in situ data collection is limited. 

Answer

The Freshwater Health Index Tool. Source: Conservation International

  • Ongoing multi-term DEVELOP Project with the objective of collecting FHI inputs by running geospatial analyses of water quality parameters such as water quality, vegetation, soil, precipitation, etc. Computer programming, NASA Earth Observations, and environmental modeling software pinpoint weakness in the Tonlé Sap Lake to effectively prioritize management.  
  • Remote sensing technologies are utilized in place of unavailable in situ data collection to gather inputs for Conservation International’s Freshwater Health Index Tool to score water parameters to pinpoint weakness / critical areas of improvement. 

Community Concern

Two women making traditional smoked fish known as prahok.

Fishing, as means of supporting local livelihoods, is soon becoming unfathomable as fishermen notice they are catching fewer fish than their grandparents did (Vann, 2016). Fewer fish catch is an indication of the imbalances and bigger ecosystem problems in the Tonlé Sap Lake. It hints towards water quality and quantity issues which the general community is aware of and is voicing their concerns.  

Fishing not only serves as a food source, but provides income to buy boat fuel, rice, and other necessities. The Tonlé Sap Lake is a critical ecosystem providing various services including freshwater supply, water supply to support crop production, and recreation. These essential services are jeopardized by climate change trends and local agricultural patterns. The impacts of climate change are threatening natural seasonal fluctuations and harming the ecosystems that support communities.   

Addressing these concerns by restoring essential ecosystems is a priority of this project in partnership with key stakeholders in the area.   

Our Project

Freshwater Health Index Breakdown

NDVI Image of the Tonlé Sap Basin

This project is aimed at developing a method for using remotely sensed products to measure the inputs for the Freshwater Health Index (FHI) which is a way for our partners at Conservation International to gauge the health of various freshwater systems. Because this project spanned 3 terms, the inputs we worked to collect were broken up by term. Term I calculated land cover naturalness, bank modification, and deviation from natural flow, Term II calculated groundwater depletion and water quality, while Term III modified calculations to collect more accurate bank modification, groundwater storage data, and water quality data. 

Water Quality data collection was made much more accurate during this term. Using the Soil and Water Assessment Tool (SWAT), we modeled the sediment and nutrient levels in the Tonlé Sap Lake Basin.  SWAT is a model that uses information about the local environment to present a picture of where nutrients, such as nitrogen, are in the water and how they move through it. In order to use SWAT, five inputs were needed to run the model: precipitation, daily minimum and maximum air temperatures, soil classification, landcover classification, and a digital elevation model for the Tonlé Sap Basin (Rossi et al., 2009). Using SWAT-CUP software, calibration of the model was simplified and using text file outputs of the SWAT as inputs for SWAT-CUP gave more accurate outputs for water quality.

Bank modification is determined using a Normalized Difference in Vegetation Index calculated from MODIS imaging. 

Terrestrial groundwater storage was calculated using the GRACE satellites. These satellites measure changes in gravity to quantify the changes in total water storage both on the surface and underground.  

Solutions

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Making a Model 

Tonlé Sap is a complex watershed. With its extreme wet and dry seasons, wide-ranging extent, and rapid human development, the area poses unique modeling challenges. Perhaps the most characteristic quality of the watershed is its flow reversal. Once every season, the river reverses direction and flows backward, moving “upstream” into the lake. 

These challenges were addressed by transitioning from SWAT 2012 to SWAT+, a more recent edition of the model that has provisions for inputting lake boundaries, computing landscape units, and differentiating between small volume channels and high-volume streams. 

Digital Elevation Model data is required to be joined with all weather data inside of SWAT text files. The preliminary software from the previous term did not account for non-rectangular study areas when performing this step. The software was updated to correctly join this information.

 

We also added additional weather data, refining the code from the previous term to output solar radiation data that bolstered the accuracy of our model. This came with other revisions to the code, including bug-fixing that allowed for the use of non-rectangular study areas. 

Dissecting Gravity

GRACE groundwater analysis posed a particular challenge. Transforming the native data, which is presented in Equivalent Liquid Water Thickness, into a meaningful groundwater statistic for the basin for the Freshwater Health Index saw many rounds of revision. In the end, we leveraged the robust design of the FHI to compute “affected areas” - locations that the GRACE satellite determined to deviate significantly from expected water storage. 

Collapsing Shores 

Banks without adequate vegetation are frequently subject to degradation, and are unable to filter pollutants from runoff before it reaches the larger watershed. Thus, the FHI requires the addition of bank modification statistics. In order to compute these statistics via NDVI, the team needed to compute data from the MOD13Q1.006 Terra Vegetation Indices 16-Day Global 250m to create a time series of MODIS derived Normalized Difference Vegetation Index (NDVI) values, spanning from January 1st 2000 to December 31st, 2020. ​With these statistics, we were able to locate potential areas of concern within Tonlé Sap that may have experienced high levels of modification. The results indicate a small decline of NDVI over time, with seasonal variation between the wet and dry seasons. ​

 

 

Conclusions

Express map highlighting conclusions of project methodology.

Project Timeline

Spring 2021

 Tonlé Sap Food Security & Agriculture.  The first term of this project created a water level time series of Tonlé Sap lake, and used it to calculate deviation from natural flow within the FHI. The team also compiled land use and land cover change maps to calculate land cover naturalness.

Summer 2021

 Tonlé Sap Food Security & Agriculture II.  The team will build on prior work by continuing to integrate and refine inputs into the FHI to calculate ecosystem vitality as a major indicator.

Fall 2021

Tonlé Sap Food Security & Agriculture III. The current team compiled the work done by previous teams, synthesized results, calibrated models, and forecasted future resource distribution of the basin by loading in all available inputs into the FHI.

References

Conservation International/Sophak Sett. (2016). ci_15384199 [JPG]. Retrieved from https://ciorg.imgix.net/images/default-source/default- 

Chandler, D. P. and Overton, . Leonard C. (2021, March 10). Cambodia. Encyclopedia Britannica.  https://www.britannica.com/place/Cambodia   

Dugan, P. (2008). Mainstream dams as barriers to fish migration: international learning and implications for the Mekong. Catch and Culture 14 (3): 9-15. 

Vann, L. (2016, March 7).  In Cambodian floating villages, a bold voice helps women boost income.  Conservation. https://www.conservation.org/blog/in-cambodian-floating-villages-a-bold- voice-helps-women-boost-income 

Varis, O., Kummu, M., Keskinen, M., Sarkkula, J., Koponen, J., Heinonen, U., & Makkonena, K. (2006). Tonle Sap Lake, Cambodia: Nature’s affluence meets human poverty. New York: United Nations Human Development Report. 

Wang, Y., Feng, L., Liu, J., Hou, X., & Chen, D. (2020). Changes of inundation area and water turbidity of Tonle Sap Lake: responses to climate changes or upstream dam construction? Environmental Research Letters, 15(9), 0940a1. 

Image Credits

Amaury Laporte (2016) Floating House and Boat Drowning in Green Plants [JPEG] Retrieved from  https://www.flickr.com/photos/alaporte/27500899953/in/album-72157669575725150/  

August Dominus (2018) Settlements in Tonle Sap. [JPEG] Retrieved from  https://commons.wikimedia.org/wiki/File:Tonle_Sap_u_sije%C4%8Dnju.jpg 

Boudewjin Huysmans (2020). Green grass on water during daytime [JPEG] Retrieved from  https://unsplash.com/photos/JxtdySglyYg 

FHI (2019) The Freshwater Health Index [Video] Retrieved from  https://www.youtube.com/watch?v=0t6FdigfVSQ  

Humphrey Muleba (2018). Man riding motor scooter [JPEG] Retrieved from  https://unsplash.com/photos/dWX7qzvPwIc 

Julia Maudlin (2014) Sorting the Catch, Cambodia [JPEG] Retrieved from  https://www.flickr.com/photos/juliamaudlin/51516262179/in/album-72157719903843539/ 

Paul Szewcyk (2019). Sunset over Mekong River [JPEG]. Retrieved from  https://unsplash.com/photos/-GAwF4y44q8 

Sorting the incoming catch, Tonlé Sap, Cambodia.  

A boat stacked high with fish netting floating on Tonlé Sap Lake.

Two women making traditional smoked fish known as prahok.

Freshwater Health Index Breakdown

NDVI Image of the Tonlé Sap Basin