TECHNICAL GUIDANCE NOTE: REMOTE SENSING June 2019 | Issue No. 1 Better Data, Better Results Remote Sensing as a Tool for Monitoring Water Quality in Lake Toba, Indonesia Lake Toba is a unique natural asset of global significance with a rich cultural heritage located in the North Sumatra Province of Indonesia. Located 904 meters above sea level and with a maximum depth of more than 500 meters, this 87-kilometer-long lake provides a wide range of economic and environmental goods and services for more than half a million people and 400 villages in the seven districts covered by the lake’s 3,658 square kilometer catchment. However, sustaining the long-term economic and environmental value of Lake Toba depends on addressing the deterioration of water quality. This technical guidance note reports on the potential benefits of using remote sensing as part of an integrated strategy to improve the monitoring and management of water quality in Lake Toba. © Mas Jono/Shutterstock. The water quality of Lake Toba has noticeably declined estimates are spatially continuous and can be repeated since the mid-1990s, threatening tourism potential and the at regular intervals. This increases data quantity and lake’s long-term sustainability. The point and non-point availability. Therefore, remote sensing can compensate for sources of water pollution (especially nutrient inputs from the limitations of in situ monitoring and will make a cost- nitrogen and phosphorus) are caused by extensive practice effective contribution to monitoring and lake management of caged aquaculture, livestock manure, and domestic decisions. As such, remote sensing is highly relevant to the wastewater runoff. Important contributing factors also government’s integrated tourism master plan for Lake Toba. include rapid change in land use and deforestation, which have exacerbated erosion in the catchment. Remote sensing using satellite earth observations has created sophisticated capabilities in the monitoring of in-land water In response, the government is preparing an integrated, quality (table 1). Combining multiple satellite sensors with cross-ministerial, and cross-sector approach for the future different types of estimation methods has improved the development of tourism at Lake Toba. This includes the retrieval accuracy of different water quality parameters. development of comprehensive solutions for improved Optically active constituents of water, which interact with management and monitoring of the lake’s water quality. These light and change the energy spectrum of reflected solar efforts are central to understanding the drivers of deteriorating radiation from water bodies, can be measured using remote water quality and to designing the most appropriate and sensing (Ritchie 2003). Four key advantages of integrating cost-effective interventions to reduce pollution. remote sensing into water quality monitoring exist (Kallio and others 2000): One of the major constraints in assessing water quality is the scarcity of primary data. Water quality data are neither • Enables a synoptic view of the entire water body, systematically collected nor available over suitable periods of which improves insights on spatial and temporal time and with appropriate spatial coverage. Ideally, monitoring variation should include several in situ measurements of parameters, • Provides a synchronized view of water quality across such as temperature, salinity, turbidity, Secchi depth and collections of lakes over large regions transparency, dissolved oxygen, and oxygen demand. For • Establishes a comprehensive historical record of water Lake Toba, understanding the processes of eutrophication quality in an area also requires monitoring of phosphorus, nitrogen, and • Helps prioritization of sampling locations and field chlorophyll-a concentrations, along with land use changes. survey timing In situ data provide point estimates of the quality of water conditions in time and space. However, obtaining spatial The water quality parameters that can be monitored, thanks and temporal variations across water bodies using in situ to technological advances and satellite launches, include: data is challenging and costly (Ritchie 2003). In situ total nitrogen, total phosphorus, chlorophyll-a, colored sampling is also dissolved organic matter (that is, dissolved organic carbon or total organic carbon), harmful algae blooms (for example, • Labor-intensive, time-consuming, and costly; cyanobacterial toxins or microcystic concentrations), total • Impractical for investigating spatial and temporal suspended sediment (that is, turbidity), and temperature. variations and water quality trends in large water Other parameters do not change the spectral properties of bodies; reflected light and have no directly detectable signals (for • Insufficient for monitoring, forecasting, and managing example, acidity, chemicals, and pathogens). However, they entire water bodies due to lack of access (for example, may be inferable from detectable water quality parameters due to topography); and with which there is strong correlation. For example, • Prone to field-sampling and laboratory errors, robust correlations have been identified between water impacting accuracy. column reflection (in some cases, emission); physical and biogeochemical constituents (for example, transparency, Remote Sensing of Water Quality chlorophyll-a, and phytoplankton); and the organic matters and mineral-suspended sediments in different water bodies With remote sensing techniques, the changes to water and (Chipman, Olmanson, and Gitelson 2009; El-Din and land can be regularly assessed and predicted over extended others 2013; Giardino and others 2014; Wang and others periods of time and over large areas. Remote sensing-derived 2006). WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 2 TABLE 1. Satellite Sensors Used in The Lake Toba Water Quality and Land Use Change Analysis REVISIT TEMPORAL SATELLITE AGENCY FREQUENCY RESOLUTION (M) NOTES COVERAGE (1/DAY) Landsat-8 (OLI) USGS/NASA 2013–present ~1/16 30 Pan~15 m Sentinel-2A (MSI) ESA 2015–present ~1/10 10 Sentinel-2B (MSI) ESA 2017–present ~1/10 10 2A+2B~5 days Envisat (MERIS) ESA 2002–2012 ~3 300 Sentinel-3A ESA 2016–present ~2 300 3A+3B~1 day (OLCI) EOS Aqua/Terra NASA 2000–present 1~2 250~500 3 hrs offset (MODIS) (Terra) 2002–present (Aqua) SNPP-JPSS NOAA/NASA 2015–present ~1 ~375 (VIIRS) HIMAWARI-8 JAXA 2015–present ~60 (10 mins) ~1000 Geostationary (AHI) Note: Satellites in orange shade indicate sensors for which atmospheric and Rayleigh corrected images and movies were produced; blue shade indicates sensors for which top of the atmosphere images and movies of example dates were produced. hrs = hours; m = meter; mins = minutes. Water quality can be monitored by means of remote and oxygen). Because different phytoplankton species sensing if it has an identifiable optical signature on the exhibit metabolic specialization in regard to preferred spectral reflectance of the water. For example, organic nutrients, phytoplankton concentrations can highlight the and inorganic matter suspended or dissolved in the water distribution and availability of incoming nutrients. changes the optical properties of the water, translating into altered reflectance of electromagnetic radiation reaching Application of Remote Sensing the satellite sensor. Measured for each available image pixel, of Lake Toba the reflectance of water can be mapped, spatial patterns can be analyzed, and concentrations of matter can be inferred. Remote sensing models must be calibrated with in situ Observation of the distribution, abundance, and movement sampling and field surveying to provide quantitative results. of colonies of aquatic plant microorganisms containing Therefore, the most optimal approach is to integrate the use chlorophyll-a pigment, of suspended sediments like silt of remote sensing, in situ measurements, and water quality and sand, and of organic matter from plants and soils is modelling. possible (figure 1). Removing the atmospheric effects and the variations in solar reflectance from the water reflectance An assessment on the potential benefits of using remote signal improves ultimate quantification. sensing to monitor water quality in Lake Toba was made as part of the development of a road map document for Mathematical models relate remote sensing observation improving the water quality of Lake Toba (World Bank with in situ measured concentrations of matter in water. 2018). Specifically, the assessment identified the options Typically, models use several reflectance bands by forming for integrating remote sensing in monitoring as well as an band differences (or ratios), which are best correlated with the application strategy to support the government’s decision measured parameter in question. For example, combinations making for reduced pollution and nutrient emissions. of the red and near-infrared bands can be used to highlight the presence and concentration of chlorophyll-a found in The combination of different sensors and satellites focusing phytoplankton and to study its population dynamics and on water quality parameters (sensors on Landsat and Sentinel metabolism. Variations in phytoplankton abundance can satellites, and MERIS sensor aboard Envisat), land cover be related to nutrient and chemical concentrations that change (MODIS sensor on Terra and Aqua satellites), and sustain them (for example, nitrogen, phosphorus, carbon, weather (sensors on HIMAWARI) allowed an integral view WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 3 FIGURE 1. Algae Blooms (Phytoplankton) and Sediment Plumes Color a Water Surface in this Landsat-8 Image Source: NASA Earth Observatory. of the dynamics in the Lake Toba basin. The tropical climate • Possible contributors to significant water quality events of the Lake Toba catchment, paired with its high altitude on January 9, 2017, in the Bakara/Baktiraja region location, present challenges to remote sensing because of the • Options to overcome data challenges related to prevailing atmospheric conditions of high cloud and haze atmospheric conditions: (a) marine layer-cloud-glint cover. A combination of multiple sensors is, therefore, needed masks and (b) combined water quality products from to obtain enough high-quality, unobstructed observations. multiple sensors Publicly available data from the relatively higher spatial • The ability to obtain multispectral imagery and resolution and lower temporal resolution “land sensors” water quality dynamics on short timescales (daily/ (Landsat and Sentinel 2A) were, therefore, combined with intraday) using a geostationary platform (for example, data from the relatively lower spatial resolution and higher HIMAWARI-8) temporal resolution sensors (Sentinel 3A, MODIS and • Required remote sensing data access, sources, HIMAWARI-8). Obtaining dense time series of usable processing algorithms, and future work for image data will increase with future expansion of earth comprehensive monitoring observation capabilities and satellites. Spatial Patterns and Temporal Variability The assessment on remote sensing of Lake Toba identified the following: The remote sensing assessment for Lake Toba focused on the parameters for turbidity and chlorophyll-a as correlated • Time series estimates of turbidity, chlorophyll-a, and to nutrient concentrations and eutrophication. The results vegetative cover (Sentinel-2A, Landsat-5, Landsat-7, indicate a presence of complex water quality dynamics driven Landsat-8, MERIS, MODIS) by nutrient-oxygen-light mixing. Surface mixing appears to • Long-term changes in water quality across the lake be driven by hydrologic transport (that is, surface runoff), (MERIS time-series ~2006) and specific land use whereas vertical mixing is limited by a weak thermocline (that changes in the Aek Manira/Silang watershed is, temperature gradient in the water body), as is typical of WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 4 tropical lakes. This impacts remobilization of sediments with periodically in the lake with complex local variability runoff events that remobilize nutrients and organic matter. (Landsat-8/Sentinel-2A). To improve the quality of data retrievals, cloud-glint data masking refinements and further A combination of data from satellite sensors was also used to regional tuning are required. The long-term MERIS time construct time-series estimates of turbidity, chlorophyll-a, series and imagery as well as high-resolution (Sentinel-2A/ and vegetation cover in the catchment. A marine layer- Landsat-8) imagery further show a significant increase in cloud-glint mask was designed and applied to address temporal and spatial variability for both chlorophyll-a and data quality challenges. For monitoring weather dynamics turbidity (figure 2). influencing water quality on a short timescale, data from geostationary HIMAWARI-8 were used. A visual analysis of land use in the Lake Toba catchment showed that there were limited large-scale changes in overall Natural and human-induced eutrophication in Lake Toba vegetative cover between 2000 and 2010 (MODIS and is apparent in long-term time-series data, starting around Landsat annual products). The quantified loss in vegetative 2006 (MERIS). An almost twofold increase in regionally cover in the Aek Manira/Silang watershed is estimated to averaged chlorophyll-a from 1~2 mg/m3 to 2~6 mg/m3 and have been approximately 1% per year between 2000 and corresponding twofold increase in light attenuation at 490 2010 and appears to have accelerated from 2013 to 2017 nm (turbidity) was observed. In localized areas, the maximum (Landsat-8; figure 3 and figure 4). turbidity and biological productivity was much higher. The impact of pollution events is also captured by remote Mesotrophic to eutrophic lake conditions with sensing. The event on January 9, 2017, in the Bakara/Baktiraja chlorophyll-a concentrations of 4~30 mg/m3 were detected region southeast of Lake Toba, led to hypoxia (oxygen FIGURE 2. Spatial and Temporal Variations of Turbidity and Chlorophyll-a in Lake Toba a. Turbidity ESA ENVISAT MERIS: L2 kd490 2.5 0.30 NW_tur SE_tur PTAN_tur 2.9 mer-2004-086-tur 2.0 2.8 0.25 1.5 tur [1/m] Latitude [deg] 2.7 0.20 2.6 1.0 0.15 2.5 0.5 0.10 2.4 0.05 0 0 an 0 ay 0 ug 0 ov 0 ar 0 un 0 ct 0 an 0 pr 0 ug 0 ov 0 ar 0 un 0 p 0 an 0 pr 0 ug 0 ov b 0 un p 0 an 0 r 0 Jul 10 v eb 20 Ap 2.3 20 -No 20 Se 20 -Fe 20 -Se 20 O 20 -M 20 -M 20 A 20 A 20 -M 20 3-J 20 -J 20 -J 20 -J 20 -N 20 -J 20 -N 20 -J 20 -N 20 -J -F 20 A 20 -A 20 -A 20 9- 4- 5- 7- 9- 6- 3- 5 7 9 4 6 8 4 6 8 8 0 3 5 7 9 3 5 7 0 0 0 20 98.5 98.6 98.7 98.8 98.9 99.0 99.1 99.2 Longitude [deg] Date b. Chlorophyll-a ESA ENVISAT MERIS L2: OC4 calculated from RRS 25 mer-2005-140-oc4 7 NW_oc4 SE_oc4 PTAN_oc4 2.9 6 20 2.8 oc4 [mg/m3] 5 Latitude [deg] 2.7 15 4 2.6 10 3 2.5 2 5 2.4 1 0 2.3 0 an 0 ay 0 ug 0 ov 0 ar 0 un 0 ct 0 an 0 pr 0 ug 0 ov 0 ar 0 un 0 p 0 an 0 pr 0 ug 0 ov b 0 un p 0 an 0 r 0 Jul 10 v eb 20 Ap 20 -No 20 Se 20 -Fe 20 -Se 20 O 20 -M 20 -M 20 A 20 A 20 -M 20 3-J 20 -J 20 -J 20 -J 20 -N 20 -J 20 -N 20 -J 20 -N 20 -J -F 20 A 20 -A 20 -A 0 20 9- 4- 5- 7- 9- 6- 3- 5 7 9 4 6 8 4 6 8 8 3 5 7 9 3 5 7 98.5 98.6 98.7 98.8 98.9 99.0 99.1 99.2 0 0 0 20 Longitude [deg] Date Source: Oregon State University. Note: In the temporal graphs, the minimum number of pixels in a quadrant required to calculate a valid median was 750. L2 Kd490 = light attenuation at the 490 nm band, a parameter for turbidity in the water column; OC4 = algorithm and band of the electromagnetic spectrum; RRS = remote sensing reflectance. WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 5 FIGURE 3. Land Use Change in the Aek Manira/Silang Watershed Region Surrounding Lake Toba, 2000-10 MODIS MOD44BVegetation L3 250m 90 MODIS Vegetation 85 Yearly L3 product (250m) Percent surface vegetation cover 80 75 70 65 60 55 am amE 50 5 6 5 6 5 6 0 0 0 0 0 0 3/ 3/ 3/ 3/ 3/ 3/ /0 0 0 0 0 /0 4/ 8/ 2/ 6/ 0 10 0 0 0 0 0 20 20 20 20 20 20 Date [YYYY/MM/DD] Aek Manira/Silang watershed am_ptc Linear (am_ptc) amE_ptc Linear (amE_ptc) Note: PTC = percentage tree cover for two parts of the Aek Manira/Silang watershed, where a more rapid reduction is observed in the smaller eastern part (am) compared to the eastern region (amE). deficiency) and fish deaths as was visible in RGB (red, green, What Drives Water Quality in Lake Toba? and blue) images from space using Landsat-8, HIMAWARI-8, and Sentinel-3A (VIIRS not verified; figure 7). Chlorophyll-a Three main drivers combine through a series of complex levels reaching more than 10mg/m3 (>30 locally) and evidence interactions to determine the prevailing water quality in of benthic sediment resuspension (figure 9) can possibly be Lake Toba. They are: aquaculture and caged fish farming; explained by significant precipitation and discharge from land use and land cover changes in the watershed; and the Aek Manira/Silang watershed. Reported meteorological short-term weather dynamics. conditions preceding the significant precipitation event are not available; however, the cloud-cover dynamics visible in Driving Factor 1: Aquaculture (Fish Farming) the HIMAWARI-8 imagery over the period of January 01 to January 08 indicate a period of potentially significant rainfall. Aquaculture started in the northern part of the lake in On January 09, 2017, HIMAWARI-8 observations also show 1996 with a series of floating fish farms at Haranggaol a significant increase in the back-scattering signal in the visible Bay. The industry consists of two commercial large- RGB wavelength bands, consistent with a harmful algae scale fish farms (one partly foreign-owned and the other bloom (HAB) and biological or biogeochemical activity. The domestically owned) and small-scale local operations timing of this event, visible from space, appears correlated with financed by domestic investors and local communities. the dissolved oxygen measurements reported at sub-1 ppm According to the Directorate General of Aquaculture level. This level is a common threshold required to support and Fisheries (2015), there were 23,000 floating fish biological respiration processes. The HIMAWARI-8 signal cages in Lake Toba in 2014. The rapid expansion of dissipates quickly the following day, and the presence of the commercial and local production has resulted in an back-scattering matter in the lake water is not visible in the estimated production capacity of 85,000 tons of fish per January 10, 2017, imagery. year (2015). WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 6 FIGURE 4. Example of Land Use Analysis in the Aek Manira/Silang Watershed Showing Erosion and Deforestation 2013–2017 (Landsat-8) Land-use (Landsat-8) Timeline — Aek Manira/Silang watershed 2013-06-07 2014-03-06 2015-02-21 2016-06-29 2017-01-09 98°30’0’E 98°45’0’E 99°0’0’E 99°15’0’E 3°0'0'' N 3°0'0'' N 2°45'0'' N 2°45'0'' N 2°30'0'' N 2°30'0'' N 2°15'0'' N 2°15'0'' N 98°30’0’E 98°45’0’E 99°0’0’E 99°15’0’E Sources: Geospatial Information Agency [Badan Informasi Geospasial, BIG] 2013; Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGA, AeroGRID, and IGN. WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 7 The process of eutrophication in Lake Toba is primarily Driving Factor 2: Land Changes and driven by nutrient emissions from aquaculture (that is, feed, Non-Point Sources carcasses, and manure from fish). Sixty-eight percent of total phosphorous loads and 76% of total nitrogen loads come Changes in water quality can also be driven by changes from aquaculture (figure 5). These are followed by livestock in land use coverage and non-point sources of pollution. manure and domestic wastewater. Between 2012 and Deforestation can have a particularly important role on the 2016, total phosphorous emissions from aquaculture nearly quantity and quality of the nutrient and sediment runoff from doubled (1,082 to 2,124 tons) according to the Provincial soil erosion into the lake. The analysis of land cover changes Environmental Agency of North Sumatra (Dinas Lingkungan in the watershed using remote sensing show limited regional Hidup-Sumatera Utara). This 2016 phosphorous load changes until 2010 and an accelerated loss of vegetative cover equated to 2.3 m people’s direct wastewater emissions. after 2013 (figure 3 and figure 4). The loss of vegetation could be attributed to deforestation, landslides, or a combination In contrast, phosphorus from domestic wastewater derived of both in the steep terrain locations in the high elevation from the 0.5 m people living in the catchment was 197 tons mountain ridge of the caldera around Lake Toba. in 2016. With some of the waste being retained on land, the domestic contribution to the lake loading is equivalent Driving Factor 3: Weather Dynamics to roughly 0.2 m people. Domestic wastewater nutrient loads remained relatively constant between 2012 and 2016. The availability of meteorological data, especially from Agricultural and other pollutant sources have even smaller high intensity weather events, is limited in the Lake contributions. Toba basin. The nearest weather monitoring station is 80 kilometers north of the lake at the Medan airport. However, remote sensing can partly address the gap in FIGURE 5. Relative Contributions of Total data. The HIMAWARI-8 low-resolution geostationary Phosphorus and Total Nitrogen Loads into satellite provides visual imagery with observations of Lake Toba in 2015 cloud cover once every 10  minutes (figure 6). These can a. Total phosphorus be used as a complement to other observations to increase our understanding of the hydrologic and water quality 0.3% dynamics in Lake Toba. 0.0% 67.8% 0.6% FIGURE 6. Weather Conditions (Cloud 1.8% Cover) over North Sumatra Captured by the 0.0% 11.2% HIMAWARI-8 Satellite, Early January 2017, 0.8% before Storm Event 19.2% 04–00 GMT Day: 09 11–00 WIB b. Total nitrogen Agriculture (crops and vegetables) 15% Sawah 5% 76% Agriculture (crops and vegetables) Aquaculture Domestic Forest Livestock Meadow Paddy field (sawah) Tourism Source: Results from model calculations of pollution loads (World Bank 2018). Source: JAXA 2017. WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 8 When Factors Act Together: Harmful FIGURE 8. Distribution of Chlorophyll-a during Algae Blooms HAB Event in Lake Toba on January 9, 2017 The onset of an HAB and its detection via satellite imagery algal_2 [log10 (mg/m3)] from several sources and sensors illustrate the applicability of remote sensing in the monitoring and forecasting of 0.0 0.1 0.52 1.0 1.48 1.99 2.0 water quality conditions in Lake Toba. An HAB on January 09, 2017, in the Bakara/Baktiraja region southeast of Lake Toba was characterized by hypoxia (oxygen deficiency) and extensive fish deaths. Landsat-8, HIMAWARI-8, and Sentinel-3A images provide evidence and illustrate the scale of the event (figure 7). Chlorophyll-a levels reached more than 10 mg/m3 and in localized areas Chlorophyll-a (algal_2) reached more than 30 mg/m3 (figure 8). Evidence for the resuspension of near-shore sediments can be explained Note: HAB = harmful algae bloom. by significant precipitation and discharge from the Aek Manira/Silang watershed into the lake, and additional FIGURE 9. Distribution of Total Suspended upwelling and vertical mixing with colder runoff water Matter during HAB Event in Lake Toba on and/or wind-driven transport in the surface layers. January 9, 2017 Given agricultural activities in the basin, runoff discharge total_susp [log10(g/m3)] from the storms in the catchment were likely rich in nutrients (nitrogen and phosphorus), organic matter, and 0.0 0.21 0.5 0.75 1.0 possibly fertilizers. The hydrodynamic resuspension of lake bottom sediments due to river inflows may also have been rich in organic matter. Landsat-8 and Sentinel-3A images of the hypoxia and HAB event in Lake Toba are shown in figure 7, figure 8, and figure 9 (total suspended matter, a proxy for turbidity). FIGURE 7. Algae Bloom and Aek Manira/Silang Runoff on January 9, 2017 Note: HAB = harmful algae bloom. In situ meteorological observations capturing storm and rainfall conditions preceding the HAB event are not available, but meteorological conditions are visible from HIMAWARI-8 during the first week of January (figure 6). This sensor thus enabled understanding of the context of meteorological conditions leading to the event. Sediment re-suspension Conclusions and Recommendations Algal-bloom (cyanobacteria? Successful management of Lake Toba will secure the hydrogen-sulfide H2S?) potential of tourism, the productivity of the catchment’s residents and farming, and the long-term economic viability Baktiraja of using the water. However, sustaining the management Aek Manira-Silang (river) runo of Lake Toba is reliant on improved and cost-effective Source: USGS/NASA Landsat-8, 2017. monitoring of water quality. WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 9 The assessment of the role remote sensing can play in water freshwater inflows, and seasonality in lake turnover, which quality monitoring for Lake Toba, and the piloted application can result in HABs. illustrated in this Knowledge Brief, show both relevance and utility for decision makers across public and private sector Establishment of a joint and integrated remote sensing agencies. The analysis of satellite-derived images can help the program as part of multiagency monitoring efforts will understanding of the highly complex and wide-reaching water improve the understanding of specific challenges facing the quality dynamics of the lake. The application confirms that water quality of Lake Toba. Regionally tuned models for high-resolution multispectral instruments (Sentinel-2A and Sentinel-2A and Sentinel-2B, Landsat-8, as well as MERIS, Landsat-8) can derive chlorophyll-a and turbidity estimates, Sentinel-3A, and VIIRS will need to be developed—all along with vegetation loss. To overcome limitations of integrated with available in situ sampling data and model atmospheric conditions, the complex geography of Lake Toba output. Operationalizing water quality monitoring through requires a multisensor approach with the implementation of remote sensing can thereby contribute to decision making advanced cloud-haze-glint filtering. for economic growth and environmental sustainability. Benefits Approach Remote sensing, modeling, and in situ sampling provide a Multispectral remote sensing using publicly available combination of cost-effective and reliable tools and sources satellite data is valuable and provides unique insight into of data to monitor water quality. Capabilities that will be the complex water quality dynamics of Lake Toba. Multiple developed for Lake Toba’s management and monitoring are sensors are needed because temporal coverage is limited transferable to other freshwater bodies and basins across by atmospheric conditions and overpass schedule (for Indonesia. example, the revisit schedule of Landsat-8 and Sentinel 2A land sensors is between five and ten days and about one day The number of publicly available remote sensing for Sentinel 3A but with lower spatial resolution). applications for water quality monitoring is increasing. The global satellite infrastructures that enable remote sensing Developing capabilities in remote sensing requires are long-term assets that are continuously being developed shared investments into computational and information to provide improvements in areal coverage, accuracy, and technology, along with building human resources capacity data availability. Examples of available platforms on the (where online training materials can provide a useful internet for accessing remotely sensed data on water quality resource). A next step should be to explore budgetary are listed here, several of which provide downloadable data requirements for the appropriate level of capital and and training materials: operational investments in servers, data communications infrastructure, and staffing. • The World Water Quality Portal managed by the United Nations Educational, Scientific and Cultural Training of government and research staff should cover: Organization (UNESCO): www.worldwaterquality​ (a) data acquisition from satellite data streams (that is, .org Sentinels 2A, 2B and 3A and Landsat-8); (b) calibration • The Global Earth Observation System of Systems of algorithm parameters to translate satellite reflectance (GEOSS) Platform of the European Space Agency data into water quality values; (c) integration with in situ and the National Research Council of Italy: www​ data sources together with centralized and online accessible .geoportal.org data management sources; and (d) interpretation, analysis, • AquaWatch: www.geoaquawatch.org and mapping techniques through Geographic Information System (GIS) platforms already in use in Indonesia. Delineating accurate nutrient concentration and distributions of pollutant sources in large watersheds such Recommendations on the Way Forward as Lake Toba requires the integration of vast amounts of information. High spatial and temporal resolution of water The development and delivery of calibrated water quality quality key parameters, such as chlorophyll-a and turbidity, data products through remote sensing observations has helps identify combinations of conditions, such as nutrient the potential to provide new and important insights into loading from aquaculture and other pollutants, rainfall, the monitoring and management of water quality in Lake WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 10 Toba. The calibration and integration of remote sensing Lukman (Center of Limnology, Indonesian Institute of data with in situ data and models will be central to the Sciences); Rudi Nugroho, Titin Handayani, Nawa Suwedi, development of a comprehensive water quality monitoring and Agung Riyadi (Centre of Environmental Technology, and management system. Agency for Assessment and Application of Technology); Budi Kurniawan (Directorate for Water Pollution Control, The Indonesian National Institute of Aeronautics and Space Ministry of Environment and Forestry); Hermono Sigit (LAPAN) has demonstrated capabilities in the application (Directorate for Watershed and Aquatic Management, of remote sensing for water quality monitoring. LAPAN’s Ministry of Environment and Forestry); Hidayati Wan, use of the USGS Landsat-8 (OLI) and ESA Sentinel-3, Siti Bayu, and Fauzi Tarigan (North Sumatra Provincial Sentinel-2A, and Sentinel-2B (OLCI/MSI) sensors to Environmental Agency); Baru Panjaitan, Novita R. and obtain turbidity and chlorophyll-a uncalibrated products Serepita Sinurat (Basin Management Centre Sumatra provides a cost-effective option for monitoring water II); Arie Prasetyo (Lake Toba Tourism Area Management quality. These could be further developed by defining a Authority Board); and Astria Nugrahany (Perum Jasa processing chain for remote sensing of a limited list of water Tirta I). The analysis for the road map document was quality products and sensors to produce regular outputs undertaken by Deltares, the World Resources Institute (including uncalibrated outputs) for broader use. The next (WRI), and Royal Haskoning DHV. list of remote sensing products should prioritize Sentinel- 3A, Sentinel-3B, Sentinel-2A, and Sentinel-2B and include The World Bank team included Marcus Wishart (Lead three-band chlorophyll-a products. Water Resource Specialist), Bertine Kamphuis (Senior Successful realization of a comprehensive water quality Private Sector Specialist), and Amy Chua Fang Lim monitoring and management system requires improved (Environmental Specialist) with support from George coordination to facilitate the collation, exchange, and Soraya (Lead Municipal Engineer), Evi Hermirasari (Senior dissemination of data and information. There are a range Urban Development Specialist), Muhammad Halik of other agencies collecting various water quality data, and Rizki (Urban Planning Analyst), and Louise Croneborg- an open access, cloud-based, integrated lake management Jones (Water and Climate Services Consultant). The platform could help realize these objectives. The remote sensing component was coordinated by Aleix development of such a system has national transferability Serrat-Capdevila (Senior Water Resources Management and application given the challenges of water quality Specialist) within the Global Remote Sensing Initiative for management and monitoring of lakes, water bodies, and Water Resources Management, in support of the Indonesia reservoirs across Indonesia. Tourism Development Project, with technical contributions from Ivan Lalovic (Remote Sensing and Water Quality Acknowledgments Specialist), Fernando Miralles-Wilhelm (Remote Sensing and Water Quality Specialist), and Stefanie Herrmann This knowledge brief is part of a broader initiative to support (Remote Sensing and Land Cover Specialist). the government of Indonesia in developing a road map document for improving the water quality of Lake Toba This work received financial support from the Australian as part of the Indonesia Tourism Development Project. Government through the Indonesia Infrastructure Support The work was coordinated by Indonesia’s Coordinating Trust Fund (INIS TF), the Swiss Confederation through Ministry of Maritime Affairs and the Ministry of Public the Sustainable Tourism Development Multi-Donor Trust Works and Housing. The Coordinating Ministry of Fund, and the Global Remote Sensing Initiative for Water Maritime Affairs was led by Ridwan Djamaludin (Deputy Resources Management of the Global Water Security and Minister for Infrastructure) and included Rahman Sanitation Partnership (GWSP). The views expressed Hidayat (Director for Infrastructure Shipping, Fishery, in this publication are the author’s alone and are not and Tourism) and Velly Asvaliantina (Head of Marine necessarily the views of the Australian government or the Tourism Infrastructure Division). The Ministry of Public Swiss Confederation. Works and Housing was led by Hadi Sucahyono (Head of Regional Infrastructure Development Agency) and Please cite this work as follows: World Bank. 2019. included Raymond Tirtoadi (Regional Infrastructure “Better Data, Better Results: Remote Sensing as a Tool Development Agency). Other  government agencies for Monitoring Water Quality in Lake Toba, Indonesia.” participating in the study included Fauzan Ali and World Bank, Washington, DC. WATER GLOBAL PRACTICE | BETTER DATA, BETTER RESULTS 11 REFERENCES Kallio, K., P. Ekholm, S. Salo, O. Pietiläinen, S. Rekolainen, Y. Laine, and M. Joukola. 2000. “Relationship between Chipman J., L. Olmanson, and A. Gitelson. 2009. “Remote Catchment Characteristics and Nutrient Concentrations Sensing Methods for Lake Management: A Guide for in an Agricultural River System.” Water Research 34 (15): Resource Managers and Decision-makers.” Developed by the 3709–16. 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It is your responsibility to determine whether permission is needed for reuse and to obtain permission from the copyright owner. If you have questions, email pubrights@worldbank.org. Ministry of Public Works and Housing of Republic of Indonesia KEMENTERIAN KOORDINATOR BIDANG KEMARITIMAN Swiss Confederation SKU W19005