Why Do We Measure River Discharge at Pontianak?

1. Where is Pontianak?

Geographical Background

Pontianak, the provincial capital of West Kalimantan in Indonesia, is situated near the equator on the island of Borneo and has been popularly known as "Equator City." The city spans approximately 175 square kilometers, within the vast Kapuas River basin—a low-lying floodplain with occasional heights seldom more than 10 meters above sea level. To the north and south, it merges into lush tropical rainforest, and to the west it merges with mangroves along the Karimata Strait. Its equatorial climate has warmth all the time (average 27°C) and lots of rain, with more than 3,000 mm of rain per year, one of Indonesia's most humid cities. Such abundant rain fuels lush vegetation, ranging from giant dipterocarp trees to vast peat swamps.

Human/Cultural Aspect

Pontianak is a cosmopolitan hub, influenced by Dayak, Malay, Chinese, and Buginese populations. Established in 1771 by Sultan Syarif Abdurrahman Alkadrie, it expanded as a trading harbor for rubber, pepper, and diamonds, with its equatorial position a strategic point for explorers in the 19th century. Cultural diversity is seen in its architecture: Sultan's Palace (Istana Kadriah) is the result of the blending of Malay and Islamic architecture, and the Hakka Chinese community's temples, Vihara Dharma Bhakti, bring colorful pagodas to the horizon. Local culture flourishes in kuda lumping (trance dance) and gamelan performances, while markets overflow with tropical fruits, hand-woven pua kumbu materials, and hot pempek (fish cakes). With over 600,000 residents, Pontianak balances modernity with sound community tradition, centered on the rivers which claim much of its landscape.

Hydrology and Overview of the River

The Kapuas River of Indonesia, 1,143 kilometers long, is Pontianak's lifeblood, originating from the Müller Mountains of central Borneo and flowing to the Karimata Strait. Along Pontianak, it bifurcates into two major channels: the Kapuas Hulu (Upper Kapuas) and Kapuas Hilir (Lower Kapuas), on which city was built between them. Tributaries like the Landak and Melawi Rivers join the system and form a massive drainage system draining 98,000 square kilometers of the interior of Borneo.

To Pontianak, the Kapuas cannot be substituted. It supplies 90% of the city's freshwater, allows river transport (critical for connecting outlying villages), and maintains fisheries that are a source of income for 30% of the populace. It is ecologically vital as the river floodplains and peat swamps are critical carbon sinks and home to endangered species like the Irrawaddy dolphin and proboscis monkey. However, palm oil expansion and basin deforestation have increased sedimentation, while climate change-coarsened season floods pose mounting threats to Pontianak's low-lying residential estates.

2. How is the River Flow near Pontianak?

Influencing Factors

Precipitation and Runoff

Pontianak has an equatorial climate with no fixed seasons, where monthly rainfall is 250–300 mm. This regular drizzle generates steady runoff from surrounding rainforests, sustaining the Kapuas' flow throughout the year—though most intensely during northwest monsoons (November–February), when rains exceed 100 mm per day. River levels rise then in Pontianak by 2–3 meters, and velocities double to 0.8–1.2 m/s, flooding over the floodplains but rarely overflowing urban defenses. Dryer months (June–August) relax currents to 0.3–0.6 m/s, but never to critical levels, because of the vast drainage basin of the Kapuas.

Ground and River Morphology

The Pontianak Kapuas River flows across a flat, alluvial plain with a gradient of barely 0.001%, creating slow, sinuous currents. Within urban areas, the river is 400–600 meters wide, and 10–15 meters deep on average—although channels can reach as much as 30 meters in depth within the main course. Tidal influence from the Karimata Strait extends up to 150 kilometers inland up to Pontianak, generating 1–2 meters of daily variation in water level and reversing currents twice a day. Tidal "bore" generates special problems: saltwater invades up to 5 kilometers into the land at high tide, affecting drinking and irrigation water quality.

Reservoir Operations

Unlike Java's river systems, the Kapuas does not have large reservoirs—its flow is naturally regulated by Borneo's rainforests and peatlands, which are sponges that absorb rain and release it gradually. There are small dams on tributaries like the Landak River (built for hydroelectric power) with localized impacts that are minimal. In 2018, a sudden release from the Landak Dam coinciding with high tides flooded Pontianak's suburbs (as reported by West Kalimantan's Disaster Agency), illustrating the need for flow monitoring to enable even minor interventions.

Historical Hydrological Events

Pontianak's biggest hydrological issue is flooding annually, with standout extreme events. The 2013 floods—spurred by 40 consecutive days of rain—saw the Kapuas level increase by 4.5 meters, flooding 60% of the city, displacing 100,000 people, and costing $50 million in losses (Antara News reported). Roads and water treatment facilities were destroyed, and cholera outbreaks followed.

Droughts are rare but intense: the 2019 El Niño reduced rainfall by 40%, reducing the Kapuas' flow and allowing saltwater to seep 10 kilometers further upstream than previously. Pontianak's water utility was forced to purchase freshwater in barges, while farmers in surrounding regencies lost 30% of their rice crops (per a 2020 Indonesian Ministry of Agriculture report).

3. How is River Flow in Pontianak Monitored?

Traditional Methods

Surface Drift Buoy Method

For centuries, the authorities have used hollowed-out tree trunks to estimate surface velocity by recording their journey time between markers. It did not account for tidal reversals and under-water currents—important in the Kapuas, where variations in depth create distinct layers of flow. Floating detritus (e.g., dead trees) additionally prejudiced results, making forecasts of floods incorrect.

Anchored Boat Method

This technique consisted of mooring ships and dipping mechanical current meters for depth velocity measurement. It provided accurate profiles but was impracticable for the breadth of Kapuas: one cross-river survey required 6 people and 8 hours, with risks from strong tidal currents. In 2010, a boat from one of the teams drifted into a submerged log, halting measurement at a flood peak.

ADCP Introduction

Acoustic Doppler Current Profilers (ADCPs) have revolutionized monitoring in Pontianak since 2015. ADCPs use sound waves to make estimates of velocity all the way through the water column, even in the broad, tidally influenced channels of the Kapuas. A 500-meter reach is surveyed in 45 minutes, with tidal reversals and eddies that cannot be detected by traditional methods. The water agency of Pontianak installed 7 ADCPs, reducing flood response by 35% and improving saltwater intrusion forecasting.

4. How Does ADCP Work?

ADCPs emit high-frequency acoustic pulses (300–1200 kHz) that bounce off suspended particles like sediment and plankton. The Doppler effect—changes in frequency of waves being reflected—discloses water velocity at different depths. Modern 4-beam configurations measure 3D currents, needed to chart the Kapuas' intricate tidal flows, which reverse twice daily in Pontianak.

5. What's Needed for High-Quality Measurements in Pontianak?

• Material Reliability: Resistant to corrosion in the Kapuas' brackish, sediment-laden water; stainless steel or titanium components are necessary.
• Portability: Light (under 8kg) to be deployed from small motorboats along narrow channels between islands in the city.
• Cost-Effectiveness: Affordable enough for West Kalimantan's budget, with long-life batteries (12+ hours) to support extended tidal cycle surveys.

6. Choosing the Appropriate Equipment

Deployment Methods

• Boat-Mounted ADCPs: Suitable for quick flood and tide measurements, plying Pontianak's city river channels.
• Bottom-Mounted ADCPs: Placed close to bridges to maintain 24/7 monitoring of tidal changes, a vital alert mechanism for saltwater intrusions.
• Cableway ADCPs: Employed upstream where the Kapuas constricts, gauging flow in wooded reaches impassable to boats.

Working Frequency

• 600 kHz ADCPs: Suit shallow, 100-meter-wide channels such as the Kapuas Hulu in central Pontianak.
• 300 kHz ADCPs: Suitable for Pontianak's wider Kapuas Hilir (400–600 meters), which measures deeper currents along the city's harbor.

Brand Recommendations

Few of the world ADCP big brands include Teledyne RDI, Nortek, and SonTek, which offer high-quality and reliable products used in various hydrological study and monitoring programs. As a budget - friendly option, the ADCP manufacturer Chinese company’s "China Sonar Panda ADCP" comes highly recommended. Made of all - titanium alloy, it is more strengthful and reliable and is a suitable option to endure the nasty river condition surrounding Thrissur. As an "affordable ADCP", it boasts decent quality performance at budget price. For more information, visit https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.