Introduction
Every rainy season, flooding has become a recurring disaster in many parts of Sumatra. From Aceh to Lampung, reports of rivers overflowing within hours, submerged settlements, and thousands of families forced to evacuate have become increasingly common. Floods now occur more frequently and with greater severity each year. This raises an important question: is this purely a natural disaster, or is human activity playing a significant role?
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| Foto By Detik.com |
In reality, Sumatra has lost more than seven million hectares of forest over the past 30 years (Warsi, 2017). Forest cover that once exceeded 50 percent has decreased dramatically. In provinces such as Riau and South Sumatra, less than 30 percent of primary forest remains (Margono, 2014). Much of this forest has been converted into oil palm plantations, industrial forest plantations, residential areas, and mining zones. Since the 1990s, this transformation has taken place at an alarming pace. Satellite imagery shows how vast stretches of natural forest have been replaced by uniform monoculture plantations.
I believe that there is a direct link between the loss of forests and the increasing frequency of floods in Sumatra. When forests are cleared, the natural system that regulates rainfall is disrupted. The canopy that intercepts rainwater disappears, the deep root networks that absorb and store water are replaced by uniform crops, and the once porous soil becomes compacted by heavy machinery. As a result, the behaviour of rainfall changes entirely. Water flows rapidly across the surface, and the soil loses its ability to retain moisture. These two processes lie at the core of the flooding crisis we are witnessing today.
We must reflect on why short term economic gains are allowed to compromise long-term safety. Timber extraction and oil palm expansion may generate profit for a few years, but the consequences, such as recurrent floods and extreme droughts will burden communities for decades. We are trading environmental stability for immediate economic growth, and now the costs are being paid in the form of destructive floods that inundate homes and claim lives. It is time to understand how forests function as natural infrastructure and what happens when we destroy them.
Hydrological Mechanisms in an Intact Forest Ecosystem
To understand why the loss of forests leads to flooding, we must first recognise how an intact forest functions as a natural water-regulating system. A forest is not merely a collection of trees, but a highly efficient and complex hydrological infrastructure.
The Role of Forests in the Water Cycle
When rainfall occurs in a forested area, the water does not immediately reach the ground. The forest canopy, composed of multiple layers of tree crowns, forms the first barrier that intercepts a significant portion of the rain. This process is known as interception. The forest canopy includes several important subsystems that help manage rainfall, such as interception, light absorption, and the cycling of nutrients and energy (Cisneros Vaca et al., 2018).
The water retained on leaves, branches, and stems partly evaporates back into the atmosphere, while another portion drips slowly onto the forest floor. The rainwater that passes through the canopy then reaches the surface of the forest soil. Here, a thick layer of leaf litter and organic material acts as an additional protective layer. This litter not only shields the soil from the direct impact of raindrops but also slows the movement of water, allowing more time for it to infiltrate the ground.
The next process is infiltration, which is the movement of water from the surface into the soil. Forests and natural vegetation enhance infiltration by reducing soil erosion and improving soil structure (Naharuddin, 2020). Tree roots and accumulated organic matter create conditions that facilitate the infiltration of rainwater into the soil and eventually into groundwater, ensuring a continuous supply of water during dry periods (Bonell and Bruijnzeel, 2005). Extensive and deep root systems create natural channels that allow water to penetrate the soil layers more effectively.
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| Rainfall to Soil Water Cycle Illustration |
In addition to absorbing water, forests also release moisture back into the atmosphere through the process of transpiration. Transpiration refers to the release of water vapour from plants through the stomata of their leaves. Leaf transpiration is the main source of evapotranspiration in forest ecosystems and contributes a significant amount of valuable moisture to the atmosphere. The combined processes of evaporation and transpiration are known as evapotranspiration, which forms one of the most essential components of the water cycle.
Soil Moisture Capacity in Natural Forests
Forest soils possess unique characteristics that enable them to store large quantities of water. Their structure, which is rich in organic matter, loose, and highly porous, allows forest soils to function like a giant sponge capable of retaining substantial amounts of moisture.
The porosity of forest soils can reach between 64 and 83 percent, with low bulk density ranging from 0.374 to 0.619 grams per cubic centimeter (Hairiah, 2001). This means that more than half the volume of forest soil consists of empty spaces that can be filled with either air or water. This condition is markedly different from compacted soils found in agricultural land or plantation areas.
The root systems of trees play a crucial role in maintaining this soil structure. Deep primary roots can extend beyond 12 centimeters, while dense secondary roots spread widely within the upper soil layers. As roots grow and eventually die, they leave behind channels that facilitate water infiltration. These root systems also bind soil particles together, preventing erosion and reducing compaction.
Forest soils enriched with organic matter retain soil moisture exceptionally well. Soil moisture levels in forests can reach between 37 and 82 percent, depending on seasonal conditions and soil type. The water stored within the soil pores is not lost immediately but is held and released gradually over time.
This capacity is known as soil moisture, or the ability of soil to retain water. Forest soils have a high permanent storage capacity, which refers to the amount of soil water at the permanent wilting point, the lowest level at which plants can still extract moisture (Hillel, 1998). In other words, forest soils are capable of storing water even during prolonged dry periods.
Gradual Water Release: The Concept of Baseflow
One of the most critical functions of a forest’s groundwater storage capacity is its ability to release water gradually into rivers. Water retained within the soil and subsurface layers does not flow out all at once; instead, it slowly percolates through aquifers before eventually reaching river channels. This sustained flow of groundwater into rivers is known as baseflow, or the basal flow of a stream.
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| Baseflow Water Release Cycle Illustration |
Baseflow is the primary source of water for rivers during the dry season, when river discharge becomes extremely limited due to the minimal input from rainfall. Upstream watershed areas play a strategic role in regulating the hydrological cycle, particularly in controlling surface runoff, enhancing baseflow, and maintaining stable river discharge.
In intact forest ecosystems, baseflow can contribute 44 percent or more of the total river flow (Nugroho et al., 2021). This means that nearly half of the water flowing in rivers during the dry season originates from groundwater reserves that are released gradually. The high infiltration capacity of forest soils allows water to easily penetrate the subsurface and reach the groundwater system, resulting in substantial storage within groundwater reservoirs. This stored water is then slowly released as baseflow into river channels.
Through this mechanism, rivers flowing through forested areas maintain relatively stable discharge throughout the year. During the rainy season, forests absorb excess rainfall and reduce flooding. During the dry season, the stored water is released gradually, preventing rivers from running dry. This illustrates the forest’s essential role as a natural regulator.
This hydrological system has functioned for thousands of years, creating a stable water balance that supports life. However, when forests are cleared, this entire mechanism collapses. The protective canopy disappears, the once-porous soil becomes compacted, and the capacity to store water declines drastically. As a result, rainfall that should have infiltrated into the soil instead flows rapidly into rivers, triggering floods during the rainy season and severe water shortages during the dry season.
Impacts of Forest Loss on Hydrology
Increase in Surface Runoff
Deforestation also triggers soil compaction due to the use of heavy machinery. Soil that was once loose and porous becomes compacted, reducing pore space and lowering its ability to absorb water. As a result, infiltration rates decline from 11 to 18 millimetres per minute in forested soils to only 8 to 13 millimetres per minute in agricultural or plantation areas (Pristianto, 2023). The impact is evident in the stark difference in runoff volume: forests generate only 8.27 millimetres of runoff, whereas agricultural land produces 127.35 millimetres, or approximately fifteen times greater (Yulianto, 2022).
Various studies confirm that deforestation increases runoff, decreases infiltration, and exacerbates the risks of flooding and erosion. Land-use change to agriculture, settlements, or open land elevates the runoff coefficient because the soil loses its ability to absorb water. Research in several watersheds, including Wailela, demonstrates that population pressure and land conversion directly increase surface runoff discharge.
Decline in Soil Moisture Capacity as a Result of Forest Loss
Forest loss causes a significant reduction in the soil’s ability to retain moisture due to the deterioration of healthy soil structure. Forest soils are highly porous and rich in organic matter, supported by continuous litter accumulation and the activity of soil organisms. Once forests are cleared, the input of organic material stops, leading to reduced porosity and increased bulk density in plantation or developed areas. As a result, the soil becomes more compact, less porous, and far less capable of storing water.
The decline in organic matter further worsens the situation, as organic material is a key component that enhances porosity and water-holding capacity. Within a few years after land clearing, organic matter content can drop by as much as half, followed by a decline in microbial activity and the weakening of soil structure. In addition, the loss of vegetative cover exposes the nutrient-rich topsoil to erosion by rainfall. This erosion transports fine particles away and leaves behind soil that is denser, nutrient-poor, and less capable of absorbing water.
Combined effects of soil compaction, organic matter loss, and topsoil erosion lead to a drastic decrease in soil moisture capacity. While forest soils can maintain high moisture levels, plantation soils retain far less, causing rainwater to no longer be stored in the ground but instead to flow rapidly across the surface. This creates a hydrological paradox in which severe flooding occurs during the rainy season and drought emerges during the dry season, driven by the loss of the soil’s natural water-storing function.
Disruption of River Baseflow
Forest loss directly undermines the soil’s ability to store water. As soil porosity declines and groundwater reserves diminish, baseflow, which is the groundwater derived component of river discharge, drops sharply. In deforested areas, baseflow can contribute more than 44 percent of river flow, but in areas that have experienced deforestation its contribution can drop to only 10–20 percent.(Pristianto et al., 2023). As a result, rivers lose a crucial source of water during the dry season, leading to significant reductions in discharge, and in some cases, streams may dry up entirely. This decline triggers further consequences, including the drying of community wells, disruptions to irrigation systems, and widespread shortages of clean water.
The loss of forest cover also drives extreme fluctuations in river flow. During heavy rainfall, the absence of vegetation prevents water from infiltrating into the soil, causing nearly all rainfall to convert into surface runoff. River discharge rises rapidly, increasing the likelihood of flooding. Conversely, during the dry season, there are no stored water reserves to be released gradually, causing rivers to shrink to critically low levels. This unstable flow regime damages river ecosystems and threatens the livelihoods of communities that depend on reliable water resources.
Moreover, the disappearance of forests eliminates the natural regulatory functions that once maintained hydrological balance. Tree roots, which stabilize soil and prevent erosion, are no longer present. Deforestation accelerates erosion and increases sedimentation in rivers, while a reduction in forest cover of 30 to 50 percent can double the risk of flooding (Restrepo et al., 2015). When forests are lost, the hydrological system shifts from stable to highly unpredictable. The most severe impacts are borne by downstream communities who do not benefit from forest conversion but must endure increasingly frequent and intense floods and droughts.
Systemic Effects of Deforestation on Hydrological Disruption
Deforestation in Sumatra has triggered widespread hydrological changes that directly affect human livelihoods and the integrity of ecosystems. The increasingly frequent flooding is only one symptom of a deeper crisis. As forest cover declines, the soil’s capacity to absorb and store water weakens, leading to higher surface runoff and reduced baseflow. Consequently, rivers experience extreme fluctuations, overflowing during the rainy season and shrinking dramatically during dry periods.
The socioeconomic consequences are substantial. Each flooding event results in financial losses amounting to billions of rupiah, inundating thousands of homes, damaging infrastructure, and disrupting community economic activities. Agricultural land degradation leads to crop failures and the loss of farmers’ income, while interruptions in distribution systems affect strategic sectors, including major commodities such as palm oil and coal. At the same time, loss of life and psychological trauma illustrate the immense social costs of ecosystem degradation.
Ecologically, deforestation accelerates the loss of biodiversity. Many endemic species have experienced sharp population declines due to the shrinking and fragmentation of their habitats, including the Sumatran tiger and elephant. Even protected areas are under pressure, with thousands of hectares of forest within national parks lost in recent years. River degradation through sedimentation further worsens the condition of freshwater ecosystems and threatens aquatic life.
This crisis also highlights a stark climate injustice. Local communities who are not involved in deforestation activities are ultimately the ones who suffer the most from its impacts, including the loss of homes, livelihoods, and access to natural resources. Meanwhile, the profits from forest conversion flow to powerful economic groups, leaving the environmental burden to be borne by vulnerable downstream communities. This situation underscores the urgent need for fair, sustainable, and community-centered forest governance.
Solutions and Recommendations
Addressing the flooding crisis in Sumatra requires an integrated approach that prioritises the restoration of ecological functions and improvements in environmental governance. A key step is to restore forest cover through reforestation and land rehabilitation programmes, particularly in watershed areas. Restoration efforts should use native tree species and be supported by data-driven evaluations to ensure long-term effectiveness. Reforestation in upstream catchments is essential for rebuilding the soil’s capacity to absorb water and stabilise river flows.
In addition to forest restoration, agroforestry offers a sustainable alternative for areas that cannot be fully returned to forest conditions. This system integrates trees with agricultural crops, helping to improve soil health, reduce erosion, and maintain the economic productivity of local communities. Stronger policy support is needed to establish agroforestry as a standard land management practice, replacing monocultures that are more likely to damage hydrological systems.
Integrated watershed management is also urgently needed by strengthening spatial planning, law enforcement, and forest cover monitoring using modern technology. Early warning systems for floods must be enhanced to protect downstream communities. Green infrastructure such as infiltration areas, constructed wetlands, and improved settlement planning in flood-prone zones represents an important adaptation strategy in the face of environmental conditions that have already changed.
Science-based policies and strong political commitment are fundamental to the success of these solutions. Environmental assessments must underpin all land-clearing permits, while incentives for conservation actors and strict enforcement against forest destruction must be strengthened. A shift towards regenerative development is essential to ensure that ecosystem health and community wellbeing progress together. The flooding crisis in Sumatra is a clear signal that systemic change is needed now, before the impacts become even more widespread and increasingly difficult to reverse.
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