Specific Characteristics and Ecosystem Types

The BS River Basin stretches from the Merapi-Merbabu-Lawu mountainous area down to its estuary in the north-east of Java-Indonesia. The original natural environment is tropical rainforest, with ecosystems ranging from the coastal mangrove forest on the north coast, rocky coastal cliff on the southern coast, low lying tropical forest, to the high altitude rainforest on the slopes of the inland mountainous region. The Java environment and climate gradually alters from west to east. It changes from wet and humid thick rainforest in the western parts to a dry savannah environment in the east, corresponding to the climate and rainfall in the regions. Beside its natural characteristics, Java is also known as the world’s most densely-populated places on the globe.

Figure 5.1. The Map of the Study Area: The BS Basin

.

Source: Hidayat et al. (Center for River Basin Organization and Management, Solo, Indonesia 2008).

Note: The BS Basin comprises three sub-basins, namely the Upper Solo River Basin, the Madiun River Basin, and the Lower Solo River Basin (shown in degraded green colours).

The people that formerly inhabited rainforests altered the natural ecosystems and shaped the landscape by creating rice paddies and terraces to support the growing population and created large settlements, since ancient times. The growing human population has put severe pressure on Java’s wildlife. Rainforests have almost disappeared and now confined to highland slopes and isolated peninsulas. Many of Java’s endemic species are critically endangered and some are already extinct (Whitten et al. 1997). It also led to increasing environmental problems. For example, the BS River Basin is categorized as one of the most critical watersheds in Indonesia (DepKimpraswil 2001).

Since Java is one of the most volcanically active islands in the world, volcanoes play a crucial role in its geological and human history. Volcanoes can be catastrophically hazardous for people living there through the flow of hot, dry particulate material or invisible emission of gasses such as carbon-monoxide, hydrogen-sulphide and sulphur-dioxide or mud-flow called

‘lahar’, and through damaging properties. Lahar causes siltation in reservoirs and ports thus raises riverbeds and causes floods in low lying areas. In contrast, volcanoes also give largely positive impacts because they create lands through lava flows, ash deposits and mud flows.

In this sense, natural erosion provides benefits by forming new lands through depositing volcanic materials as alluvium plains with a thick layer of fertile sediments (Whitten et al.

1997). Based on observation, mud-beds of some rivers are used as paddy-fields during dry season, particularly in Ngawi Regency, as a meeting point from upstream rivers.

Fine volcano ash is transported over great distances from the erupting crater, providing a top dressing of soil-enriching material over a wide area. Therefore these benefits are not limited to the immediate vicinity of the active volcano. Basically, the soil fertility is particularly high in Central and East Java because the volcanoes produce basaltic lavas, and lower in

Mt. Merapi Ngawi

Mt. Mebabu

& Mt.Merapi Mt. Lawu

Mt.Wilis JAVA

West Java because the volcanoes produce more silica–rich andesitic lavas (Whitten et.al.

1997 and World Bank 1990). The study Basin has an advantage in terms of nutrient input from the surrounding active volcanoes.

However, volcano activities can also be an agent for major landscape change.

Geological disasters like volcanic eruptions and earthquakes as short term hazards cannot be avoided, but certain areas are clearly more at risk than others. Erosion is an example for long–term hazards which ultimately have a greater impact than the more dramatic short term hazards, albeit with fewer deaths. In contrast, erosion is relatively avoidable, particularly erosion relating to land practices by people (Whitten et al. 1997).

The BS Basin drains a watershed area of around 1.610.000 ha. It is divided into three sub-basins, namely the Upper Solo River Basin in the west, the Madiun River Basin in the south, and the Lower Solo River Basin in the east (Figure 5.1).

Rivers from these upper streams flow gathering tributaries from steep slopes of volcanic cones of Mt. Merapi (2.914 m a.s.l.), Mt. Merbabu (3.142 m a.s.l.) and Mt. Lawu (3.265 m a.s.l.) meeting in the Ngawi Regency. On the upper land, the soil is volcanic and covered by natural mountain tropical forest. In general, the upstream Basin is characterized by steep slopes (>45%) and about 20% of the total land is very sensitive to soil erosion, about 24% is rather sensitive and the rest is less sensitive. A continuous carriage of a large quantity of eroded volcanic material contributes to a high sediment load in the BS River (DepKimpraswil 2001). According to Erftemeijer and Djuharsa (1988 in Whitten et al. 1996), the annual sediment deposited in this river is estimated to be about 11 million m³. To monitor the level of sedimentation, tapped behind large dams, checks of dams and sluices have been introduced. Meanwhile, the estuary has become very flat. During the dry season tidal influence can be detected 100 km upstream.

The lowland ecosystem from Ngawi Regency starts where the estuary forms an alluvial-plain-ecosystem (DepKimpraswil 2001). For centuries this lowland-ecosystem has been dominated by teak plantations (BPKH IX 2004). The channel of the BS River is prominently low and can have an extensive inundation during floods. Approaching the estuary, vast marshy and swampy areas form the Jero- and Jabung Swamps (DepKimpraswil 2001). The intrusion of sea water to the inland is used for fishponds and salt production (BPKH IX 2004).

Figure 5.2. Natural vegetation Types of the BS Basin

Source: DepKimpraswil (2001) and BPKH IX (2004)

Note: In the BS Basin consists mainly of six natural vegetation types (the locations are indicated with arrows), extending from the mountainous areas (circles) to the estuary (mangrove forest); and climate variation from moist to seasonally dry (deciduous).