Lake Rawa Pening

Rawa Pening is located in the volcanic Ambarawa Basin in Central Java, ^[1] between Ambarawa and Salatiga. It covers a total area of 2,500 to 2,670 hectares (25.0 to 26.7 km²; 9.7 to 10.3 sq mi).^[2][3] It is shallow and swamplike,^[1] with muddy banks.^[4]

Rawa Pening is estimated to have been formed between 18,000 and 13,500 BC after a period of increased precipitation. It reached its largest size from 11,000 to 9,000 BC but shrank until it reached its current size around 6,000 BC.^[5]

To protect Rawa Pening, the local government has enacted a green belt policy. Several establishments, such as the tourist attraction Kampoeng Rawa, were controversial owing to their construction within this belt.^[6]

One of the major roles of Rawa Pening is as a source of hydroelectricity, with a power station located on the Tuntang River near the lake. Fishing is common as a source of supplementary income, and the water from the lake irrigates nearby rice fields. It has also been cited as being used for flood control.^[4]

Numerous species of aquatic plants live in Rawa Pening, including Panicum repens,^[7] Eichhornia crassipes, Salvinia cucullata,^[8] and Hydrilla verticillata.^[3] Estimates of the total number of species vary, from 20 in 1949 to 19 aquatic and 71 semi-aquatic species in 1972.^[9] According to Soenarto Hardjosuwarno, P. repens is one of the main contributors to the formation of floating islands in the lake,^[7] with M. Soerjani noting that E. crassipes and S. cucullata also contribute.^[8] The aquatic plants, such as H. verticillatta, take root in a large amount of peat (in some places measuring 2 metres (6.6 ft) thick), causing more peat to gather. Eventually, terrestrial plants take root in the floating islands as well.^[10]

According to Tri Retnaningsih Soeprobowati, a researcher at Diponegoro University in Semarang, Rawa Pening could become dry by 2021 due to increasing sedimentation. She reports a total rate of deposition of 270 to 880 kilograms (600 to 1,940 lb) per day, averaging 780 tonnes yearly,^[2] with the water level dropping 29% since 1989.^[3] The overabundance of aquatic plants has been blamed, especially E. crassipes. As such, there are calls for a culling of the E. crassipes population to 20% by 2030.^[3] Other suggestions include the introduction of grass carp to control the plant population and the use of herbicides as last resorts.^[2][3] The destruction of H. verticillatta had earlier been attempted beginning in 1932, originally using mechanical methods but later switching to herbicides, These efforts were discontinued in the 1960s.^[11]