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Asian Journal of Agriculture and Development (AJAD) - Call for papers!

Geologic, climatic, and anthropogenic controls on the morphology of the Progo River in Indonesia, and their implications for river management strategies

(Indonesia), Doctor of Philosophy in Environmental Science (University of the Philippines Los Baños)

Dissertation Abstract:

The study examined the geologic, climatic, and anthropogenic factors that influence morphological changes along the Progo River, in the Daerah Istimewa Yogyakarta (DIY) Province, Indonesia, highlighting the impacts of river morphological changes on farming communities, and proposing river management strategies based on river styles framework and climate change projections. The Progo River drains an area of around 17,432 square kilometers, and it is responsible for irrigating the DIY Province region. The morphology of rivers is quite dynamic. Its frequent shifts translate to variable impacts on communities that rely on the river's many services. Therefore, it is essential to understand the drivers of recent river morphological changes.

CAESAR-Lisflood is a modeling approach to correlating the relative impact of climate and land cover changes on watershed morphology and sediment discharge. This modeling software was applied to the study area, and its results were validated by analyzing the effects of geologic, climatic, and anthropogenic changes in river morphology using Google Earth Engine (GEE). A combination of GEE images, a geological map, the Shuttle Radar Topography Mission Digital Elevation Model (SRTM DEM), fieldwork, and petrographic analysis were used to identify river morphological changes ascribed to geological variables. Comparing GEE images of the study area for 2002 and 2022 allowed for the creation of a morphology change map that identified erosion and accretion areas and illustrated changes in river morphology over the twenty-year period. To demonstrate how climate affected the study area throughout the investigation, a normalized difference vegetation index (NDVI) analysis was carried out and compared with annual rainfall information over the same twenty-year timeframe. A correlation graph was produced to assess the degree of causation between the amount of riverbank erosion and the annual precipitation number. Validation was done through fieldwork and interviews. Semi-structured interviews (SSIs) with farmers who worked along the banks of the Progo River were conducted to assess the impacts of river morphological changes on farming communities. GEE images, drone aerial photos, and field observations were used to design the River Styles Framework. The World Bank Climate Change Knowledge Portal's climatic forecasts were used to evaluate the possible effects of climate change in DIY Province.

The results suggest that four geological parameters, namely lithology, geological structure, slope, and grain size of river sediments, influence the morphological changes of the Progo River. Large boulder distributions generally indicate locations more resistant to morphological changes, as larger grain sizes make it harder for river currents to take away from upstream locations. Even in moderately sloping sections, loose sedimentary materials erode more quickly in the lower­reach areas than boulders and bedrock, especially during periods of intense rainfall. As drainage areas expand, the channel slopes become flatter and less erosion-resistant. Limestone, loose sedimentary rocks, and volcanic breccia were among the lithologies found in the study area. Compared to volcanic breccia, limestone is less resistant to river erosion. Ejecta from the Merapi Volcano that erupted in 2010 also deposited significant amounts of loose materials in the river system, contributing to the sediment budget and effecting a relative increase in sediment transport immediately after the episode.

Although climatic factors clearly influence river morphology, the data on the Progo River is very coarse. The only accessible information on precipitation for the period covering 2002 to 2022 is on an annual basis. Therefore, seasonal variability is unaccounted for. Analysis of the change in precipitation for the same period also did not yield any significant increase or decrease. Additionally, based on climate change projections, the expected change in precipitation projected for 2050 will only result in a 0.01 mm increase. Anthropogenic influences such as dam construction, changes in land use and land cover, and quarrying operations appear to influence the morphological variations of the Progo River more significantly across time and space. The GEE images from 2019, which followed the completion of the dam's construction, showed some erosion but no additional accretion compared to the previous year. Accretion on the Progo River negatively impacted farmers in the upper-reach area but benefited farmers in the lower-reach area. Farmers in the upper-reach area typically experience accretion losses. The upper-reach area's use of sand and gravel as accretion materials has decreased soil fertility, resulting in poorer crop yields and revenue. Farmers from the midstream to the downstream areas chose accretion land for growing crops because of the higher fertility of riverbank deposits. The Merapi Volcano eruption resulted in the deposition of these fertile materials due to the combination of organics and fine sediments, such as mud. On the other hand, riverbank erosion has resulted in significant negative effects on farmers due to losses in arable areas and difficulty in obtaining water for irrigation. As an adaptation to such sudden changes, shifts in livelihood options were reported. This mostly involved movement from farming to quarrying. Other adaptation strategies include clearing away sand and gravel that impede plant growth, boosting the fertility of agricultural land by increasing farm inputs, lowering the kolonjono grass cutting height to match the level of floodwaters, excavating wells to irrigate the land, and creating "patusan," or holes, in the land to allow water to seep in during a flood.

This research expands adaptation strategies to include planning farming areas at some distance and elevation relative to the river's normal flood height to minimize contact with accretion materials utilizing geotechnical methods to assess rockfall-prone areas, and implementing sustainable quarrying practices. Additionally, local governments and community leaders can be oriented to understand and consider management responses based on the River Styles Framework. These include adaptation strategies for climate change projections such as river and floodplain rehabilitation and restoration, riparian buffer establishment and restoration, surface and groundwater management adaptation, conservation agriculture, and drought early warning systems.