SWAT for Land Vulnerability Assessment in Wonogiri Dam Catchment

Author: Eko Priyanto

Priyanto, Eko, 2018 SWAT for Land Vulnerability Assessment in Wonogiri Dam Catchment, Flinders University, College of Science and Engineering

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Wonogiri Dam is one of the most important reservoir in Indonesia that plays a key role in flood prevention, irrigation, hydro energy and tourism. However, this multi- purpose dam face a severe silting because of sedimentation from soil erosion and run-off in its catchment. During 30 years period, the capacity of the dead storage decreases significantly less than 40 % and it is predicted that the age of this dam less than 20 years from now. Therefore, identification of soil erosion-prone locations and estimate quantitatively of soil loss with reliable accuracy are important for planning and applying suitable soil and water conservation practices as well as controlling soil erosion. It is also important to consider the source of erosion by using vegetative conservation treatment, which can have a positive impact on the long-term preservation of nature. Moreover, the development of a decision support tool in support of catchment management plans is required that is based on scientific research in land use change and the effect of rainfall intensity on sediment-yield. This plan includes hydrological models and soil and water conservation measures, as can for instance be simulated with the Soil and Water Assessment Tool (SWAT). The main aim of this thesis is to locate and identify areas that are prone to soil erosion in the Wonogiri Dam catchment using a SWAT model and also to estimate runoff and soil loss potential as well as map the vulnerable areas. This map is important for integrated soil erosion reduction planning programme. This study concluded that the severity of the situation in the Wonogiri Dam catchment especially from Keduang watershed is reflected in the model's estimation of soil erosion. An average estimated soil loss of 7.9 t/ha from the land surface of the Keduang watershed was determined for the period from 2007 to 2016, which is below the soil formation rate in the watershed. However, areas of severe erosion are dominated in the north upland sub watersheds where the slopes are steeper than in the southern part of the watershed. Extreme soil erosion rates of more than 60 t/ha/year are estimated for many sub watershed of the Keduang watershed. In order to determine the impact of topography, land use and soils on the estimated erosion, several HRUs were selected and average monthly soil loss was compared. It was determined that slope is the most dominant factor for erosion, followed by land use, rock fragment and soil type. However, for future studies, it is important to include crop species in the SWAT model because most published studies on erosion modelling in Keduang watershed have so far not been including specific crops. In combination with the use of a detailed land use map, this might improve the accuracy of model predictions significantly. Well maintained reverse side bench terraces were implemented and simulated in SWAT by setting the USLE_P factor to 0.2 for agricultural land and rice field sloping more than 8 %. This practice is already used in the Keduang watershed and parts of Wonogiri Dam catchment. The model output suggests that this conservation practice could decrease average erosion rates from the land surface (based on HRU) of the basin from 7.9 to 3.2 t/ha. Increasing efforts in soil conservation are in any case essential to improve the livelihood of the Wonogiri Dam catchment farmers by improving food security.

Keywords: Soil erosion, SWAT, Erosion sensitivity, Land vulnerability

Subject: Hydrology thesis

Thesis type: Masters
Completed: 2018
School: College of Science and Engineering
Supervisor: Prof. Okke Batelaan