Estimating the Useful Life of the Sempor Reservoir Using Erosion Modelling

Figures & Tables

Susceptibility level	Erosion rate [t · ha^-1 · a^-1]	Area [km²]	Percentage [%]
Very low	<15	16.098	39.62
Low	15–60	3.820	9.40
Moderate	60–180	5.002	12.31
Severe	180–480	6.157	15.15
Very severe	>480	9.559	23.52
Total		40.635	100.00

Month	Monthly discharge [mln m³ · month^-1]
Month	2017	2018	2019	2020	2021	Average
January	16.550	14.309	17.586	13.622	17.695	15.952
February	11.238	15.589	9.324	12.242	15.549	12.788
March	4.765	14.807	10.392	20.843	9.117	11.985
April	11.047	8.404	2.685	6.525	7.566	7.245
May	1.168	0.716	1.713	8.857	9.502	4.391
June	0.884	2.237	0.094	2.039	3.228	1.696
July	0.009	0.922	0.033	0.386	5.191	1.308
August	0.292	0.010	0.596	0.581	0.317	0.359
September	2.000	0.009	–	0.973	1.482	1.116
October	4.415	0.003	0.033	24.668	4.568	6.737
November	10.140	11.840	1.071	26.127	10.569	11.949
December	13.430	0.229	3.768	18.865	15.547	10.368
Annually discharge [mln m³·a^-1)						85.897

Data	Data source
Depth of reservoir (2023)	Field survey
Sediment’s specific gravity (2023)	Field survey
Digital elevation model	Indonesia Geospatial Information Agency
Land-use data (2020)	Indonesia Geospatial Information Agency
Rainfall data at Sempor, Sampang, Somagede and Kedungwringin Stations (2009–2020)	Water Resources Management Center for Progo-Bogowonto-Luk Ulo; Main Station of Serayu-Opak River Basin

Parameter	Equation
Trap efficiency TE [%]	TE=96.67+(17.82×(lnCI))+(0.569×(lnCI)2)1+(0.154×(lnCI))+(0.0153×(lnCI)2)${\rm{TE}} = {{96.67 + (17.82 \times (\ln {{\rm{C}} \over {\rm{I}}})) + (0.569 \times {{(\ln {{\rm{C}} \over {\rm{I}}})}^2})} \over {1 + (0.154 \times (\ln {{\rm{C}} \over {\rm{I}}})) + (0.0153 \times {{(\ln {{\rm{C}} \over {\rm{I}}})}^2})}}$
Sediment yield G [t·a^-1]	G = SDR × Ea

Reservoir capacity [m³]	Annually discharge [mln m³ · a^-1]	TE [%]
23,090,500.45	85.897	90.09

Dead storage in 2013 [m³]	Dead storage in 2023 [m³]	Dead storage change [10 years m³]	Actual Sedimentation Rate [m³·a^-1]
494,712.98	155,680.22	339,032.75	33,903,275

Landform	Texture [%]			M	a	b	c	K
Landform	Sand	Silt	Clay	M	a	b	c	K
River valley	31	37	32	4,624	4.31	3	4	0.39
Colluvial plain	27	64	7	8,463	3.12	2	4	0.76
Lower slope	25	36	39	3,691	4.05	3	1	0.23
Middle slope	20	72	8	8,464	2.62	2	5	0.83
Upper slope	44	31	25	5,625	3.86	2	2	0.38
Hillcrest	10	20	70	873	4.50	3	4	0.12

Slope [%]	Slope classification	Length and steepness LS	Area [ha]	Percentage [%]
5–15	II	1.20	307.59	7.57
15–35	III	4.25	1,178.64	29.01
35–50	IV	9.50	1,501.23	36.94
>50	V	12.00	1,076.03	26.48
Total			4,063.50	100.00

Land use	C	Conservation management	P	Crop management and soil conservation CP	Area [ha]	Percentage [%]
Conservation forest	0.001	No conservation	1.00	0.001	1,373.11	33.8
Plantation forest	0.2	No conservation	1.00	0.2	627.66	15.4
Mixed cropland	0.1	No conservation	1.00	0.1	957.50	23.6
Settlement	1	No conservation	1.00	1	378.06	9.3
Paddy field	0.01	Traditional terraces	0.40	0.004	291.32	7.2
Paddy field	0.01	No conservation	1.00	0.01	37.92	0.9
Dryland farming	0.7	Contour farming (9–20%)	0.75	0.525	130.47	3.2
Dryland farming	0.7	Contour farming (>20%)	0.90	0.63	267.46	6.6
Total					4063.50	100.0

Rain station	Rainfall erosivity [R]	Area [km²]	R × Area
Kedungwringin	2,467.21	16.18	39,915.92
Sampang	2,437.48	18.27	44,527.47
Sempor	2,176.53	3.85	8,390.13
Somagede	2,828.84	2.33	6,601.82
Total		40.63	99,435.35
Rainfall erosivity (R) of the reservoir catchment area [MJ mm ha⁻¹ h⁻¹ a⁻¹]			2,447.04