1. Subbasin Loss Methods
HEC-HMS

2. Seven Methods Deficit and Constant Green and Ampt
Gridded SCS Curve number
Gridded Soil Moisture Accounting
Initial and Constant
SCS Curve Number
Soil Moisture Accounting

3. Green and Ampt
Theory
Combines unsaturated flow from Darcy’s law with requirements of mass conservation
Initial loss is included to model interception and depression storage
Excess precipitation is computed using Green and Ampt equations after initial loss is satisfied

4. Green and Ampt Input Initial loss Volumetric moisture deficit
Wetting front suction
Hydraulic conductivity

5. SCS Curve Number Theory Equation Empirical method developed by SCS
Estimates excess precipitation as a function of cumulative precipitation, soil cover, land use, and antecedent moisture.
Equation
Pe = (P-Ia)2 / (P – -Ia + S)

6. SCS Curve Number Equation parameters (metric unit)
Pe = Excess Precipitation
P = Accumulated rainfall
S = Potential maximum retention
S = (25,400 – 254 * CN) / CN
Ia = Initial abstraction = 0.2 * S
CN = Curve Number
CNcomposite = sum (Ai * CNi) / sum Ai
CN = 30 (very permeable)
CN = 100 (impervious cover)

7. SCS Curve Number
Required input
Initial loss
Curve number

8. Initial and Constant Basic Concepts and Equations
- The maximum potential rate of precipitation loss is constant througout an event

9. Initial and Constant
The constant loss rate can be viewed as the ultimate infiltration capacity of the soils

10. Deficit and Constant Quasi-continuous model of precipitation loss
Initial loss can recover after a prolonged period of no rainfall

11. Soil Moisture Accounting
Basic Concept and Thory
- Continuous model that simulates both wet and dry weather behavior
- The SMA model represents the watershed with a series of storage layers

12. Pros and cons of HEC-HMS loss models
Initial and constant rate
- ‘Mature’ model that has been used successfully.
- Easy to set up and use
- Model is parsimonious (kurang baik)
- Difficult to apply to ungaged area
- Model may be too simple to predict losses within event

13. Deficit and constant rate
Similar to initial and constant rate
Geen and Ampt
- Parameters can be estimated for ungaged watersheds from information about soils
- Not widely used, not as much experience in professional community

14. SCS CN
- Simple, predictable and stable
- Relies on only one parameter
- Well established, widely accepted
- Predicted values not in accordance with classical unsaturated flow theory
- Rainfall intensity not considered
- Infiltration rate will approach zero during a storm of long duration
- Default initial abstraction does not depend upon storm

15. Perhitungan Losses

17. I a =0.2 S S =254 (100/CN -1) Pe = hujan lebih kumulatif pada waktu t,
P = ketebalan hujan kumulatif pada waktu t,
Ia = abstraksi awal (kehilangan awal), dan
S = potensi retensi maksimum,
Nilai CN bergerak antara 100 (tubuh air) dan 30 (tanah kedap air dengan laju infiltrasi tinggi).

18. Bagaimana Cara Menentukan Curve Number

20. Jenis Tanah

21. Peta Jenis Tanah DAS Waduk Wlingi
Peta Hydrologic Soil Group Waduk Wlingi

22. Peta Hydrologic Soil Group DAS Cimanuk-Cipasang

23. TABLE 5.1 Curve Numbers for Antecedent Soil Moisture Condition II
Hydrologic Soil Group
Land Use Description A B C D
Commercial, row houses and townhouses 80 85 90 95
Fallow, poor condition 77 86 91 94
Cultivated with conventional tillage 72 81 88
Cultivated with conservation tillage 62 71 78
Lawns, poor condition 58 74 82
Lawns, good condition 39 61
Pasture or range, poor condition 68 79 89
Pasture or range, good condition
Meadow 30
Pavement and roofs
100
Woods or forest thin stand, poor cover 45 66 83
Woods or forest, good cover 25 55 70
Farmsteads 59
Residential quarter-acre lot, poor condition 73
Residential quarter-acre lot, good condition 75 87
Residential half-acre lot, poor condition 67
Residential half-acre lot, good condition 53
Residential 2-acre lot, poor condition 63 84
Residential 2-acre lot, good condition 47
Roads 92
Source: From NRCS, 1984.

24. Peta Land Use DAS Brantas-Waduk Wlingi
Peta CN DAS Brantas-Waduk Wlingi

25. Peta Land Use DAS Cimanuk-Cipasang
Peta CN DAS Cimanuk-Cipasang

26. Factors to Convert Curve Number for AMC II to AMC I or AMC III
TABLE 5.2
Adjustments to Runoff Curve Number (CN) for Dry
or Wet Antecedent Soil Moisture Conditions
Factors to Convert Curve Number
for AMC II to AMC I or AMC III
Curve Number (AMC II)
AMC I (dry)
AMC III (wet) 10
0.40
2.22 20
0.45
1.85 30
0.50
1.67 40
0.55
1.50 50
0.62
1.40 60
0.67
1.30 70
0.73
1.21 80
0.79
1.14 90
0.87
1.07
100
1.00