Subbasin Loss Methods

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 • 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

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