Forecast Model Results

All models exhibit significant skill in the forecast, summarized in Table 12.2. Two measures of forecast skill are used—Nash-Sutcliffe E and LEPS scores— for illustration and broadly to reflect the performance objectives of the modeling (Bennett et al. 2013).

TABLE 12.1

Summary of Characteristics for Catchments Used in the Analysis

Catchment

Catchment and Rainfall—Runoff Characteristics

Lat.

Long.

Area

(km2)

Rainfall

(mm)

Runoff

(mm)

Runoff Coef. (%)

% Days Flow >0.1 mm

Percentile Flows (mm)

20%

50%

410033 Murrumbidgee River at Mittagang Crossing

36.17

149.09

1891

882

134

10-15

71

0.55

0.28

410047 Tarcutta Creek at Old Borambola

35.15

147.66

1660

818

110

10-15

50

0.68

0.31

410061 Adelong Creek at Batlow Road

35.33

148.07

155

1138

256

>20

89

0.97

0.44

412080 Flyers Creek at Beneree

33.50

149.04

98

915

106

10-15

50

0.65

0.29

412082 Phils Creek at Fullerton

34.23

149.55

106

821

124

10-15

62

0.58

0.27

418025 Halls Creek at Bingara

29.91

150.58

156

755

44

6

24

0.22

0.14

421036 Duckmaloi River at below dam site

33.77

149.94

112

967

244

>20

80

0.95

0.40

TABLE 12.2

Summary of Forecast Skills for Catchments Used in the Analysis

Catchment

Forecast Skill

Case

FLOW

SOI

FLOW+SOI

E

LEPS

E

LEPS

E

LEPS

410033 Murrumbidgee

Days >20%

0.35

27.1

0.23

11.6

0.58

41.7

River at Mittagang Crossing

Days >50%

0.36

23.1

0.19

12.2

0.60

39.7

410047 Tarcutta Creek at

Days >20%

0.41

32.8

0.23

17.6

0.57

46.4

Old Borambola

Days >50

0.39

26.2

0.18

11.2

0.50

36.0

410061 Adelong Creek

Days >10%

0.54

41.4

0.16

12.0

0.64

49.5

at Batlow Road

Days >20%

0.63

42.0

0.17

11.1

0.71

50.4

412080 Flyers Creek at

Days >20%

0.34

25.8

0.22

10.2

0.54

37.6

Beneree

Days >50%

0.42

28.8

0.22

10.9

0.56

40.0

412082 Phils Creek at

Days >20%

0.40

19.2

0.22

12.3

0.59

32.1

Fullerton

Days >50%

0.54

30.0

0.22

12.2

0.64

39.7

418025 Halls Creek at

Days >20%

0.13

12.4

0.16

11.7

0.29

26.3

Bingara

Days >50%

0.26

15.3

0.16

13.0

0.44

31.5

421036 Duckmaloi River

Days >20%

0.16

12.3

0.24

13.5

0.45

28.1

at below dam site

Days >50%

0.24

16.7

0.27

17.7

0.51

34.0

Nash and Sutcliffe (1970) provide a measure of the agreement between the "mean" forecast (close to the 50 percent exceedance probability forecast) and the actual number of days in October-February that the daily flow exceeds a threshold. A higher E value indicates a better agreement between the forecast and actual values, with an E value of 1.0 indicating that all the "mean" forecasts for all years are exactly the same as actual values.

The LEPS score (Piechota et al. 2001) attempts to compare the distribution of forecast (forecast for various exceedance probabilities) with the number of days in October-February that the daily flow exceeds a threshold. A LEPS score of 10 percent generally indicates that the forecast skill is statistically significant. A forecast based solely on climatology (same forecast for every year based on the historical data) has a LEPS score of 0.

The LEPS scores in all the forecast models are greater than 10 percent, indicating significant skill in the forecast. The SOI model has similar skill in the seven catchments, with E values of about 0.2 and LEPS scores of 10-15 percent. The FLOW model is considerably better than the SOI model in five catchments (410033, 410047, 410061, 412080, 412082; E generally greater than 0.35 and LEPS generally greater than 25 percent), whereas at the gauge sites of the other two catchments (418025, 421036), the FLOW and SOI models have similar skill. In all seven catchments, the FLOW+SOI model has greater skill than the FLOW or SOI model alone. In the five catchments where the FLOW model has greater skill than the SOI model, the E and LEPS for the FLOW+SOI model are generally greater than 0.5 and 40 percent, respectively

(compared to 0.35 and 25 percent in the FLOW model). In the two catchments where the FLOW model and SOI model have similar skill, the E and LEPS for the FLOW+SOI model are generally greater than 0.3 and 25 percent (compared to less than 0.25 and 20 percent in the FLOW or SOI model alone).

 
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