Forecast Model Results

All models exhibit significant skill in the forecast, summarized in Table 2. Two measures of forecast skill are used—Nash-Sutcliffe E and LEPS scores.

The Nash-Sutcliffe E (Nash and Sutcliffe, 1970) provides a measure of the agreement between the "mean" forecast (close to the 50% 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 exceed-ance probabilities) with the number of days in October-February that the daily flow exceeds a threshold. A LEPS score of 10% 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.

Table 2 Summary of Forecast Skills for Catchments Used in the Analysis

Catchment

410033 Murrumbidgee R @ Mittagang Crossing

410047 Tarcutta Ck @ Old Borambola

410061 Adelong Ck @ Batlow Road

412080 Flyers Creek @ Beneree

412082 Phils Creek @ Fullerton

418025 Halls Creek @ Bingara

421036 Duckmaloi River @ Below Dam Site

Forecast Skill FLOW SOI FLOW+SOI

E

LEPS

E

LEPS

E

LEPS

Days

>20%

0.35

27.1

0.23

11.6

0.58

41.7

Days

>50%

0.36

23.1

0.19

12.2

0.60

39.7

Days

>20%

0.41

32.8

0.23

17.6

0.57

46.4

Days

>50

0.39

26.2

0.18

11.2

0.50

36.0

Days

>10%

0.54

41.4

0.16

12.0

0.64

49.5

Days

>20%

0.63

42.0

0.17

11.1

0.71

50.4

Days

>20%

0.34

25.8

0.22

10.2

0.54

37.6

Days

>50%

0.42

28.8

0.22

10.9

0.56

40.0

Days

>20%

0.40

19.2

0.22

12.3

0.59

32.1

Days

>50%

0.54

30.0

0.22

12.2

0.64

39.7

Days

>20%

0.13

12.4

0.16

11.7

0.29

26.3

Days

>50%

0.26

15.3

0.16

13.0

0.44

31.5

Days

>20%

0.16

12.3

0.24

13.5

0.45

28.1

Days

>50%

0.24

16.7

0.27

17.7

0.51

34.0

The LEPS scores in all the forecast models are greater than 10%, 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%. 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%), 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%, respectively (compared to 0.35 and 25% 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% (compared to less than 0.25 and 20% in the FLOW or SOI model alone).

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