Seasonal Forecast Models

The relationship between streamflow and ENSO and the serial correlation in streamflow can be exploited to forecast streamflow several months ahead. These relationships are well described in Chiew and McMahon (2003) and demonstrate the statistical significance of the lag correlation of the linear relationship between 3-month streamflow (in Oct-Nov-Dec and in Jan-Feb-Mar) and the SOI value in the previous 3 months in catchments throughout Australia. Using this relationship, we can forecast summer streamflow throughout most of eastern Australia from spring indicators of ENSO. Serial correlation in streamflow must also be considered when forecasting streamflow because it is generally stronger than the streamflow-ENSO relationship and is persistent throughout the year.

To make risk-based management decisions, we must express forecasts as exceedance probabilities (e.g., probability of getting at least 10 pumping days). In this study, exceedance probability forecasts are derived at tributary scale for seven unimpaired catchments in the Murray-Darling basin. The derivation of the forecasts is based on categorization and consequent nonparametric modeling of streamflow distributions and their antecedent conditions (e.g., discrete SOI categories) (see, for example, Sharma, 2000, and Piechota et al., 2001). Catchments were selected because of their relative proximity to the Namoi basin (all within the Murray-Darling basin in New South Wales) and to reflect a range of rainfall-runoff conditions and forecast skills. Proximity to the Namoi basin is to support coupling with the decision-making models that have been developed by Letcher (2002) within the water management regulatory framework in the Namoi basin, although they simulate representative farmer behavior.

Daily streamflow data from the period 1912-1997 are used. The data include extended streamflow estimates using a conceptual daily rainfall-runoff model (Chiew et al., 2002). The catchment locations and long-term average rainfall-runoff characteristics are summarized in Table 1.

Forecasts are made for the number of days in October-February that the daily flow exceeds the two pumping thresholds under consideration. The thresholds are calculated based on flow days only, defined as days when the daily flow exceeds 0.1 mm. The forecast is derived by relating the number of days in October-February that the daily flow exceeds a threshold to explanatory variables available at the end of

Table 1 Summary of Characteristics for Catchments Used in the Analysis

Catchment

410033 Murrumbidgee R < Mittagang Crossing

Catchment and Rainfall-Runoff Characteristics

Runoff % Days

Area Rainfall Runoff Coef.

Lat. Long. (km2) (mm) (mm) (%) 36.17 149.09 1891 882 134 10-15 71

Flow Percentile >0.1 mm Flows (mm)

to On Kj

410047 Tarcutta Ck @ Old Borambola

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