Soil Moisture and Runoff Processes at Tarrawarra

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Soil Moisture and Runoff Processes at Tarrawarra

240 A Western and R Grayson

Figure 9.14. Simulated and observed runoff hydrographs at Tarrawarra during 1996 for run 5. ‘‘A’’, ‘‘B’’

and ‘‘C’’ identifyevents referred to in the text.

9.14 – ‘‘B’’) and ending on September 12 (Figure 9.14 – ‘‘C’’), the model systematicallyoverestimates

the magnitude of the runoff events.

It should be noted that it is not possible to correct the error in mean runoff by

calibrating the evapotranspiration parameters, as these control the spatial distribution

of evapotranspiration rather than its absolute value during high soil

moisture periods when the runoff occurs. Also it is unlikelythat the potential

evapotranspiration is underestimated during this period, since this was a wetter

than average winter and the estimates used are alreadyslightlygreater than the

available average monthlyregional estimates of potential evapotranspiration

(Wang et al., 1998). Obviously, it is possible to simulate the annual runoff correctlybyadjusting

the total storage in the model, but this results in a poorer

simulation when the timing of runoff events is considered.

The qualityof the runoff predictions can be assessed using the prediction

efficiency(Nash and Sutcliffe, 1970); however, this is verysensitive to anybias

in the runoff predictions. For run 5 the predictive efficiencyis only41%. For run

10, when the deep seepage was calibrated such that the model correctlysimulated

the annual runoff, the predictive efficiencyis 63%. The events in April that the

model failed to simulate (Figure 9.14 – ‘‘A’’) account for half of the remaining

errors. If the runoff events on 17–19 April are ignored, and the deep seepage is

calibrated such that the model correctlypredicts the total runoff in the remaining

events, the predictive efficiencycan be increased to 82%.

9.10 DISCUSSION AND CONCLUSIONS

In this chapter, the Thales model framework has been applied to simulate the

hydrologic behaviour of the Tarrawarra catchment for a period of one year.

Detailed spatial soil moisture patterns, saturation deficit, soils, meteorological

and runoff data are available for this catchment. The simulations have been

compared to spatial patterns of soil moisture in the top 30 cm of the soil profile,

to saturation deficit measurements at individual neutron moisture meter tubes

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