Soil Moisture and Runoff Processes at Tarrawarra

Soil Moisture and Runoff Processes at Tarrawarra

228 A Western and R Grayson

10 looks at how the simulations are affected by adding deep seepage to match

the mean simulated and observed runoff volumes (i.e. to close the water



The soil moisture pattern results of what we judge to be the most realistic simulation

(run 5) are presented first. Run 5 has spatially uniform soil depths and

parameters and includes potential evapotranspiration that is spatially weighted

according to the potential solar radiation index. Then, the patterns from the

other simulations are compared to this run. Finally, we return to run 5 and

compare simulated and observed saturation deficit time series and catchment

runoff. The runs are described in some detail to highlight how the measured

patterns and other data were used to identify model problems and successes.

Run 5 was judged to be the best by making visual comparisons of the simulated

and observed soil moisture patterns and visual examinations of the spatial pattern

of simulation errors (see Chapter 3, pp. 78–9). The winter and spring periods

were emphasised in this comparison because strong spatial organisation was

observed during these periods and because there was little difference between

the simulations during the summer period. It should be noted that the differences

between the runs were often subtle and that no one run was the best on every

occasion. Some indication of the differences between the runs can be obtained

from Table 9.3, which summarises the simulation error statistically. Three statistics

are shown: the mean error; the root mean squared error; and the error

standard deviation. The root mean squared error incorporates the effect of both

bias and random error, while the error standard deviation is a measure of the

random error.

9.9.1 Soil Moisture Patterns – Run 5

Figure 9.9 shows the simulated soil moisture patterns for run 5 for 1996. Some

care in interpreting the small-scale variability is required when comparing

Figures 9.6 and 9.9. It is clear that the simulated moisture patterns are smoother

than the observed patterns. There are three key reasons for this. First, the soil

parameters are assumed to be spatially uniform in the model, whereas some

small-scale variability would be expected in reality. Second, the observed patterns

contain some measurement error. Third, the model support scale is of the order

of 20 m while the measurement support scale is only 0.1 m. If the observation

support scale was 20 m, the small-scale (< 20 m) variability apparent in the

observed patterns would be averaged out and the observed pattern would appear

much smoother. Thus some of the small-scale variability in the observed data is

not relevant to the model formulation and can be ignored when comparing the

observations and simulations. However, there are some small-scale features that

are critical to the runoff response of the catchment. These are the narrow bands

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