## 3 Results
### Data-Model Comparisons and Model Adjustment
#### 3.1.2 Ecosystem CO\({}_{2}\) Fluxes  
Annual totals of GPP, TER and NEP for 1999-2007 derived from EC were compared to simulations using either the PnET or PSIM modules after parameter adjustment (Table 4). The differences between mean measured annual values and those simulated by PnET were less than 15% (\(-8.5\)%, \(-6.7\)% and \(-14.1\)%, for GPP, TER and NEP, respectively). Interestingly, underestimation of GPP (and consequently NPP) occurred particularly in the years 2003 and 2006 and did not fully recover in the subsequent years. Closer agreements were obtained with PSIM, with differences between mean measured and simulated values of less than 1% for each CO\({}_{2}\) flux component, with no particular high divergence in specific years. While PSIM standard deviations were smaller than those for EC measurements, indicating a low sensitivity to inter-annual variations in environmental influences, PnET deviation was similar for TER and NEP but higher for GPP, reflecting an overestimation of drought susceptibility. Figure 2: Daily soil moisture measured at the Tower Site, Straits Inclosure during 1999–2003 and simulated by LandscapeDNDC (with PnET and PSIM as the vegetation modules). Measured data are from a fixed probe at the Tower Site. Statistical analysis of annual GPP, TER and NEP totals for 1999-2007 using ModEval [46] showed lower RMSE values when using the PSIM module (7.8, 10.7 and 19.5% for GPP, TER and NEP, respectively) than when using PnET (14.8, 10.9 and 37.4%, respectively) (Table 5a). However, the RMSE values for both modules were all within the 95% confidence intervals of the measured values, and the t-test showed no significant difference between the mean simulated and measured values. Modelling efficiency (ME) values were negative for these annual PnET simulations and coefficients of determination (CD) \(<\) 1, which indicates that the PnET-simulated annual totals described the data less well than the mean of the observations, and emphasises that the measured inter-annual variations were not well simulated. PSIM also had negative ME values (though closer to zero) but CD values > 1, which indicates that this model described the measured data better than the mean of the measurements. Simulated monthly totals for GPP, TER and NEP show a better fit to measured equivalents with high positive ME, CD > 1, no significant bias and high correlation (Figure 3, Table 5b) than annual data (Table 5a). Monthly GPP was particularly well simulated with an ME of 0.94 using PSIM and 0.92 using PnET. RMSE% values were large, particularly for NEP, but were all within the measured 95% confidence intervals, and t-test of mean difference also showed no significant bias in each case. Whereas PnET simulated a reduced GPP and TER in 2003, which was a dry year with Feb-Oct rainfall 50% of the average for that period and high summer temperatures (maximum 33.8 \({}^{\circ}\)C), the EC measurements indicated that GPP and TER were higher than average. Although there was a reduction in moisture in the upper soil layers (Figure 2), the site has heavy clay soil (see Table 2) and the previous year was wet with an above average annual rainfall of 1094 mm, suggesting trees had access to subsurface water not represented in the model. PSIM simulations showed little change in GPP for 2003 over the previous year. The seasonal patterns of the daily GPP averaged over 1999-2007 were quite well simulated by both PnET and PSIM (Figure 4a). Understorey plants produce leaves earlier than canopy trees, and PSIM is able to model the two plant cohorts separately. However, the averaged data from EC suggest that PSIM overestimated this GPP contribution by a factor of approximately 2 during March, although the overestimation is small in comparison with total annual GPP. For TER (Figure 4b), averaged values from the PnET module were closest to measurements in spring and early summer when respiration increases rapidly during canopy growth (April-July), whereas averaged PSIM values were closest in late autumn and winter (November-March).