Results

Interannual and spatial variability of peak streamflow and its timing

The variability of the spring specific streamflow magnitude (Qmax) observed in the twelve basins is shown in Fig. 3a, while volumetric streamflow values are given in Table 4. The highest specific Qmax values are observed in the more agricultural basins (Acadie, Nicolet and Beaurivage), while the forested basins (Batiscan, Matawin, York and Godbout) have lower specific flows, except Bras-du-Nord (#9) and Ouelle (#8) (Fig. 3a). Qmax is also more variable in more agricultural compared to forested basins, again with the exception of the Bras-du-Nord and Ouelle basins. The seasonality of the spring peak flow is shown in Table 4. For the two northernmost basins (Godbout and York), 90% of the spring flow peaks occurred the latest, in May. For the two southernmost basins (Nicolet and Acadie), melting occurred earlier with 40% of peakflow events occurring in March and 40% in April. For the remaining basins, 65% of the peakflow events occurred during April. The peakflow timing shows pronounced interannual variability as well as spatial variability between basins (Fig. 3b). The general increasing trend from south to north in the peakflow timing also appears clearly. Also, for the three completely forested basins located on the Canadian Shield, i.e., Matawin (#7), Bras du Nord (#9) and Batiscan (#10), and melting occurs later compared to basins at the same latitude with less forested areas, such as Beaurivage (#5: 60% forest cover) and Etchemin (#6: 74% forest cover). Therefore, the spatial distribution of the Qmax timing appears to respond primarily to latitude and secondarily to land cover.

Contribution of snowmelt and rain to flood volumes

The contribution of pre-flood water volumes (snowmelt and rainfall) available for runoff to the total flood volume estimated from the multivariate regression method is illustrated in Fig. 4 for the twelve basins. The rainfall contribution is high for the southernmost Acadie basin (#1) compared to the other basins. While the median rainfall contribution in Acadie (0.25) is only slightly higher than that of the other basins, the interannual variability is large, with the third quartile of the distribution reaching near 0.75, and in some extreme years, rainfall was the sole contributor. For the other basins, the median rain contribution is around 0.20, but can be as high as 0.60, which shows that the rain contribution to the spring flood volume in all basins can be important.
Annual variations in water volume (snowmelt + rainfall) available for runoff explain between 67% and 93% of the interannual variability in flood volumes (Table 5). Rainfall and snowmelt volume variability had a comparable effect on flood streamflow volume for five basins (Nicolet, Acadie, Batiscan, Matawin and Bras du Nord), whereas for the other seven basins, the interannual variability in flood volume is more controlled by snowmelt volume than rainfall (Table 5), without a clear relation with latitude or land cover.

Correlation between antecedent factors and spring flow peak and timing

Correlations between Qmax and the six antecedent factors for the 12 basins are displayed on a correlogram (Fig. 5). Snowmelt intensity (Meltint) is positively correlated with peak flow in all basins (r = 0.23 to 0.64); the correlation is significant (p  < 0.05) in most basins, except for Ouelle, Batiscan and Bécancour. Peak SWE (SWEmax) is also positively correlated with Qmax in all basins; correlations are significant in all basins but Ouelle, Bras du Nord, Beaurivage and Bécancour. Thus, years with higher snow accumulation and faster melting rate (intensity) generally tended to result in higher peak flow. The pre-flood accumulated snowmelt (Meltsum) is positively correlated with Qmax in all basins (r = 0.07 to 0.50) but only significant in five of them. Pre-flood accumulated rainfall and its mean intensity do not show any significant univariate correlation with Qmax, except for the two basins Bras du Nord and Godbout, where Qmax is positively correlated with rainfall intensity (Rainint) and in Famine where Qmax is positively correlated with Rainsum. Soil moisture (Smean) is significantly and positively correlated with Qmax in only four basins (Godbout, Batiscan, Famine and Matawin) and negatively correlated in Bécancour.
The correlation coefficient for Meltint is stronger than for SWEmax in six basins, while SWEmaxis a better predictor in only two basins, Matawin and Batiscan. Overall, the correlation analysis shows that the pre-flood melt rate (Meltint) is the best overall univariate predictor of the spring peakflow, followed by the maximum SWE (SWEmax) and accumulated snowmelt (Meltsum), which is logical given the strongly nival character of the hydrological regime of rivers in Quebec. On the other hand, the correlation coefficients are overall only moderate, suggesting that a combination of several factors would be required to better explain the variability in spring flow magnitude.
For the peakflow timing, the pre-flood accumulated rainfall (Rainsum) significantly controls QmaxTin all basins, except for the three basins Famine, York, and Godbout (Fig.  6). This means that spring flood peaks occur later during years with high rainfall volumes during the pre-flood period. A larger amount of accumulated snowmelt (Meltsum) and a slower melt rate (Meltint) also seem to favor a later occurrence of flood peaks, however with varying levels of statistical significance (Fig.  6). The correlation with soil moisture is not spatially coherent, being significantly anti-correlated with flow timing in two basins (Famine, and Beaurivage) and positively correlated in Ouelle.

Multivariate prediction of spring flow peak and timing

The stepwise multivariate regression models explain 40 to 74% of the variation in Qmax in all basins except in Beaurivage and Batiscan, where the models only explain 20-28% of the variation (Table 6). The snowmelt intensity (Meltint) was the predictor most often retained in the regression models (7/12 basins). It has a positive effect on Qmax, i.e., more rapid melting led to higher peak flow, in all basins but York, where the effect of Meltint was negative. Meltin was notably the sole significant predictor of Qmax in two basins, Acadie and Beaurivage, where it explained 60% of the variability in Qmax for the former but only 20% for the later. Either SWEmax or Meltsum was retained in 7 basins, but never together, as these two variables are partly collinear (r = 0.45-0.69), i.e., thicker snowpacks led to larger pre-flood snowmelt volumes, with a positive effect on Qmax in both instances, i.e., leading to higher flood peaks.
Among the rainfall-related variables, the rainfall intensity (Rainint) was the most frequent predictor (5/12 basins) and the one with the largest overall effect among all predictors. The accumulated rainfall amount (Rainsum) contributed to Qmax variability in four basins, with an overall lesser effect than Rainint. Pre-flood soil moisture was significant in only three basins, with a small and positive effect in Matawin and a counter-intuitive, negative effect on Qmax, (Bécancour and York), i.e., drier pre-flood soils leading to higher peak flows in these two basins.
In order to try improving the prediction performance in basins where the initial predictor set led to a low R2 (e.g., Nicolet, Beaurivage, Ouelle, Batiscan), the maximum rainfall (Rainintmax) and snowmelt (Meltintmax) intensity during the pre-flood period were added as potential predictors, to see if these or any of the other basins were sensitive to the most extreme pre-flood rainfall and snowmelt events. Meltintmax significantly and positively contributed to explain Qmax in two basins only, Nicolet and Ouelle, while Rainintmax also had a large positive effect in Ouelle only. The Beaurivage and Batiscan models were not improved, remaining with low R2 values of 0.20 and 0.28, respectively.
Interannual variations in peakflow timing (QmaxT) were comparatively well explained by a different combination of factors between basins, with adjusted R2 varying between 0.27 and 0.65. (Table 7). Accumulated rainfall and its mean intensity explain most of the variation in all the basins, however with opposites effects. In southern basins, larger rainfall volumes led to earlier flood peaks, while the opposite occurred in more northern basins. Inversely, more intense rainfall events tended to delay flood peaks in southern basins while the opposite occurred in northern basins. Accumulated snowmelt (Meltsum) had a positive effect in five basins across the latitudinal gradient, i.e., more snowmelt delayed flood peaks, except in Ouelle where the effect was opposite. The snowmelt intensity Meltint had a noticeable negative influence on flood timing in southern basins (5/12 basins), i.e., slower melt rates led to delayed flood peaks.