## 3 Change Detection Approaches
Static thresholds were used for snow-off day definitions and it is assumed that relevant melt periods are longer than two days. If multiple events occur, the longest period has been selected. This does not account for short term interruptions and thus classifies the end of melt only with respect to the entire spring melt period. More advanced scatterometer-based products include snow covered area and melt area. The snow covered area can be determined with all above mentioned approaches as well as with optical data [37]. The melt area can be derived with the methods of [15] or [12] as they consider beginning and end of spring thaw. This differs from glaciological applications where single days or consecutive days with surface melt need to be identified for melt season length determination (e.g., [8]) as any interruption of melt may impact the mass balance. Surface melt of seasonal snow cover, especially in relation to \begin{table}
\begin{tabular}{l l l l} \hline
**Reference** & **Sensor** & **Method** & **Parameters** \\ \hline
[31] [PERSON] \& \& \& \& \\ [PERSON] 1996 & (C-band) & Location specific summer (July) and winter (February) backscatter & Thaw timing \\
[11] [PERSON] _et al._ & NSCAT & Five day average backscatter and location specific difference from overall mean & Thaw timing \\
[15] [PERSON] _et al._ & NSCAT & Application of similar method as in [PERSON] _et al._ 1996 & Thaw timing \\
[18] [PERSON] _et al._ 2003, & SeaWinds & Diurnal differences with respect to noise and multiple thaw periods & Start and end of major thaw period \\
[16] [PERSON] _et al._ & NSCAT & Extension of [PERSON] _et al._ 1999 & Start, end, primary thaw date \\
[33] [PERSON] _et al._ & SeaWinds & As in [PERSON] _et al._ 2004a & Start, end, primary thaw \\
2004b & QuikSCAT & As in [PERSON] _et al._ 2004a & Start, end, primary thaw date plus autumn \\
[23] [PERSON] _et al._ 2007 & SeaWinds & Fixed threshold for deviation from winter (February) backscatter level & Thaw timing \\
[24] [PERSON] _et al._ 2008 & SeaWinds & Application of method from [PERSON] _et al._ 1999 to average & Snow-off date \\  & QuikSCAT & [PERSON] _et al._ 1999 to average & \\  & & evening backscatter with respect to summer mean values (August) & \\ \hline \end{tabular}
\end{table} 
Table 2: Methods and parameters of spring snowmelt products from spaceborne scatterometers. rain-on-snow events, affects thermal properties of the snow pack and the soil beneath [38]. Even single days of thaw during spring can cause an increase in heterotrophic soil respiration [12]. The primary thaw day [15] extracted for the year 2000 in the circum-boreal and -arctic regions showed good correlations (R = 0.75) with modelled timing of water content increase in the snowpack [34]. The temporal evolution of the area which undergoes snowmelt at a certain time influences the spring river discharge patterns of high latitude basins [39]. Current approaches are not applicable in regions where no continuous snow cover/frozen ground conditions during the winter time exists as they are designed to identify one seasonal thaw event or period only. The presence of snow itself is not considered in all approaches. The presence of melting snow causes decreased backscatter similar to water in both C-band and Ku-band. Multiple thaw periods can occur within the same winter season at all latitudes. This needs to be accounted for independent whether ground thaw or snow thaw should be detected. This has been so far considered in two mapping approaches only [12, 24]. The for QuikSCAT typical variations in noise are only directly accounted for by the TUW method [12, 18]. Noise is in general reduced by averaging over several days. As shown in Figure 3, melt events can also occur during mid winter and are followed by freeze up with Ku-band backscatter increase. These backscatter events are described by [11] for NSCAT.