## 4 Results
### Analysis of SZA Effect in the Expanded Five Subwatersheds  
Based on the above result, we applied this technique to five subwatersheds of the Tarim River Basin during snow-melting periods using the same paired-date difference method (Figure 4). In Figure 4, the abscissa represents the SZA difference between the second day (15 August 2004) and the first day (14 August 2004), while the ordinate represents the valid SCP difference between the second and first days. Each scatter plot shows the change in the SCP difference based on the SZA difference for each of the subwatersheds over 10 hydrological years. Each scatter plot was divided similarly into four quadrants, where spots in the first quadrant represent suitable paired dates that showed an increase in both SZA and valid SCP from Day 1 to Day 2. A similar procedure was applied to the other quadrants. Generally, there is no possibility of snow cover increasing during a snow-melting period. The valid SCP difference between the second and first days at such a time should therefore be negative, and data should plot at negative values on the ordinate. Reassuringly, most spots are distributed within the third and fourth quadrants in Figure 4. However, even though these points indicate that snow cover decreased, the cause cannot be identified with confidence, as the reduction might have been the result of snow cover shrinkage during the depletion period or a decrease in the SZA value. Because it is difficult to distinguish the relative contribution of each mechanism, points plotted within these two quadrants were not analyzed further in this study. Nonetheless, some spots with positive ordinate values highlight an atypical increase in snow cover during the snow-melting period, but without new snowfall. Further investigation revealed more data points in the first quadrant than in the second quadrant, which showed that most of the increases in snow cover occurred when the SZA increased, verifying our initial hypothesis. Points in the second quadrant were interpreted as representing statistical errors or anomalies. Average values and linear-fitting equations were calculated for valid SCP differences between the second and first days in each quadrant. We identified similarities in these data for all scatter plots, with the average first-quadrant values being consistently higher than second-quadrant values, indicating that the valid SCP increased more with increasing values of SZA. The absolute gradients of the fitting lines in the first quadrants were all larger than those in the second quadrants; indeed, the average values or gradients of the slopes for the first quadrant data were typically twice of those in the second quadrant. The linear relationship observed between the increases in valid SCP differences and SZA values in the first quadrant indicated that larger values of SZA lead to overestimation of SCP. Figure 3: Changes in: (**a**) snow cover area calculated from MOD10A1 and HJ-1A/B data; and (**b**) the sensor zenith angle (SZA) value in a typical area of the Tarim River Basin on 14–17 May 2011. Figure 4: Scatter plots of snow cover percentage (SCP) difference versus sensor zenith angle (SZA) difference in five subwatershed areas of the Tarim River Basin over a 10-year period. Average values for the SCP differences and equations for lines of best fit are also given. All the scatter plots contain groups of points that are aligned in columns at distinctly different distances along the abscissa. This is because of the change in the value of the SZA at the same location (i.e., the same pixel in the MOD10A1 data), which is related to the variation in the satellite's orbital pattern. However, the SZA difference between two consecutive days was fixed, and therefore, the pixels in the same subwatershed were aligned by the same process that formulated the columns described above. The spatial areas of the different subwatersheds and the locations of areas where snowfall frequently occurred both affect the distance between these columns and their widths.