4.0 Estimation of SWE index from Accumulaton and depletion curves :
Accumulation and Depletion curve for any given year is a graphical representation depicting the gradual increase and decreseas of the areal extent of snow cover in the basin due to winter snowpack buildup and summer ablation ( 4), . An Accumulation and Depletion curve is obtained by plotting % SCA on different dates ( during the period February to July ) Against time ( Fig. 3) . Tis % SCA is computed from the digital analysis of all relatively cloud free NOAA- AVHRR data during this period after appropriate geometrical corretion are made. Spectral bands I ( visible ) 3 ( mid IR ) and 4 ( thermal IR ) are used for this purpose . A 1 Km Resolution Digital Terrain Model ( DTM ) is used to extrapolate snow cover under clouds. A limited amount of Landsat MSS/ TM and IRS LISS I/ II data is used to verfy the AVHRR data analysis result since no real time ground verification is possible in snow applications. The percentage SCA statistics of different sub -basin and the entire basin are computed and are added to the already exiting data base.
Figure 3 Snow cover Depletion Curves of Sutlej basin. The initial position and pattern of each curve is indicative of the SWE.
In general , the depletion curve is the cummulative effict of the SWE in the basin the temperature regime , and the under lying terrain. . While developing these curves , care is to be taken to account for possible temporary increase in snow covered area due to snowfall just prior to the date of satellite overpass . Therefore , the contribution of such snowfall to the seasonal snowpack has to be assessed. This years relatively easier in the case of depletion curves for the past years since the hydro- meterological data ( daily snowfall , rainfall, temprature ) is readily available to aid the develoment of these curves. Region - wise depletion of snow cover is also studied to understand the regional distribution of new snowfall and its influence on the SCA of the entire basin which will assist in finalising the depletion curve of the entire basin . It is especially useful in forecast situation in the absence of ground data ( Fig. 4)
Figure 4 Spatial depletion curves of Sutlej basin for the year 1993, showing the differences in snow accumulation and depletion in
differences portions of the basin.
As mentioned in the earlior paragraph, the temperature regime in the basin also affects the depletion of the snow cover , in addition to the the SWS of the basin , In order to study , modified depletion curves are drawn by replacing " Time" by " Accumulated Degree Days " along the X -axis . However, in the case of Sutleg Basin, It has been found that the general trend and relative position of the depletion cruves of the various years do not change compared to the original depletion curves. This may be attributed to the fact that during the initial depletion period , the temperatures in the basin are still quite low, and the differences in the curves are primarily due to the differences in the SWS of the basin.
The actual depletion curves of sutleg basin depleted in Figure in show that the trend of the curves is quite complex compared to the hypothetical curves in Figure 2. However, these curves still confirm the concept that the relative placement of
