I am loading grid dynamically using smart rendering, so on vertical scroll click how can i get start row position.
and how can i change my server requesting URL.
In any moment of time you can call
var state = grid.getStateOfView();
state - current top row
There is an onScroll event, so you can catch the moment when scroll state of grid was changed.
Also, there is onDynXLS event, it occurs when grid request new data from server.
mygrid.loadXML(any custom server side url here);